Soil Carbon Input Research Articles

Variations in Soil Respiration at Different Soil Depths and Its Influencing Factors in Forest Ecosystems in the Mountainous Area of North China

An in-depth understanding of the dominant factors controlling soil respiration is important to accurately estimate carbon cycling in forest ecosystems. However, information on variations in soil respiration at different soil depths and the influencing factors in forest is limited. This study examined the variations in soil respiration at two soil depths (0–10 and 10–20 cm) as well as the effects of soil temperature, soil water content, litter removal, and root cutting on soil respiration in three typical forest types (i.e., Pinus tabulaeformis Carrière, Platycladus orientalis (L.) Franco, and Quercus variabilis Bl.) in the mountainous area of north China from March 2013 to October 2014. The obtained results show that soil respiration exhibited strong seasonal variation and decreased with soil depth. Soil respiration was exponentially correlated to soil temperature, and soil respiration increased with soil water content until reaching threshold values (19.97% for P. tabulaeformis, 16.65% for P. orientalis, and 16.90% for Q. variabilis), followed by a decrease. Furthermore, interactions of soil temperature and water content significantly affected soil respiration at different soil depths of forest types, accounting for 68.9% to 82.6% of the seasonal variation in soil respiration. In addition to soil temperature and water content, aboveground litter and plant roots affected soil respiration differently. In the three forest types, soil respiration at two soil depths decreased by 22.97% to 29.76% after litter removal, and by 44.84% to 53.76% after root cutting. The differences in soil respiration reduction between the two soil depths are largely attributed to variations in substrate availability (e.g., soil organic content) and soil carbon input (e.g., litter and fine root biomass). The obtained findings indicate that soil respiration varies at different soil depths, and suggest that in addition to soil temperature and water content, soil carbon input and dissolved organic substances may exert a strong effect on forest soil respiration.

Simulating soil CO2 emissions under present and climate change conditions in selected vegetation covers of a semiarid region

Changes in soil organic carbon (SOC) can lead to increased concentration of carbon dioxide (CO2) in the atmosphere and eventually contribute to climate change. Simulation models are useful tools to evaluate the effect of future scenarios of climate change on soil CO2 emission in the long term; this study aimed to: (1) evaluate the performance of RothC model using SOC data measured from five different vegetation covers in a semiarid region (southern Iran); (2) assess the impacts of present climate and climate change scenarios on cumulative CO2 emissions from soil; and (3) assess the net effect of climate change on soil CO2 emissions by comparing the two scenarios. The following vegetation covers were studied: rangeland, cypress trees, almond trees, cypress understory and almond understory. Model validation indicated that RothC accurately simulated SOC measured data (R2 = 0.97; Pearson correlation = 0.98; performance efficiency = 0.96). Results showed that climate change effect on soil cumulative CO2 emissions increased by 2050 under all vegetation covers, although not significantly in each vegetation cover in comparison with the present climate scenario. The extent of soil cumulative CO2 emissions may be related to the different decomposability of plant materials and soil carbon input (plant material quantity) in tree covers and rangeland/understory covers, respectively. However, vegetation covers with the highest and lowest soil cumulative CO2 emissions did not correspond with the highest and the lowest values of soil CO2 emissions under the net effect of climate change. In addition, trends of the soil CO2 emissions were decreasing in all vegetation covers during the 2014–2050 period. We argue that under the net effect of climate change SOC will be resistant against further decomposition over time. In fact, easily decomposable materials are fully or partially depleted, and microbial population and decomposition rate of litter materials will decline. This also implies a chemical change of organic matter and the formation of humus complex compounds showing a high resistance to decomposition. Therefore, the study of organo-mineral complex and humus complex compounds in the soils of this region is recommended for future researches.

