Proceedings of the REGIONAL COLLOQUIUM ON SOIL ORGANIC MATTER STUDIES. Anais do COLOQUIO REGIONAL SOBRE MATÉRIA ORGÂNICA DO SOLO

Transcrição

1 Proceedings of the REGIONAL COLLOQUIUM ON SOIL ORGANIC MATTER STUDIES Anais do COLOQUIO REGIONAL SOBRE MATÉRIA ORGÂNICA DO SOLO October, 18-22,1982 Piracicaba, SP-Brasil cena Centro de Energia Nuclear nu Agricultura CENA/USP PROMO Companhia de Promoção de Pesquisa Científica e Tecnológica do Estado de São Paulo - PROMOCET

2 ?*%,* *., * HT- J» ^"ffi L» October, 18-22,1982 Piracicaba, SP-Brasil Editors CARLOS CLEMENTE CERRI DIVA ATHIÉ DÉCIO SODRZEIESKI cena Centro de Energia Nuclear no Agricultura CENA/USP PROMO Companhia de Promoção de Pesquisa Científica e Tecnológica do Estado de São Paulo - PROMOCET

3 GOVERNO DO ESTADO DE SAO PAULO Governador JOSÉ MARIA MA.RIN Secretário da Indústria. Comércio. Ciência e Tecnologia OSVALDO PALMA UNIVERSIDADE DE SAO PAULO Reitor ANTONIO HÉLIO GUERRA VIEIRA Centro d* Emrgla Nuclear na Agricultura Diretor ENEAS SALATI The Organizing Committee has sent tor publication only the papers received within the established time. Contents and review of papers published In these Proceedings ere of the entire responsibility of the respective authors. Copy of re Proceedings can be requested to: Region! * t 'loqulum on Soil Organic Matter Studies CENA i i ro de Energia Nuclear na Agricultura Av. C i> ti -to a/n' - Caixa Postal 96 Phot «$) Telex: (019) 1097 CENA BR 134Í JC caba, SP BRASIL C f «'nits Organizador do Coloquio encaminhou, para pi."' cação, apenas os trabalhos enviados dentro do prtr.j pré-estabelecldo. O iteúdo e a revisão dos textos dos trabalho» putf cados, nestes Anais, sâo de inteira responsabilidade dor k 'jspectivos autores. Cófi? destes Anais podem ser obtidas mediante soli- mçâo ao: Colòouio Regional sobre Matéria Orgânica do Solo CENA Centro de Energia Nuclear na Agricultura i.i. Centenário s/n. - Caixa Postal 96 Tel: (0194) Telex: (019) 1097 CENA BR Piracicaba, SP - BRASIL Companhia de Promoção de Pesquisa Cientifica e Tecnológica do Estado de Slo Paulo - PROMOCET Printed m Sâo Paulo, October, Impresso em Sio Paulo em outubro de PROMOCET Av. Angélica, andar Sâo Paulo - SP - BRASIL Telex: (011) SEAI BR

4 SPONSORED BY: / PATROCINADO POR: Programa das Nações Unidas para o Desenvolvimento / Agência Internacional de Energia Atômica / Secretaria de Cooperação Econômica e Técnica Internacional PNUD/AIEA/SUBIN Comissão Nacional de Energia Nuclear - CNEN Conselho Nacional de Desenvolvimento Cientifico e Tecnológico - CNPq Empresa Brasileira de Pesquisa Agropecuária - EMBRAPA Secretaria da Indústria, Comércio, Ciência e Tecnologia do Estado de São Paulo Companhia de Promoção de Pesquisa Cientifica e Tecnológica do Estado de São Paulo - PROMOCET PROMOTION OF: / PROMOÇÃO DE: Universidade de São Paulo - USP Centro de Energia Nuclear na Agricultura - CENA ORGANIZING COMMITTEE: / COMITÊ ORGANIZADOR: CARLOS CLEMENTE CERRI Centro de Energia Nuclear na Agricultura CENA/USP PETER B. VOSE Agência Internacional de Energia Atômica AIEA/BRA/5/009 CENA DIVA ATHIÉ Agência Internacional de Energia Atômica AIEA/BRA/5/009 CENA DÉCIO SODRZEIESKI Companhia de Promoção de Pesquisa Científica e Tecnológica do Estado de São Paulo PROMOCET

5 PREFACE Soil organic matte, is an espec'al, complex and little kncwr subject; however, it plays an essential role h plant and animal life development. To take appropriate measures aiming a; the preservation of natural environments or the achievement of a rational handling o* ei.vironi?rsts modified by agricultural c. industrial practices, it will be necessary 'or us to know the subject well. Chemists, biologists, agronomists, ecologists and scientists cf other fields have been studying scil organic matter in the functioning of ecosystems either natural cr disturbed by man. In the last few years advanced laboratories, generally in developed countries, have produced new d=ita on soil organic matter phenomena and its component substances. The chemical structure of these substances and their properties have been investigated by, notably, modelling research. In the field, global balanço caículalions have become even more precise and, dus to the use of isotopes, there has been «b3»ter understanding of organic nv.;ter dynamics. However, what is the situation with regard to the tropics? Are tiitsre specific problems connected with tropical scilr' 1 And, what is the importance of soil organic matter to cultural systems taking.. 3 ^r.r.sideratiun the present economic situation? The objective of th's Regional Culloquium on Soil Organic Matte* Studies /a3 to find answers to the above questions r,nd make an appraisal of the present ntoatior,. Discussions centred on wide top. s: oil organic matter in natural o? lithe -lítturbâd ecosystems; the relationship fxjiwtfon c "íanic matter and agricultural products?'; isotope techniques as a means to improve the study of soil organic matter; how to make better use of organic residues (ayricuftinv,;. industrial and urban). The organization o' this Meeting haci been possible due to the comprehension «nd support from several institutions? i persons. We would like to express our ttonfâ: to the United Nations Development Programme/ International Atomic Energy Agency and Secretaria de Cooperação Econômica e Técnica Internacional - SUBIN -, which have given us the necessary support to gather together scientists from Latin-American countries and lecturers from France, Nigeria and the United States; to the Comissão Nacional de Energia Nuclear (National Commission for Nuclear Energy) - CNEN -. the Conselho Nacional de Desenvolvimento Científico e Tecnológico (National Technological and Scientific Research Council) - CNPq -, and the Empresa Brasileira de Pesquisa Agropecuária (Brazilian Institution for Agricultural Research) - EMBRAPA -, for their financial support; and to the Companhia ds Promoção de Pesquisa Científica e Tecnológica do Estado de São Paulo - PROMOCET -, for their kindness in helping us with the publication of the Proceedings. Our especial thanks go to the British Council for the participation of two lecturers from the United Kingdom. To Dr. Peter B. Vose, of the IAEA Project BRA/5/009-CENA, to Dr. Eneas Salati, Director of CENA, for their understanding, suppct and incentive, and to Dr. Boris Volkoff, of ORSTOM, whose great scientific knowledge has markedly changed the study and knowledge of humus of Brazilian soils and who has also been of invaluable help, our sincere thanks. To Mrs. Diva Athié, Senior Administrative Assistance of the IAEA Project BRA/5/009-CENA, whose active participation in all phases and in every respect, has positively influenced the complete success of the Colloquium, our most especial thanks. Carlos Clemente Cerri IV

