Free Light Fraction Research Articles

Decadal Nitrogen Fertilization Decreases Mineral‐Associated and Subsoil Carbon: A 32‐Year Study

AbstractCrop residues and manure are important sources of carbon (C) for soil organic matter (SOM) formation. Crop residue return increases by nitrogen (N) fertilization because of higher plant productivity, but this often results only in minor increases of SOM. In our study, we show how N fertilization and organic C additions affected SOM and its fractions within a 32‐year‐long field‐experiment at Puch, Germany. Five organic additions, no‐addition (control), manure, slurry, straw and straw + slurry, were combined with three mineral N fertilization rates (no, medium and high fertilization), which resulted in 1·17–4·86 Mg C‐input ha‐1 y‐1. Topsoil (0–25 cm) SOM content increased with N fertilization, mainly because of the C in free light fraction (f‐LF). In contrast, subsoil (25–60 cm) SOM decreased with N fertilization, probably because of roots' relocation in Ap horizon with N fertilization at the surface. Despite high inputs, straw contributed little to f‐LF but prevented C losses from the mineral‐associated SOM fraction (ρ > 1·6 g cm‐3) with N fertilization, which was observed without straw addition. Above (straw) and belowground (roots) residues had opposite effects on SOM fractions. Root C retained longer in the light‐fractions and was responsible for SOM increase with N fertilization. Straw decomposed rapidly (from f‐LF) and fueled the mineral‐associated SOM fraction. We conclude that SOM content and composition depended not only on residue quantity, which can be managed by the additions and N fertilization, but also on the quality of organics. This should be considered for maintaining the SOM level, C sequestration, and soil fertility. Copyright © 2016 John Wiley & Sons, Ltd.

Addition of nitrogen enhances stability of soil organic matter in a temperate forest

Physicochemical protection of soil carbon provided by soil aggregates is critical to carbon (C) sequestration in terrestrial ecosystems. However, the stability of soil organic matter (SOM) in terrestrial ecosystems in response to atmospheric nitrogen (N) deposition is unclear. In this study, N was added to a forest soil dominated by deciduous trees on Changbai Mountain, China, at three different rates (0, 25 and 50 kg N ha−1 year−1) from 2007 to 2012. Its effect on C content and stabilization was evaluated by soil fractionation and stable isotope (δ13C) analyses. The results showed that large macroaggregates (2–8 mm) decreased and small macroaggregates (0.25–2 mm) increased with increasing rates of N addition, whereas soil C content remained unchanged. Irrespective of the N treatments, the C content of soil organic matter (SOM) fractions differed significantly between large and small macroaggregates, which suggests that the size of aggregate classes regulates C content in the SOM fractions. A slight increase in the C content of microaggregates within macroaggregates (Mm) and that of silt and clay fractions was recorded with the addition of N at 50 kg N ha−1 year−1. This increase also occurred in the silt and clay fraction within microaggregates (Intra‐SC). Unprotected C (comprising the free light fraction (Free‐LF) and coarse particulate organic matter (CPOM)) accounted for 18.9% only of the total C and decreased in response to the addition of N. The δ13C signature and C/N ratios obtained for SOM fractions showed that newly formed C was transferred from POM to Intra‐SC. Overall, our results suggested that long‐term addition of N might promote stabilization of C by increasing small macro‐ and micro‐aggregation within macroaggregates in temperate forest soil.Highlights Investigated stability of soil organic matter in response to long‐term N addition in a deciduous forest. Physicochemical protection of soil organic matter by aggregates changed after N addition. Soil organic carbon in microaggregates within macroaggregates increased with N addition. Long‐term N addition might promote C stabilization in temperate forest soil.

