Light Organic Matter Research Articles

Soil carbon and nitrogen stocks, light organic matter, and remaining phosphorus under a crop-livestock integration system

Crop-livestock integration (CLI) is a management system that has been developed for a variety of soil and climatic conditions, which combined with the no-tillage system (NTS), has many variations in Brazil, particularly regarding grazing intensity. The objective of this study was to assess the effects of different grazing intensities and soybean (Glycine max L.) crops on carbon (Cst) and nitrogen (Nst) stocks, light organic matter (LOM), root content of the LOM (RLOM), and remaining phosphorus (Prem) in an Oxisol under CLI. The treatments consisted of three different heights (0.25 m [P25], 0.35 m [P35], and 0.45 m [P45]) of pasture grown in a CLI system. Two control areas, an area without grazing (AWG) and a native vegetation area of the Cerrado biome (NCA), which were adjacent to the experimental area, were compared with the cultivated areas. Soil samples were collected from 0.00-0.05, 0.05-0.10, and 0.10-0.20 m beneath each area in a completely randomized design with six replications. Soil characteristics were evaluated in the post-pasture cycle period (PP) and the post-soybean crop period (PC), which coincided with the wet and dry seasons, respectively. The NCA had more LOM in the 0.00-0.05 m layer during the PC and the 0.05-0.10 m layer during both the PP and the PC, C in the LOM in the 0.00-0.20 m layer during the PP and the PC, N in the LOM in the 0.00-0.05 m layer in the PC and the 0.05-0.10 m layer in the PP and the PC, and Cst in the 0.00-0.20 m layer in the PP and the PC than the cultivated areas, but a lower soil density (SD) and Prem in the 0.00-0.20 m layer during both the PP and the PC. Treatment P35 had more Nst in the PC and RLOM in the PP than the other cultivated treatments and the AWG. The absence of grazing in the AWG decreased the SD from the PP to the PC, while the treatments with different grazing intensities had similar values throughout this period. Our results suggest that production could be intensified under light and moderate grazing intensities in the CLI system.

SOIL ORGANIC MATTER FRACTIONS UNDER SECOND-ROTATION EUCALYPTUS PLANTATIONS IN EASTERN RIO GRANDE DO SUL

ABSTRACT The aim of the present study was to evaluate the effect of eucalyptus plantations on total organic C (TOC) and total N (TN) stocks as well as the C and N in the fulvic acid (FA), humic acid (HA), humin (HU), light organic matter (LOM) and microbial biomass (MB) fractions in soils with different textures in the eastern part of the state of Rio Grande do Sul, Brazil. Soil samples were collected from the 0-10-cm, 10-20-cm, 20-40-cm, 40-60-cm and 60-100-cm soil layers in a completely randomized experimental design with subdivided plots. Under the eucalyptus plantation, clay loam soil presented lower HA, HU and MB C stocks in the 0-100-cm soil layer compared to reference vegetation, whereas sandy soil presented higher and C and N in HA and LOM as well as C in MB. The observed increase in TOC in eucalyptus plantations was more pronounced in the surface soil layer (0-10 cm), i.e., approximately 150% higher than under native vegetation, which was probably due to the high contribution of the eucalyptus litter. Differences in C and N stocks in soil organic matter (SOM) fractions between eucalyptus plantations and areas with reference vegetation were more pronounced in sandy soil, showing the capacity of the clay fraction to protect SOM.