6 PREFACIO A matéria orgânica do solo constitui um objeto particular, complexo, ainda não muito conhecido, mas que sempre concorre para um desenvolvimento harmonioso das plantas e da vida animal. É preciso, pois, conhecê-la bem, para se tomar medidas certas, que visem a preservação dos meios naturais ou para se chegar ao manejo racional dos meios transformados pelas práticas agrícolas ou industriais. Quimicos, biólogos, agrônomos, ecólogos e cientistas de outras áreas vêm estudando a matéria orgânica do solo por vários anos e o progresso que vem sendo alcançado ressalta o papel da matéria orgânica no funcionamento dos ecossistemas, tanto naturais como modificados pelo homem. Nos últimos anos, laboratórios avançados, geralmente de países desenvolvidos, trouxeram dados novos a respeito dos fenômenos e das substâncias constituintes da matéria orgânica do solo. As estruturas químicas dessas substâncias e suas propriedades têm sido investigadas através, notadamente, de pesquisas em modelos. No campo, os balanços globais são cada vez mais precisos e, graças ao uso de isótopos, tem havido melhor compreensão de sua dinâmica. Qual seiá, porém, a situação no que diz respeito aos meios tropicais? Existirão problemas específicos ligados aos solos tropicais? E qual a importância da matéria orgânica do solo nos sistemas culturais levando em consideração a atual conjuntura econômica? Este Colóquio Regional Sobre Matéria Orgânica do Solo objetivou encontrar respostas a essas perguntas e fazer um ba'anco da situação atual. Foi feita uma -.aliação dos conhecimentos sübre a matéria orgânica dos solos tropicais, mais particularmente da América Latina, e t luz dos resultados obtidos foram definidas linhas prioritárias de pesquisa. Os debates centraram-se em temas amplos: matéria orgânica dos solos nos ecossistemas naturais ou pouco perturbados; relação matéria orgânica e produtividade agrícola; técnicas isotópicas como meio de estudo da matéria orgânica do solo; como tirar um melhor proveito dos resíduos agrícolas, industriais e urbanos. A realização desta reunião foi possível graças à compreensão e ajuda de diversas entidades e pessoas. Queremos registrar nossos agradecimentos, de maneira simples, mas sincera, ao Programa das Nações Unidas para o Desenvolvimento/Agência Internacional de Energia Atômica e à Secretaria de Cooperação Econômica e Técnica Internacional - SUBIN -, que nos deram o apoio necessário para que pudéssemos reunir neste Colóquio cientistas de países latino-americanos, e conferencistas da França, Estados Unidos e Nigéria; à Comissão Nacional de Energia Nuclear - CNEN -, ao Conselho Nacional de Pesquisa Científica e Tecnológica - CNPq -, e à Empresa Brasileira de Pesquisa Agropecuária - EM3RAPA -, pelo apoio financeiro; à Companhia de Promoção de Pesquisa Cientifica e Tecnológica do Estado de São Paulo - PROMOCET -, que nos deu valioso auxílio com a publicação dos Anais. Nosso reconhecimento especial ao Conselho Britânico por ter patrocinado a vinda de dois conferencistas do Reino Unido da Grã-Bretanha. Ao Dr. Peter B. Vose do projeto AIEA/BRA/5/009-CENA, ao Dr. Eneas Salati, Diretor do CENA, pelo seu elevado espírito de compreensão, apoio e incentivo e ao Dr. Boris Volkoff, da ORSTOM, que com seu elevado conhecimento científico revolucionou o estudo e o conhecimento do húmus dos solos do Brasil, tendo também nos apoiado e incentivado, os nossos agradecimentos. A Sra. Diva Athíé, Assistente Administrativa Superior do Projeto AIEA/BRA/5/009, cuja participação ativa em todas as fases do evento, influiu decisivamente no êxito do nosso Colóquio, o nosso agradecimento muito especial. Carlos Clemente Cerri

7 CONTENTS/SUMARIO Preface Prefácio iv v Session I SOIL ORGANIC MATTER IN NATURAL ECOSYSTEMS 1 Recent observations on humus composition and properties ANDERSON, G 3 Grupos funcionales y propiedades especlrales del humus de suelos de Argentina PANUSKA, E. N.; ROSELL, B. A 11 Estúdio potenciometrico del humus en solventes acuo-orgánicos ROSELL, R. A.; MIGLIERINA, A. M. 19 Relações entre o humus do solo e do ambiente aquático TOLEDO, A. P. P. de 29 Descomposición de gramineas tropicales en sabanas inundables de los llanos venezolanos BASTARDO, H.; RIVERA, C. J.; SANTANA, A 35 Mineralización de fósforo orgânico por actividad microbiana en suelos de sabana y de un bosque de Pimm caribawi var. honduransii, en Venezuela FLORES, D. A.; MALAVE, V.; BASTARDO, H 45 Comparação do humus de capoeira e de floresta natural em Latossolos Amarelos da região Amazônica, Brasil MANAHINO, R.P.; VOLKOFF, B.; CERRI, C. C 51 Descomposición de componentes lignocelu'osicos en sabanas inundables de los llanos venezolanos RIVERA, C. J.; BASTARDO, H 59 Desaparición de matéria orgânica en un bosque cultivado de Pimm carlbam var. hondurtntfs, en Venezuela BASTARDO, H.; ESTEVES. A., rlores, D. A 65 Session II ORGANIC MATTER AND SOIL PRODUCTIVITY 73 Carbon flow in the plant-soil system: a comprehensive approach WAREMBOURG, F. R 75 The need to integrate associative N,-flxation and soil organic matter studies VOSE, P. B 81