Soil organic carbon in cropping sequences with predominance of soya bean in the argentinean humid Pampas

AbstractSoya bean (Glycine max (L.) Merr.) monoculture can lead to a decrease in labile fractions of soil organic carbon (SOC). This study sought to evaluate the effects of cover crops (CC), application of fertilizer, and crop rotation on SOC, particulate organic carbon (POC), and soil carbon input in soya bean‐based crop sequences under a no‐till cropping system in the Argentinean Humid Pampas. Five crop sequences at two sites differing in initial SOC were evaluated: continuous soya bean (Sb), continuous soya bean fertilized with phosphorus (P) and sulphur (S) (Sbf), grass CC / PS‐fertilized soya bean (CC/Sbf), nitrogen (N)‐fertilized CC / PS‐fertilized soya bean (CCf/Sbf) and NPS‐fertilized crop rotation with high intensification sequence index (ISI) (Rot). At 0–5 cm, SOC and POC were higher (p < .05) in the sequences with higher residue‐C supply (CC/Sbf; CCf/Sbf and Rot) at both sites. Changes in SOC at 0–20 cm simulated by AMG model closely tracked measured results at 0–20 cm. Findings from this study suggest that the inclusion of CC or crop rotation with high ISI improved C balance in soils under crop sequences with soya bean predominance.

Ecological agriculture intensification through crop-pasture rotations does improve aggregation of Southeastern-Pampas Mollisols

Soil physical health is strongly dependent on aggregate stability (AS) given it determines most soil physical properties. Aggregate stability is associated with total (SOC) and particulate (POC) soil organic carbon and all of them are very sensitive to management practices. Crop-pasture rotations and tillage systems could be combined pursuing ecological agriculture intensification (EAI) to manage soil manipulation and carbon input to improve or maintain AS and SOC and POC. We hypothesized that 1) EAI through rotating pasture periods with cropping improves AS through macroaggregate (MA) formation and stabilization regardless of the tillage system, and 2) reduced disturbance due to no-tillage (NT) during cropping periods allow reducing pasture period frequency without affecting MA formation and stabilization. The aim of this work was to evaluate the effect of different crop-pasture rotations and tillage systems (NT and conventional tillage (CT)) on soil AS and MA proportion and stability, and SOC and POC content of a mollisol of the Southeastern Argentinean Pampas (37° 45' 09’’ S, 58° 18' 48’’ W). Soil samples were taken at 0–5 and 5–20 cm in a long-term experiment (since 1976) to determine aggregate size distribution and stability and SOC and POC content. The sequence intensification index on a monthly basis (SIIm) was calculated and carbon input through crops and pastures was estimated. The increase of SIIm due to crop-pasture rotations led to higher AS and MA stability in the arable layer and especially in the surface 5 cm. This was due to the greater aboveground and root carbon input and lower frequency of tillage associated with pastures respect to cash crops. Cropping systems under NT failed to sequester organic carbon as compared to CT except for the uppermost soil layer, but they successfully improved AS and MA stability. Anyway, regardless of the tillage system, two years of pasture after a cropping period were not enough to take AS up to the level shown by permanent pasture, except under NT at 0–5 cm. The results suggest that despite carbon sequestration failure, cropping periods under NT could be extended (<SIIm) without compromising soil physical health. Contrarily, under CT soil health decreases sharply with the decrease of SIIm. Therefore, the only way to grant better soil functioning under CT is through increasing SIIm with more frequent or longer pasture periods and/or introducing other crops between cash crops. Evidence collected in this loam high-organic-matter soil was not enough to reject the hypotheses.

CO2 emission and its relation to soil temperature, moisture, and O2 absorption in the reforested areas of Cerrado biome, Central Brazil

To characterise the temporal variability in soil CO2 emissions (FCO2), soil O2 influx (FO2), soil water content (SWC), and soil temperature (Ts) and their relations in long-term reforested areas (30 years of conversion) in an Oxisol, Cerrado biome, Brazil. Methods The following land-use changes (Luces) were evaluated: pine (PI), eucalyptus (EU), and native species (NS) reforested areas. The molar ratio between FCO2 and FO2 (respiratory quotient, RQ) was calculated to better understand the process of soil metabolism. Soil CO2 emission was 28% less in PI than in the other LUCs. A model including Ts, SWC, and FO2 could explain 91 and 62% of the FCO2 temporal variability in NS and PI, respectively. The total FCO2 (November 2015 to May 2016) were 11.26, 10.99, and 7.97 Mg ha-1 for EU, NS, and PI areas, respectively (p 1 is associated with the effect of root system-mediated soil respiration. Our results suggest that LUCs influence soil carbon input and output—soil carbon dynamics—by changing soil attributes.