Assessment of soil degradation through soil aggregation and particulate organic matter following conversion of riparian buffer to continuous cultivation

SummaryAgriculture in the temperate zone of the Midwestern USA has removed most riparian buffer zones from the landscape, which impairs the soil functioning of the soil in these ecosystems. The objective of this study was to assess soil degradation in a non‐buffered riparian zone with an annual row crop (ARC) and in a longer established (more than 85 years) riparian cool season grass (CSG) buffer in central Iowa, USA. To achieve our objective, we compared the functioning of the target system, ARC, with a system that has been undisturbed since 1990, CSG. We measured the aggregate size distribution and stability of macroaggregrates, aggregate‐associated soil organic carbon and particulate organic matter (POM). The categorization of aggregates by size and water stability (slaking resistance) showed a significantly larger (P < 0.05) proportion of water‐stable macroaggregates (> 250 µm) under CSG (53%) than under ARC (26.3%). On the other hand, soil under ARC had the larger proportion of large and small water‐unstable macroaggregates. The results indicate that the concentration of organic C associated with the total water‐stable aggregates is 1.9 times greater under CSG (23.9 mg C g−1) than for the ARC (12.4 mg C g−1). The concentration of carbon in light particulate organic matter (LPOM‐C) under CSG accounted for 12% of total soil organic carbon (TSOC), but only 6% under ARC management. The chemical composition of LPOM in the ARC soil had a large alkyl‐C/O‐alkyl C ratio, which suggests more organic matter decomposition than with the CSG. Electronic micrographs showed evidence that under CSG roots and fungal hyphae form an extensive network around the large macroaggregates (> 2000 µm), and thus maintain the stability of the macroaggregates. Preserving the CSG buffer reduces soil erosion and the processes associated with nonpoint source pollutants, which improves water quality in an intensively managed agricultural system.Highlights How do soil aggregation and POM change after conversion of CSG buffer to continuous cultivation? Improved understanding of soil degradation through water‐stable and ‐unstable aggregates and SOC. Lack of disturbance of CSG buffer favours formation of macroaggregates and SOC stabilization. Preserving CSG buffer improves water quality in an intensively managed agricultural system.

Effects of treatments on biomass bottom ash applied to the manufacture of cement mortars

The use of biomass for power generation is increasingly common. However, one of the most important problems when using biomass is the amount of waste produced in the combustion process such as ash, which must be transported to landfills for deposition.Biomass bottom ash may contain a large amount of organic matter which is the most restrictive property for reusing this by-product. The potential reuse is determined by chemical and physical properties. The presence of light particles and organic matter confer to biomass bottom ash certain physical and chemical characteristics which significantly reduce the possibility of reuse. Previous studies have demonstrated the possibility of using biomass bottom ash in cement-treated materials, but its properties imply a decrease of compressive and flexural strength.Therefore, this paper reported the study of the mechanical and durability properties of mortars containing biomass bottom ash applying different processing methods. Cement was replaced with different rates of substitution of biomass bottom ash and several samples were treated applying different techniques, such as grinding, elimination of lightweight particles and combustion.The results showed significant improvements in the mechanical and durability properties of mortars in which some treatment were applied. Lightweight particle extraction and organic matter removal by burning considerably improved the mechanical behaviour of the manufactured mortars. Therefore, mortars with crushed biomass bottom ash without floating particles and without organic matter led to a 10% of compressive strength decrease with respect to the control mortar.Thus, this study displays a possibility of waste valorisation by means of to the reuse of this type of by-product in mortars and concretes manufacture.

ORGANIC MATTER LABILE FRACTIONS AND CARBON STOCKS IN A TYPIC QUARTZIPSAMMENT CULTIVATED WITH SUGARCANE HARVESTED WITHOUT BURNING

ABSTRACT The permanence of sugarcane straw on the soil surface, in systems without the pre-harvest straw burning practice, directly affects the soil organic matter dynamics. The objective of this work was to evaluate the changes in total organic carbon (TOC), carbon in the light organic matter (CLOM) and particulate organic carbon (POC), and their carbon stocks in a typic Quartzipsamment cultivated for nine years with sugarcane crops, which were conducted without the pre-harvest straw burning practice, in Paraipaba, State of Ceará, Brazil. Disturbed and undisturbed soil samples were collected at depths of 0.0-0.025, 0.025-0.05, 0.05-0.10, 0.10-0.20 and 0.20-0.30 m, in the sugarcane crop area and in an adjacent native forest area, in order to quantify the TOC, CLOM and POC, as well as the carbon stocks accumulated in the layer 0.0-0.30 m related to these fractions (TOCSt, CLOMSt and POCSt). TOC content changes after nine years of sugarcane crops, conducted without pre-harvest straw burning, were found only in the layers 0.10-0.20 and 0.20-0.30 m. The CLOM varied only in the layer 0.025-0.05 m. The POC content changes were more noticeable than the changes in TOC and CMOL. The CLOM of the sugarcane crop area presented high similarity with TOC, which may affect their quantification in studies related to the soil organic matter dynamics. The sugarcane crop increased the TOCSt, POCSt and CLOMSt in the layer 0.0-0.30 m, compared with the adjacent native forest area.

