Enhance Soil Organic Carbon Stocks Research Articles

Effect of the irregular shelterwood system on soil organic carbon stock and soil quality of Shorea robusta Gaertn. f. forest in Nepal

The effective management of forests relies on the crucial role played bysilvicultural systems. However there exist a significant knowledge gap regarding impact of these systems in Nepalese forests. Therefore, this research was conducted to assess the effects of the forest management activities under irregular shelterwood system on soil organic carbon (SOC) stock and the overall soil quality of Sal (Shorea robusta Gaertn. f.) forests in Terai region of Nepal. Stratified random sampling method with 1.67 % sampling intensity was adopted in this study where management of stands was used as basis of strata. A total of 30 composite soil samples (15 each from managed and unmanaged forest stands) were collected from a depth of 0–30 cm, taken from the four corners and the center of each plot. Soil quality index (SQI) method was used for soil quality assessment using indicators on the basis of prior studies conducted in Nepal. Our study found significant difference in soil parameters except organic carbon, pH, silt, and clay among the managed and unmanaged forest stands (p < 0.05). SOC stock of unmanaged forest stands (48.87 ± 1.34 ton ha−1) was significantly greater than managed forest stands (27.76 ± 1.27 ton ha−1). Similarly, unmanaged forest stands demonstrated better soil quality with higher SQI value (0.66) than managed forest stands (0.50). This negative impact of irregular shelterwood silviculture system highlights the necessity for management interventions to enhance SOC stock and overall soil quality. To establish a robust conclusion, further replication of similar studies at different soil depths and in other management regimes, along with longitudinal studies, is essential.

Tree species identity drives soil carbon and nitrogen stocks in nutrient-poor sites

The establishment of mixed forest stands is an option to enhance soil organic carbon stocks and to protect forest ecosystems from various impacts of climate change. Increasing temperatures and drought potentially affect the vitality of the native coniferous Norway spruce (Picea abies), often used in mixed forests. We investigated the effects of a replacement of Norway spruce by Douglas fir (Pseudotsuga menziesii) admixed to European beech (Fagus sylvatica) on carbon (C) and nitrogen (N) concentrations and stocks, as well as the vertical distribution and changes in forest floor and mineral soil (down to 30 cm depth). Two regions with different soil conditions (loamy vs sandy soils textures) were selected, considering four sites in each region. Each site included a quintet of neighboring forest stands (>85 yrs) of European beech, Douglas fir, and Norway spruce stands as well as mixtures of beech with either Douglas fir or spruce. Our results showed that the C stocks of the organic layer were significantly influenced by tree species, while the C stock of the mineral soil varied among soil types. Total soil organic C stocks demonstrated notable species-specific characteristics, primarily driven by the elevated C stocks in the organic layer. In sandy soils, conifers and mixed forests store 10 % more C and N in the organic layer compared to loamy soils, whereas the C and N stocks under beech remained unaffected, regardless of the site condition. The interaction between species and sites was significant only for Douglas fir and mixed Douglas fir/beech, indicating that the impact of species on C and N varied across sites and was notably pronounced in sandy soils. The higher potential for carbon and N storage in mixed-species forests compared to pure stands emphasizes the capacity of mixed forests to provide valuable ecosystem services, enhancing C sequestration in sandy soils.

Mulch application as the overarching factor explaining increase in soil organic carbon stocks under conservation agriculture in two 8-year-old experiments in Zimbabwe

