Light Organic Matter Fractions Research Articles

Land-use perturbations in ley grassland decouple the degradation of ancient soil organic matter from the storage of newly derived carbon inputs

Abstract. In a context of global change, soil has been identified as a potential carbon (C) sink, depending on land-use strategies. To detect the trends in carbon stocks after the implementation of new agricultural practices, early indicators, which can highlight changes in short timescales, are required. This study proposes the combined use of stable isotope probing and chemometrics applied to solid-state 13C nuclear magnetic resonance (NMR) spectra to unveil the dynamics of the storage and mineralization of soil carbon (C) pools. We focused on light organic matter fractions isolated by density fractionation of soil water stable aggregates because they respond faster to changes in land use than the total soil organic matter (SOM). Samples were collected from an agricultural field experiment with grassland, continuous maize cropping, and ley grassland under temperate climate conditions. Our results indicated contrasting aggregate dynamics depending on land-use systems. Under our experimental conditions, grassland returns larger amounts of C as belowground inputs than maize cropping, evidencing a different distribution of light C fractions between aggregate classes. Coarse aboveground inputs from maize contributed mostly to larger macroaggregates. Land-use changes with the introduction of ley grassland provoked a decoupling of the storage and/or degradation processes after the grassland phase. The newly derived maize inputs were barely degraded during the first 3 years of maize cropping, whereas grassland-derived material was depleted. As a whole, results suggest large microbial proliferation as shown by 13C NMR under permanent grassland, then reduced within the first years after the land-use conversion, and finally restored. The study highlighted a fractal structure of the soil, determining a scattered spatial distribution of the cycles of storage and degradation of soil organic matter related to detritusphere dynamics. As a consequence, vegetal inputs from a new land use are creating new detritusphere microenvironments that may be disconnected from the dynamics of C cycle of the previous land use. The formation of those different and unconnected microenvironments may explain the observed legacy effect of the previous land use, since each microenvironment type contributes separately to the overall soil C cycle. The effects of the new land use on the soil C cycle are delayed until the different detritusphere microenvironments remain unconnected, and the ones from the previous land use represent the predominant microenvironment type. Increasing knowledge of the soil C dynamics at a fine scale will be helpful in refining the prediction models and land-use policies.

Evaluation of the Impact of Citrus Cultivation on Carbon and Nitrogen Stocks of Light Organic Matter Fractions of Amazonian Dark Earth (ADE) and Surrounding Soils (Oxisoil) from Central Amazonia

As Terras Pretas de Indio (TPI) sao solos eficientes em reter grande quantidade de materia orgânica, mas ainda sao pouco conhecidas quanto aos efeitos sobre as fracoes fisicas do solo nestes ambientes tropicais. Diante disso, o objetivo do presente estudo foi avaliar as distribuicoes espaciais e em profundidade das fracoes orgânicas leves e o estoque de carbono e nitrogenio em Latossolo Amarelo com A antropico (TPI) comparado a um Latossolo Amarelo distrofico tipico (LA), sob o mesmo cultivo de laranja Pera Rio (Citrus sinensis (L.) Osbeck) na Costa do Laranjal, Manacapuru – AM. O fracionamento fisico foi determinado pelo metodo densimetrico, onde foram extraidas as fracoes leves livres e intra-agregadas em solucao de iodeto de sodio. Foram realizadas analises de granulometria, fertilidade e carbono orgânico total (C) dos solos. Os resultados demonstraram que o carbono, ao se comparar TPI e LA, foi maior na TPI, tendo sido expresso tambem nos estoques de carbono. Finalmente, as concentracoes de carbono da Fracao Leve Livre foram maiores que os teores de carbono da Fracao Leve Intra-agregada na TPI e no LA.