Indicadores de qualidade de Latossolo relacionados à degradação de pastagens

Resumo O objetivo deste trabalho foi selecionar os indicadores de qualidade de solo mais relacionados à degradação de pastagens, e estabelecer os limites de diferentes níveis de degradação a partir desses indicadores. Cinco pastagens sobre Latossolo Vermelho-Amarelo foram selecionadas, em terço médio da encosta, com diferentes níveis visuais de degradação, tendo-se considerado as variáveis: perda de vigor, presença de espécies espontâneas, solo exposto e erosão. Avaliou-se a cobertura do solo, e coletaram-se amostras nas profundidades de 0,00-0,05, 0,05-0,10 e 0,10-0,20 m, para as análises físicas, químicas e de matéria orgânica. Diferentemente da classificação visual, a análise de agrupamento das variáveis de cobertura indicou apenas dois níveis de degradação: de baixo a moderado e de alto a muito alto, com percentual de classificações corretas acima de 87% na análise discriminante, em que o solo exposto foi a variável mais assertiva para discriminar os níveis de degradação. A matéria orgânica leve e o componente Ca+Mg foram os indicadores de qualidade mais relacionados aos índices de degradação de pastagens à profundidade de 0,00-0,05 m; enquanto o teor de potássio foi o indicador mais relacionado aos índices às profundidades de 0,05-0,10 e 0,10-0,20 m.

Estoques e frações da matéria orgânica do solo sob os sistemas plantio direto e convencional de repolho

Resumo O objetivo deste trabalho foi avaliar as frações húmicas, granulométricas e oxidáveis da matéria orgânica do solo, o conteúdo de matéria orgânica leve no solo, e os teores e estoques de C e N em Latossolo Vermelho, manejado pelos sistemas plantio direto e convencional, para o cultivo de repolho. Áreas de pastagem e de cerrado nativo foram utilizadas como controle. As amostras foram coletadas em 2014, às profundidades 0,0-0,05 e 0,05-0,10 m, após um ciclo de cultivo do repolho sob plantio direto e convencional, em área com longo histórico de cultivo convencional de hortaliças. O sistema plantio direto aumenta os teores de matéria orgânica leve, C e N, assim como os estoques de C e os valores de C orgânico particulado, de C orgânico associado a minerais, e de frações húmicas e oxidáveis da matéria orgânica. O sistema plantio direto melhora a qualidade do solo, em comparação ao plantio convencional, além de proporcionar, para a maioria das variáveis analisadas, maiores valores do que os observados em área de pastagem e valores semelhantes aos da área sob cerrado nativo.

Phosphorus fractions in an agricultural chronosequence under tillage regimes in the Cerrado area in Goiás, Brazil