Timescales of carbon turnover in soils with mixed crystalline mineralogies

Abstract. Organic matter–mineral associations stabilize much of the carbon (C) stored globally in soils. Metastable short-range-order (SRO) minerals such as allophane and ferrihydrite provide one mechanism for long-term stabilization of organic matter in young soil. However, in soils with few SRO minerals and a predominance of crystalline aluminosilicate or Fe (and Al) oxyhydroxide, C turnover should be governed by chemisorption with those minerals. Here, we correlate mineral composition from soils containing small amounts of SRO minerals with mean turnover time (TT) of C estimated from radiocarbon (14C) in bulk soil, free light fraction and mineral-associated organic matter. We varied the mineral amount and composition by sampling ancient soils formed on different lithologies in arid to subhumid climates in Kruger National Park (KNP), South Africa. Mineral contents in bulk soils were assessed using chemical extractions to quantify Fe oxyhydroxides and SRO minerals. Because of our interest in the role of silicate clay mineralogy, particularly smectite (2 : 1) and kaolinite (1 : 1), we separately quantified the mineralogy of the clay-sized fraction using X-ray diffraction (XRD) and measured 14C on the same fraction. Density separation demonstrated that mineral associated C accounted for 40–70 % of bulk soil organic C in A and B1 horizons for granite, nephelinite and arid-zone gabbro soils, and > 80 % in other soils. Organic matter strongly associated with the isolated clay-sized fraction represented only 9–47 % of the bulk soil C. The mean TT of C strongly associated with the clay-sized fraction increased with the amount of smectite (2 : 1 clays); in samples with > 40 % smectite it averaged 1020 ± 460 years. The C not strongly associated with clay-sized minerals, including a combination of low-density C, the C associated with minerals of sizes between 2 µm and 2 cm (including Fe oxyhydroxides as coatings), and C removed from clay-sized material by 2 % hydrogen peroxide had TTs averaging 190 ± 190 years in surface horizons. Summed over the bulk soil profile, we found that smectite content correlated with the mean TT of bulk soil C across varied lithologies. The SRO mineral content in KNP soils was generally very low, except for the soils developed on gabbros under more humid climate that also had very high Fe and C contents with a surprisingly short, mean C TTs. In younger landscapes, SRO minerals are metastable and sequester C for long timescales. We hypothesize that in the KNP, SRO minerals represent a transient stage of mineral evolution and therefore lock up C for a shorter time. Overall, we found crystalline Fe-oxyhydroxides (determined as the difference between Fe in dithionate citrate and oxalate extractions) to be the strongest predictor for soil C content, while the mean TT of soil C was best predicted from the amount of smectite, which was also related to more easily measured bulk properties such as cation exchange capacity or pH. Combined with previous research on C turnover times in 2 : 1 vs. 1 : 1 clays, our results hold promise for predicting C inventory and persistence based on intrinsic timescales of specific carbon–mineral interactions.

Soil Carbon Stocks under Amazonian Forest: Distribution in the Soil Fractions and Vulnerability to Emission

Transformations of natural ecosystems in tropical regions, which are usually covered by high-biomass forests, contribute to increased atmospheric CO2. Much of the carbon in forest ecosystems is stored in the soil. This study estimates soil carbon stock in a dense forest in central Amazonia from sets of soil samples collected in three topographic positions (plateau, slope and valley bottom). Soil organic matter (SOM) was fractionated by density and particle size, thus obtaining the free light fraction (FLF), intra-aggregated light fraction (IALF), sand fraction (F-sand), clay fraction (F-clay) and silt fraction (F-silt). Soil organic carbon (SOC) stocks on the plateaus (Oxisol), slopes (Ultisol) and valley bottoms (Spodosol) were 98.4 ± 7.8 Mg·ha-1, 72.6 ± 5.4 Mg·ha-1 and 81.4 ± 8.9 Mg·ha-1, respectively. Distribution of carbon in soil fractions was: 112.6 ± 15 Mg·ha-1 (FLF), 2.5 ± 0 Mg·ha-1 (ILAF), 40.5 ± 1.5 Mg·ha-1 (F-silt), 68.5 ± 4.2 Mg·ha-1 (F-clay) and 28.3 ± 1.4 Mg·ha-1 (F-sand), totaling 252.4 ± 22.1 Mg·ha-1 of carbon. Carbon is largely in labile form and near the soil surface, making it liable to release from deforestation or from climate change. Spodosols are more susceptible to soil carbon losses, demonstrating the need to preserve forested areas close to Amazonian rivers and streams.

Erosional redistribution of topsoil controls soil nitrogen dynamics

In recent years, the role of soil erosion on terrestrial carbon sequestration had been the focus of a growing number of studies. However, relatively little attention has been paid so far to the role of erosion on the lateral distribution of soil nitrogen (N) and the role of geomorphic processes on soil N dynamics. Here, we present primary data on the stock of nitrogen in soil and its rate of erosion at a relatively undisturbed, zero-order watershed in northern California. Erosion transports 0.26–0.47 g N m−2 year−1 from eroding slope positions (Summit and Slope), and about two-thirds of the eroded N enters depositional landform positions (Hollow and Plain). Our results show that depositional-position soil profiles contain up to 3 times more N than soil profiles in the eroding positions. More than 92% of all soil nitrogen was chemically bound to soil minerals in all the landform positions, compared to 2–4% each found in the free light and occluded light fractions. Nitrogen associated with the free light fraction in topsoil is particularly susceptible to loss by soil erosion. By comparison, soil N associated with the aggregate-protected occluded light fractions and the mineral-associated dense fractions is likely to be protected from gaseous and dissolved losses. On average, we found that soil N has mean residence time of 694 years in eroding landform positions, compared to 2951 years in depositional landform positions. Our results also show that microbial processing of organic matter exerts strong control on overall soil N storage and N stabilized through sorptive interactions with soil minerals only in poorly drained depositional landform positions. Soil erosion exerts important control on stock, distribution, and long-term fate of soil N in dynamic landscapes.