Nitrogen immobilization in paddy soils as affected by redox conditions and rice straw incorporation

Biotic and abiotic processes controlling nitrogen (N) immobilization in paddy soils may significantly affect nutrient availability for plant uptake during the rice cropping season, as well as the efficiency of applied N fertilizers. Understanding the influence of water and crop residue management practices on N availability, however, requires detailed insight into the mechanisms and factors controlling N immobilization in these soils. We evaluated changes in fertilizer-15N immobilization in a paddy topsoil incubated for 160d under flooded or non-flooded conditions, with or without rice straw incorporation. The distribution of immobilized N between different soil fractions and interlayer N fixation was assessed by combining aggregate-size, density and chemical fractionation with stable isotope analysis, while compound-specific δ15N analysis of individual amino sugars was used to evaluate microbial utilization of applied N. Fast immobilization of applied N (≈48% applied N) was observed in both flooded (Eh=+0.4 to −0.2V) and non-flooded (Eh=+0.4 to +0.6V) soils, however in the latter most of this N was released during incubation. The finer soil fractions served as the greatest sink of immobilized N, retaining 5–36% of the added N. Although biotic processes were mainly responsible for N retention, about 4–11% of N applied to flooded soils was weakly fixed within the interlayer of clay minerals, primarily associated with microaggregates. Straw addition further enhanced N immobilization under both oxic and anoxic conditions, with ≈12% of total immobilized N (2–4% of applied N) associated with the light organic matter fraction. The increasing incorporation of applied N into microbial residues suggested that addition of rice straw to paddy soils may lead to effective microbial-mediated immobilization and stabilization of significant portions of N inputs.

Effect of cover plants on soil C and N dynamics in different soil management systems in dwarf cashew culture

Inappropriate soil management practices are one of most important constraints on cashew production in Northeast Brazil. This work aimed to evaluate the changes in soil C and N dynamics caused by different management of cover plants (spontaneous or leguminous) in dwarf cashew culture in Northeast Brazil. The evaluated management systems for control of spontaneous vegetation included: disc harrowing (DH), mechanical mowing (MeM), manual mowing (MaM), leguminous cover crop (LEG), leguminous cover crop+mulch of Carnaúba palm (LEG+m), and application of herbicide (HERB). The floristic composition, plant density, biomass production, and the nutrient content of the spontaneous plants that grew between cashew rows were evaluated. Total soil C and N, mineral- and organic-N, light organic matter fraction, microbial biomass, CO2-C emission rates and humic substances were determined. HERB treatment promoted a substantial change in the floristic composition of spontaneous species. Richness index was highest in the LEG treatment in the fifth year of the experiment. Dry matter amount of spontaneous plants was quite similar in MeM, MaM, LEG, and LEG+m treatments, with average values of 2245kgha−1. DH and HERB treatments reduced dry matter by 69 and 80%, respectively. The MeM system resulted in the highest soil C stock, whereas the DH treatment tended to intensify the mineralization process of soil organic matter due to soil disturbance. LEG treatment resulted in the highest mineral-N contents. Our results suggest that the maintenance of soil organic C levels in DH and MeM treatments is strongly dependent on maintenance of light organic matter levels. Soil from treatments which favored input of organic matter (e.g., LEG, LEG+m and HERB) presented greater C-humic/C-fulvic acids ratio, indicating the predominance of humic acids. Our findings reinforce that cover plants (e.g., spontaneous vegetation and leguminous species) play an important environmental role, contributing to organic matter and nutrient cycling in the soil. Management systems that promote soil covering and the use of leguminous species increased soil organic matter stocks.

Matéria orgânica leve do solo em sistema agroflorestal de corte e trituração sob manejo de capoeira

O objetivo deste trabalho foi avaliar os impactos do ciclo pousio‑cultivo sobre as frações leves da matéria orgânica do solo (MOS), em sistema agroflorestal sequencial de corte e trituração, com diferentes manejos de capoeira, na Amazônia Oriental. A amostragem foi realizada nas fases de pré‑pousio, pós‑pousio e pós‑cultivo de milho. Foram determinados os estoques de matéria orgânica leve livre (MOL‑L) e matéria orgânica leve oclusa (MOL‑O), carbono orgânico total (COT), nitrogênio total (NT) e estoques de C e N na MOL‑L e na MOL‑O. Não houve efeito do manejo da capoeira nas variáveis estudadas, exceto quanto ao NT a 30-50 cm e à MOL‑O a 10-20 cm de profundidade do solo. As frações da matéria orgânica leve foram afetadas pelas fases do sistema agroflorestal sequencial de corte e trituração. Os maiores estoques de C e N foram observados na MOL‑L, nos períodos pós‑pousio e pós‑cultivo. Embora os estoques de C e N totais do solo não tenham aumentado, o aumento da MOL‑L e de seus estoques de C e N indica melhoria e manutenção da qualidade do solo no sistema de corte e trituração da biomassa acumulada em 23 meses de pousio, mesmo após período de cultivo.