The increase in the amount and quantity of soil organic matter (SOM), as well as the use of phosphorus-based fertilizers in the superficial soil layer in areas under tillage regimes (TR), may affect phosphorus (P) dynamics in the soil. Therefore, the aims of the present work were as follows: to evaluate the inorganic and organic fractions of P and its lability levels (labile, moderately labile, and moderately resistant) in a Distroferric Red Latosol under tillage regimes (TR) 3, 15, and 20 years after implementation, and to compare them with those of areas of native Cerrado and pastures. We also focus on analyzing the correlations of the P fractions in these areas with other soil attributes, such as total carbon and nitrogen levels, light organic matter (LOM), chemical and physical granulometric fractions of the SOM, maximum phosphate adsorption capacity (MPAC), and the remaining phosphorus (Prem). In each of these areas, samples were collected from the 0.0-0.05 and 0.05-0.10 m soil layers. An entirely randomized experimental design was used. After TR implementation, the constant use of phosphorus-based fertilizers as well as the incremental addition of SOM resulted in an increase in the levels of labile, moderate labile, and moderately resistant organic and inorganic P, with a tendency for total P accumulation to be mostly in the inorganic, moderately labile form. The native Cerrado soil presented high levels of labile and moderately labile inorganic P. Pasture areas presented the lowest levels of labile organic and inorganic P, as well as moderately labile and moderately resistant organic P. By principal component analysis (PCA), it was possible to observe alterations in all soil attributes and P levels of the fractions analyzed.

Carbon cycle history through the Jurassic–Cretaceous boundary: A new global δ13C stack

We present new carbon and oxygen isotope curves from sections in the Bakony Mts. (Hungary), constrained by biostratigraphy and magnetostratigraphy in order to evaluate whether carbon isotopes can provide a tool to help establish and correlate the last system boundary remaining undefined in the Phanerozoic as well provide data to better understand the carbon cycle history and environmental drivers during the Jurassic–Cretaceous interval. We observe a gentle decrease in carbon isotope values through the Late Jurassic. A pronounced shift to more positive carbon isotope values does not occur until the Valanginian, corresponding to the Weissert event. In order to place the newly obtained stable isotope data into a global context, we compiled 31 published and stratigraphically constrained carbon isotope records from the Pacific, Tethyan, Atlantic, and Boreal realms, to produce a new global δ13C stack for the Late Oxfordian through Early Hauterivian interval. Our new data from Hungary is consistent with the global δ13C stack. The stack reveals a steady but slow decrease in carbon isotope values until the Early Valanginian. In comparison, the Late Jurassic–Early Cretaceous δ13C curve in GTS 2012 shows no slope and little variation. Aside from the well-defined Valanginian positive excursion, chemostratigraphic correlation durSchning the Jurassic–Cretaceous boundary interval is difficult, due to relatively stable δ13C values, compounded by a slope which is too slight. There is no clear isotopic marker event for the system boundary. The long-term gradual change towards more negative carbon isotope values through the Jurassic–Cretaceous transition has previously been explained by increasingly oligotrophic condition and lessened primary production. However, this contradicts the reported increase in 87Sr/86Sr ratios suggesting intensification of weathering (and a decreasing contribution of non-radiogenic hydrothermal Sr) and presumably a concomitant rise in nutrient input into the oceans. The concomitant rise of modern phytoplankton groups (dinoflagellates and coccolithophores) would have also led to increased primary productivity, making the negative carbon isotope trend even more notable. We suggest that gradual oceanographic changes, more effective connections and mixing between the Tethys, Atlantic and Pacific Oceans, would have promoted a shift towards enhanced burial of isotopically heavy carbonate carbon and effective recycling of isotopically light organic matter. These processes account for the observed long-term trend, interrupted only by the Weissert event in the Valanginian.

Physical quality of a Luvisol under agroforestry systems in a semi-arid region, Brazil

Agroforestry systems provide many environmental benefits in semi-arid regions; however, trampling by cattle, and agricultural practices, can degrade physical quality of the soil. The aim of this study was to evaluate the degree of compaction and the compression behaviour of a Luvisol after 14 years under agro-silvopastoral (ASP) and silvopastoral systems (SILV), compared with a soil under natural vegetation (NV). We evaluated the degree of soil compactness, compaction index, pre-consolidation pressure, maximum bulk density, critical water content and the stocks of total organic carbon (C), organic matter (OM), C in light organic matter (LOM), and nitrogen (N) in LOM. The results indicated that agroforestry systems change the compressive behaviour of the soil, increasing maximum bulk density (P < 0.05) and reducing critical water content (P < 0.05). Despite the increases in maximum bulk density, changes induced by agroforestry system did not affect the pore space, as shown by similar values for degree of compactness (P > 0.05) under ASP, SILV and NV. This suggests a change in the quality of OM, as evidenced by the lower values for stocks of N in LOM (P < 0.10) and total organic C (P < 0.05) under ASP and SILV, whereas pre-consolidation pressure was unaffected (P > 0.05). After 14 years, the degree of compactness and load-bearing capacity of the soils under ASP and SILV had not increased, being suitable for plant growth.