Molecular composition of several soil organic matter fractions from anthropogenic black soils (Terra Preta de Índio) in Amazonia — A pyrolysis-GC/MS study

The stability of soil organic matter (OM) in Amazonian anthropogenic soils, Terra Preta de Índio (TPI), is still not completely understood. The large contribution from black carbon (BC) and minerals to these soils is well-known; OM stability is therefore frequently explained by these properties, providing intrinsic recalcitrance (BC) and chemical protection against decomposition (OM-mineral associations), but the relative importance of both mechanisms that underlie the stability are not yet understood. In order to better understand the sources and decay processes of soil OM in these soils, we studied the molecular composition using pyrolysis gas-chromatography/mass-spectrometry. To this end, several soil OM fractions of samples from two TPI soils and an adjacent Xanthic Acrisol that served as a reference soil (REF) were studied (0–2m depth). A sequential fractionation was applied in order to better understand the composition of soil OM pools with different association to the soil matrix. The studied soil OM fractions included the free light fraction (FLF; i.e., charcoal and litter), the occluded light fraction (OLF; charcoal and litter within aggregates), 0.1M NaOH extractable OM (EX; comparable with the combined humic acid and fulvic acid fractions), and the remaining residue (RES; treated with HF/HCl; comparable with the humin fraction). The largest contribution to the total soil OM was found for EX and/or RES, the sum of which varied between 79% and 98% (Ave. 90.8%, n=18). The minor contribution from the OLF (Ave. 4.4%) indicates that stabilisation within aggregates was not a major factor that contributed to OM stability in the studied profiles. Differences in molecular composition between the fractions were larger than those between profiles or with depth. The EX fraction showed the lowest contribution from plant (lignin phenols) and BC (PAHs, indenes, aromatics) derived products. The RES fraction showed a significantly larger contribution from BC related pyrolysis products compared to the EX. This non-extractable BC that is present in the RES after removal of the light fractions may reflect preferential binding of PAHs to minerals or, alternatively, the presence of highly condensed macromolecular polyaromatic structures. Changes with depth within the EX and RES fractions showed a larger contribution from products derived from BC in the anthropogenic horizons. Furthermore, the OM composition in EX and RES indicated that the degree of degradation of OM in anthropogenic horizons was relatively low. Apart from these similarities for anthropogenic horizons, the chemical composition of soil OM also showed large differences between both TPI soils. TPI-1 showed large variation between horizons while TPI-2 showed only minor changes with depth; this may be related to different phases of human occupation with time and/or a different soil usage.

Qualidade física e frações da matéria orgânica de um Planossolo sob sistema plantio direto

Resumo O objetivo deste trabalho foi avaliar a qualidade física e as frações da matéria orgânica de um Planossolo sob sistema plantio direto (SPD), com diferentes tempos de implantação, no Sul do Brasil. Amostras de solo foram coletadas nas camadas de 0,00-0,03, 0,03-0,06, 0,06-0,10 e 0,10-0,20 m, tendo-se determinado: a porosidade total (Pt); a macroporosidade (Ma); a microporosidade (Mi); a resistência do solo à penetração (Rp); a densidade (Ds); os macro e microagregados; o diâmetro médio ponderado dos agregados (DMP); o carbono orgânico total (COT); o carbono das frações leve livre (FLL), leve oclusa (FLO) e pesada (FP); os estoques de carbono (EstC), os índices de labilidade (ILC), de estoque (IEC) e de manejo do carbono (IMC). O tempo de implantação SPD aumentou a Pt, a Ma, os EstC, e diminuiu a Mi, a Rp e a Ds. O SPD promove os atributos DMP, ILC, IEC e IMC a partir do quinto ano de implantação, e, portanto, melhora a qualidade física do solo.