Abstract. Conservation agriculture (CA), combining reduced or no tillage, permanent soil cover, and improved rotations, is often promoted as a climate-smart practice. However, our understanding of the impact of CA and its respective three principles on top- and subsoil organic carbon stocks in the low-input cropping systems of sub-Saharan Africa is rather limited. This study was conducted at two long-term experimental sites established in Zimbabwe in 2013. The soil types were abruptic Lixisols at Domboshava Training Centre (DTC) and xanthic Ferralsol at the University of Zimbabwe farm (UZF). The following six treatments, which were replicated four times, were investigated: conventional tillage (CT), conventional tillage with rotation (CTR), no tillage (NT), no tillage with mulch (NTM), no tillage with rotation (NTR), and no tillage with mulch and rotation (NTMR). Maize (Zea mays L.) was the main crop, and treatments with rotation included cowpea (Vigna unguiculata L. Walp.). The soil organic carbon (SOC) concentration and soil bulk density were determined for samples taken from depths of 0–5, 5–10, 10–15, 15–20, 20–30, 30–40, 40–50, 50–75 and 75–100 cm. Cumulative organic inputs to the soil were also estimated for all treatments. SOC stocks at equivalent soil mass were significantly (p&lt;0.05) higher in the NTM, NTR and NTMR treatments compared with the NT and CT treatments in the top 5 cm and top 10 cm layers at UZF, while SOC stocks were only significantly higher in the NTM and NTMR treatments compared with the NT and CT treatments in the top 5 cm at DTC. NT alone had a slightly negative impact on the top SOC stocks. Cumulative SOC stocks were not significantly different between treatments when considering the whole 100 cm soil profile. Our results show the overarching role of crop residue mulching in CA cropping systems with respect to enhancing SOC stocks but also that this effect is limited to the topsoil. The highest cumulative organic carbon inputs to the soil were observed in NTM treatments at the two sites, and this could probably explain the positive effect on SOC stocks. Moreover, our results show that the combination of at least two CA principles including mulch is required to increase SOC stocks in these low-nitrogen-input cropping systems.

Interactive effects of agricultural management on soil organic carbon accrual: A synthesis of long-term field experiments in Germany

Crop production often leads to soil organic carbon (SOC) losses. However, under good management practice it is possible to maintain and even re-accumulate SOC. We evaluated how different cropland management techniques affected SOC stocks in the topsoil (0–30 cm depth) of 10 long-term experiments (LTE) in Germany. We found that SOC stocks were particularly enhanced by mineral fertilization and organic amendments like straw incorporation and to a smaller degree by irrigation, but only slightly affected by the choice of preceding crops. In agreement with global meta-analyses, liming and reduced tillage had little or even negative effects on SOC storage, but effects also depended on fertilization. Management effects on SOC stocks were dependent on soil texture: sandy soils showed the lowest SOC stocks of 20.9 ± 2.3 (standard error of the mean) Mg ha−1, but exhibited the largest relative response to different management options. Annual changes in SOC stocks ranged from −3.0 ‰ with no mineral N fertilization, to + 6.1 ‰ with farmyard manure application, using the mineral-fertilized and limed treatment as reference. Even higher rates of up to + 10.6 ‰ yr−1 were reached with the combination of irrigation and straw incorporation. Note that the contribution of organic amendments to SOC accrual and thus to climate change mitigation must be adjusted for reduction in SOC at sites from which straw was removed. Overall, the potential of agricultural management to influence and enhance SOC stocks is significant. This potential is controlled by soil type and land-use duration, is largest for sandy soils with overall lowest SOC stocks, and is characterized by antagonistic and synergistic effects of different management practices.

Particulate organic carbon is more sensitive to nitrogen addition than mineral-associated organic carbon: A meta-analysis

Soil organic carbon (SOC) is heterogeneous and consists of multiple pools differing in physical and chemical properties. SOC stocks are expected to increase in response to nitrogen (N) enrichment. However, the comprehensive understanding and general conclusions about the responses of different SOC pools such as particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) to N addition are still lacking, in spite of the fact that this knowledge is of valuable for the accuracy of predictions of the vulnerability of SOC to N deposition. Here, we conducted a meta-analysis to explore the effects of N addition on POC and MAOC, as well as their relative proportions in bulk SOC. We found that N addition increased POC (20.6%) to a greater extent than MAOC (4.5%). Consequently, N addition significantly increased the proportion of POC in bulk SOC by 10.4%, but significantly decreased the proportion of MAOC in bulk SOC by 2.6%. The response of POC to N addition was negatively correlated with the response of pH and microbial biomass carbon (MBC), while the response of MAOC to N addition was positively correlated with the response of pH and MBC. Notably, the positive effect of N addition on the proportion of POC in bulk SOC decreased as the initial proportion of POC increased. Inversely, the negative effect of N addition on the proportion of MAOC in bulk SOC increased with the initial proportion of MAOC. Overall, our findings suggest that although N could enhance SOC stocks, the larger increment of POC would make the SOC pools more susceptible to future global changes. Our results also highlight the need to explicitly incorporate the differential responses of POC and MAOC into ecosystem models to improve predictions of SOC stock responses to global change.