Microbial carbon recycling: an underestimated process controlling soil carbon dynamics – Part 2: A C<sub>3</sub>-C<sub>4</sub> vegetation change field labelling experiment

Abstract. The mean residence times (MRT) of different compound classes of soil organic matter (SOM) do not match their inherent recalcitrance to decomposition. One reason for this is the stabilization within the soil matrix, but recycling, i.e. the reuse of "old" organic material to form new biomass may also play a role as it uncouples the residence times of organic matter from the lifetime of discrete molecules in soil. We analysed soil sugar dynamics in a natural 30-year old labelling experiment after a wheat-maize vegetation change to determine the extent of recycling and stabilization by assessing differences in turnover dynamics between plant and microbial-derived sugars: while plant-derived sugars are only affected by stabilization processes, microbial sugars may be subject to both, stabilization and recycling. To disentangle the dynamics of soil sugars, we separated different density fractions (free particulate organic matter (fPOM), light occluded particulate organic matter (≤ 1.6 g cm−3; oPOM1.6), dense occluded particulate organic matter (≤ 2 g cm−3; oPOM2) and mineral-associated organic matter (> 2 g cm−3; mineral)) of a silty loam under long-term wheat and maize cultivation. The isotopic signature of neutral sugars was measured by high pressure liquid chromatography coupled to isotope ratio mass spectrometry (HPLC/IRMS), after hydrolysis with 4 M Trifluoroacetic acid. While apparent MRT of sugars were comparable to total organic carbon in the bulk soil and mineral fraction, the apparent MRT of sugar carbon in the oPOM fractions were considerably lower than those of the total carbon of these fractions. This indicates that oPOM formation was fuelled by microbial activity feeding on new plant input. In the bulk soil, MRT of the mainly plant-derived xylose were significantly lower than those of mainly microbial-derived sugars like galactose, rhamnose, fucose, indicating that recycling of organic matter is an important factor regulating organic matter dynamics in soil.

INDICADORES DE QUALIDADE DO SOLO SOB DIFERENTES SISTEMAS DE USO NA MESORREGIÃO DO AGRESTE PARAIBANO

RESUMO: Os sistemas de cultivo modificam os atributos do solo, podendo alterar sua qualidade. O trabalho teve como objetivo avaliar as alterações nos atributos físicos, químicos e biológicos de solos sob sistemas de cultivo orgânico (solos cultivados organicamente há mais de sete anos), em conversão para orgânico e convencional, tendo como referência o solo sob mata, assim como identificar os atributos a serem utilizados como indicadores de qualidade do solo. Foram selecionadas três áreas de propriedades agrícolas, localizadas nos municípios de Areia, Remígio e Lagoa Seca, Paraíba, para a coleta de amostras de solo deformadas, nas camadas de 0-10 e 10-20 cm. Os dados foram submetidos à ANOVA, Teste de Tukey e Análise de Componentes Principais. Houve redução da densidade do solo (Ds) e aumento da porosidade total (PT) no sistema de cultivo orgânico comparado aos sistemas de cultivo em conversão para orgânico e convencional. Os teores de fósforo e de carbono orgânico (CO), a soma de bases (SB), a capacidade de troca catiônica (CTC), a matéria orgânica particulada leve (MOPL), o carbono da matéria orgânica particulada leve (CMOPL), a respiração edáfica (RE) e o pH dos solos sob cultivo orgânico foram superiores aos obtidos nos solos sob cultivo convencional. O cultivo em bases orgânicas proporcionou a manutenção da qualidade do solo em condições semelhantes e/ou melhores que a condição de mata e para as condições deste trabalho os indicadores mais adequados para analisar o cultivo em bases orgânicas foram Ds, PT, CO, MOPL, CMOPL, pH, SB, CTC e RE.