Frações físicas da matéria orgânica em Latossolo cultivado com cana-de-açúcar no Rio Grande do Sul

Resumo O objetivo deste trabalho foi avaliar as frações físicas da matéria orgânica do solo (MOS) e o índice de manejo de carbono (IMC), após diferentes períodos de preparo do solo para a implantação de cana-de-açúcar. O estudo foi realizado em Latossolo Vermelho, de textura argilosa, depois de 1, 3 e 5 anos do preparo do solo para a implantação da cultura, com queima da palhada e colheita manual. Como referência, utilizou-se o solo de mata nativa adjacente à área de estudo. Amostras de solo foram coletadas nas camadas de 0,00-0,05, 0,05-0,10 e 0,10-0,20 m, para quantificar o IMC, bem como o C orgânico total, o C particulado, o C associado a minerais e o C das frações leve livre, leve oclusa e pesada da MOS. O preparo do solo associado à queima da palhada na colheita diminuiu o aporte de matéria orgânica fresca ao solo e acelerou a decomposição do compartimento lábil da MOS, com reflexos negativos sobre os estoques das frações físicas do solo. As maiores proporções de C foram verificadas na fração associada a minerais. As frações densimétricas da MOS foram mais sensíveis na detecção de alterações dos estoques, labilidade, índice de labilidade e IMC, nos diferentes tempos de implantação da cultura. As áreas com cana-de-açúcar apresentam IMC inferior a 100, o que indica impacto negativo do cultivo sobre os estoques de matéria orgânica e sobre a qualidade física do solo.

Changes of soil properties and carbon fractions after long-term application of organic amendments in Mollisols

Land degradation threatens soil fertility as well as environmental security. To assess the extent of degradation of black Mollisols in northeast China, chemical and microbial indicators of soil quality were determined after long-term application of organic amendments combined with chemical fertilizers. The application of high amounts of organic manure plus chemical fertilizer (NPK) significantly increased soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), and mineral nitrogen (Nmin), as well as microbial biomass carbon (MBC), extractable dissolved organic carbon (DOC), microbial quotient (qMIC), carbon (C) in free light fraction (fLF) and its percentage of total soil C. Soil C/N ratio and C portion in heavy fraction (HF) both decreased significantly, which might have resulted from accelerated mineralization of stable C. Application of straw at a low rate plus NPK significantly increased Nmin and MBC concentrations and qMIC. Compared with the unamended control, no negligible differences were found for most of the selected parameters under the treatments NPK alone, high-rate straw plus NPK, and low-rate manure plus NPK. These results indicated that NPK along with either a high rate of manure or a low rate of straw profoundly changed certain key soil chemical and microbial properties. Despite these apparent benefits in soil properties, however, high rates of manure application is likely contributing little to soil C stabilization. Low rates of organic amendments, especially crop straw, are therefore recommended, as this practice can improve or at least maintain soil properties in degraded Mollisols, while minimizing the adverse impacts of more intense applications.

Soil organic carbon dynamics under long-term fertilization in a black soil of China: Evidence from stable C isotopes.

Effects of different fertilizers on organic carbon (C) storage and turnover of soil fractions remains unclear. We combined soil fractionation with isotope analyses to examine soil organic carbon (SOC) dynamics after 25 years of fertilization. Five types of soil samples including the initial level (CK) and four fertilization treatments (inorganic nitrogen fertilizer, N; balanced inorganic fertilizer, NPK; inorganic fertilizer plus farmyard manure, MNPK; inorganic fertilizer plus corn straw residue, SNPK) were separated into four aggregate sizes (>2000 μm, 2000–250 μm, 250–53 μm, and <53 μm), and three density fractions: free light fraction (LF), intra-aggregate particulate organic matter (iPOM), and mineral-associated organic matter (mSOM). Physical fractionation showed the iPOM fraction of aggregates dominated C storage, averaging 76.87% of SOC storage. Overall, application of N and NPK fertilizers cannot significantly increase the SOC storage but enhanced C in mSOM of aggregates, whereas MNPK fertilizer resulted in the greatest amount of SOC storage (about 5221.5 g C m2) because of the enhanced SOC in LF, iPOM and mSOM of each aggregate. The SNPK fertilizer increased SOC storage in >250 μm aggregates but reduced SOC storage in <250 μm aggregates due to SOC changes in LF and iPOM.

Initial Recovery of Organic Matter of a Grass-Covered Constructed Soil after Coal Mining

Revegetation is essential to improve the quality of constructed soils. This study investigated effects of perennial grasses on total organic carbon (TOC) stock, organic matter (OM) fractions and on quality of a recently constructed soil, after coal mining. Soil samples were collected from the 0.00-0.03 m layer two years after the beginning of the experiment. The treatments consisted of Cynodon dactylon cv vaquero (T1); Urochloa brizantha (T2); Panicum maximun (T3); Urochloa humidicola (T4); Hemarthria altissima (T5); Cynodon dactylon cv tifton (T6); bare constructed soil (T8); and natural soil (T9). The treatments with grass species increased the TOC stock by 57 % and increased the OM lability in comparison with T8. Higher C accumulation in the coarse and free light fractions and a higher C management index were observed in T2 and T3, indicating greater suitability of these species for the initial recovery of OM of the constructed soil.