8 Relação entre a evolução dos compartimentos húmicos sob cultura e os fatores ffsicos e químicos da fertilidade em diferentes solos tropicais DABIN. B 87 The value of mulches in the management of organic matter in tropical soils AYANABA, A 97 Soil organic matter considerations in agricultural systems of the humid tropics MILLER, R. H.; NICHOLASES, J. J.; SANCHEZ, P. A.; BANDY, D. E 105 Efeito residual do nitrogênio mineral e do biológico fixado por forrageiras, sobre o rendimento do sorgo forrageiro (Sorghum vulgare, Pers.) PAULINO, V. T.; CUNHA, P. G. da; FIGUEIREDO, L. A ; MATTOS, H. B. de 111 Produccíón de trigo en ei noroeste de Uruguay en funcion de la fertilización NPK, contenido en matéria orgânica del suelo y condiciones climáticas RABUFFETTI, A.; LABUONORA, D 117 Matéria orgânica en suelos alofanicos y su effecto sobre Ia disponibilidad de fósforo y microelementos ZUNINO V., H 123 Session III USE OF ISOTOPE TECHNIQUES IN SOIL ORGANIC MATTER STUDIES 131 Use of isotopes in organic matter studies: a discussion illustrated by recent applications WAREMBOURG, F. R Decomposição de matéria orgânica marcada com "C e formação de biomassa microbiana em solo ácido do município de Piracicaba, SP, Brasil CERRI, C. C; VOLKOFF, B.; EDUARDO, B. de P ( 137 J Decomposition of 14 C-labelled plant residues in different soils and climates of Costa Rica GONZALEZ A., M. A.; SAUERBECK, D. R ( 141 Discriminação isotópica do carbono nos húmus de latossofo e podzol da região Amazônica do Brasil.. VOLKOFF, B.; MATSUI, E.; CERRI, C. C.(. 147, Relação "C/ 12 C nos horizontes humfferos superficiais e nos horizontes escuros profundos dos solos de campo e mata da região de Campos do Jordão, São Paulo, Brasil MODENESI, M. C; MATSUI, E.; VOLKOFF, B i ^ 155

9 Contanto Sumário Influência da adição de fontes de carbono sobre a degradação do carbaril em solos HIRATA, R.; LUCHINI, L. C; MESQUITA, T. B.; RÜEGG. E. F Photosynthate consumption and carbon turnover in the rhizosphere depending on plant species and growth conditions SAUERBECK, D. R.; HELAL, H. M.; NONNEN, S.; ALLARD, J.-L ( 165 Avaliação da fixação simbiótica de nitrogênio em soja por marcação da matéria orgânica do solo RUSCHEL, A. P.; FREITAS, J. R. de; VOSE, P. B Utilização de nitrogênio pela soja (Glycin* max) influenciada pela adição de bagaço de cana-de-açúcar BONETTI, R.; SAITO, S. M. T Soil organic matter and nitrogen interaction in a tropical agrosystem. Study by size organic matter fractionation and isotope techniques FELLER, C ; GUIRAUD, G.; GANRY, F Destino do nitrogênio aplicado [(' NH 4 ).SO 4 ] como complementação à vinhaça em um solo ácido de Piracicaba, São Paulo ALMEIDA, M. T.; VICTORIA, R. L; CERRI, C C Estudo da mineralização do "N-orgânico num oxisol (L.E.) e sua absorção por uma gramínea (Melinit minutiflora Beauv.) URQUIAGA C, S.; LIBARDI, P. L.; REICHARDT, K.; PADOVESE, P. P.; MORAES, S. O.; VICTORIA, R. L Ensayo de aprovechamiento de Ia residualidad de un fertilizante nitrogenado [(' NH,),SO,] por un cultivo de trigo mediante ei empleo de una técnica isotopica BUJAN, A.; QUITEGUI, M. I.; QUITEGUI, M. C; GHELFI, L. E.?. de; DEYBE, D fí71 ^^ MS5 Ç7w_ (209 J Secsion IV USE OF ORGANIC RESIDUES IN AGRICULTURE 215 Tecnologia de fabricação de fertilizantes orgânicos CORRÊA, D.; PRESSI, F. P.; JACOMETTI, M. L. G.; SPITZNER Jr., P. I., PAREDES, E. A 217 Utilização do lodo de esgoto na agricultura CARVALHO, P. de C. T, de 223 Utilização de lodo de esgoto como fertilizante BETTIOL, W, 227 Produção de fertilizantes orgânicos a partir do tratamento de efluentes de curtume SANTOS, A. C. dos 233

10 Contonts/Sumtrio Efeito da adição de matéria orgânica e de vermiculita expandida em algumas propriedades de um oxisol com graminea (Melinis minutffiora Beauv.) URQUKGA C, S.; LIBARDI, P. l_; REICHARDT, K ; MORAES, S. O.; PADOVESE, P. P.; COSTA, A. C. S. da 237 Las investigaciones sobre abonos orgânicos en Mexico FERNANDEZ G., R 245 ÍNDICE DE AUTORES 253

11 SESSION I Monday, Oct. 18 SOIL ORGANIC MATTER IN NATURAL ECOSYSTEMS Recent observations on humus composition and properties ANDERSON, G. Grupos funcionales y propiedades espectrales del humus de suelcs de Argentina PANUSKA, E. N.: ROSELL, R. A. Estúdio potenciometrico del humus en solventss acuo-orgánicos ROSELL, R. A.; MIGLIERINA, A. M. Relações entre o humus do solo e do ambiente aquático TOLEDO, A. P. P. de Descomposición de gramineas tropicales en saaanas inundables de los llanos venezolanos BASTARDO.».; RIVERA, C. J.: SANTANA, A. Mineralización de fósforo orgânico por actividad microbiana en suelos de sabana y de un bosque de Pinut caribna var. hondurensls, en Venezuela FLORES, D. A.; MALAVE, V.; BASTARDO, H. Comparação do humus de capof ira e de floresta natural em Latossolos Amarelos da região Amazônica, Brasil MANARINO. R. P.; VCLKOFF, B.; CERRI. C. C. Descomposición de componentes lignocelulosicos en sabanas inundables de los llanos venezolanos RIVERA, C. J ; BASTARDO, H. Desaparición de matéria orgânica en un bosque cultivado de Pinus caribaea var. hondurensis, en Venezuela BASTARDO, H.; ESTEVES. A.; FLORES, D.A. COLOQUIO REGIONAL SOBRE MATÉRIA ORGÂNICA DO SOLO 1