Effect of different vegetation restoration on soil organic carbon dynamics and fractions in the Rainy Zone of Western China

Vegetation restoration on purple soil (Eutric Leptic Regosols) slopes aiming at reducing soil erosion in the Rainy Zone of Western China has significantly altered soil organic carbon (SOC) storage and distribution. A better understanding of the effects of different vegetation restoration types on SOC dynamics and fractions is critical in devising better policy to protect or enhance SOC stocks to improve soil quality and ecosystem function. In the present study, total, labile, and non-labile organic carbon (TOC, LC, and NLC), and carbon management index (CMI) of Cryptomeria fortunei (CF), mixed C. fortunei and Betula luminifera (MF), Neosinocalamus affinis (NA), and Camellia sinensis (CS) were compared with those of Zea mays field (ZM) on purple soil slopes in the Rainy Zone of Western China in order to develop more effective ways to implement vegetation restoration in the future. Different vegetation restoration types (CF, MF, NA and CS) increased TOC stock by 47.79%–118.31% and NLC stock by 56.61%–129.52% in the 0–50 cm soil layer compared with that of ZM. The direction and magnitude of changes in LC stock and CMI, however, depended strongly on the vegetation restoration type. Compared with ZM, CF had the largest increase of LC stock and CMI, whereas NA had the largest decrease of LC stock and CMI in the 0–50 cm soil layer. The LC:TOC ratio in four reforested species all declined significantly compared with that of ZM (p < 0.01), indicating decreased SOC activity after afforestation. The vegetation type and soil depth together explained more than 90% of the changes of TOC and its fractions in the plantations on purple soil slopes. Our study demonstrates that transforming the ZM into the CS is optimal to achieve the sustainable development goal, whereas transforming the ZM into the NA reduces the SOC activity and availability.

Soil carbon and nitrous oxide dynamics in corn (Zea mays L.) production under different nitrogen, tillage and residue management practices

Reducing tillage frequency and intensity and returning crop residues to the soil are two agricultural management practices to increase soil organic carbon (SOC). Timing and rates of nitrogen fertilizer, coupled with tillage methods and residue incorporation, can also influence nitrous oxide emissions. In this study, we used measured data from a long term (23 years) tillage and residue management experiment, under intensive grain corn production, together with the DNDC (DeNitrification-DeComposition) process-based model, to ascertain the most effective fertilizer and tillage management practices to reduce nitrous oxide emissions and nitrate leaching, and increase soil carbon stocks. The field experiment consisted of no-till [NT], reduced [RT], and conventional tillage [CT], with and without corn residue return. After calibration and validation, using field observed data, the DNDC model was run with 108 management scenarios (six N application rates, spring single N fertilization, spring split N fertilization and fall fertilization, three tillage practices and two residue-return options). N application rate was predicted to be a significant driver for corn yield, SOC sequestration, N2O emissions and nitrate leaching, indicating the need to identify a critical N rate to both optimize production and achieve sustainability goals. Split (vs. single) N application did not significantly affect corn yield, nitrate leaching, N2O emissions or SOC, while fall fertilization should be avoided due to greater nitrate leaching and higher N2O emissions as well as lower crop yield. Although stimulating N2O emissions, both NT and residue return are recommended for their ability to enhance SOC stocks. Results of this study can be used for policy development regarding carbon sequestration and GHG emissions for intensive grain cropping systems in the humid regions of North America.

