Changes In Total Soil Organic Carbon Research Articles

Can landscape restoration improve soil carbon stock? A study from Sero Watershed, Northern Ethiopia

Climate change due to anthropogenic induced land degradation is a real challenge to the Ethiopian ecosystem. Hence, Landscape Restoration also called Integrated Watershed Management (IWM) was practiced to rehabilitate the degraded lands. However, its contribution to climate change adaptation and mitigation was not well studied. Hence, this work has examined the role of IWM on improving climate change adaptation and mitigation, taking SOC stock as an indicator. IWM impact time series (1999, 2009 and 2018) data on land use and land cover (LULC) and total soil organic carbon changes were collected at Sero Watershed of northern Ethiopia. Soil samples (84 disturbed and 84 undisturbed) were collected from three soil depths (0–30, 30–60 and 60–90 cm) for soil carbon, bulk density, gravel content analysis and for overall SOC stock estimation. Research findings indicated that vegetation cover increased by 37.2% from 1999 to 2018 at the expenses of bare land and cropland. Thus, in turn has resulted to an overall increase in total SOC stock by 8.5%. It can be concluded that IWM has a significant role to adapt and mitigate the impacts of climate change through increased soil carbon stock in addition to its contribution to landscape restoration. • A two decades landscape restoration led to increased vegetation cover by 37.2%. • The increase in vegetation cover increased total SOC stock by 8.5%. • Improved land restoration implies a significant role to adapt the impacts of climate change.

Fungi determine increased soil organic carbon more than bacteria through their necromass inputs in conservation tillage croplands

Stover mulching over no-till soil is regarded as a promising practice to increase soil organic carbon (SOC) in croplands against climate change. Microbial necromass is a significant source of SOC stock and unequivocally controlled by the microbial community. Yet, a complete link that spans from agricultural practices to microbial community features, to soil necromass C, and eventually to SOC is poorly understood. Here, we conducted a 10-y corn field experiment with five treatments, which included conventional tillage (CT), no-tillage without stover (NT–0), and no-tillage with low, medium, and high amounts of stover mulching (NT–low, NT–medium, and NT–high) in a Molisol of northeastern China. We investigated the stocks and changes in total SOC and its microbial necromass C along a soil depth down to 40 cm, and we evaluated how SOC dynamics and stabilization processes were associated with microbial community features. We characterized microbial community diversity and structure using 16S rRNA and internal transcribed spacer (ITS) sequencing, and we characterized microbial biomass and necromass using phospholipid fatty acid and amino sugar biomarkers. Compared with conventional tillage, no-tillage with medium and high amounts of stover mulching increased SOC stocks in the upper 0–40 cm of soil by > 0.4% per year. No-tillage treatments (without and with stover) had almost no effect on the proportion of total microbial necromass C to SOC, but greatly modified the ratio of fungal necromass C to bacterial necromass C, which increased in top layers (0–5 cm) and decreased in deep layers (10–40 cm). SOC was governed mainly by fungal necromass C, which was correlated positively with fungal biomass. Fungal necromass C, not bacterial necromass C, was more closely associated with microbial community composition. Our results suggested that no-tillage with medium stover mulching was the optimal treatment to achieve the best trade-off between stover input and SOC storage. Differentiating microbial C pools from total SOC and, notably, separating fungal and bacterial necromass C pools can refine our mechanistic understanding of SOC storage as well as its association with microbial biota.

Spatial distribution of soil microbial activity and soil properties associated with

Although much work has been completed in Australia to examine the effects on aboveground ecology of environmental plantings using mixed species of native trees, only limited attention has been focused on their effects on soils and soil microbial population. A study was conducted to determine the spatial distribution of microbial activity, total soil organic carbon (TOC), total nitrogen (TN) and extractable phosphorus (P) in soils under Eucalyptus camaldulensis and Acacia pendula. A 13-year-old environmental planting with mixed native tree species at Gunnedah, New South Wales, was used as a study site. Soil samples were taken from both inside and outside the tree canopy at each of the four compass points (N, S, E and W) at depths of 0–5, 5–10, 10–20, 20–30 and 30–50 cm. The soil was tested for heterotrophic respiration (MicroRespTM), TOC and TN (LECO) and P (Colwell). Microbes were more active inside compared with outside the tree canopy in both A. pendula and E. camaldulensis. The basal respiration rate was significantly higher under A. pendula canopy compared with E. camaldulensis canopy. The relative activity of the microbes and concentrations of TOC, TN and P declined with soil depth. Further, TOC, TN and P contents under the canopy of A. pendula were higher than those of E. camaldulensis and showed a significant positive correlation with basal respiration. However, no difference was detected in the various soil properties measured and microbial activity at four compass points around trees. Changes in soil TOC, TN and extractable P due to the tree plantings were significant only for the 0–5 cm soil depth and changes in microbial activity were mostly confined to the upper 20 cm depth. The improved levels of soil microbial activity and soil nutrients under the tree canopy could be used to measure restoration success of environmental plantings.