Shifts in soil organic carbon and nitrogen dynamics for afforestation in central China

The afforestation has been proposed as a primary means of sequestering carbon (C) from the atmosphere, thereby mitigating climate change. However, consequences of afforestation on soil organic C and nitrogen (N) dynamics due to spatial heterogeneity are not fully understood. The objectives of this study were to identify the consequences of 18 years of afforestation on C and N dynamics in the Danjiangkou Reservoir area of central China. Soil samples from the woodland, shrubland, cropland and adjacent open area soils (i.e. the control) were separated into four aggregate sizes (>2000μm, 250–2000μm, 53–250μm and <53μm), and three density fractions [free light fraction (LF), intra-aggregate particulate organic matter (iPOM) and mineral-associated organic matter (mSOM)]. All fractions were analyzed for their C and N content, and δ13C and δ15N values. Afforestation enhanced the soil C and N storage, primarily due to increases in soil C and N storage of macroaggregates (>2000μm) with the largest (65–87%) fraction in iPOM. The C: N ratios generally increased from the cropland to shrubland to woodland across all fractions. The 13C values indicated that the fastest decay rates (k=0.024yr−1) in LF of macroaggregates (>2000μm) was in woodland, but the fastest decay rates (0.114–0.137yr−1) in iPOM of all aggregates were observed in cropland. Meanwhile, the most enriched δ15N values (0.87–3.81) were found in cropland soil. Our results suggest that afforestation could alter weight distribution of soil fractions and increase the soil organic C and N storage, thereby affecting the turnover and stability of SOM.

Soil formation and its implications for stabilization of soil organic matter in the riparian zone

Riparian woodlands consist of different landscape units characterized by different hydroecomorphological site conditions that are reflected in the distribution of soils and tree species. These conditions are determined by flooding frequency and duration, distance to river channels, elevation and water flow velocity. The influence of these environmental drivers on the stabilization of soil organic matter (SOM) has as yet not been investigated. Hence, the aim of our study is to link soil formation and its drivers with stabilizing processes of SOM in riparian floodplain forests. We investigated soils and sediments at two sites in the ash–maple–elm–oak alluvial forest zone (AMEO sites) and two sites in the willow-poplar alluvial forest zone (WiP sites) within the riparian zone of the Danube near Vienna (Austria). Sediments and soils were characterized based on texture, contents of organic carbon (OC), nitrogen, Fe oxides, and soil pH. Density fractionation was used to separate OC fractions in terms of stabilization process and resulting organic matter (OM) turnover time: the free light fraction (fast turnover), the light fraction occluded in aggregates (intermediate turnover) and the heavy fraction of OM associated tightly to mineral surfaces (slow turnover).At both sites, soil and sediment properties reflect the hydroecomorphological site conditions for formation of the landscape units in the riparian zone: Soils at AMEO sites develop during constant deposition of fine-textured sediment while water flow velocity is low. Progressing soil development causes a continuous decrease in OC content with increasing soil depth, mainly from fractions with fast and intermediate turnover. As a consequence the heavy fraction clearly dominates with around 90% of OC. Temporally variable flooding conditions with occurring turbulences found at WiP sites result in a discontinuous change of soil properties with increasing soil depth. Former topsoil horizons buried by huge amounts of sediments seem to keep the OC fractionation typical for topsoil horizons with extraordinarily high amounts of light fraction OM (free and occluded) representing 20–40% of total OC. The presented results confirm that sedimentation and soil formation are simultaneous processes at AMEO sites. At WiP sites both processes seem uncoupled with alternate phases of sedimentation and soil formation. Thus, the frequent burial of topsoil material formed at WiP sites seems to enable the conservation of unstable organic matter fractions at this part of active floodplains.