Additional carbon inputs to reach a 4 per 1000 objective in Europe: feasibility and projected impacts of climate change based on Century simulations of long-term arable experiments

Abstract. The 4 per 1000 initiative aims to maintain and increase soil organic carbon (SOC) stocks for soil fertility, food security, and climate change adaptation and mitigation. One way to enhance SOC stocks is to increase carbon (C) inputs to the soil. In this study, we assessed the amount of organic C inputs that are necessary to reach a target of SOC stocks increase by 4 ‰ yr−1 on average, for 30 years, at 14 long-term agricultural sites in Europe. We used the Century model to simulate SOC stocks and assessed the required level of additional C inputs to reach the 4 per 1000 target at these sites. Then, we analyzed how this would change under future scenarios of temperature increase. Initial stocks were simulated assuming steady state. We compared modeled C inputs to different treatments of additional C used on the experimental sites (exogenous organic matter addition and one treatment with different crop rotations). The model was calibrated to fit the control plots, i.e. conventional management without additional C inputs from exogenous organic matter or changes in crop rotations, and was able to reproduce the SOC stock dynamics. We found that, on average among the selected experimental sites, annual C inputs will have to increase by 43.15 ± 5.05 %, which is 0.66 ± 0.23 MgCha-1yr-1 (mean ± standard error), with respect to the initial C inputs in the control treatment. The simulated amount of C input required to reach the 4 ‰ SOC increase was lower than or similar to the amount of C input actually used in the majority of the additional C input treatments of the long-term experiments. However, Century might be overestimating the effect of additional C inputs on SOC stocks. At the experimental sites, we found that treatments with additional C inputs were increasing by 0.25 % on average. This means that the C inputs required to reach the 4 per 1000 target might actually be much higher. Furthermore, we estimated that annual C inputs will have to increase even more due to climate warming, that is 54 % more and 120 % more for a 1 and 5 ∘C warming, respectively. We showed that modeled C inputs required to reach the target depended linearly on the initial SOC stocks, raising concern on the feasibility of the 4 per 1000 objective in soils with a higher potential contribution to C sequestration, that is soils with high SOC stocks. Our work highlights the challenge of increasing SOC stocks at a large scale and in a future with a warmer climate.

Soil type and cropping system as drivers of soil quality indicators response to no-till: A 7-year field study

Conventional agricultural practices (CAP) highly impacted soil functions involved in the provision of multiple ecosystem services. No-till (NT) has been repeatedly indicated as a key way to enhance soil fertility and biodiversity, while having a positive effect on the environment and climate change. However, experimental evidence to date shows contrasting effects of NT on physical parameters of soil fertility, as well as on soil organic carbon (SOC) storage and soil biodiversity especially if different soil types and cropping systems are considered.The objectives of the present 7-year field study were: (i) to unravel the effect of NT on SOC, soil bulk density, and water stability index (WSI) of soil aggregates across five different soil types (Silty Clay, Silty Clay Loam, Clay Loam, Silt Loam, and Sandy Loam) and cropping systems (silage vs grain production), (ii) to examine how soil fauna (i.e. microarthropods and earthworms) is affected by NT practices under those soil-crop conditions, and (iii) to assess relationships among responses of soil physical indicators (i.e. soil bulk density and WSI), SOC, and soil fauna.Our results showed that soil bulk density was generally not affected by 7-year NT across all soil types. At the same time, NT increased WSI. It follows that NT may increase stable rather than artificial tillage-derived porosity due to reduced soil disturbance, and increased SOC and biological activity. However, reduced importance for boosting aggregates stability should be attributed to NT where initial SOC is high, clay and silt are predominant soil fractions, and the rate of crop residue is low.Our findings suggest that NT may enhance SOC stock (on average 0.66 Mg C ha−1 year−1) in the 0–30 cm soil layer. However, we found (i) a tendency of NT to decrease SOC concentration in the 15–30 cm soil layer (on average −1.18 g C kg−1), and (ii) no SOC increase induced by NT if crop residues were not left onto the soil surface.Nevertheless, NT practices promoted (i) the microarthropods adaptation measured with the QBS-ar index (105.5 vs 64.7), and (ii) the increase of earthworm abundance (412 vs 123 individuals m−2), which are fundamental for enhancing nutrient cycling and soil porosity.Correlations among soil fauna (i.e. QBS-ar and earthworm density), soil physical parameters (i.e. soil bulk density and WSI), and SOC (i.e. concentration and stock) corroborates the hypothesis that soil fauna adaptation and proliferation are main detectors of soil quality and sensitive indicators of changes in soil tillage.