Maize root distributions strongly associated with water tables in Iowa, USA

Root distributions determine crop nutrient access and soil carbon input patterns. To date, root distribution data are rare but needed to improve knowledge and prediction of cropping system sustainability. In this study, we sought to (i) quantify variation in maize (Zea mays) and soybean (Glycine max) roots by depth and environment across Iowa, USA and (ii) identify environmental factors explaining the most variation. Over three years we collected soil cores from 0 to 210 cm in 16 maize and 12 soybean field experiments at grain filling. Root mass, length, carbon (C) and nitrogen (N) were determined at 30 cm increments, coupled with crop, soil, management, and weather-related measurements. Percentage of root mass located in the top 30 cm varied from 52 to 94% in maize and 54–84% in soybean. Variation in maize root distributions was strongly associated with depth to water tables, variation in soybean with soil physical attributes. Root C:N ratios were highly variable with no depth-pattern, averaging 20 and 30 for soybean and maize, respectively. In both crops, specific root lengths increased with depth to 60 cm, and thereafter remained constant. Field studies of roots should consider depth to water tables and soil moisture measurements, as they influence vertical root distributions.

A Bayesian analysis of longitudinal farm surveys in Central Malawi reveals yield determinants and site-specific management strategies.

Understanding the challenges to increasing maize productivity in sub-Saharan Africa, especially agronomic factors that reduce on-farm crop yield, has important implications for policies to reduce national and global food insecurity. Previous research on the maize yield gap has tended to emphasize the size of the gap (theoretical vs. achievable yields), rather than what determines maize yield in specific contexts. As a result, there is insufficient evidence on the key agronomic and environmental factors that influence maize yield in a smallholder farm environment. In this study, we implemented a Bayesian analysis with plot-level longitudinal household survey data covering 1,197 plots and 320 farms in Central Malawi. Households were interviewed and monitored three times per year, in 2015 and 2016, to document farmer management practices and seasonal rainfall, and direct measurements were taken of plant and soil characteristics to quantify impact on plot-level maize yield stability. The results revealed a high positive association between a leaf chlorophyll indicator and maize yield, with significance levels exceeding 95% Bayesian credibility at all sites and a regression coefficient posterior mean from 28% to 42% on a relative scale. A parasitic weed, Striga asiatica, was the variable most consistently negatively associated with maize yield, exceeding 95% credibility in most cases, of high intensity, with regression means ranging from 23% to 38% on a relative scale. The influence of rainfall, either directly or indirectly, varied by site and season. We conclude that the factors preventing Striga infestation and enhancing nitrogen fertility will lead to higher maize yield in Malawi. To improve plant nitrogen status, fertilizer was effective at higher productivity sites, whereas soil carbon and organic inputs were important at marginal sites. Uniquely, a Bayesian approach allowed differentiation of response by site for a relatively modest sample size study (given the complexity of farm environments and management practices). Considering the biophysical constraints, our findings highlight management strategies for crop yields, and point towards area-specific recommendations for nitrogen management and crop yield.

Drivers of soil carbon stabilization in oil palm plantations

AbstractIncreasing soil organic carbon (SOC) in agroecosystems is necessary to mitigate climate change and soil degradation. Management practices designed to reach this goal call for a deeper understanding of the processes and drivers of soil carbon input stabilization. We identified main drivers of SOC stabilization in oil palm plantations using the well‐defined spatial patterns of nutrients and litter application resulting from the usual management scheme. The stabilization of oil palm‐derived SOC (OP‐SOC) was quantified by δ13C from a shift of C4 (savanna) to C3 (oil palm) vegetations. Soil organic carbon stocks under frond piles were 20% and 22% higher compared with harvest paths and interzones, respectively. Fertilization and frond stacking did not influence the decomposition of savanna‐derived SOC. Depending on management zones, net OP‐SOC stabilization equalled 16–27% of the fine root biomass accumulated for 9 years. This fraction was similar between frond piles and litter‐free interzones, where mineral NPK fertilization is identical, indicating that carbon inputs from dead fronds did not stabilize in SOC. A path analysis confirmed that the OP‐SOC distribution was largely explained by the distribution of oil palm fine roots, which itself depended on management practices. SOC mineralization was proportional to SOC content and was independent on phosphorus availability. We conclude that SOC stabilization was driven by C inputs from fine roots and was independent of alteration of SOC mineralization due to management. Practices favouring root growth of oil palms would increase carbon sequestration in soils without necessarily relying on the limited supply of organic residues.

