Changes In Soil Organic Carbon Stocks Research Articles

Soil carbon stock changes under different land uses in the Amazon

Historically, the Amazon region has been undergoing significant land use changes due to the introduction of cattle ranching. However, over the few last years, soybean plantations have increased in importance, with consequences for soil organic carbon (SOC) stocks. We measured the SOC stock changes in a chronosequence of native forest, pasture and soybean sites established during different periods on a farm in Rondônia State (Brazilian Amazon). The native forest was converted to pasture and later to soybean. The pasture sites were 11, 15 and 26years old, while the soybean sites were one (two sites) and 3years old. SOC stocks (0 to 0.3m) of the native forest and the six sites were measured for comparison. Overall SOC stocks (up to 0.3m depth) did not change when forest was converted to pasture. While the overall SOC stock quickly decreased immediately after the pasture was converted to soybean. This information about the increase (pasture) or immediate decrease (pasture-soybean) of SOC stocks after conversion from forest to agricultural areas is important for future studies on soil C dynamics.

Influence of nitrogen fertilization on the net ecosystem carbon budget in a temperate mono-rice paddy

In temperate rice paddy fields, mono-rice is cultivated under flooding for <120days during summer, and thereafter, soil is maintained under the dried upland condition during the cold fallow season. In this region, rice is generally only cultivated by chemical fertilization without organic amendment. Furthermore, almost all rice straw is removed as a livestock feeding material, and then, soil organic carbon (SOC) stock is rapidly depleted with cropping practices. However, the seasonal and annual variations of the soil C balance have not been evaluated in this paddy field. To investigate the nitrogen (N) fertilization effect on SOC stock changes in mono-rice paddy fields, urea was applied at different levels (0–180kgNha−1) as a N fertilizer and the annual C balances were determined by analyzing the net ecosystem C budget (NECB) for 2years. The annual NECB was a negative value (minus 1192 to minus1434kgCha−1year−1), irrespective of the N fertilization rates. This means that these levels of SOC stock could be depleted with cropping practices. The negative NECB was mainly influenced by the highly mineralized C loss during the fallow season and harvest removal during the rice cultivation season. More specifically, the seasonal NECB was largely negative (minus 1679 to minus 1969kgCha−1) during the dried fallow season, but slightly positive (375–661kgCha−1) during flooded rice cultivation. However, the annual and seasonal NECBs were changed by N fertilization according to a quadratic response model. The annual and seasonal NECBs increased with the increasing N fertilization level, maximized at 113–127kgNha−1, and thereafter, decreased. This indicates that the optimum level, not the excess level, of N fertilization is favorable to increase the soil C stock in a temperate mono-rice paddy soil.

Long-term (≥20 years) application of fertilizers and straw return enhances soil carbon storage: a meta-analysis

Increasing soil carbon (C) storage is crucial to addressing climate change and ensuring food security. The C sequestration potential of the world’s cropland soil is 0.4–0.8 Pg soil C year−1, which may be achieved through the adoption of recommended management practices (RMPs), including fertilizer management. This study aimed to quantitatively evaluate the influence of long-term application of different fertilizers and straw retention on soil organic carbon (SOC) storage, to compare the calculated response ratios with Intergovernmental Panel on Climate Change (IPCC)-recommended default relative stock change factors, and to propose recommendations for enhancing SOC sequestration. The meta-analysis indicated that the long-term application of chemical fertilizers (CF), organic fertilizers (OF), combined chemical and organic fertilizers (CFOF), and straw return (SR) significantly enhanced the SOC storage. Response ratios varied significantly (p < 0.05) across different fertilization measures and climatic zones, and was sensitive to the initial SOC content. The mean response ratio was 0.94 for no fertilizer (NF), 1.08 for CF, 1.48 for OF, 1.38 for CFOF, and 1.28 for SR. When IPCC default values for response ratios were applied, SOC storage with OF and CFOF treatments in warm temperate regions with a dry climate was underestimated by 26%, and in the cool temperate region with a moist climate was overestimated by 25% (p < 0.05). Analysis showed that sustained application of organic fertilizers and straw return could be a beneficial measures to mitigate climate change and ensure food security in China. Our findings highlight the importance of deriving SOC stock change factors for a detailed classification of cropland by fertilizer management, climate, and soil types in order to more accurately reflect the effects of policy measures.