Effect of Land use Types on Soil Organic Carbon Stock at Sire Morose Sub Watershed, Hidabu Abote District of North Shoa Zone, Central Highland of Ethiopia

Information about effects of different land use types on soil organic carbon stock is crucial for best land management practices and combating climate change and enhancing ecological restoration. The study was aimed to estimate the effect of land use types on soil organic carbon stock at sire morose sub watershed Hidbuabote district Ethiopia. Three land use types were selected from the sub watershed (Forest, grazing and cultivated land). Undisturbed core and disturbed composite soil samples were collected randomly from three sites with three replications from each land use type at two varying depths (0-20cm and 20-40cm) and subjected to laboratory soil analysis. Heterogeneity in soil C storage was observed across land use types and along soil depth due to disparity in spatial distribution of soil C densities arising from the influences of variations in land use types and management practices. Accordingly, the total mean values soil organic carbon stocks (SOCS) for forest land was 85.97Mg/ha, which was higher than that of grazing land (83.45Mg/ha) and the lowest being that of cultivated land (49.54Mg/ha). Moreover, the average CO2e sink was 315.51 Mg ha-1, 306.26 Mg ha-1 and 181.81Mg ha-1 in soil of the forest, grazing and cultivated land, respectively. Relatively the result shows potential contribution of forestland use types to enhance soil organic carbon stocks and environmental protection.

Potential of Small Holder Farmers Agricultural Practices in Enhancing Soil Organic Carbon Stock and Other Selected Soil Physico Properties at Akaki District, Ethiopia

Land degradation and extensive use of agricultural lands have led to the decline in soil fertility. To reverse the nutrient deterioration of cultivated lands, farmers have started employing various conservation agriculture practices. This study was initiated to examine the potential of conservation agriculture (integrated practices such as inter cropping, crop rotation, residue retention and minimum tillage) by smallholder farmers to enhance soil organic carbon stock and other selected soil physicochemical properties in Akaki district Bilbilo micro watershed. A systematic sampling method was employed for data collection. Totally 96 composite soil samples (8 plots x 2 systems x 2 replication of site x 3 depth: 0-10cm, 10-20cm and 20-30cm) were collected for analysis. Results showed that soil bulk density (BD) was significantly (p<0.05) varied with practices and depth (p<0.001). It was lower in soil under conservation (0.78 g m-3) than under conventional practice (1.48 gm-3); and in the top layer 0-10 cm (1.21±0.05) than the rest depths. BD showed increasing trend with soil depth across the practices: lower on the top 0-10cm depth (0.78 ±0.03) compared with the rest. The pH was higher (7.28) in conservation than conventional (5.75). The CEC was higher (14.6Cmol (+)/kg) in conservation practice than in the conventional (10.3 cmol (+)/kg). Both pH and CEC had shown increase in the two practices and soil depth due to the leaching of base cation from upper to lower layers. The mean SOC stocks decrease with increasing soil depth about the significant variations with treatments and depth. The SOC stock was higher (110.6t c /h) in conservation practice than in conventional practice (50.22 t/ha). Similarly, total N stocks was also higher (19.5 t c/ha) in conservation practice than in conventional practice (17.4 t c/ha). SOC and total N in both practices had decreased with soil depth due to lower accumulation of organic residue in the lower layer. Likewise, C: N ratios had increases with depth due to similar reason the decrease the amount of soil organic carbon and TN pool (e.g. root biomass) with depth. However, the C: N ratio has higher value in conservation practice (6.51) than in conventional practice (6.05). Most of the measured soil properties were improved in CA aided followed by soil depth compared with conventional agriculture and at soil depth layers ones. The interaction of farming practice types and soil depth also significantly affected all parameters.