Unexpected increases in soil carbon eventually fell in low rainfall farming systems

Understanding the drivers of soil organic carbon (SOC) change over time and confidence to predict changes in SOC are essential to the development and long-term viability of SOC trading schemes. This study investigated temporal changes in total SOC, total nitrogen (N), and carbon (C) fractions (particulate organic carbon - POC, resistant organic carbon - ROC and humus organic carbon - HOC) over a 16-year period for four contrasting farming systems in a low rainfall environment (424 mm) at Condobolin, Australia. The farming systems were 1) conventional tillage mixed farming (CT); 2) reduced tillage mixed farming (RT); 3) continuous cropping (CC); and 4) perennial pasture (PP). The SOC dynamics were also modelled using APSIM C and N modules, to determine the accuracy of this model. Results are presented in the context of land managers participating in Australian climate change mitigation schemes. There was an increase in SOC for all farming systems over the first 12 years (total organic C, TOC% at 0–10 cm increased from 1.33% to 1.77%), which was predominately in the POC% fraction (POC% at 0–10 cm increased from 0.14% to 0.5%). Between 2012 and 2015, there was a decrease in SOC back to starting levels (TOC = 1.22% POC = 0.12% at 0–10 cm) in all systems. The PP system had higher TOC%, POC% and HOC% levels on average and higher SOC stocks to 30 cm depth at the final measurement in 2015 (PP = 30.43 t C ha−1; cropping systems = 23.71 t C ha−1), compared to the other farming systems. There was a decrease in TN% over time in all farming systems except PP. The average C:N increased from 14.1 in 1999 to 19.7 in 2012, after which time the SOC levels decreased and C:N dropped back to 15.8. The temporal change in SOC was not able to be represented by the AusFarm model. There are three important conclusions for policy development: 1) monitoring temporal changes in SOC over 12 years did not indicate long-term sequestration, required to assure “permanence” in SOC trading (i.e. 25–100 years) due to the susceptibility of POC to degradation; 2) without monitoring SOC in reference land uses (e.g. CT cropping system as a control in this experiment) it is not possible to determine the net carbon sequestration, and therefore the true climate change mitigation value; and 3) modelling SOC using AusFarm/APSIM, does not fully represent the temporal dynamics of SOC in this low rainfall environment.

Long-term impact of no-till conservation agriculture and N-fertilizer on soil aggregate stability, infiltration and distribution of C in different size fractions

Soil degradation associated with the loss of soil organic carbon (SOC) has been a major concern in sub-Saharan Africa because of the subsequent yield reduction. It is not fully understood how long-term additional C through biomass and N-fertilizers impact on C distribution in soil aggregates and its effects on soil aggregate stability and infiltration in sub-tropical maize monocropping system. The study, therefore, assessed long-term changes in total SOC (TSOC), aggregate-associated C, particulate organic C (POC), aggregate stability and infiltration in the 0–10, 10–20 and 20–30 cm depths under different tillage systems after 13 years of implementation of the trial. The three tillage systems were no-till (NT), rotational tillage (RT) both with permanent residue cover and conventional tillage (CT) with residue removed. N-fertilizer was applied at a rate of 0, 100 and 200 kg/ha. On average TSOC did not vary (p > 0.05) across the tillage treatments, 27.1 t/ha NT vs 26.0 t/ha RT and 26.6 t/ha CT, but varied with depth where it was stratified in the 0–10 cm depth in NT and RT. Particulate organic C, however, varied significantly (p < 0.05) across the treatments where it decreased with increase in tillage intensity but only in the 0–10 cm depth. Carbon associated with large aggregates (>2000 μm) differed marginally (p = 0.085) with tillage treatment with NT having 38.0 t/ha, RT 36.6 t/ha and CT 29.7 t/ha. However, differences (p < 0.05) were observed in small macroaggregates (250–2000 μm) with NT having 37.8 t/ha, RT 33.5 t/ha and CT 30.4 t/ha in the surface depth. Fertilizer application rate did not seem to affect soil aggregate stability & SOC. The results found a strong effect of residue retention in NT and RT in the soil surface with aggregate stability which, was correlated with the high rate of infiltration rate in these treatments. The results of this study indicate that reduced soil disturbance improves physical protection of SOC, soil structure and infiltration, however, it also indicated that TSOC takes time to improve in maize continuous monocrop system in the studied soil.