Towards complete and harmonized assessment of soil carbon stocks and balance in forests: The ability of the Yasso07 model across a wide gradient of climatic and forest conditions in Europe

Accurate carbon-balance accounting in forest soils is necessary for the development of climate change policy. However, changes in soil organic carbon (SOC) occur slowly and these changes may not be captured through repeated soil inventories. Simulation models may be used as alternatives to SOC measurement. The Yasso07 model presents a suitable alternative because most of the data required for the application are readily available in countries with common forest surveys. In this study, we test the suitability of Yasso07 for simulating SOC stocks and stock changes in a variety of European forests affected by different climatic, land use and forest management conditions and we address country-specific cases with differing resources and data availability. The simulated SOC stocks differed only slightly from measured data, providing realistic, reasonable mean SOC estimations per region or forest type. The change in the soil carbon pool over time, which is the target parameter for SOC reporting, was generally found to be plausible although not in the case of Mediterranean forest soils. As expected under stable forest management conditions, both land cover and climate play major roles in determining the SOC stock in forest soils. Greater mean SOC stocks were observed in northern latitudes (or at higher altitude) than in southern latitudes (or plains) and conifer forests were found to store a notably higher amount of SOC than broadleaf forests. Furthermore, as regards change in SOC, an inter-annual sink effect was identified for most of the European forest types studied. Our findings corroborate the suitability of Yasso07 to assess the impact of forest management and land use change on the SOC balance of forests soils, as well as to accurately simulate SOC in dead organic matter (DOM) and mineral soil pools separately. The obstacles encountered when applying the Yasso07 model reflect a lack of available input data. Future research should focus on improving our knowledge of C inputs from compartments such as shrubs, herbs, coarse woody debris and fine roots. This should include turnover rates and quality of the litter in all forest compartments from a wider variety of tree species and sites. Despite the limitations identified, the SOC balance estimations provided by the Yasso07 model are sufficiently complete, accurate and transparent to make it suitable for reporting purposes such as those required under the UNFCCC (United Nations Framework Convention on Climate Change) and KP (Kyoto Protocol) for a wide range of forest conditions in Europe.

Toward inventory-based estimates of soil organic carbon in forests of the United States.

Soil organic carbon (SOC) is the largest terrestrial carbon (C) sink on Earth; this pool plays a critical role in ecosystem processes and climate change. Given the cost and time required to measure SOC, and particularly changes in SOC, many signatory nations to the United Nations Framework Convention on Climate Change report estimates of SOC stocks and stock changes using default values from the Intergovernmental Panel on Climate Change or country-specific models. In the United States, SOC in forests is monitored by the national forest inventory (NFI) conducted by the Forest Inventory and Analysis (FIA) program within the U.S. Department of Agriculture, Forest Service. The FIA program has been consistently measuring soil attributes as part of the NFI since 2001 and has amassed an extensive inventory of SOC in forest land in the conterminous United States and southeast and southcentral coastal Alaska. That said, the FIA program has been using country-specific predictions of SOC based, in part, upon a model using SOC estimates from the State Soil Geographic (STATSGO) database compiled by the Natural Resources Conservation Service. Estimates obtained from the STATSGO database are averages over large map units and are not expected to provide accurate estimates for specific locations, e.g., NFI plots. To improve the accuracy of SOC estimates in U.S. forests, NFI SOC observations were used for the first time to predict SOC density to a depth of 100cm for all forested NFI plots. Incorporating soil-forming factors along with observations of SOC into a new estimation framework resulted in a 75% (48±0.78Mg/ha) increase in SOC densities nationally. This substantially increases the contribution of the SOC pool, from approximately 44% (17Pg) of the total forest ecosystem C stocks to 56% (28Pg), in the forest C budget of the United States.

Detecting small-scale spatial heterogeneity and temporal dynamics of soil organic carbon (SOC) stocks: a comparison between automatic chamber-derived C budgets and repeated soil inventories

Abstract. Carbon (C) sequestration in soils plays a key role in the global C cycle. It is therefore crucial to adequately monitor dynamics in soil organic carbon (ΔSOC) stocks when aiming to reveal underlying processes and potential drivers. However, small-scale spatial (10–30 m) and temporal changes in SOC stocks, particularly pronounced in arable lands, are hard to assess. The main reasons for this are limitations of the well-established methods. On the one hand, repeated soil inventories, often used in long-term field trials, reveal spatial patterns and trends in ΔSOC but require a longer observation period and a sufficient number of repetitions. On the other hand, eddy covariance measurements of C fluxes towards a complete C budget of the soil–plant–atmosphere system may help to obtain temporal ΔSOC patterns but lack small-scale spatial resolution. To overcome these limitations, this study presents a reliable method to detect both short-term temporal dynamics as well as small-scale spatial differences of ΔSOC using measurements of the net ecosystem carbon balance (NECB) as a proxy. To estimate the NECB, a combination of automatic chamber (AC) measurements of CO2 exchange and empirically modeled aboveground biomass development (NPPshoot) were used. To verify our method, results were compared with ΔSOC observed by soil resampling. Soil resampling and AC measurements were performed from 2010 to 2014 at a colluvial depression located in the hummocky ground moraine landscape of northeastern Germany. The measurement site is characterized by a variable groundwater level (GWL) and pronounced small-scale spatial heterogeneity regarding SOC and nitrogen (Nt) stocks. Tendencies and magnitude of ΔSOC values derived by AC measurements and repeated soil inventories corresponded well. The period of maximum plant growth was identified as being most important for the development of spatial differences in annual ΔSOC. Hence, we were able to confirm that AC-based C budgets are able to reveal small-scale spatial differences and short-term temporal dynamics of ΔSOC.