12

13 Recent observations on humus composition and properties ANDERSON, G. INTRODUCTION Comparisons of the chemical nature of the organic matter of soils have rot so far indicated any marked effect of climate. For example such properties as elemental and functional group composition, the nature and quantities of degradation products and infrared spectra were found to be comparable in soils of the West Indies, central Argentina and Canada representing trop'cal, sub-tropical and temperate climates*. More recently Chen, Senesi and Schnitzer 2 have examined the oxidative degradation products of soils from the Mediterranean area and, as with organic fractions from soils in other climatic zones, the major aliphatic products were straight chain fatty acids, and di and tri carboxylic acids. The aromatic products included a range of phenolic and benzene carboxylic acids. There tended to be a slightly lower level of phenolic acids compared with soils from other regions, but it was concluded that the general chemical structures must have been similar. In 1979 Saiz-Jimenez and co-workers 3 examined a soil with prairie vegetation from southern Spain by pyrolysis-mass spectrometry and looked also at the Isolated organic fractions. They, too, found the same degradation products that had been identified in soils from other zones. The pyrogram of the whole soil showed a predominance of peaks derived from complex poiysaccharides. Peaks from proteins and from phenolic and other aromatic-containing materials were also present, but in lesser amounts. The most likely difference between zones, therefore, is not in the nature of the materials present, but in the rate at which transformations occur. This was well illustrated by tests carried out by Jenkinson and Ayanaba who compared the rates of decomposition of 14 C-1abe11ed plant residues under tropical and temperate conditions. Whan labelled ryjgrass was mixed in the soil under field conditions in the forest zone of Nigeria, the decomposition pattern wts virtually the same as that obtained for the same material 1n England, except that It wis four times faster, In Nigeria there was Macaulay Institute for Soil Research, Craigiebuckler, Aberdeen, Scotland. little difference in the rates under shade or in the open, even although soil temperatures were considerably greater in the open. The mean between March and September was 32.9 C in the open and 23.7 C in the shade, but at the lower temperature there was probably less desiccation so that decomposition may have been more continuous but at a lower intensity. About 20% of the labelled carbon remained in the soil after one year. In another test they took a sample of an English soil to Nigeria to carry out the decomposition experiment, and found that the rates were then the same for the soils from both zones. Some time earlier the rate of decomposition of labelled wheat straw in 12 Costa Rican soils was examined by Sauerbeck and Gonzalez 5. The rate was somewhat lower than with the ryegrass in Nigeria, with 23 to 36% of the labelled carbon remaining after 13 months. The soils were more acid, with ph values ranging from 4.8 to 6.1, compared with 6.2 to 7.0 for the Nigerian soils, and this may have been one of the faclors influencing rate. In considering some recent investigations which have thrown new light on the chemical nature and properties of humus, it is therefore assumed that they are relevant to all climatic zones. HUMIC SUBSTANCES AND HUMIFICATION Much useful information about the nature of the constituent building blocks of humic substances has been obtained by degradation studies of the kind carried out by Schnitzer 1 in Canada and Hayes 6 in the U.K. For example, a wide range of aromatic and aliphatic car'boxy- 11c acids have been Identified after oxidation with reagents such as permanganate, and in the case of fulvic add Schnitzer 1 ha:, suggested a structure in which aromatic carboxylic acids are linked together by hydrogen bonds. An earlier suggestion put forward by Haworth 7 suggested that humic add consisted of a wry stable polycychc aromatic core to which was attached proteins, carbohydrates, phenolic adds and othei* components of recent plant and microbial origin. Many aromatic structures reach the soil from decomposing plant and microbial remains COLÔQUIO REGIONAL SOBRE MATÉRIA ORGÂNICA DO SOLO 3

14 and, even before Mich humification has occurred, will be found fn th«usual HA, FA and hunin fractions after alkaline extraction. Evan in well-humified organic matter, the nature of the vegetation from which It Is derived can be deduced by such factors as the Identity»nd proportions of the phenolic adds released on degradation reflecting, for example, the structure of Hgnins originally present. In a review article published in 1978 Paul and van Veen 8 made the statement tiiat aromatic compounds such as complexed phenolic and ca'- boxylic adds comprise 50Í of the total soil carbon. But recent results from various workers have indicated that the proportion of aromatic groupings in the organic metier of some soils is not as high as was former^ thought. The technique which 1s proving most useful in - studies of this kind is nuclear magnetic resonance spectroscopy (NHR). Initially proton NMR was used extensively on soil extracts of various kinds. Grant 9, for example, dissolved most of the organic matter in several British soils by exhaustive extraction and dttected a considerable proportion of polymethylene chains, amounting in one soil to 3ÚÍ of the total. Most workers examining soil humic substances have detected protons of aromatic rings but amounts have been very variable and even in the most highly aromatic humic substances, aliphatic protons are present in higher proportions than aromatic protons 10. More recent investigations have used 13 C NMR and in particular cross polarisation NHR which allows solid samples tc be examined. One advantage of this technique Is the fact that organic materials do not need to be extracted and fractionated, with the consequent risk of artifact formation. In the first reported use of this technique for whole soils by Barron and co-workers 11 in New Zealand, spectra from 6 soils varied from highly aliphatic to moderately aromatic. But although this technique allows a broad differentiation between aromatic and aliphatic carbons, the peaks obtained have very little detail in them. However, resolution can be greatly improved if the sample is spun at a certain angle to the applied field - the so-called magic angle, which is 54 44'. The technique 1s called cross polarisation-magic angle spinning or CP-MAS. Wilson and co-workers 12 ' 13 have examined several soils in New Zealand and concluded that the aromaticity can vary widely even when the parent material and vegetation are similar. At first sight one of the factors which seemed to Make i difference was age (Table 1). In the Reefton chronostquence the Hokitika soil was quite aromatic but Its nuch older coun'erparts were highly aliphatic. But the young Franz X soil was also very aliphatic, and the authors fait that one of the key factors might be related to rainfall ard to the rate of nullification. They suggested that where rainfall was high the soluble phenolics would be quickly leached before extensive nullification had time to occur, thus implying that highly aliphatic soils are not well humified. An alternative explanation is that humification proceeds with or without the presence of aromatic precursors. If they are present, the latter will te incorporated and may constitute a high proportion of the total initially. But the fact that humification in aquatic environments proceeds without them, or with very little contribution from them, seems to indicate that they are not necessary for the process. Since the rate of humification is likely to be one of the factors which depend i-ery much on climate, 1t would be very interesting now to use this technique to compare the aromaticity of a range of tropical and temperate soils, taking into account also such factors as age, vegetation and soil type. Another very useful technique for giving information about some of the structural units in humic substances is pyrolysis, either followed directly by mass spectrometry of the pyrolysis products, or by gas chromatography to separate the components. In the latter case, mass spectrometry can still be used as a further step in the characterisation of the products. As a general rule, C-0, C-N and C-haiogen bonds break most readily during pyrolysis 14. Aromatic rings on the other hand are tery stable. This means that aromatic condensation polymers fragment at the heteroatom between monomers, and electronwithdrawing side-groups on an aliphatic chain such as OH and Cl are readily eliminated as H 2 0 and HC1 with the formation of double bonds. Lignins give a range of phenols and methoxyphenols, the number of methoxy groups depending on the source of the lignin. They are common in pyrolysates of peat and surface organic horizons but decrease as humification proceeds. During pyrolysis of poiysaccharides the chain breaks at C-0 bonds, and the ON groups on the rings eliminate 1n turn as H 2 0 giving double bonds. Furan ring compounds are ultimately formed whether the original ring was furanose or pyranose. Prominent ions have the Table 1. Aroma ic carbon in soils of two chronosequences in New Zealand (From Wilson et at. 12 ). REEFTON CHRONOSEQUENCE AGE PARENT MATERIAL AROMATIC CARBON HOKITIKA IKAMATU 300 years GLACIAL GRAVELS OKARITO FRANZ JOSEF CHRONOSEQUENCE FRANZ X MAPOURIKA GLACIAL MORAINES REGIONAL COLLOQUIUM ON SOIL ORGANIC MATTER STUDIES