The Millennial model: in search of measurable pools and transformations for modeling soil carbon in the new century

Soil organic carbon (SOC) can be defined by measurable chemical and physical pools, such as mineral-associated carbon, carbon physically entrapped in aggregates, dissolved carbon, and fragments of plant detritus. Yet, most soil models use conceptual rather than measurable SOC pools. What would the traditional pool-based soil model look like if it were built today, reflecting the latest understanding of biological, chemical, and physical transformations in soils? We propose a conceptual model—the Millennial model—that defines pools as measurable entities. First, we discuss relevant pool definitions conceptually and in terms of the measurements that can be used to quantify pool size, formation, and destabilization. Then, we develop a numerical model following the Millennial model conceptual framework to evaluate against the Century model, a widely-used standard for estimating SOC stocks across space and through time. The Millennial model predicts qualitatively similar changes in total SOC in response to single factor perturbations when compared to Century, but different responses to multiple factor perturbations. We review important conceptual and behavioral differences between the Millennial and Century modeling approaches, and the field and lab measurements needed to constrain parameter values. We propose the Millennial model as a simple but comprehensive framework to model SOC pools and guide measurements for further model development.

Effects of soil map scales on simulating soil organic carbon changes of upland soils in Eastern China

Digital soil maps with different scales have been widely used to quantify soil organic carbon (SOC) dynamics in the upland-crop fields. However, most of the regional SOC modeling often uses soil maps with a fixed scale, thus how varying map scales influence modeled SOC dynamics has rarely been quantified. Different map scales have different levels of details in describing the soil properties (e.g., soil texture, SOC content, bulk density, and pH) for a specific area, with the high resolution map having finer spatial representation. In this study, six digital upland soil databases of the northern Jiangsu Province in Eastern China at scales of 1:50,000 (P5), 1:250,000 (P25), 1:500,000 (P50), 1:1,000,000 (P100), 1:4,000,000 (P400), and 1:10,000,000 (P1000) have been used to drive the DeNitrification-DeComposition (DNDC, version 9.5) model to quantify SOC changes for the period of 1980–2009. This suite of scales selections covers all basic soil map scales in China. Our simulations indicate that the regional total SOC changes from 1980 to 2009 in the top layer (0–50cm) for upland soils using P5, P25, P50, P100, P400, and P1000 soil maps were 37.89, 34.91, 36.04, 37.05, 39.28 and 38.46 Tg C, respectively. Taking the regional total SOC changes based on the most detailed soil map, P5, as a reference, the relative deviation of those derived from the P25, P50, P100, P400, and P1000 were 7.86%, 4.86%, 2.21%, 3.67%, and 1.51%, respectively. Although the relative deviation of regional total SOC changes for most soil maps are low, disparities exist among SOC changes for different soil groups (i.e. 4.8 to 981%) and for administrative areas level (i.e. 0.32 to 151%). The results indicate that SOC estimates are significantly influenced by soil map scale. Lack of detailed soil information, like representation of many soil types and spatial variations in soil types, in coarse-scale maps result in the large uncertainties in the estimates of SOC changes. This study stresses the needs of detailed soil digital maps for accurate quantification of regional SOC changes.

Evaluating the Potential of Marginal Land for Cellulosic Feedstock Production and Carbon Sequestration in the United States.