Deleterious interaction of light impairment and organic matter enrichment on Isoetes lacustris (Lycopodiophyta, Isoetales)

Light reduction and increased organic matter load often occur together in lakes undergoing eutrophication. We aimed at experimentally testing the relative importance of light availability, organic matter enrichment and their interactions in the collapse of healthy populations of Isoetes lacustris. We conducted an in situ shading (65 and 35% of incident light) and organic matter enrichment (10% enrichment) experiment in a Pyrenean pond (NE Spain). We followed plant performance using growth indicators, carbon balance indicators and individual survival. Severe light reduction (35%) resulted in a lengthening of the new leaves and no effects on mortality. Organic matter enrichment resulted in lower production and increased individual mortality. The combination of both stresses resulted in stronger negative effects and the highest mortality rate, which helps explaining I. lacustris die-offs observed after damming. Under severe light reduction (35%), plants used corm starch to keep growing. Consequently, starch percentage decreased and sucrose percentage increased as it was transported to the growing leaves. The most extreme changes were observed under severe light reduction and fertilization, which extremely increased the ratio between sucrose and total non-structural carbohydrates (TNC). Thus, I. lacustris sucrose:TNC ratio is a good indicator of light reduction and organic matter enrichment stresses.

Satellite-Based Management Tool for Oak Savanna Ecosystem Restoration

Abstract The structure and function of oak Quercus spp. savanna ecosystems in the North American Midwest were originally maintained by an active disturbance regime (often fire). Subsequent reductions in the frequency of disturbance after European settlement have facilitated rapid, regional conversion of these ecosystems to more closed-canopy forest. Hence, regional-scale management strategies are now needed to restore critical spatial gradients of light, temperature, soil moisture, and soil organic matter for recovery and sustenance of the unique mosaic of understory grass and forb species assemblages that define oak savannas. Tree species composition, distribution, mortality, basal area, and canopy cover are important forest structural parameters that are intrinsically linked to oak savanna restoration ecology. In this benchmark study, we seek to determine whether Landsat-based monitoring protocols can be developed as a tool to guide and monitor regional-scale restoration and management efforts. Using the Sherburne National Wildlife Refuge in central Minnesota as a test case, ground-based forest-structure data were collected and used with multitemporal Landsat sensor data and iterative exclusion partial least-squares regression to calibrate six predictive overstory structure models. Model calibrations produced moderate- to high-accuracy results with respective adjusted coefficient of determination and root mean-squared error values as follows: 0.859, 9.3% (canopy cover); 0.855, 2.95 m2 ha−1 (total basal area); 0.741, 11.6% (red oaks relative basal area); 0.781, 11.9% (bur oak relative basal area); 0.861, 3.20 m2 ha−1 (living oak basal area); and 0.833, 9.1% (dead oak relative basal area). We used the resulting structure models for the Sherburne test site to demonstrate how these data could be applied to help managers prioritize areas within management zones for restorative treatments. Although our Sherburne oak savanna test ecosystem is small (12,424 ha) compared with the size of a full Landsat scene (3.4 million ha), resulting structure models can be extended to the whole Landsat scene, which demonstrates how such modeling protocols can be used for repeated (e.g., annual to decadal), regional-scale analysis and assessment to improve management, planning, and implementation of oak savanna restoration efforts elsewhere.

Why are the δ13Corg values in Phanerozoic black shales more negative than in modern marine organic matter?