15 molecular mass of the appropriate monomer, less 2 or 3 water Molecules, but their amounts decrease as hunification proceeds. In contrast, furan derivatives persist into more humified organic matter, possibly derived from an already modified polysaccharide structure. Pyrolysis has been used by Braceweil and co-workers 15 to examine whole soils and various organic fractions from a number of areas 1n Scotland. One of the fractions analysed by pyrolysis/gc was the brown polymeric material which occurs in the soil solution, and resembles the fulvic acid fraction of the organic matter 15. Several benzene derivatives were obtained, probably derived for the most part from aromatic components in the original structure but it must be remembered that where an aliphatic chain is heavily substituted by electron-withdrawing groups, eliminations lead to a heavily unsaturated chain which can readily cyclise to give aromatic products. For example, aromatic hydrocarbons are the main pyrolysis products of PVC. There were also furan derivatives and these were probably derived from polysaccharide. But one peak of special interest was identified as cyclopentencne. a compsund which has not yet been obtained by pyroly:is of any bio-polymer so far tested. It is, however, formed by pyrolysis of aliphatic polycarboxylic acids having at least four carboxyl groups on consecutive carbon atoms. It is given, for example, by butane tetracarboxylic add. the other main product from this compound being dimethylmaleic anhydride. Both cyclopenten <>. and dimethylmaleic anhydride are formed curing pyrolysis 0'" the water-soluble polymer and of fulvic acij and this is very good evidence for the existence of chains of four or more carbons with carboxyls attached. The amount of cyclopentenone obtained on pyrolysis increases with increasing r.umification. These findings give excellent support to observations made by H.A. Anderson and colleagues in their investigations of synthetic model compounds. Some years ago he synthesized a model polymer, polymaleic acid (PMA), which shows many of the properties characteristic of fulvic acids, and it contains chains of carbon atoms linked to carboxyls. It, too, releases cyclopentenone on pyrolysis 15, its elemental analysis and infra-red spectrum are similar to those of fulvic acid 16, and Malcolm and Vaughan 1? have shown that, like fulvic add, PMA inhibits the activity of a plant phosphatase. Potent1ometr1c tifation data obtained by Young and co-workers 18 suggest that in both fulvic add and PMA the carboxyl groups are organized into clusters, more so In fulvic acid than in PMA, whereas in huntie acid they are much more Isolated. Close proximity of carboxyl croups, as well as providing possible metal-chelate sites, also increases solubility because of the increased ease of 1orizat1on. The stability constant of one complex, with copper, has been examined 19 and Found to be comparable with published values for the Cu-FA complex. One essential property of any model of a humic substance is that it should be very stable in the soil. PMA labelled with C has been used in incubation studies and the loss of CO, monitored with time». When the soil/pma mixture, containing 0.1* PMA, was incubated aerobieally in the dark at about ZO C for 5 weeks, the release of the PMA-bound carbon closely followed that of the Indigenous soil carbon. In a similar experiment 1t was found that only about 6% of the "HA carbon was lost fro» the system 1n one year, showing that the material is Indeed extremely stable. The occurrence of considerable amounts of aliphatic carboxyls in the soil organic matter as a whole has been indicated by the «MR Investigations of Wilson and co-workers 13 in New Zealand (Table 2). They pointed out that much of the aromatic carbon is likely to be linked to hydrogen and oxygen, and therefore the relatively high rarboxyl values suggest that many carboxyl groups must be attached to aliphatic carbons. Among the other components of fulvic acids are phenolic add moeities and poiysaccharides, bui much of the litter can be isolated by chromatography on charcoal or 'Polyclar'2i. After such removal the residual fulvic add does not release any further simple sugars on acid hydrolysis. However, pyrolysis releases furan derivatives, which are characteristic pyrolysis products Of carbohydrates, and in addition, hydrolysis with hydrochloric add releases laevulinic add, a characteristic hydrolysis product of hexose sugars, so It is possible that carbohydrates or closely related structures are still present. If so, they do not respond to the usual colour tests for carbohydrates 22. REACTION OF ORGANIC MATTER WITH METALS The importance of iron and aluminium gels in stabilising organic compounds in the soil cannot be overemphasized. Whether they are present on their own or as coatings on more crystalline clay minerals, they readily adsorb or form complexes with organic components in the soil and protect them from attack. The part they play in Scottish e able soils became evident many years ago when a high correlation was noted between the total organic matter content and the amount of iron and aluminium extractable with an add oxaiate solution 23. This suggested that the so-called 'active' iron and aluminium were more important than crystalline clay minerals in tneir stabilising effects. Later on evidence was obtained of their effect OR one particular class of compound, the inositol phosphates which constitute about one half of the organic phosphorus in Scottish Table 2. HOKITIKA IKAMATU FRANZ X MAPOURIKA TIRAU Aromatic and carboxyl carbon in New Zealand soils (Wilson et al. 13 ) X AROMATIC - C i COOR - C COLÔQUIO REGIONAL SOBRE MATÉRIA ORGÂNICA DO SOLO 5