Land availability for growing feedstocks at scale is a crucial concern for the bioenergy industry. Feedstock production on land not well-suited to growing conventional crops, or marginal land, is often promoted as ideal, although there is a poor understanding of the qualities, quantity, and distribution of marginal lands in the United States. We examine the spatial distribution of land complying with several key marginal land definitions at the United States county, agro-ecological zone, and national scales, and compare the ability of both marginal land and land cover data sets to identify regions for feedstock production. We conclude that very few land parcels comply with multiple definitions of marginal land. Furthermore, to examine possible carbon-flow implications of feedstock production on land that could be considered marginal per multiple definitions, we model soil carbon changes upon transitions from marginal cropland, grassland, and cropland-pastureland to switchgrass production for three marginal land-rich counties. Our findings suggest that total soil organic carbon changes per county are small, and generally positive, and can influence life-cycle greenhouse gas emissions of switchgrass ethanol.

No-till with high biomass cover crops and invasive legume mulches increased total soil carbon after three years of collard production

ABSTRACTStudies have shown that conversion to conservation tillage from conventional tillage does not increase total soil organic carbon (SOC) significantly in the short term (<5 yrs). While no-till increases total SOC in the medium to long term, it was hypothesized that the inclusion of high biomass cover crops and organic mulches may increase total SOC in the short term. The use of invasive, perennial leguminous species as mulch may sustainably increase SOC during limited-input fall vegetable production. The objective of this study, conducted in Alabama (USA), was to quantify total SOC changes due to organic mulches and forage soybean (Glycine max (L.) Merr. cv. Derry) as a summer cover crop after conversion to no-till during limited-input fall collard (Brassica oleracea L.) production. Forage soybean as a summer cover crop did not increase SOC. No-till without mulch (control treatment) significantly increased SOC at 0–5 cm from 6.3 to 14.0 g kg−1 soil in 3 yrs, whereas inclusion of cut-and-carry mulches increased SOC to ≥22.6 g kg−1 soil. Treatments did not affect collard yield, which averaged 17,863 kg ha−1 yr−1. This represents a novel, limited-input system that may help control on-farm invasive species while sustainably increasing SOC in the short term.

Spatially governed climate factors dominate management in determining the quantity and distribution of soil organic carbon in dryland agricultural systems.

Few studies describe the primary drivers influencing soil organic carbon (SOC) stocks and the distribution of carbon (C) fractions in agricultural systems from semi-arid regions; yet these soils comprise one fifth of the global land area. Here we identified the primary drivers for changes in total SOC and associated particulate (POC), humus (HOC) and resistant (ROC) organic C fractions for 1347 sample points in the semi-arid agricultural region of Western Australia. Total SOC stock (0–0.3 m) varied from 4 to 209 t C ha−1 with 79% of variation explained by measured variables. The proportion of C in POC, HOC and ROC fractions averaged 28%, 45% and 27% respectively. Climate (43%) and land management practices (32%) had the largest relative influence on variation in total SOC. Carbon accumulation was constrained where average daily temperature was above 17.2 °C and annual rainfall below 450 mm, representing approximately 42% of the 197,300 km2 agricultural region. As such large proportions of this region are not suited to C sequestration strategies. For the remainder of the region a strong influence of management practices on SOC indicate opportunities for C sequestration strategies associated with incorporation of longer pasture phases and adequate fertilisation.

Uncertainty of organic carbon dynamics in Tai-Lake paddy soils of China depends on the scale of soil maps

Agro-ecosystem models have been widely used to quantify soil organic carbon (SOC) dynamics based on digital soil maps. However, most of the studies use soil data of single or limited choices of map scales, thus the influence of map scales on SOC dynamics has rarely been quantified. In this study, six digital paddy soils databases of the Tai-Lake region in China at scales of 1:50,000 (P005), 1:200,000 (P02), 1:500,000 (P05), 1:1,000,000 (P1), 1:4,000,000 (P4), and 1:14,000,000 (P14) were used to drive the DNDC (DeNitrification & DeComposition) model to quantify SOC dynamics for the period of 2001⿿2019. Model simulations show that the total SOC changes from 2001 to 2019 in the top layer (0⿿30cm) of paddy soils using P005, P02, P05, P1, P4, and P14 soil maps would be 3.44, 3.71, 1.41, 2.01, 3.57 and 0.10 Tg C, respectively. The simulated SOC dynamics are significantly influenced by map scales. Taking the total SOC changes based on the most detailed soil map, P005, as a reference, the relative deviation of P02, P05, P1, P4, and P14 were 7.9%, 58.9%, 41.6%, 3.9%, and 97.0%, respectively. Such differences are primarily attributed to missing soil types and spatial variations in soil types in coarse-scale maps. Although the relative deviation of P4 soil map for the entire Tai-Lake region is the lowest, substantial differences (i.e., 22⿿1010%) exist at soil subgroups level. Overall, soil map scale of P02 provides best accuracy for quantifying SOC dynamics of paddy soils in the study region. Considering the soil data availability of entire China, P1 soil map is also recommended. This study suggested how to select an appropriate scale of input soil data for modeling the carbon cycle of agro-ecosystems.