Impacts of Spartina alterniflora invasion on soil organic carbon and nitrogen pools sizes, stability, and turnover in a coastal salt marsh of eastern China

Plant invasion may impact ecosystem structure and function, and further affect soil organic matter (SOM) dynamics. However, the influence of plant invasion on soil organic carbon (C) and nitrogen (N) pools sizes, stability, and turnover in SOM of invaded ecosystems is not fully understood. In this study, soil C and N contents, and δ13C and δ15N values of free light fraction (LF), intra-aggregate particulate organic matter (iPOM) and mineral-associated organic matter (mSOM) were investigated in an invasive Spartina alterniflora community, adjacent bare flat and native Suaeda salsa and Phragmites australis communities. Short-term S. alterniflora invasion significantly enhanced organic C and N contents in SOM, free LF, iPOM, mSOM compared with bare flat and increased the proportion of allocated C in iPOM compared with S. salsa and P. australis soils (0–0.30m depth). The proportion of the S. alterniflora-derived C in free LF and iPOM were significantly higher than that in mSOM, and the highest S. alterniflora-derived C content was found in iPOM of S. alterniflora soil. The most enriched δ15N values were found in S. alterniflora soil. Increased δ15N values and decreased C:N ratios from the free LF to iPOM to mSOM in S. alterniflora soil indicated a greater degree of decomposition. The results suggest that 10-year S. alterniflora invasion significantly alters soil organic C and N pools sizes and stability through changing plant residuals input, physical distribution of S. alterniflora-derived C and C turnover in SOM fractions.

Organic carbon dynamics in soils with pyrogenic organic matter that received plant residue additions over seven years

The effect of repeated application of plant residues on mineralization of different organic carbon (OC) pools in a pyrogenic organic matter (PyOM) amended soil was determined using an incubation study conducted over 7.1 years. At five occasions during this period, sugarcane residues (C4) were mixed with the soil (C4) with or without PyOM (C3) amendments. Organic C mineralized during the incubation period or remaining in different physical soil fractions after 7.1 years was partitioned into PyOM carbon (PyOM-C) and native soil organic matter C (nSOM-C) or sugarcane C plus nSOM-C (SC-C + nSOM-C). When compared to the control, total cumulative OC (comprising both nSOM-C and PyOM-C) mineralized in the presence of PyOM was 40% higher after the first 2.5 years, but equal by 6.2 years and 3% lower by the end of the incubation period. The cumulative nSOM mineralization after 7.1 years was 2.57 mg CO2–C g−1 soil with PyOM compared to 3.16 mg CO2–C g−1 soil without PyOM addition (p = 0.13; n = 3). More than 60% of the added PyOM-C was present in the free-light fraction by the end of the 7.1 years. In total, 93% of the added PyOM-C remained in soil compared to 25–28% of SC-C + nSOM-C. Sugarcane residues increased the remaining PyOM-C in the occluded-light fraction by 3% (p < 0.05) and in the organo-mineral fraction by 4% (p < 0.1), suggesting a possible preferential use of SC-C or accumulation of metabolites of decomposed PyOM. However, the addition of sugarcane had no significant effect on overall mineralization of PyOM. The presence of PyOM accelerated the mineralization of SC-C + nSOM-C by 9% (p < 0.001). This is probably due to enhanced mineralization of sugarcane residues rather than native SOM. Although PyOM was likely to accelerate mineralization of added plant residues throughout a 7-year period, PyOM did not increase cumulative nSOM mineralization when plant residues were absent (p > 0.05), so PyOM may reduce nSOM mineralization in the long term.

Beneficial effects of reduced tillage and green manure on soil aggregation and stabilization of organic carbon in a Mediterranean agroecosystem

Semiarid Mediterranean agroecosystems need the implementation of sustainable land management (SLM) practices in order to maintain acceptable levels of soil organic matter (SOM). The application of SLM practices helps to maintain soil structure and physical-chemical protection of soil organic carbon (SOC), hence improving soil carbon sequestration and mitigating CO2 emissions to the atmosphere. In an organic, rain-fed almond (Prunus dulcis Mill., var. Ferragnes) orchard under reduced tillage (RT), as the habitual management practice during the 14 years immediately preceding the experiment, we studied the effect of two agricultural management practices on soil aggregate distribution and SOC stabilization after four years of implementation. The implemented practices were (1) reduced tillage with a mix of Vicia sativa L. and Avena sativa L. as green manure (RTG) and (2) no-tillage (NT). Four aggregate size classes were differentiated by wet sieving (large and small macroaggregates, microaggregates, and the silt plus clay fraction), and the microaggregates occluded within small macroaggregates (SMm) were isolated. In addition, three organic C fractions were separated within the small macroaggregates and microaggregates, using a density fractionation method: free light fraction (free LF-C), intra-aggregate particulate OM (iPOM-C), and organic C associated with the mineral fraction (mineral-C). The results show that the combination of reduced tillage plus green manure (RTG) was the most-efficient SLM practice for SOC sequestration. The total SOC increased by about 14% in the surface layer (0–5cm depth) when compared to RT. Furthermore, green manure counteracted the effect of tillage on soil aggregate rupture. The plant residue inputs from green manure and their incorporation into the soil by reduced tillage promoted the formation of new aggregates and activated the subsequent physical-chemical protection of OC. The latter mechanism occurred mainly in the fine iPOM-C occluded within microaggregates and mineral-C occluded within small macroaggregates fractions, which together contributed to an increase of up to 30% in the OC concentration in the bulk soil. No-tillage favored the OC accumulation in the mineral-C within the small macroaggregates and in the fine iPOM-C occluded within microaggregates in the surface layer, and in the mineral-C occluded within the small macroaggregates and microaggregates at 5–15cm depth, but four years of cessation of tillage were not enough to significantly increase the total OC in the bulk soil.