16 I o CD u c n < Figure 1. Table 3. ji, rptio* 1 of added IHP Content &F IHP * IPP in Acety'acetone extract (9:1) G200 G Concentrated aliquot 300 Fractionation of an acetylacetone soil extract on Sephadex gel columns (Scott and Anderson 25 ). Correlation coefficients between oxalate-soluble Fe and Al in six acidic soils and inositol hexaphosphate (IHP) and pentaphosphate (IPP) values (Anderson et al. 24 ). PH 3 ph 6 Soluble Fe Soluble Al Soluble Fe Al 0.56 U.S7 0.91' Significant ai S.0' level love'. Table *** O.SO '" 0.82' 0.78 Molar ratios of C to Fe and Al in organic fraction:, separated by gel chromatography (See Figure 3). 'Fe 'Al Soil Series CD B C D FOUOIANO (Slate) COUNTESSHELLS (Granite) INSCh (Basic Igneous) soils 24. These are broken down quickly in sand, reieasir; Inorganic phosphate to a growing crop, but in soil they are mineralised very slowly, and accumulate to high levels. The degree of adsorption of added inositol hexaphosphate at ph 3 and ph 6 *as therefore examined in a range of soils whic-i differed in their contents of Fe and Al 25. Up to a certain level of addition the ester was completely sorbed in every case, leaving nothing in solution, and that level varied with the soil. Above that point, soluble complexes were formed so that the absolute level of adsorption decreased. The level of sorption was less at ph 6 than ph 3 but the pattern remained the same. The maximum amounts sorbed were well correlated with acid-oxalate extractable Fe and Al, but Al seemed to be the agent primarily responsible (Table 3) But the total amounts of natural inositol phosphates in these soils were better correlated with iron and it seems likely that the formation of iron complexes is the key to the long-term stability of these compounds in Scottish soils. Because so much of the organic matter is associated with iron and aluminium, many investigations have been made of the effects of romplexing agents as extractants. One example is 0.2 M acetylacetone at ph 3 - which proved effective in extracting organic sulphur components with little risk of artifact formation under such mild conditions 25. The centrifuged extract was passed through Sephadex columns to fractionate the components (Fig. 1). The first was G-200 and this removed, by exclusion, any very high molecular weight components and colloidal material not removed by centrifuging. The soluble material was then put through G-25, yielding a number of fractions, three of which, B, C and D, were analysed for carbon and sulphur 25, but also for iron and aluminium 26. Most of the iron occurred in fraction B and there were two observations which may be of interest. The molar amounts of iron and aluminium were similar in each soil, and the C to metal ratios were also much the same in the three soils, all values being between 15 and 21 (Table 4). This is equivalent to the narrow ratio of 1 metal atom to every 8 to 10 carbon atoms. The stabilising action of aluminium has been demonstrated in some soils in Galicia by Carballas and co-workers 27 ' 28. The organic matter in these soils has a great resistance to microbial attack and it was suggested that this Is due to the formation of organicaluminium complexes from freshly released organic compounds and aluminium from the weathering of the granite from which the soils are derived. To test the effect of aluminium, the soils were incubated with added glucose and maize straw with and without the addition of an aluminium gel. As predicted, the gel brought about a striking drop in the mineralisation rate, amounting to 30 to 40% after 22 days at 28 C. Another part played by metal-organic complexes 1s n the development of podzois. Among the first to suggest a mechanism for the formation of B horizons in podzols was Tairm 29 in 1931 who considered that the Iron and aluminium moved down the profile in the form of inorganic colloids and were precipitated to form the B horizon. Then, by a different mechanism, organic colloids moved down and were precipitated at the top of the sesquioxide layer to form the B n horizon. Tamil's 6 REGIONAL COLLOQUIUM ON SOIL ORGANIC MATTER STUDIES

17 views were not generally accepted, however, because it was felt that such inorganic colloids would not be sufficiently stable or mobile. Subsequently, therefore, mechanisms involving organic matter were postulated, for example, that simple organic completing agents, removed by the rain from living or recently dead vegetation, mobilise iron and aluminium and transport them down the profile. They are then, for one reason or another, precipitated in the B horizon. Another related suggestion is that organic polymers, primarily futvates, are responsible for the coroplexing, transporting and deposition. In podzoi B horizons the ratio of iron and aluminium to organic matter is higher, sometimes very much higher, than can be accounted for in this way but it has been suggested that this can be explained by the subsequent decomposition of some of the organic.natter. Many of the suggested mechanisms were critically reviewed in 1976 by Petersen 30 who concluded that metalorganic complexes were of prime importance. Over the last few years, however, Fô.mer anj his colleagues 31 ha'-e made some observations about the properties of aluminium compounds which have led him to believe that an inorganic migration may in fact predominate during the early stages of podzolisation. He has shown that hydroxyaluminium species react with orthosilicic acid at ph values below 5 to give stable sols or solutions and the material formed has been called protoimogolite, because its infra-red spectrum shows it to be closely related structurally to the iiineral imogolite, and on heating in solution it forms a synthetic imogolite. The clay fractions in Scottish podzol B2 horizons do in fact contain imogolite, sometimes in major amounts. Farmer has also shown that mixed A^C^-F^OySiOj-f^O sols, of comparable stability to prctoimogoiite, can be formed for Fe:Al ratios up to 1:1. He has therefore suggested a possible ssquence of events in podzol formation, occurring in 3 stages. Stage 1: Aluminium and iron are relea'ed from weatherable minerals under attack by simple organic acids and chelating agents. A little may be transported in organic form but such simple organic molecules are quickly broken down by microorganisms. Under suitably acidic conditions, stable protoimogolite or inorganic colloids also containing iron will be formed, moving down the profile and then being precipitated in the B horizon. These solutions cannot, he argues, transport negatively charged organic matter except as minor components adsorbed on the positively charged inorganic colloid. Stage 2: Then with further development of the organic surface layers, colloidal and soluble organic acids, more stable to biological attack, move down through the unreactiye A 2 hirizon until they encounter the sesquioxide coated surfaces in the B horizon and there the humates and some of the fulvates are quickly precipitated, while the remainder of the fulvates penetrate more deeply before being adsorbed. Stage 3: During very wet periods reducing conditions can occur in the Bf, horizons with reduction of Iron to the ferrous form, and some leaching. The iron can then be reoxidised further down the profile forming a thin iron pan at the top of the B 2 horizon, with occasional reducing conditions above it and permanently oxidising conditions below. The resulting impeded drainage gradually changes the iron humus podzol to a peaty podzol. EFFECTS ON PLANT NUTRITION Soil organic matter can affect plant growth and nutrition in a number of ways, both direct and indirect, one examp 1 ; being the effect of organic ligands on the uptake of nutrient cations. In one series of experiments32, wheat seedlings were grown in nutrient solutions to which were added various amounts of HA, FA, a water soluble soil polymer and PMA, and the effect on the uptake of iron by the roots and by the shoots was measured after seven days. With all these materials, uptake by the roots decreased as the ligand concentration increased (Fig. 2). The ph of the nutrient solution was 6.5, at which much of the iron Is in polynuclear form, readily adsorbed on the root surface and the values do not distinguish adsorbed and absorbed iron. The ligands were in all cases competing for iron and reducing the amount adsorbed by the root. With the shoots (Fig. 3), however, the valtis represent iron absorbed and translocated within the plant. Here the fulvate, the watersoluble polymer and PMA all enhanced uptake. The iron was initially complexed by the ligands p 1 é r. k _ i LIGMD COKENTMTKM nj/1 Figure 2. Effect of natural and synthetic organic ligands on the uptake of Fe by wheat seedling roots (Linehan and Shepherds 2 ). Figure 3, Effect of natural and synthetic organic ligands on the uptake of Fe by wheat seedling shoots (Linehan and Shepherd 32 ). COLÔQUIO REGIONAL SOBRE MATÉRIA ORGÂNICA DO SOLO 7