Dynamics of soil aggregate-associated organic carbon along an afforestation chronosequence

The objectives of this study were to determine the dynamics of aggregate-associated organic carbon (OC) along an afforestation chronosequence on abandoned farmland of China, and to examine the contributions of changes in aggregate-associated OC to changes in total soil OC. We investigated the dynamics of OC associated with aggregates along an afforestation chronosequence. Water-stable aggregates were isolated, and the OC concentrations in total soil and the aggregates were measured. Averaged across the entire chronosequence, afforestation led to 116, 128 and 108 % average increases in OC concentrations in macroaggregates, microaggregates and the <0.053 mm size class, respectively, in the top 20 cm of soil. The OC stocks in macroaggregates increased by averages of 651 and 473 % at 0–10 and 10–20 cm depths, respectively, mostly within the first 24 years. The OC stocks in microaggregates decreased during the first 35 years and then increased during 48–200 years of afforestation. Averaged across the entire chronosequence, the increases in OC stocks in macroaggregates accounted for 83 and 100 % of the total increase in OC stocks in soils at 0–10 and 10–20 cm depths, respectively. Our results indicated that the accumulation of OC in soils after afforestation on abandoned farmland was mainly due to the accumulation of OC in macroaggregates.

Changes in total soil organic carbon stocks and carbon fractions in sugarcane systems as affected by tillage and trash management in Queensland, Australia

The use of sugarcane trash (tops and residue) retention systems has been reported to lead to increases in total soil organic carbon (TOC) stocks. However, these increases have generally been small and confined to the top 0.05 m of the soil profile. It has been hypothesised that the amount of TOC sequestered could be increased if the intensive tillage that occurs at the end of a sugarcane ratoon cycle, which is known to decrease TOC, could be eliminated. This research examined the effect of no-till management and/or trash retention on four trial sites throughout Queensland, to assess the ability of this management to increase TOC stocks. Management effects on particulate organic carbon (POC), humus organic carbon (HOC), and resistant organic carbon (ROC) stocks were also assessed using mid-infrared spectroscopy. No significant changes in TOC, POC, HOC, or ROC were observed over either 0–0.1 or 0–0.3 m depth at any of the sites examined, when sites were considered as a whole. The results indicate that these management practices currently have limited capacity to increase TOC stocks on these soil types over 0–0.1 or 0–0.3 m depth for the purposes of carbon sequestration.

Soil organic carbon losses due to land use change in a semiarid grassland

Knowledge about the effect of land use change on soil organic carbon (OC) in semiarid grassland is essential for understanding C cycles and for forecasting ecosystem C sequestration. Our objectives were (1) to study the effect of land use change on aggregate size distribution, aggregate-associated OC concentrations, and aggregate-associated stocks in a semiarid grassland area and (2) to relate changes in the aggregate fractions to changes in total soil OC. Cropland and shrubland plots were established in a semiarid grassland area in 1982. We collected soil samples from adjacent grassland, cropland, and shrubland plots 27 years later and measured OC concentrations in the macroaggregate (>0.25 mm), microaggregate (0.25–0.053 mm) and silt+clay (<0.053 mm) fractions. Total soil OC concentrations and stocks decreased significantly after the grassland was converted to cropland or shrubland. Soil microbial biomass C, root biomass, and root C also declined. The proportion of soil in the macroaggregate fraction decreased after conversion to cropland or shrubland. Decreases in macroaggregate-associated OC stocks accounted for more than half of the OC losses that occurred when grassland was converted to cropland. The decreases in macroaggregate-associated OC stocks were due to declines in both macroaggregation and macroaggregate-associated OC concentrations after conversion to cropland. In contrast, decreases in microaggregate-associated OC stocks accounted for more than half of the OC losses when grassland was converted to shrubland. The declines in microaggregate-associated OC stocks were primarily due to a decrease in microaggregate-associated OC concentrations after conversion to shrubland. Land use changed caused significant decreases in soil OC stocks. Conversion to cropland soil resulted in large decreases in macroaggregate-associated OC stocks whereas conversion to shrubland resulted in large decreases in microaggregate-associated OC stocks. Any changes in land use in semiarid grasslands could cause the grassland soil to become a source of atmospheric CO2; therefore extreme caution should be taken to avoid this hazard.