Soil Organic Carbon Depletion from Forests to Grasslands Conversion in Mexico: A Review

Land use change from forests to grazing lands is one of the important sources of greenhouse gas emissions in many parts of the tropics. The objective of this study was to analyze the extent of soil organic carbon (SOC) loss from the conversion of native forests to pasturelands in Mexico. We analyzed 66 sets of published research data with simultaneous measurements of soil organic carbon stocks between native forests and pasturelands in Mexico. We used a generalized linear mixed effect model to evaluate the effect of land use change (forest versus pasture), soil depth, and original native forest types. The model showed that there was a significant reduction in SOC stocks due to the conversion of native forests to pasturelands. The median loss of SOC ranged from 31.6% to 52.0% depending upon the soil depth. The highest loss was observed in tropical mangrove forests followed by highland tropical forests and humid tropical forests. Higher loss was detected in upper soil horizon (0–30 cm) compared to deeper horizons. The emissions of CO2 from SOC loss ranged from 46.7 to 165.5 Mg CO2 eq. ha−1 depending upon the type of original native forests. In this paper, we also discuss the effect that agroforestry practices such as silvopastoral arrangements and other management practices like rotational grazing, soil erosion control, and soil nutrient management can have in enhancing SOC stocks in tropical grasslands. The results on the degree of carbon loss can have strong implications in adopting appropriate management decisions that recover or retain carbon stocks in biomass and soils of tropical livestock production systems.

Stocks, composition and vulnerability to loss of soil organic carbon predicted using mid-infrared spectroscopy

Developing a routine and cost effective capability for measuring soil organic carbon (SOC) content and composition will allow identification of land management practices with a potential to maintain or enhance SOC stocks. Coupling SOC content data and mid-infrared (MIR) spectra through the application of partial least-squares regression (PLSR) analyses has been used to develop such a prediction capability. The objective of this study was to determine whether MIR/PLSR analyses provide accurate estimates of the content and composition of SOC that can be used to quantify SOC stocks and its potential vulnerability to loss. Soil was collected from a field trial incorporating a range of land use (pasture, arable cropping and bare fallow) and tillage (intensive, minimum and no tillage) treatments over a nine-year period. The SOC content was measured by dry combustion analysis. Particulate organic carbon was separated from other forms of carbon on the basis of particle size (SOC in the &gt;50 µm fraction). Resistant organic carbon was quantified using solid-state 13C nuclear magnetic resonance. The MIR/PLSR algorithms were successfully developed to predict the natural logarithms of the contents of SOC and POC in the collected soils. With initial calibration, a single MIR analysis could be used in conjunction with PLSR algorithms to predict the content of SOC and its allocation to component fractions. The MIR/PLSR predicted SOC contents provided reliable estimates of the impact of agricultural management on the 0–25-cm SOC stocks, as well as an indication of the vulnerability of SOC to loss. Development of this capability will facilitate the rapid and cost effective collection of SOC content data for detecting the impact of agricultural management treatments on SOC stocks, composition and potential vulnerability to change.

A farm level approach to explore farm gross margin effects of soil organic carbon management

This paper investigates farm gross margin effects of management measures aimed at enhancing soil organic carbon (SOC) stocks to maintain soil fertility while providing important ecosystem services. An optimising farm level model, ScotFarm, is used to investigate the farm gross margin effects of selected SOC management measures for arable farms in Scotland (UK) and Aragon (Spain). The sensitivity of model results to effects on crop yields and costs of production is tested for each measure. The results suggest that considerable regional differences in the financial viability of SOC measures exist. Tillage management is the only measure with positive effects on farm gross margins of Scottish farms at baseline levels of yield effects and input costs. In the case of farms in Aragon, Spain, fertiliser management, crop rotations (with legumes) and tillage management (in later years) show improvements in gross margins. Residue management is estimated to have a negative effect on farm gross margins for both Scottish and Spanish crop farms. Results of the sensitivity analysis indicate that effects of SOC management on farm gross margins are more sensitive to a change in crop yields than to changes in input costs. The findings point to further research needs with respect to the trade-offs between yield effects and changes in input costs arising from the adoption of SOC management measures, and have implications for agricultural policy design aimed at enhancing SOC stocks under a changing climate.