Long‐term fertilization and manuring effects on physically separated soil organic‐matter pools under continuous wheat cropping at a rainfed semiarid site in China

AbstractAn essential prerequisite for a sustainable soil use is to maintain a satisfactory soil organic‐matter (OM) level. This might be achieved by sound fertilization management, though impacts of fertilization on OM have been rarely investigated with the aid of physical fractionation techniques in semiarid regions. This study aimed at examining changes in organic C (OC) and N concentrations of physically separated soil OM pools after 26 y of fertilization at a site of the semiarid Loess Plateau in China. To separate sensitive OM pools, total macro‐OM (> 0.05 mm) was obtained from bulk soil by wet‐sieving and then separated into light macro‐OM (< 1.8 g cm–3) and heavy macro‐OM (> 1.8 g cm–3) subfractions; bulk soil was also differentiated into light OM (< 1.8 g cm–3) and mineral‐associated OM (> 1.8 g cm–3). Farmyard manure increased concentrations of total macro‐OC and N by 19% and 25%, and those of light fraction OC and N by 36% and 46%, compared to no manuring; both light OC and N concentrations but only total macro‐OC concentration responded positively to mineral fertilizations compared to no mineral fertilization. This demonstrated that the light‐fraction OM was more sensitive to organic or inorganic fertilization than the total macro‐OM. Mineral‐associated OC and N concentrations also increased by manuring or mineral fertilizations, indicating an increase of stable OM relative to no fertilization treatment, however, their shares on bulk soil OC and N decreased. Mineral fertilizations improved soil OM quality by decreasing C : N ratio in the light OM fraction whereas manuring led to a decline of the C : N ratio in the total macro‐OM fraction, with respect to nil treatment. Further fractionation of the total macro‐OM according to density clarified that across treatments about 3/4 of total macro‐OM was associated with minerals. Thus, by simultaneously applying particle‐size and density separation procedures, we clearly demonstrated that the macro‐OM differed from the light OM fraction not only in its chemical composition but also in associations with minerals. The proportion of the 0.5–0.25 mm water‐stable aggregates of soil was higher under organic or inorganic fertilizations than under no manure or no mineral fertilization, and increases in OC and N concentrations of water‐stable aggregates as affected by fertilization were greater for 1–0.5 and 0.5–0.25 mm classes than for the other classes. Results indicate that OM stocks in different soil pools can be increased and the loose aggregation of these strongly eroded loess soils can be improved by organic or inorganic fertilization.

Efficiency of an agrosystem designed for family farming in the pre-Amazon region

AbstractIn the humid tropics, the continuous use of the same area reduces nutrient availability and increases the incidence of weeds. To circumvent these obstacles, farmers practice itinerant agriculture associated with slashing and burning with negative effects on the local and global environment. In search for a suitable system for humid tropical agriculture, the objective of this study was to investigate the performance of no-till alley cropping in conjunction with the use of annual legume crops grown during the off-season. The experiment was implemented in a one-hectare alley cropping system in which the leguminous tree clitoria (Clitoria fairchildiana R.A. Howard) was used. The experimental design consisted of randomized blocks with four replications of the following treatments: Stylosanthis (Stylosanthis capitata), showey rattlebox (Crotalaria spectabilis), sunn hemp (Crotalaria juncea), jack bean (Canavalia ensiformis) and a control with clitoria alone, without an annual legume. In January 2007 and 2008, maize was planted in each alley. One hundred and twenty days after annual legumes were sown, the total biomass was recorded. Weed incidence was assessed 35 days after maize planting. Analyses of the C, N, P, K, Ca and Mg contents of the legumes were carried out. To assess soil organic matter (SOM), composite soil samples from the surface 0–5 cm were collected from experimental plots. Two adjacent areas were also sampled for comparison: a 10-year-old secondary forest and an area of conventional tillage. The SOM was fractionated using a densitometric and a granulometric method. Conventional systems reduce the silt and free light organic matter fractions more than no till. The use of annual legumes changes the composition of the weed community, replacing the more aggressive types with those less competitive. The use of showy rattlebox (C. spectabilis) may be an effective strategy for reducing weed density in the long-season crop. Furthermore, relative to the use of leguminous trees alone, higher yields of maize can be obtained with the use of showy rattlebox (C. spectabilis) and sunn hemp (C. juncea) without the application of additional N.