Coupled stalagmite – Alluvial fan response to the 8.2 ka event and early Holocene palaeoclimate change in Greece

We explore the expression of early Holocene climatic change in the terrestrial Mediterranean of southern Greece. A regional palaeoclimate record from stable isotope and trace element geochemical proxies in an early Holocene (~12.4 ka to 6.7 ka) stalagmite is compared to the timing of palaeosol (entisol) development on an early Holocene alluvial fan located <100 km from the stalagmite site. Radiocarbon dated entisol development records fan abandonment surfaces, which can be coupled to the stalagmite climate signal. Variations in δ13C best record the main elements of palaeoclimatic change, more negative values indicating soil carbon input to karst groundwater under wetter conditions. The wettest conditions begin around 10.3 ka, coincident with the start of sapropel 1 deposition in the eastern Mediterranean. The widely documented northern hemisphere ‘8.2 ka event’ of cooler and drier conditions has a muted δ18O climatic signal in common with other stalagmite climate records from the wider Mediterranean. However, less negative δ13C values do record a period of episodic dryness between ~8.8 and ending at 8.2 ka. Wetter conditions re-established after 8.1 ka to the end of the record. The oldest alluvial fan entisols were developing by ~9.5 ka, and a prominent rubified entisol developed ~8.3 to 8.4 ka, indicating pedogenesis within dating error of the 8.2 ka event. The speleothem record of episodic dryness between ~8.8 and 8.2 ka, combined with other regional proxies, is consistent with the notion that precipitation patterns in Greece may have changed from predominantly winter frontal to summer convective during this period. Palaeosol formation on the alluvial fan may have been an allocyclic response to this change. It is plausible that fan-channel incision, driven by temporary development of a ‘flashier’ summer rainfall regime, isolated large areas of the fan surface allowing onset of prolonged pedogenesis there.

Carbon balance and soil carbon input in a poplar short rotation coppice plantation as affected by nitrogen and wood ash application

The increasing importance of short rotation coppice (SRC) plantations for bioenergy production makes it crucial to evaluate their carbon (C) sink potential and to understand which sustainable practices could increase productivity and C sequestration. This study examined the combined effect of woody ash and nitrogen (N) fertilization on the productivity, net C balance (NECB) and soil C sink capacity of a poplar SRC plantation established on a former arable land in northern Italy. Above- (ANPP) and below-ground (BNPP) primary productivity, including both root litter and rhizodeposition (Croot litter), were estimated by a combination of inventory methods, soil respiration measurements and C budgeting approaches. NECB was estimated as changes in both plant and soil C pools, and the portion of soil organic carbon (SOC) derived from poplar leaf and root litter was estimated using a combination of in-growth cores and isotopic measurements. Nitrogen application at 100 kg ha−1 increased ANPP in terms of C accumulation only in the first year, while NECB was not affected by treatments and was 9.96 Mg C ha−1 year−1 on average. Irrespective of the treatment, the plantation accumulated on average 0.7 Mg C ha−1 year−1 of SOC and the poplar-derived soil C input was 3.0 Mg ha−1 year−1. Positive NECB shows that the studied SRC resulted in an important C sink even in the first years after the establishment. However, long-term studies are needed to better assess the effect of N and ash application on C and nutrient cycles over more than one rotation period.