Temporal dynamics of soil organic carbon after land-use change in the temperate zone - carbon response functions as a model approach

Land-use change (LUC) is a major driving factor for the balance of soil organic carbon (SOC) stocks and the global carbon cycle. The temporal dynamic of SOC after LUC is especially important in temperate systems with a long reaction time. On the basis of 95 compiled studies covering 322 sites in the temperate zone, carbon response functions (CRFs) were derived to model the temporal dynamic of SOC after five different LUC types (mean soil depth of 30±6 cm). Grassland establishment caused a long lasting carbon sink with a relative stock change of 128±23% and afforestation on former cropland a sink of 116±54%, 100 years after LUC (mean±95% confidence interval). No new equilibrium was reached within 120 years. In contrast, there was no SOC sink following afforestation of grasslands and 75% of all observations showed SOC losses, even after 100 years. Only in the forest floor, there was carbon accumulation of 0.38±0.04 Mg ha−1 yr−1 in afforestations adding up to 38±4 Mg ha−1 labile carbon after 100 years. Carbon loss after deforestation (−32±20%) and grassland conversion to cropland (−36±5%), was rapid with a new SOC equilibrium being reached after 23 and 17 years, respectively. The change rate of SOC increased with temperature and precipitation but decreased with soil depth and clay content. Subsoil SOC changes followed the trend of the topsoil SOC changes but were smaller (25±5% of the total SOC changes) and with a high uncertainty due to a limited number of datasets. As a simple and robust model approach, the developed CRFs provide an easily applicable tool to estimate SOC stock changes after LUC to improve greenhouse gas reporting in the framework of UNFCCC.

Implications for Soil Properties of Removing Cereal Straw: Results from Long‐Term Studies1

Bioenergy developments could lead to large‐scale removal of cereal straw from fields, with consequences for soil organic carbon (SOC) and related properties. In 25 experiments of 6 to 56 yr duration there was a trend for SOC and total soil N content to increase where straw was incorporated annually. However the increases were only significant in six experiments and were &lt;10% in the majority of cases. Increases in microbial biomass C or N were always proportionately greater than for SOC or N. In simulations of annual straw incorporation using the RothC model, 90% of the microbial biomass C increase in 100 yr was reached within 20 yr as biomass C moves toward a new equilibrium value more rapidly than total SOC. Simulations also showed that if straw was removed in 50% of years, SOC and biomass C increases were about 50% of those with annual straw incorporation. There is considerable evidence that small changes in total SOC have disproportionately large impacts on soil physical properties such as aggregate stability, water infiltration rate, and plow draft and that microbial activity is crucial in the formation of stable aggregates. We conclude that, although changes in SOC resulting from addition or removal of straw are small, it would be unwise to remove straw every year as this is likely to lead to deterioration in soil physical properties. Local assessments are required to determine the frequency of straw removal that is acceptable for soil functioning; this will influence the capacity of bioenergy installations.

Changes of soil organic carbon and its fractions in relation to soil physical properties in a long-term fertilized paddy

Soil organic carbon (SOC) has an important role in improving soil quality and sustainable production. A long-term fertilization study was conducted to investigate changes in SOC and its relation to soil physical properties in a rice paddy soil. The paddy soils analyzed were subjected to different fertilization practices: continuous application of inorganic fertilizers (NPK, N–P–K = 120–34.9–66.7 kg ha −1 yr −1 during 1967–1972 and 150–43.7–83.3 kg ha −1 yr −1 from 1973 to 2007), straw based compost (Compost, 10 Mg ha −1 yr −1), a combination of NPK + Compost, and no fertilization (control). Soil physical properties were investigated at rice harvesting stage in the 41st year for analyzing the relationship with SOC fraction. Continuous compost application increased the total SOC concentration in plough layers and improved soil physical properties. In contrast, inorganic or no fertilization markedly decreased SOC concentration resulting to a deterioration of soil physical health. Most of the SOC was the organo-mineral fraction (<0.053 mm size), accounting for over 70% of total SOC. Macro-aggregate SOC fraction (2–0.25 mm size), which is used as an indicator of soil quality rather than total SOC, covered 8–17% of total SOC. These two SOC fractions accumulated with the same tendency as the total SOC changes. Comparatively, micro-aggregate SOC (0.25–0.053 mm size), which has high correlation with physical properties, significantly decreased with time, irrespective of the inorganic fertilizers or compost application, but the mechanism of decrease is not clear. Conclusively, compost increased total SOC content and effective SOC fraction, thereby improving soil physical properties and sustaining production.