18 and then released either at the root surface or within tht root tissues. It seems unlikely that much of It was translocated 1n complexed form because when labelled 59 Ft and ^-labelled PHA were used, the ratio of iron to ligand was vtry much higher in the shoot than in the original complex. With the kymate, a maxinun was quickly reached; one explanation night be that at low concentrations much more labile complexes are formed than at higher concentrations. Another explanation might be that when excess ligand is present, the dissociation of the complex is depressed. The other ligands - fulvate, the water-extractable polymer and PMA - did not produce a maximum uptake of Fe at the levels used in the experiment. The organic ligands complex many other metals besides iron and aluminium, including biologically important trace elements. The amounts of the polymers do not seem to be dependent on the total amounts of soil organic matter, but rather on the pedological drainage conditions and also on ph. With poorer drainage the amounts increase ard there is some evidence that they also increase with ph over the range 5-7. The prime controlling factor is possibly again the active aluminium, which is lower in poorly drained soils, thus retaining less o.ganic matter in an insoluble form, and its sorting power is greatest under acid conditions 33. There are a number of trace element problems in north-east Scotland and one of the elements concerned is copper. As is the case with iron, translocation of copper from the roots to the shoots of wheat seedlings in nutrient solution is enhanced if organic ligands suet as the water-soluble polymers or PMA are present. Copper complexed by these liganc's can be characterized by EPR spectroscopy and this technique has been used to "stiffly the mechanism of uptake of copper, from such complexes, by a living root 34. By examining the spectrum at intervals it is possible to see the transition of the spectrum from that of soil organic IT tter-bound copper to an ami no acid complexed form, suggesting that amino acids in the root exchange with the soil polymer. It seems likely that the copper ami no acid complex is formed within the cytoplasm, and it is possibly translocated in this form also. However, i1 water culture, some of the ami no acid copper complexes diffuse quickly away from the root whereas if the roots are held in moist air, the complexes build up to high levels in the free space of the root. In agreement with this, if plant roots are briefly exposed to labelled micro-nutrients, the adsorbed nutrients are much more quickly translocated to the shoots if the roots are held in moist air rather than in water culture 35. Rate of uptake may thus be very much dependent on soil physical conditions. RELATIONSHIPS BETWEEN POLYSACCHARIDES AND SOIL STRUCTURE There is no doubt that soil organic components play an Important part 1n binding soil particles together into stable aggregates or crumbs which provide a well ordered, well drained environment for plant growth, make the land easier to cultivate and reduce the risk of erosion. This 1s supported by the fact that high positive correlation coefficients have been found between aggregate stability and organic carbon 21. Several workers have produced evidence over the years that polysaccharides are among the most important components having this effect, but others disagree 21. Some of the arguments are concerned with the effects on aggregation of chemical treatments which are intended to destroy carbohydrates, for example period*te, a reagent whicn oxidises compounds containing cis-hydroxyl groups. In some cases such treatment was found to cause virtually complete degradation of crumb structure, while in others it had little effect. But then it was noticed that carbohydrate in soils is nuch more resistant to periodate attack than a 'pure* product, possibly due in part to its chemical structure, but more likely due to the protective action of inorganic components. Longer periodate treatments were then used, but again with mixed effects on aggregation. But few people seem to have examined the soils for carbohydrate residues after treatment. Cheshire and co-workers 36 have now done this. Water-stable aggregates were measured after shaking for 25 mins. by plotting turbidity against time. The sane was done after ultrasonic treatment for 5 mins., and the data taken as representing 100% disaggregation. Samples were also tested in this way after oxidation with periodate and the degree of disaggregation related to these results. The first thing they did was to test some of the oxidation treatments used by other authors, which had only partially affected aggregation. Using such treatments on one of their own soils, they found the disruption varied from 495! to 73%, but the sugar residue was consiaerable, from 65% to 39% of the original. They then went on to test aggregate stability and sugar residues after various periods of periodate treatment. The first soil tested,of the Craigiebuckler series, gave a good relationship between the residual reducing sugar content of the soil and the extent of disruption of the aggregates. Other soils gave similar relationships, regardless of cropping history. Some were in cereals, whereas others had been under grass for nany years. The aggregates tested by Cheshire and coworkers were less than 50ym in diameter, that is they fell into the microaggregate class as defined by Edwards and Bremner 37 who considered that the basic structural units in soils were microaggregates less than 250ym in diameter and that these consisted of clay-polyvalent metal-organic matter complexes. The aggregating effect of humic substances will be of this nature, but in the case of polysaccharides it is thought to be due to direct sorption of the polymer itself, acting like a kind of glue. In a recent article reviewing work in this field, Tisdall and Oades^e from Australia considered the factors affecting the stability of micro and macro-aggregates. They concluded that micro-aggregates are stabilized against disruption due to wetting and drying, and mechanical ef.fects, by several mechanisms in which organo-minerai complexes play a dominant role, but polysaccharides also play a part. The binding of micro-aggregates is not Influenced by changes In the organic matter content of the soil caused, for example, by different crop rotations. In contrast, they concluded that the water stability of macroaggregates depends largely on growing roots and fungal hypha», and thus on living root systems. When the roots and hyphae decompose and are not replaced, the organic matter content and the numbers of stable macroaggregates decline. The stabilization of macro-aggregates 1s therefore controlled by S REGIONAL COLLOQUIUM ON SOIL ORGANIC MATTER STUDIES