Effects of Soil Management on Aggregation and Organic Matter Dynamics in sub-Saharan Africa

Maintenance of soil organic matter (SOM) is important for soil quality and agricultural productivity. However, little is known about the effects of management practices of different intensities on soil aggregation and SOM dynamics in tropical arable cropping systems of sub-Saharan Africa. We investigated the influence of land use practices and management intensity on soil aggregation and SOM dynamics across 12 long-term field experiments in eastern and western Africa. Aggregate size distribution and SOM were measured in arable systems under contrasting management intensities of high carbon, low carbon and a fallow. Aggregate stability indices and SOM were generally higher in the fallow compared to the arable systems. Fallowing and high carbon inputs in arable soils, significantly improved aggregate stability and carbon (C) and nitrogen (N) stabilization in whole soil, and in aggregate fractions. In contrast, no significant improvements in soil aggregation and C and N stabilization were found when organic inputs were either applied in low quantities or not applied at all, thus resulting in low carbon in soils. Our study showed that fallowing and long-term application of organic amendments alone or in combination with mineral fertilizers were the best among the practices tested in this study, for enhanced C and N stabilization in soils with the subsequent benefits of improving soil physical and chemical properties. These results emphasize the importance of management for sustaining soil quality. It is recommended that fallowing be an integral part of sustainable soil management strategies in these regions. Keywords: Soil aggregation, soil organic matter, carbon, nitrogen, management intensity

Organic carbon dynamics in soils of Mid-Atlantic barrier island landscapes

Geomorphic processes associated with coastal wetlands often result in high carbon accumulation rates and relatively large carbon stocks. Barrier islands are an important component of coastal ecosystems and freshwater and brackish wetlands on these islands provide an important habitat for a number of animal and plant species. However, organic carbon dynamics in these landscapes have received relatively little attention. The objective of this study was to document the accumulation of organic carbon and understand the factors influencing soil carbon dynamics on barrier islands. Ten topographic transects were established on different landforms on Assateague Island National Seashore, MD, USA. Soil organic carbon stocks, aboveground carbon inputs, and decomposition rates were compared among landforms (representing differing degrees of landform stability and soil age) and drainage conditions. Soil organic carbon stocks (0–1.0 m) ranged from 0.49 to 18.8 kg C m−2, and increased in magnitude with soil age. Higher carbon stocks in the older soils were partly attributed to the increased time over which carbon had accumulated. Additionally, a shift from herbaceous dominated to forest dominated plant communities led to greater carbon inputs in older soils. Carbon stocks were also greater in the very poorly and poorly drained soils (relative to drier soils) where high levels of carbon inputs (plant biomass) exceed decomposition rates, which were slowed under anaerobic conditions. While rates of carbon accumulation (average 0.021 kg C m−2 yr−1) are somewhat low compared to other more productive systems, barrier island soils have the potential to store large amounts of organic carbon (relative to other soils) as organic-rich surface horizons are buried during overwash events.

Combinations of soil properties, carbon inputs and climate control the saturation deficit dynamics of stable soil carbon over 17-year fertilizaiton

The soil organic carbon (SOC) saturation deficit (Csd) of silt and clay fractions represents the potential for SOC sequestration in a stable form and can influence organic C stabilization efficiency. Little is known, however, about temporal changes of stable soil Csd and how it is affected by soil properties, climate and C inputs. We investigated the temporal changes in the Csd of fine fractions (<53 μm) and examined the factors controlling these changes at three dry-land sites with 17-year fertilizer management histories in China. The rates of change in the stable soil Csd under manure treatments varied from −0.72 to −1.24% yr−1 after 17 years of fertilization, indicating that stable C levels under manure treatments were significantly higher than those under other treatments. Stable soil Csd was controlled by a combination of soil properties, temperature, and C inputs at all sites, and the higher variance of Csd of fine fractions can be explained by the soil properties (up to 50%). Furthermore, the quantity of C inputs was the most influential variable for stable soil Csd. These results revealed key controls on stable C sequestration potential and indicated the need to develop management strategies to promote stable C sequestration under long-term intensive fertilization.

Long-chain n-alkane and n-fatty acid characteristics in plants and soil - potential to separate plant growth forms, primary and secondary grasslands?