AbstractThe δ13Corg values of Phanerozoic black shales average −27‰, whereas those of modern marine organic matter average −20‰. The black shale isotopic values mimic those of continental organic matter, yet their organic geochemical properties mandate that they contain predominantly marine organic matter. Hypotheses that proposed to explain the low δ13C values of black shales include diagenetic losses of isotopically heavier organic matter components, releases of isotopically light carbon from methane clathrates or extensive magmatic events, greater photosynthetic discrimination against 13C during times of higher atmospheric pCO2, and greenhouse climate stratification of the surface ocean that magnified photic zone recycling of isotopically light organic matter. Although the last possibility seems contrary to the vertical mixing that leads to the high productivity of modern oceanic upwelling systems, it is consistent with the strongly stratified conditions that accompanied deposition of the organic carbon‐rich Pliocene‐Pleistocene sapropels of the Mediterranean Sea. Because most Phanerozoic black shales contain evidence of photic zone anoxia similar to the sapropels, well‐developed surface stratification of the oceans was likely involved in their formation. Existence of isotopically light land plant organic matter during several episodes of extensive magmatism that accompanied black shale deposition implies massive release of mantle CO2 that added to the greenhouse conditions that favored oceanic stratification. The 13C depletion common to most Phanerozoic black shales apparently resulted from a greenhouse climate associated with elevated atmospheric pCO2 that led to a strongly stratified ocean and photic zone recycling of organic matter in, augmented by magmatic CO2 releases.

Organic matter pools and nutrient cycling in different coffee production systems in the Brazilian Cerrado

Agroforestry and organic systems have been used to reduce the negative effects that conventional coffee cultivation has on soils. In this work, 13C-CPMAS-NMR, Fourier transform infrared spectroscopy, elemental composition, classical humus fractionation and the soil fertility status were used to evaluate the impact of these three systems on a Latosol from the Brazilian Cerrado. Continuous input of tree residues promoted changes to the soil organic matter with increase in total organic carbon, humic acids (HA) and light organic matter, mainly in the topsoil. Available P and cation exchange capacity were also increased and the acidity status decreased in the agroforestry system. Moreover, HA from the agroforestry were enriched in O-alkyl C, O-di-alkyl C and alkyl C groups and the organic system resulted in HA richer in carboxyl groups. The conventional system resulted in greater aromatic and methoxyl participation, and lower phenol groups. HA from the agroforestry system were richer in easily degradable structures and the chemical fractionation demonstrated a decrease in both recalcitrant fractions, allowing for a more conservative and sustainable management of soil fertility. The modifications were not as evident in the organic system, probably due to the low organic fertilizer input.

Dinâmica da matéria orgânica de um neossolo quartzarênico de cerrado convertido para cultivo em sucessão de soja e milheto

O uso e o manejo de Neossolos Quartzarênicos podem influenciar a matéria orgânica do solo (MOS), mas a magnitude dessas alterações ainda é pouco pesquisada nas condições de Cerrado. O objetivo deste trabalho foi estudar as variações que ocorrem na MOS em um Neossolo Quartzarênico, por consequência da conversão da vegetação original para o uso agrícola e cultivo sequencial de soja e milheto em sucessão. Em uma área de cultivo comercial em Alto Garças, MT, foram coletadas amostras de solo nas profundidades de 0-5, 0-10, 0-20 e 0-40 cm, em talhões que estavam sob sucessão soja-milheto durante 1, 2, 6, 8 e 10 anos e, como referência da vegetação original, amostras foram tomadas em área adjacente sob Cerrado nativo. Foram determinados os teores de carbono orgânico total (COT), carbono e nitrogênio total e o teor de carbono nas frações ácidos húmicos, ácidos fúlvicos e na matéria orgânica leve (MOL), além da relação isotópica do carbono na MOS. A conversão do Cerrado para cultivo agrícola elevou o teor de COT do solo, sobretudo em razão do aporte de MOL e fragmentos de carvão. O manejo desse solo com a sucessão soja-milheto, mesmo com o uso de gradagens pesadas, não causou reduções nos teores de COT, ao longo do tempo. Contudo, observou-se variação na distribuição do carbono nas diferentes frações da MOS ao longo do tempo de cultivo, indicando alterações significativas na qualidade da MOS. Notou-se aumento no percentual de carbono na forma de MOL e redução dos teores de carbono na forma de ácidos fúlvicos ao longo do tempo de cultivo, sugerindo a perda seletiva dos compostos mais solúveis. A relação isotópica do carbono do solo aumentou com o tempo de cultivo, indicando maior contribuição do carbono derivado do milheto para a manutenção dos teores de MOS. Os atributos da MOS relativos à camada de 0-20 cm foram mais sensíveis às variações em razão do tempo de cultivo do que os mesmos atributos avaliados nas demais camadas estudadas. As variáveis relacionadas ao fracionamento da MOS evidenciaram-se mais sensíveis às variações por causa do tempo de cultivo do que simplesmente do teor de COT do solo, apresentando o potencial de uso dessas variáveis como indicadores de qualidade do solo.