Mudanças na matéria orgânica do solo causadas pelo tempo de adoção de um sistema agrossilvopastoril com eucalipto

Sistemas agrossilvopastoris (SASPs) representam práticas de manejo agroflorestal que têm como principal objetivo permitir maior diversidade e sustentabilidade do sistema. A matéria orgânica do solo (MOS) é uma das principais propriedades indicadoras da sustentabilidade de um sistema de cultivo, principalmente em solos sob condições tropicais. Os objetivos deste trabalho foram estudar o impacto dos SASPs de cultivo do eucalipto em diferentes frações de C e N da MOS ao longo do tempo. Para isso, foi amostrada uma cronossequência composta por seis tempos de implantação dos SASPs. O tempo zero foi representado pela área de referência (Cerrado), e os demais, com dois, três, quatro, sete e 10 anos de implantação. Foram coletadas amostras de solo das camadas de 0-10, 10-20, 20-40, 40-60 e 60-100 cm, na linha e na entrelinha de cultivo do eucalipto. Foi feita a caracterização física e química desses solos, assim como as determinações de C orgânico total (COT), N total (NT), C na matéria orgânica leve (MOL), N na MOL e C oxidável por permanganato de potássio, como uma medida de labilidade do C (CL). A partir desses dados foram calculados o IMC CL, que é um Índice de Manejo do C, e o IMC CMOL, que é uma modificação do índice original. O estoque total de C orgânico no solo (COS) (0-100 cm) na linha de plantio do eucalipto foi mantido ao longo do tempo, enquanto na entrelinha houve tendência de perda inicial de C, sem que esses estoques retornassem aos valores originais (Cerrado) no final do décimo e último ano estudado. O estoque total de N mostrou comportamento semelhante na linha e na entrelinha de cultivo do eucalipto. Após perda inicial acentuada de N, houve tendência de acúmulo desse elemento ao longo do tempo de implantação dos SASPs. Contudo, esse acúmulo não foi sustentável e, ao final do período estudado, esses valores eram menores que os originais. A fração da MOL foi a que apresentou as maiores alterações, confirmando sua sensibilidade a alterações de manejo. As perdas de C e N da MOL também foram maiores para a entrelinha de plantio de eucalipto do que para a linha. O C lábil (CL), assim como o índice calculado a partir dele (IMC CL), não possibilitou ajustes dos seus resultados como variável do tempo de implantação dos SASPs. Entretanto, o IMC modificado, calculado a partir do C da MOL (IMC CMOL), apresentou bons ajustes para todas as camadas estudadas; além disso, apresentou correlação mais elevada com o COT, mostrando-se mais indicado para a comparação dos sistemas de manejo estudados. Pôde-se perceber a importância de estudar camadas mais profundas do solo, já que elas podem apresentar perdas de C significativas e de difícil recuperação. A implantação dos SASPs acarretou diminuição dos estoques de COT das camadas superficiais, e o tempo necessário para a recuperação dos estoques originais ficou além do período máximo estudado. A fração mais sensível ao manejo adotado foi a MOL livre.

Efeito da solução de separação densimétrica na quantidade e qualidade da matéria orgânica leve e na quantificação do carbono orgânico da fração pesada