Variability of soil organic carbon stocks and soil CO2 efflux across urban land use and soil cover types

Urban soils are an important soil organic carbon (SOC) store, but soils are also a natural source of carbon dioxide (CO2) due to soil CO2 efflux derived from decomposition and root related respiration. Little is known about the temporal and spatial variability of SOC stocks and soil CO2 efflux in urban areas, in particular in the southern hemisphere. The objectives of our study were to quantify SOC stocks and soil CO2 efflux in tree (urban forests) and grass dominated (parklands) urban ecosystems and to investigate the underlying processes which determine urban SOC stocks and soil CO2 efflux. Soil organic C stocks were quantified to 30cm depth and soil CO2 efflux was determined using a closed chamber technique at 16 sites across Auckland, New Zealand. Vegetation and soil characteristics were measured to 30cm depth at each study plot. In addition, the diurnal and seasonal variability in soil CO2 efflux was assessed at six sites. Median soil organic C stocks (0–10cm) were significantly higher in parkland soils (4.8kgm−2, IQR: 0.9) compared to urban forest soils (2.7kgm−2, IQR: 0.1), which was largely due to higher bulk densities in parkland soils. In contrast, median soil CO2 efflux was not significantly different between parklands (5.2μmolm−2s−1, IQR: 0.8) and urban forests (4.5μmolm−2s−1, IQR: 1.9) sites. This suggests that increasing soil bulk densities without restricting root growth may enhance SOC stocks without increasing soil CO2 efflux. Soil temperature and soil water content explained 54% (urban parkland) and 71% (urban forests) of the temporal variation in soil CO2 efflux. Urban SOC stocks and soil CO2 efflux were similar to non-urban forests and grasslands in New Zealand, highlighting the importance of considering urban soils when assessing SOC stocks and soil CO2 efflux.

Differences in fertilization impacts on organic carbon content and stability in a paddy and an upland soil in subtropical China

The stability of soil organic carbon (SOC) is of great importance in controlling long-term carbon (C) sequestration and feedbacks of soil C pools to climate change. It has been well documented that rice cropping and organic amendments could enhance SOC stocks, while the stability of the sequestered C has not been well understood yet. The objective of this study was to examine the difference in SOC content and stability between a paddy and an upland field under long-term fertilization and to explore the potential link between SOC chemical and biological stability. Soils were collected from two adjacent long-term paddy (annually double-rice cropping) and upland (annually double-corn cropping) experiments with the same soil parent material, where different fertilization regimes were initiated in 1981 and 1986, respectively. The chemical and biological stability of SOC were measured by acid hydrolysis and aerobic incubations, respectively. The C concentration was significantly higher in the paddy soil than in the upland soil, regardless of fertilization regimes. As compared to the initial level, long-term rice cropping without any fertilization (CK) significantly enhanced SOC concentration, whereas corn cropping led to a slight decline. Manure amendments combined with inorganic NPK fertilizers (NPKM) significantly increased SOC content relative to the inorganic NPK application alone treatment (NPK) in both the upland and paddy fields. The paddy soil had significantly greater cumulative C released per gram of soil C (i.e., the biological stability of SOC) than the upland soil. Cumulative C released per gram of soil C was significantly higher in NPKM than in NPK in both the paddy and upland soils, whereas no significant difference was observed between the CK and NPK treatments. Acid hydrolysis could reveal the difference in C stability between the paddy and upland soils, but could not characterize the effect of fertilization. Rice cropping facilitates SOC accumulation relative to upland cropping, whereas the inherent SOC stability is lower under the former than the latter in the site. The lower SOC turnover in the paddy than in the upland is mainly due to anaerobic conditions. Organic amendments significantly enhance SOC, but reduce its stability in both the paddy and upland fields. The sensitivity of acid hydrolysis is relatively low in determining SOC stability.