Organic Matter Fractions and Quality of the Surface Layer of a Constructed and Vegetated Soil After Coal Mining. II - Physical Compartments and Carbon Management Index

Soils constructed after mining often have low carbon (C) stocks and low quality of organic matter (OM). Cover crops are decisive for the recovery process of these stocks, improving the quality of constructed soils. Therefore, the goal of this study was to evaluate the effect of cover crops on total organic C (TOC) stocks, C distribution in physical fractions of OM and the C management index (CMI) of a soil constructed after coal mining. The experiment was initiated in 2003 with six treatments: Hemarthria altissima (T1), Paspalum notatum (T2), Cynodon dactylon (T3), Urochloa brizantha (T4), bare constructed soil (T5), and natural soil (T6). Soil samples were collected in 2009 from the 0.00-0.03 m layer, and the TOC and C stocks in the physical particle size fractions (carbon in the coarse fraction - CCF, and mineral-associated carbon - MAC) and density fractions (free light fraction - FLF; occluded light fraction - OLF, and heavy fraction - HF) of OM were determined. The CMI components: carbon pool index (CPI), lability (L) and lability index (LI) were estimated by both fractionation methods. No differences were observed between TOC, CCF and MAC stocks. The lowest C stocks in FLF and OLF fractions were presented by T2, 0.86 and 0.61 Mg ha-1, respectively. The values of TOC stock, C stock in physical fractions and CMI were intermediate, greater than T5 and lower than T6 in all treatments, indicating the partial recovery of soil quality. As a result of the better adaptation of the species Hemarthria and Brizantha, resulting in greater accumulation of labile organic material, the CPI, L, LI and CMI values were higher in these treatments, suggesting a greater potential of these species for recovery of constructed soils.

EFICIÊNCIA DE SOLUÇÕES DENSAS NO FRACIONAMENTO FÍSICO DA MATÉRIA ORGÂNICA DO SOLO

Os mecanismos de estabilização da matéria orgânica (MO) têm sido estudados em solos tropicais e subtropicais brasileiros; no entanto, poucos trabalhos avaliaram a influência da parte metodológica do uso das soluções nos resultados obtidos. Objetivou-se avaliar a eficiência de duas soluções salinas (iodeto de sódio - NaI 1,8 kg dm-3 e politungstato de sódio - PTS 2,0 kg dm-3) na separação de frações densimétricas da MO em dois solos (Argissolo Vermelho do sul do Brasil - 220 g kg-1 argila e Latossolo Vermelho do Cerrado - 630 g kg-1 argila) e o reflexo dessa eficiência na magnitude e importância do mecanismo de proteção física por oclusão em agregados no acúmulo de MO em solos brasileiros. Amostras de solo coletadas nas camadas de 0,00-0,05; 0,05-0,10; e 0,10-0,20 m foram submetidas ao fracionamento físico densimétrico e separadas as frações leve livre (FLL), leve-oclusa (FLO) e pesada (FP) da MO do solo. O uso de PTS 2,0 kg dm-3 aumentou o rendimento de carbono orgânico (CO) das FLL e FLO em ambos os solos em relação à solução de NaI 1,8 kg dm-3, sendo o efeito mais pronunciado na FLO. A utilização do sistema plantio direto (PD) aumentou os estoques de CO total na camada de 0,00-0,05 m do Argissolo em relação ao solo em preparo convencional (PC). O mecanismo de proteção física da MO por oclusão em agregados foi efetivo no aumento dos estoques de C do solo, sendo responsável por aproximadamente ⅓ do acúmulo na camada superficial do Argissolo (0,00-0,05 m). Para o Latossolo, não houve diferenças entre o estoque de CO total do PD e do PC, porém o solo sob PD apresentou acúmulo de aproximadamente ⅔ do estoque de C como FLO na camada superficial do solo. A proteção física por oclusão em agregados é um mecanismo expressivo na estabilização e sequestro de C em solos tropicais e subtropicais, cuja importância pode ser mascarada pela baixa eficiência da solução de NaI em estudos de fracionamento densimétrico da MO do solo.