19 manaoenent, is increased under pasture and declines under arable cropping. As far as the stability of raicro-ao^r^tc is concerned, however, th? data of Che5*iioanJ co-xorkers3é indicates that the role of polysaccharides >as been underestimated. SUMMARY Comparisons of the chenical nature of the organic na';ter in soils from tropical, sub-tropical aid temperate legions have mt shown any marked effect of cl.mate, the ttain differences being in the rate at which' transformations occur. Among the products released on oxidative degrada'ion of scil organic matter are a range of phenolic and aromatic carboxylic acids and ipuch of the total carbon has been attributed to avnutic ring structures in the past, ivt fiw application of nuclear magnetic resmwtt. Spectrpmetry has shown that in many S7i<> ihe amount of hydrogen and carbon in aliphatic form is much higher than in aromatic structures. Another useful technique for assessing composition is pyrolysis followed by gas chromatography or mass spectrometry which gives characteristic breakdown products from, for example, carbohydrates and lignins.and make it possible to follow some of the change which occur as humification proceeds. Among the pyrolysis products of fulvic acids are compounds indicating the presence of chains of carbon atoms with carboxyl groups attached. This is in agreement with modelling investigations which have shown close similarity between polymaleic acid and water soluble humus, - indicating that many of the characteristic properties of the natural soil materials are due to the presence of such chains. The stability of organic matter in soils owes much to reactions with amorphous iron and aluminum oxides and an example is given of their effect on some individual organic compounds. Attempt; have been made to fractionate soluble complexes by gel chromatography giving some fractions of high molecular size with metal to carbon ratios of about 1 to 10. The role of metal organic complexes in the development of podzols has also been reexamined. Recent work on the occurrence and distribution of imogolite and protoimogolite suggests that inorganic sols initially play a predominant role in the early stages of podzoiisation, resulting in the precipitation of aluminium and iron in the B horizon. Only at a later stage is organic matter transported and precipitated by the resquioxides. Organic ligands in the soil solution can have an effect on the availability of some trace metals to plant and can, for example, increase the uptake of iron and copp-r by young wheat seedlings. The use of electron paramagnetic resonance spectroscopy indicates the likely form in which copper is absorbed by the roots. One of the most important properties of soil organic matter is its role in stabilising soil structure and among the components thought to be responsible are the polysaccharides. Attempts hav%i been made in the past to assess their contribution by measuring aggregates before and afer chemical destruction of the polysaccharide, but recent investigations have shown that misleading results can be obtained because of the chemical stability of the soil material. REFERENCES 1. Schnitzer, M., In: Soil organic matter studies II, pp Vienna: I.A.E.A. 2. Chen, Y., Senesi, N., and Schnitzer, M., J. Soil Sci. 29, Saiz-Jimenez, C., Haider, K., and Meuzelaar, H.L.C., Geoderma 2, Jenkinson, D.S., and Ayanaba. A., J. Soil Sci. Soc. *ner., [, Sauerbeck, D.R., and Gonzalez, M.A., In: Soil organic matter studies I, pp. i Vienna: I.A.E.A. 6. Hayes, f '.B., Stacey, M., and Swift, R.S., Fuel 51, Haworth, R.D., J. Soil Sci. Ill, Paul, E.A., and Van Veen, J.A., Trans. 11th Congr. Soil Sci Grant, D., Nature, London 270, 10. Wilson, M.A., J. Soil Sci. 32, 11. Barron, P.F., Wilson, M.A., Stephens, J.F., Cornell, B.A., and Tate, K.R., Nature, London 286, Wilson, M.A., Barron, P.F., and Goh, K.M., J. Soil Sci. 32, Wilson, M.A., Pugmire, R.J., Zilm, K.W., Goh, K.H., Heng, S., and Grant, D.M., Nature, London 294, Bracewell, J.M., and Robertson, G.W., Anal. Proc. 18, Bracewell, J.M., Robertson, G.W., and Welch, D.I., J. Anal. Appi. Pyrol. I, Anderson, H.A., and Russell, J.D., Nature, London 260, Malcolm, R.E., and Vaughan, D., Plant and Soil Young, S.D., Bache, B.W., Welch, O.I., and Anderson, H.A., J. Soil Sci. 32, ~~ 19. Edmonds, T.E., Pu Guogang, and West, T.S., Anal. Chim. Acta ]29, Welch,.D.I., Ph.D thesis, University of Aberdeen. 21. Cheshire, M.V., Nature and origin of carbohydrates in soils. London, Academic Press. 22..Macaulay Institute Annual Report, 1978/79, No. 49, pp Williams, E.G., Scott, N.M., and Macdonald H.O., 1958: J. Sc1. Fd. Agric. 9, Anderson, G. ( Williams, E.G., and Moir, J.O., J. Soil Sd. 25, Scott, N.M.. and Anderson G., J. Soil Sci. 27, COLÔQUIO REGIONAL SOBRE MATÉRIA ORGÂNICA DO SOLO 9

20 26. Anderson, G. Unpublished r,ults. 27. Carballas, T., Carballas, M., and Jacquin, F., Anal. Edafol. y Agrohiol. 37, Carballas. M., Carballas, T., and Jacquin, F Anal. Edafol. y Agrobiol. 38, ~~ 29. Tanin, 0., Hedd. Stat. Skogsfors. Anst. 26, Petersen, L., Podzols and podzolization. Copenhagen. DSR Forlag. 31. Anderson, H.A., Berrow, M.L., Farmer, V.C., Hepburn, A., Russell, J.D., and Walker, A.D., J. Soil Sci. 33, Linehan, D.J., and Shepherd, H., Plant and Soil 52, Linehan, D.J., J. Soil Sci. 29, ~ 34. Goodman, B.A., and Linehan, D.J., In: The soil-root interface; Eds. J.L. Harley and R. Scott Russell, pp London, Academic Press. 35. Nacaulay Institute Annual Report, 1979/80, No. 50, p Cheshire, M.V., Sparling, G.P., and Mundie C.M.H., J. Soil Sci. (in the press). 37. Edwards, A.P., and Bremner, J.H., J. Soil Sci. 18, Tisdall, J.H., and Oades, J.M., J. Soil Sci. 33, REGIONAL COLLOQUIUM ON SOIL ORGANIC MATTER STUDIES