Plant-derived lipid molecular proxies can provide insight into present-day soil carbon input and to what extent organic carbon is degraded within soil. To explore whether soil characteristics of ‘primary’ (i.e., native grasslands above the historic treeline) and ‘secondary’ (i.e., human-modified grasslands below the historic treeline) grasslands reflect recent and/or past vegetation input we compared lipid characteristics in modern vegetation and soil from 30 locations around the historic treeline (1150 m) in Central Otago, New Zealand. At each location the dominant plant species and the mineral topsoil (0–10 cm) were analyzed for a range of n-alkane and n-fatty acid proxies. Grass/herbs and trees differed in their total n-alkane concentration (88.9 vs 232.6 μg/g plant dry weight), even-over-odd predominance (EOP, 10.2 vs 18.3) and n-C20/n-C24 ratio (2.2 vs 6.4). Soil samples under all plant growth forms had considerably lower n-alkane and n-fatty concentrations than the corresponding plant samples. Soil under grass/herbs had lower (4.1) odd-over-even predominance (OEP) values compared to soil under trees (10.1) indicating that lipid degradation under grass/herbs cover was higher. Principle component analysis demonstrated that soil lipid characteristics reflected the current vegetation cover. Principal component analysis also revealed a separation between ‘primary’ and ‘secondary’ grasslands. However, further evaluations are required to confirm the validity of lipid proxies in reconstructing the vegetation history of secondary grasslands.

Carbon dynamics in surface and deep soil in response to increasing litter addition rates in an agro-ecosystem

Increasing atmospheric CO2 concentration and nitrogen deposition are predicted to increase fresh carbon (C) inputs in soils because of higher net primary productivity. However, relationship between soil C cycling and increased inputs of fresh C in soils is not clear. Moreover, most of the studies on soil organic carbon (SOC) focus on upper 20 cm although >50% SOC is present below this depth. We designed this study to unravel the response of SOC to 0% (control), 0.3%, 0.6%, 1.2% and 2.4% addition of maize litter (C4) to a surface (0–15 cm) and deep layers (55–75 cm) of a C3 soil. The isotopic difference between the maize litter and soil allowed distinguishing soil-derived (native) CO2-C from litter-derived CO2-C and calculating the priming effect (PE). Total respiration was measured over 78 d incubation whereas the PE was determined 39 d and 78 d after incubation. Our results showed that all the litter addition levels induced PE -stimulation of mineralization of the extant SOC in response to availability of fresh organic matter (FOM)- in surface and deep soil layers. Moreover, the PE was significantly higher in the deep than surface soil layer in response to all litter additions, except 2.4% treatment during the 2nd harvest. This result suggests that the deep SOC would mineralize at higher rates under similar supply of FOM to surface and deep soils in this agroecosystem. We also showed a significantly positive linear relationship between PE and litter addition rates for both soils implying continuous loss of SOC under increasing organic matter inputs. In conclusion, our study reveals that the deep soil C would be more vulnerable to priming effect in response to litter addition than the surface soil.

Effects of N fertilization and maize straw on the transformation and fate of labeled (15NH4)2SO4 among three continuous crop cultivations