Molecular chemistry in humic Ferralsols from Brazilian Cerrado and forest biomes indicates a major contribution from black carbon in the subsoil

Soil organic matter (OM) stability plays an important role in the global carbon cycle. The molecular characterisation of soil OM may contribute to an understanding of the feedback mechanisms between soil OM and climate. Umbric Ferralsols with humic properties (humic Ferralsols) are characterised by a thick dark carbon-rich A horizon, and co-occur next to Umbric Ferralsols without humic characteristics. They are concentrated in south-eastern Brazil which suggests an environmental control. In order to understand OM stability in these soils, humic Ferralsols from three Brazilian biomes were studied, including Cerrado (savanna), subtropical forest and tropical forest. The studied soils were developed on several geological substrates, including basalt, gneiss, chamokites, phyllite and tertiary sediments. The molecular composition of the free light fraction (FLF), occluded light fraction (OLF) and 0.1M NaOH extractable OM of samples from topsoil and subsoil from eight profiles was examined using analytical pyrolysis (pyrolysis-GC/MS). The light fraction reflects litter and charcoal, obtained by density fractionation before (FLF) and after (OLF) ultrasonic disruption; the NaOH extractable fraction (humic acid+fulvic acid) reflects more decomposed material. The chemical differences among soils from different Brazilian biomes were much smaller than the vertical gradient within each profile. The results showed that, within the light fractions, the content of which varied between 2% and 22% of the total carbon content (Ct), black carbon (BC) derived pyrolysis products showed a higher abundance in the topsoil. In the extractable OM, which varied between 46% and 88% of Ct, BC derived pyrolysis products showed a higher abundance in the subsoil. This suggests a continuous input of BC and an increase in the degree of decomposition with depth. Pyrolysis products associated with BC contributed up to 10% to the extractable pyrolysable fraction in the subsoil. The results suggest that degradation of charcoal, which is translocated to the subsoil by macroscopic soil organisms, results in a relative high contribution from naphthalene, 2-ethenylnaphthalene, phenanthrene, benzene, dibenzofuran and benzonitrile (subsoil) compared to (other) polyaromatic hydrocarbons (PAHs; C1-, C2- and C3-naphthalenes and fluorene) in relatively fresh charcoal (topsoil). The results highlight the importance of the subsoil in studies of the long term effects of fire on soil OM and suggest that BC is an important factor that underlies carbon stability in humic Ferralsols.

DISTRIBUTION OF ORGANIC CARBON IN DIFFERENT SOIL FRACTIONS IN ECOSYSTEMS OF CENTRAL AMAZONIA

Organic matter plays an important role in many soil properties, and for that reason it is necessary to identify management systems which maintain or increase its concentrations. The aim of the present study was to determine the quality and quantity of organic C in different compartments of the soil fraction in different Amazonian ecosystems. The soil organic matter (FSOM) was fractionated and soil C stocks were estimated in primary forest (PF), pasture (P), secondary succession (SS) and an agroforestry system (AFS). Samples were collected at the depths 0-5, 5-10, 10-20, 20-40, 40-60, 60-80, 80-100, 100-160, and 160-200 cm. Densimetric and particle size analysis methods were used for FSOM, obtaining the following fractions: FLF (free light fraction), IALF (intra-aggregate light fraction), F-sand (sand fraction), F-clay (clay fraction) and F-silt (silt fraction). The 0-5 cm layer contains 60 % of soil C, which is associated with the FLF. The F-clay was responsible for 70 % of C retained in the 0-200 cm depth. There was a 12.7 g kg-1 C gain in the FLF from PF to SS, and a 4.4 g kg-1 C gain from PF to AFS, showing that SS and AFS areas recover soil organic C, constituting feasible C-recovery alternatives for degraded and intensively farmed soils in Amazonia. The greatest total stocks of carbon in soil fractions were, in decreasing order: (101.3 Mg ha-1 of C - AFS) &gt; (98.4 Mg ha-1 of C - FP) &gt; (92.9 Mg ha-1 of C - SS) &gt; (64.0 Mg ha-1 of C - P). The forms of land use in the Amazon influence C distribution in soil fractions, resulting in short- or long-term changes.