A matéria orgânica leve tem se mostrado um indicador precoce das mudanças na matéria orgânica do solo (MOS) acarretadas por diferentes usos e sistemas de manejo. Várias soluções são empregadas na sua separação, mas pouco se sabe sobre o efeito delas na quantidade e qualidade da fração extraída, tampouco se conhecem as possíveis interferências que a solução de separação pode ocasionar na posterior quantificação do carbono (C) orgânico. Assim, foram selecionados 20 solos com diferentes teores de argila e que se encontravam sob diferentes usos, onde foram coletadas amostras, em três repetições, na profundidade de 0-20 cm. Depois de secas ao ar, destorroadas, passadas em peneira com malha de 2 mm e homogeneizadas, retiraram-se subamostras para determinação do teor de argila e para realização do fracionamento densimétrico da MOS. Do fracionamento densimétrico com água ou NaI (1,8 kg L-1), foram obtidas a fração leve (FL) e a fração pesada (FP) da MOS. O teor de C da FL foi determinado por combustão via seca, em analisador elementar (CHN), e o teor de C da FP foi determinado por oxidação via úmida com aquecimento externo, segundo Yeomans & Bremner (1988) (Y & B), e em analisador elementar (CHN). Adicionalmente, foi determinado o teor equivalente em C orgânico por via úmida em soluções-padrão com NaI em diferentes concentrações. Foi observado que a quantidade da FL extraída com NaI foi, em média, 4,86 vezes maior em relação à extraída com água destilada. A relação C:N da fração extraída com NaI foi, em média, 1,85 vez superior à apresentada pela extração com água destilada, mas com grande variação entre solos. Com relação à FP da MOS, o teor de C determinado por Y & B foi similar quando se procedeu à separação densimétrica com água, mas foi cerca de 1,5 vez superior ao determinado pelo CHN quando esta fração foi separada com NaI, indicando que houve superestimação do teor de C orgânico devido à interferência do NaI. Esse fato foi confirmado pelo aumento linear no consumo de Cr2O7-2 quando da presença de concentrações crescentes de NaI em soluções isentas de C. Dessa forma, a extração com NaI recupera maior proporção da FL, mas que difere em qualidade daquela extraída com água. Além disso, algum possível resíduo de NaI usado na separação pode interferir na quantificação do C orgânico quando este for determinado por dicromatometria. Resultados de estudo adicional sugerem que cada mol de I- consome aproximadamente 1 mol de Cr2O7-2.

Labile soil organic matter pools under a mixed grass/lucerne pasture and adjacent native bush in Western Australia

Total C and N were measured in whole soils (0–0.15, 0.15–0.35, and 0.35–0.65 m), light organic matter fractions (<1 g/cm3 (LF 1.0) and 1.0–1.7 g/cm3 (LF 1.7)) in surface soils, and in leaf litter collected from a mixed grass/lucerne pasture and adjacent native bush at Moora, Western Australia. The C content of the plant material and light fractions was characterised by 13C cross-polarisation/magic angle spinning nuclear magnetic resonance (13C CP/MAS NMR) spectroscopy. Water-extractable organic C (WEOC) and N (WEON) were measured in soil, and dissolved organic C (DOC) and N (DON) were measured in soil solutions. In addition, both NO3-N and NH4-N (SMN) were measured in soil solutions and water extracts. Total soil C (0–0.65 m) did not differ significantly between land uses, but there was clear evidence of N enrichment under the pasture system, which contained significantly (P < 0.05) more total N in the surface soil (0–0.15 m) compared with that under native bush. The significantly (P < 0.05) smaller C/N ratios of the surface soil, plant litter, and light fractions (LF 1.0 and 1.7) under the pasture provided further evidence of N enrichment. The 13C CP/MAS NMR spectra for plant material and light fractions did not differ greatly between landuses, but in both cases the O-alkyl : alkyl carbon ratio declined with increasing density. The decomposition and subsequent mineralisation of the relatively N-rich organic matter fractions in the pasture system may have contributed to the significantly (P < 0.05) greater DOC, DON, and SMN concentration measured in soil solutions under pasture compared with those under native bush.

Evaluating Chemical and Physical Indices of Nitrogen Mineralization Capacity with an Unequivocal Reference

After decades of searching for a rapid method to estimate the N mineralization capacity of soil, there is still no consistent recommendation. It is legitimate to examine the causes for the often‐conflicting results in literature. The efficacy of various references that have been used as benchmarks for assessing chemical and physical indices in the literature is critically reviewed in this paper. Gross N mineralization and consumption during waterlogged and aerobic incubations were estimated in a wide range of soils. It was found that equivalent to 17 to 90 and 23 to 59% of the mineralized N was consumed during the waterlogged and aerobic incubations, respectively. As net N production rate represents the balance between N‐producing and N‐consuming processes, it appears difficult to find a simple method that could be used to predict the net effect of several concurrent processes. We used the gross N mineralization as a reference criterion for N mineralization ability. Total organic N, water‐soluble organic N, alkali‐hydrolyzable N, acid‐hydrolyzable N, hot salt‐hydrolyzable N and N in the light organic matter fraction were assessed against this reference criterion. All indices except light fraction N were significantly related to gross N mineralization. Water‐soluble organic N had the highest correlation of all the indices tested. None of the chemically hydrolyzed N fractions consistently showed closer relationships with N mineralization than total organic N, suggesting that these chemical methods are ineffective in extracting a biologically labile fraction of soil organic N.