Organic carbon additions: effects on soil bio‐physical and physico‐chemical properties

SummaryThe effects of organic carbon (OC) additions from farm manures and crop residues on selected soil bio‐physical and physico‐chemical properties were measured at seven experimental sites, on contrasting soil types, with a history of repeated applications of farm manure or differential rates of inorganic fertilizer nitrogen (N). Repeated (&gt; 7 years annual additions) and relatively large OC inputs (up to 65 t OC ha−1) were needed to produce measurable changes in soil properties, particularly physical properties. However, over all the study sites, there was a positive relationship between OC inputs and changes in total soil OC and ‘light’ fraction OC (LFOC), with LFOC providing a more sensitive indicator of changes in soil organic matter status. Total soil OC increased by an average of 3% for every 10 t ha−1 manure OC applied, whereas LFOC increased by c. 14%. The measured soil OC increases were equivalent to c. 23% of the manure OC applied (up to 65 t OC ha−1 applied over 9 years) and c. 22% of the crop residue OC applied (up to 32 t OC ha−1 over 23 years). The manure OC inputs (but not crop residue OC inputs) increased topsoil porosity and plant available water capacity, and decreased bulk density by 0.6%, 2.5% and 0.5% with every 10 t ha−1 manure OC applied, respectively. Both OC sources increased the size of the microbial biomass (11% increase in biomass C with 10 t OC ha−1 input), but only manure OC increased its activity (16% increase in the soil respiration rate with 10 t OC ha−1 input). Likewise, the potentially mineralizable N pool only increased with manure N inputs (14% increase with 1 t manure total N ha−1). However, these soil quality benefits need to be balanced with any potential environmental impacts, such as excessive nutrient accumulation, increased nitrate leaching and phosphorus losses and gaseous emissions to the atmosphere.

Microaggregate-associated carbon as a diagnostic fraction for management-induced changes in soil organic carbon in two Oxisols

Carbon stabilization by macroaggregate-occluded microaggregates (Mm) has been proposed as a principal mechanism for long-term soil organic carbon (SOC) sequestration in temperate alternative agricultural and (af)forested systems. The aim of this study was to evaluate the importance of the Mm fraction for long-term C stabilization in Oxisols and to validate its diagnostic properties for total SOC changes upon changes in land use. Soil samples were taken from the 0–5 and 5–20 cm soil layers of native forest vegetation (NV), conventional tillage (CT) and no-tillage (NT) systems at an experimental site near Passo Fundo and one near Londrina in Southern Brazil. After aggregate-size separations by wet-sieving, macroaggregate-occluded water-stable microaggregates (53–250 μm) (Mm) were isolated from large (>2000 μm) and small (>250 μm) macroaggregates. Particulate organic matter located inside the Mm (intra-Mm-POM) and the mineral fraction (< 53 μm) associated with the Mm (mineral-Mm) were separated from the POM fraction located outside the Mm (inter-Mm-POM) by density flotation followed by mechanical dispersion. Sand-free Mm-C concentrations on a macroaggregate basis were generally greater under NV and NT compared to CT in the 0–5 cm depth at both sites. Our findings support the importance of Mm (especially the mineral-Mm fraction) as long-term C-stabilization sites in highly weathered tropical soils under sustainable agricultural and natural systems. At both sites, significant differences in total SOC stocks (g C m −2) among different land use systems were always accompanied by parallel Mm-C stock differences. Though total SOC did not differ among land use systems in the 0–20 cm depth at both sites, Mm-C stocks were greater under NT compared to the CT treatment in the 0–20 cm depth at the Londrina site. We concluded that in these highly weathered tropical soils the Mm-C fraction is a more responsive fraction to management changes than total SOC and represents a diagnostic fraction for present as well as potential total SOC changes upon land-use change.