How will organic carbon stocks in mineral soils evolve under future climate? Global projections using RothC for a range of climate change scenarios

Abstract. We use a soil carbon (C) model (RothC), driven by a range of climate models for a range of climate scenarios to examine the impacts of future climate on global soil organic carbon (SOC) stocks. The results suggest an overall global increase in SOC stocks by 2100 under all scenarios, but with a different extent of increase among the climate model and emissions scenarios. The impacts of projected land use changes are also simulated, but have relatively minor impacts at the global scale. Whether soils gain or lose SOC depends upon the balance between C inputs and decomposition. Changes in net primary production (NPP) change C inputs to the soil, whilst decomposition usually increases under warmer temperatures, but can also be slowed by decreased soil moisture. Underlying the global trend of increasing SOC under future climate is a complex pattern of regional SOC change. SOC losses are projected to occur in northern latitudes where higher SOC decomposition rates due to higher temperatures are not balanced by increased NPP, whereas in tropical regions, NPP increases override losses due to higher SOC decomposition. The spatial heterogeneity in the response of SOC to changing climate shows how delicately balanced the competing gain and loss processes are, with subtle changes in temperature, moisture, soil type and land use, interacting to determine whether SOC increases or decreases in the future. Our results suggest that we should stop looking for a single answer regarding whether SOC stocks will increase or decrease under future climate, since there is no single answer. Instead, we should focus on improving our prediction of the factors that determine the size and direction of change, and the land management practices that can be implemented to protect and enhance SOC stocks.

Variation in organic‐carbon concentration and bulk density in Flemish grassland soils

AbstractThe different management regimes on grassland soils were examined to determine the possibilities for improved and/or changed land management of grasslands in Flanders (Belgium), with respect to article 3.4 of the Kyoto Protocol. Grassland soils were sampled for soil organic carbon (SOC) and for bulk density.For all grasslands under agricultural use, grazing and mowing + grazing led to higher SOC stocks compared with mowing, and grazing had higher SOC stocks compared with mowing + grazing. Overall, 15.1 ± 4.9 kg OC m–2 for the clayey texture, 9.8 ± 3.0 kg OC m–2 for the silty texture, and 11.8 ± 3.8 kg OC m–2 for the sandy texture were found for grassland under agricultural use to a depth of 60 cm. For seminatural grasslands, different results were found. For both the clayey and silty texture, mowing and mowing + grazing led to higher SOC stocks compared with grazing. The clayey texture had a mean stock of 15.1 ± 6.6, the silty texture of 10.9 ± 3.0, and the sandy texture of 12.1 ± 3.9 kg OC m–2 (0–60 cm). Lower bulk densities were found under grazed agricultural grassland compared with mown grassland but for seminatural grassland, no clear trends for the bulk density were found. The best management option for maintaining or enhancing SOC stocks in agricultural grassland soils may be permanent grazed grassland. For seminatural grassland, no clear conclusions could be made. The water status of the sampled mown fields was influencing the results for the clayey texture.Overall, the mean SOC stock was decreasing in the order clay &gt; sand &gt; silt. The higher mean SOC concentrations found for the sandy texture, compared to the finer silty texture, may be explained by the historical land use of these soils.

Forest soils and carbon sequestration

Soils in equilibrium with a natural forest ecosystem have high carbon (C) density. The ratio of soil:vegetation C density increases with latitude. Land use change, particularly conversion to agricultural ecosystems, depletes the soil C stock. Thus, degraded agricultural soils have lower soil organic carbon (SOC) stock than their potential capacity. Consequently, afforestation of agricultural soils and management of forest plantations can enhance SOC stock through C sequestration. The rate of SOC sequestration, and the magnitude and quality of soil C stock depend on the complex interaction between climate, soils, tree species and management, and chemical composition of the litter as determined by the dominant tree species. Increasing production of forest biomass per se may not necessarily increase the SOC stocks. Fire, natural or managed, is an important perturbation that can affect soil C stock for a long period after the event. The soil C stock can be greatly enhanced by a careful site preparation, adequate soil drainage, growing species with a high NPP, applying N and micronutrients (Fe) as fertilizers or biosolids, and conserving soil and water resources. Climate change may also stimulate forest growth by enhancing availability of mineral N and through the CO 2 fertilization effect, which may partly compensate release of soil C in response to warming. There are significant advances in measurement of soil C stock and fluxes, and scaling of C stock from pedon/plot scale to regional and national scales. Soil C sequestration in boreal and temperate forests may be an important strategy to ameliorate changes in atmospheric chemistry.