The transformation and fate of fertilizer N applied to soil is particularly important for increasing N retention in soil-crop systems and minimizing N loss from agroecosystems. However, it is obscure how the transformation and fate of fertilizer N will be affected by N input rates and crop residue amendment. In present study, an outdoor experiment was conducted in Mollisols of Northeast China throughout three crop cultivations using 15N-labeled tracer techniques to evaluate the effects of N fertilization levels and maize straw application on the conversion of applied fertilizer N to newly synthesized organic N, recently fixed NH4+, and mineral N in soil-crop systems. Generally, six treatments were set up: no N fertilization (CK), application of only maize straw (M, 16.49 mg N kg−1 soil), low N input levels (N1, 44.64 mg N kg−1 soil), N1 plus maize straw (N1+M, 61.13 mg N kg−1 soil), high N input levels (N2, 89.28 mg N kg−1 soil), and N2 plus maize straw (N2+M, 105.77 mg N kg−1 soil). The results showed that the majority of applied fertilizer N was rapidly transformed into newly synthesized organic N, followed by recently fixed NH4+. Compared with the N1 and/or N2 treatments, maize straw amendment in the N1+M and N2+M treatments significantly increased the residual amount of applied fertilizer N in soil by accelerating the conversion of fertilizer N to newly synthesized organic N or recently fixed NH4+. Among three crop cultivations, the percentages of recovery of applied fertilizer N in crops (PR-Ncrop) and in soil (PR-Nsoil) were averagely 61.1% and 15.8%, respectively, while the percentage of N loss (PL-N) was 23.1%. Applying maize straw significantly elevated the PR-Ncrop and PR-Nsoil but decreased the PL-N compared to those without maize straw. These results suggested that the application of maize straw with high C/N ratio is important to retain applied fertilizer N in soil-crop systems. Balancing carbon and nitrogen inputs in soil through proper combination of fertilizer N and maize straw will be a potentially practical management to increase fertilizer N recovery in crop and soil, and therefore reduce fertilizer N loss from crop-soil systems.

Comprehensive analysis of grazing intensity impacts soil organic carbon: A case study in typical steppe of Inner Mongolia, China

Overgrazing is a primary cause of grassland degradation, including the loss of soil carbon, but comprehensive analysis of the mechanism by which grazing affects soil organic carbon (SOC) is limited. To investigate this mechanism, we measured the contents of total soil organic carbon (TSOC) and active SOC fractions in grazed and ungrazed soil, and we examined the influence of grazing on soil microenvironments, soil carbon input, transformation and utilization, and soil respiration. The experiment was conducted in typical steppe in Inner Mongolia, China, under light, moderate, and heavy grazing plus a no-grazing control. Grazing changed the contents and stocks of the TSOC and active SOC fractions as well as the composition of the SOC. After three years, light and moderate grazing increased TSOC content by 3.44% and 5.43%, respectively, while heavy grazing decreased it by 4.30%. The changes in active SOC fractions were not uniform under different grazing intensities. All grazing intensities increased the stocks of microbial biomass carbon (MBC) and dissolved organic carbon (DOC) compared with the control, but their magnitudes did not rise with increased grazing intensity. Moderate grazing increased the stocks of potentially mineralizable carbon (PMC) and particulate organic carbon (POC), whereas light and heavy grazing decreased them; this may indicate likely long-term changes in SOC following changes in grazing intensity. Light grazing resulted in a larger C input to soil and faster transformation, more emission, and less accumulation of SOC. Moderate grazing led to a smaller C input to soil, and its transformation, utilization and emission rates were between those associated with light grazing and heavy grazing, and there was more accumulation. Heavy grazing resulted in the smallest C input, faster transformation and utilization, less change in emission, and less accumulation. Based on these results, moderate grazing is likely the most practical grazing intensity for the sustainable utilization of this grassland ecosystem to feed livestock. However, because this was a short-term study, the results did not reach significance, and long-term studies are needed.

Lime application lowers the global warming potential of a double rice cropping system

Liming is a common practice to alleviate soil acidification in agricultural systems worldwide. Because liming affects soil microbial activity and soil carbon (C) input rates, it can affect soil greenhouse gas (GHG) emissions as well. However, little is known about the effect of liming on GHG emissions from rice agriculture, one of the main sources of anthropogenic methane (CH4). Here, we report on the first experiment to measure the effect of liming on GHG emissions from rice paddy fields. We studied a double rice cropping system in an acid paddy for two years and measured the impacts of liming on GHG emissions and rice growth with or without straw incorporation. We found that liming reduced CH4 emissions in the early rice season, but it did not affect nitrous oxide (N2O) emissions. Over the two-year study, lime application reduced total CH4 emissions by 12.5% and 15.4% in plots without and with straw incorporation, respectively. Lime application significantly enhanced rice aboveground biomass, while reducing the area- and yield-scaled global warming potential of CH4 and N2O emissions. Lime application stimulated soil respiration during the fallow season and reduced the abundance of methanogens during the early rice growing season. Together, these results suggest that liming reduces CH4 emissions by promoting the decomposition of organic matter during the fallow season, thereby reducing C availability for methanogens. We conclude that in the short term, liming is an effective practice to reduce greenhouse gas emissions from acidic paddy soils.