Biology and air–sea gas exchange controls on the distribution of carbon isotope ratios (δ<sup>13</sup>C) in the ocean

Abstract. Analysis of observations and sensitivity experiments with a new three-dimensional global model of stable carbon isotope cycling elucidate processes that control the distribution of δ13C of dissolved inorganic carbon (DIC) in the contemporary and preindustrial ocean. Biological fractionation and the sinking of isotopically light δ13C organic matter from the surface into the interior ocean leads to low δ13CDIC values at depths and in high latitude surface waters and high values in the upper ocean at low latitudes with maxima in the subtropics. Air–sea gas exchange has two effects. First, it acts to reduce the spatial gradients created by biology. Second, the associated temperature-dependent fractionation tends to increase (decrease) δ13CDIC values of colder (warmer) water, which generates gradients that oppose those arising from biology. Our model results suggest that both effects are similarly important in influencing surface and interior δ13CDIC distributions. However, since air–sea gas exchange is slow in the modern ocean, the biological effect dominates spatial δ13CDIC gradients both in the interior and at the surface, in contrast to conclusions from some previous studies. Calcium carbonate cycling, pH dependency of fractionation during air–sea gas exchange, and kinetic fractionation have minor effects on δ13CDIC. Accumulation of isotopically light carbon from anthropogenic fossil fuel burning has decreased the spatial variability of surface and deep δ13CDIC since the industrial revolution in our model simulations. Analysis of a new synthesis of δ13CDIC measurements from years 1990 to 2005 is used to quantify preformed and remineralized contributions as well as the effects of biology and air–sea gas exchange. The model reproduces major features of the observed large-scale distribution of δ13CDIC as well as the individual contributions and effects. Residual misfits are documented and analyzed. Simulated surface and subsurface δ13CDIC are influenced by details of the ecosystem model formulation. For example, inclusion of a simple parameterization of iron limitation of phytoplankton growth rates and temperature-dependent zooplankton grazing rates improves the agreement with δ13CDIC observations and satellite estimates of phytoplankton growth rates and biomass, suggesting that δ13C can also be a useful test of ecosystem models.

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.

Carbono, matéria orgânica leve e frações oxidáveis do carbono orgânico sob sistemas de aléias

The objective of this study was to evaluate the levels of total organic carbon (TOC) of the soil, light organic matter (LOM) in water and oxidizable fractions of organic carbon in a cultivated area of maize in the alleys Flemingia macrophylla under different managements of their shoot. The study area is located in “Agroecological Fazendinha in Seropedica city, Rio de Janeiro, Brazil. The experimental design was randomized blocks with three treatments and eight replications, consisting of the following treatments: control (corn cultivation with no alleys); cultivation of maize in alley cropping with pruning to 0,6 m high and growing maize in alley cropping system without pruning. In each area composite samples were collected at 0-5 cm. The presence of alleys and, in the pruning Flemingia macrophylla found that this area is connected with the growing season does not change the contents of the TOC soil. However, the use of stems and leaves as green manure originating from the treatment was carried out where pruning of alleys Flemingia macrophylla, provided the soil increasing concentrations of LOM, C content of LOM and fraction F1.