Biogas production and changes in soil carbon input - A regional analysis

The inclusion of biogas production into the agricultural system has modified crop management and as a result the soil organic carbon (SOC) cycle of the agricultural landscape. To evaluate the effects for the German federal state of Saxony this study determines: (1) the share of agricultural land required for biogas production, (2) the change in regional carbon input fluxes to soil during the time of the establishment of the biogas production considering also the quality of sources of different fresh organic matter (FOM) for the formation of SOC and (3) the differences in carbon input to SOC between the area influenced by biogas production (here “biogas fingerprint area” (BFA)) and the surrounding arable land.Based on the location of biogas plants the region was subdivided into biomass providing units (BPUs) where a part of the arable land was considered as affected by biogas production (BFA). We hypothesized that each biogas plant uses a specific substrate mix according to its capacity. The carbon fluxes for each BPU were estimated for the years 2000 (without biogas plants) and 2011 (with biogas plants). For the year 2011, the analysis included the area demand for production of biogas feedstock and digestate recycling.On average 17.6% of the BPU agricultural land was required to supply the biogas plants and dispose of their digestate. Per kilowatt installed electrical capacity this equates to 2.0 ha, including inter alia 0.4 ha for energy crops. Highest area requirements have been observed for biogas plants with <500 kW installed capacity. Between 2000 and 2011 the total carbon flux into soil increased by 2.1%. When considering the quality of different FOM sources the gain in carbon input was 2.8%. The BFAs showed higher carbon input to soil than the surrounding agricultural land due to high contributions from digestate and crop residues (esp. agricultural grass). This compensated the low carbon input from crop by-products (e.g. straw).

Changes in soil carbon inputs and outputs along a tropical altitudinal gradient of volcanic soils under intensive agriculture

Volcanic soils contain a large stock of soil organic carbon (SOC) which is highly vulnerable to changes in land use and climate warming. In this study we examine the changes of SOC stocks along a tropical elevation gradient (100 m–700 m; +437 mm yr−1 of rainfall and −0.7 °C every 100 m), which is subject to two intensive agricultural systems (banana monoculture and vegetable crops) characterised by the heavy use of fertilisers and pesticides. We hypothesise that in these systems SOC is mainly controlled by soil mineralogy, climatic factors and soil tillage. We used a process-based approach to determine soil C inputs and outputs along the elevation gradient. The banana monoculture systems (198 plots) are characterised by a temporal steady-state of SOC, and the vegetable crop systems (55 plots) present high annual SOC losses. The banana systems were used to determine for the first time the altitudinal change in the in situ rate constant of SOC mineralisation (kSOC). Under banana monoculture, SOC stock increased by 218%, C input from crop residues decreased by 44%, and kSOC decreased by 570% across the altitudinal gradient. These results indicated that the SOC gradient was mainly induced by changes in C outputs. Allophane content increased with altitude (R2 = 0.61; 8 g kg−1 every 100 m) and was positively correlated with SOC content (R2 = 0.51) and negatively correlated with kSOC (R2 = 0.53). We hypothesise that the physical protection of SOC within amorphous allophanic minerals was the main factor responsible for the kSOC and SOC stock gradients. Strictly, the effects of allophane content and rainfall cannot be separated because soil mineralogy in the studied area is mainly determined by the level of rainfall. Our process-based analysis indicated that changes in temperature along the elevation gradient could only explain 22% of the observed change in kSOC. SOC stocks under vegetable crops also increased with altitude; they were 57% lower than under banana at low altitude but only 9% lower at high altitude. This reflected simultaneously the higher C outputs under vegetable crops due to more intensive soil tillage, and the greater resilience to soil disturbance of protected SOC in altitude. The results suggested that neither soil nutrient content nor soil biological properties contributed to the SOC and kSOC gradients under the intensive cropping systems analysed during this study. Overall, we conclude that SOC might be more vulnerable to soil tillage and warming in low-altitude soils.