Soil carbon sequestration due to post-Soviet cropland abandonment: estimates from a large-scale soil organic carbon field inventory.

The break-up of the Soviet Union in 1991 triggered cropland abandonment on a continental scale, which in turn led to carbon accumulation on abandoned land across Eurasia. Previous studies have estimated carbon accumulation rates across Russia based on large-scale modelling. Studies that assess carbon sequestration on abandoned land based on robust field sampling are rare. We investigated soil organic carbon (SOC) stocks using a randomized sampling design along a climatic gradient from forest steppe to Sub-Taiga in Western Siberia (Tyumen Province). In total, SOC contents were sampled on 470 plots across different soil and land-use types. The effect of land use on changes in SOC stock was evaluated, and carbon sequestration rates were calculated for different age stages of abandoned cropland. While land-use type had an effect on carbon accumulation in the topsoil (0-5cm), no independent land-use effects were found for deeper SOC stocks. Topsoil carbon stocks of grasslands and forests were significantly higher than those of soils managed for crops and under abandoned cropland. SOC increased significantly with time since abandonment. The average carbon sequestration rate for soils of abandoned cropland was 0.66MgCha-1 yr-1 (1-20years old, 0-5cm soil depth), which is at the lower end of published estimates for Russia and Siberia. There was a tendency towards SOC saturation on abandoned land as sequestration rates were much higher for recently abandoned (1-10years old, 1.04MgCha-1 yr-1 ) compared to earlier abandoned crop fields (11-20years old, 0.26MgCha-1 yr-1 ). Our study confirms the global significance of abandoned cropland in Russia for carbon sequestration. Our findings also suggest that robust regional surveys based on a large number of samples advance model-based continent-wide SOC prediction.

Increased uncertainty in soil carbon stock measurement with spatial scale and sampling profile depth in world grasslands: A systematic analysis

There is an important need to better understand how the soil organic carbon (SOC) stocks affect the global atmospheric C balance. Grasslands represent 40% of the earth’s land surface but efficiently and effectively quantifying their SOC stocks and their changes is a challenge. Although factors influencing the variability of grassland SOC stocks such as climate, management, or topography have been investigated, very few studies have quantified the uncertainty of SOC stock measurement. Quantifying this uncertainty is critical to determine our ability to detect changes in space or time, and ultimately to develop guidance to help designing appropriate measurement strategies for quantifying carbon stocks and stock changes. SOC stock measurement may be performed at various spatial scales, from local (≤1km2) to broader scale (≥100km2) applications. In addition, the recommended sampling depth for SOC measurement varies according to project purposes, national circumstances, and land use. To our knowledge, the assessment based on world literature of the effect of spatial scale (i.e. size of the measured area) and sampling depth on the uncertainty of SOC stock measurement in grasslands has never been performed. We quantified the uncertainty of SOC stock measurement from a global analysis of 51 research articles meeting strict requirements of rigour of SOC measurements, totaling 177 grasslands from 19 countries, to assess the effects of the spatial scale and the sampling depth of soil profile on this uncertainty, and to explore the implications for SOC stock change detection involving future sampling.We observed that spatial scale and soil profile depth combined to explain 44% of the variability of SOC stock uncertainty, as measured by the coefficient of variation (CV). More specifically, the CV increased with the spatial scale and soil profile depth measured. However, the sampling depth effect is less certain due to lack of data at deeper depths. This uncertainty associated with SOC stock measurements has ramifications for SOC stock change detection. For example, at a fixed 0–30cm soil profile depth, the minimum detectable change (MDC) of SOC stocks between two sampling dates (50 samples) was 14, 20 and 29% at 0.1, 10 and 10 000km2, respectively. At a fixed spatial scale of 0.1km2, the MDC was 12, 14 and 18% for soil profiles of 0–10, 0–30 and 0–100cm, respectively. Finally, this study provides global estimates of the uncertainty that will be useful for planning sampling strategies for SOC stock measurement in various projects across the world and to evaluate the feasibility of SOC stock measurement for different investment levels and timescales.

Global warming potential and greenhouse gas emission under different soil nutrient management practices in soybean-wheat system of central India.

Soil nutrient management is a key component contributing to the greenhouse gas (GHG) flux and mitigation potential of agricultural production systems. However, the effect of soil nutrient management practices on GHG flux and global warming potential (GWP) is less understood in agricultural soils of India. The present study was conducted to compare three nutrient management systems practiced for nine consecutive years in a soybean-wheat cropping system in the Vertisols of India, in terms of GHG flux and GWP. The treatments were composed of 100% organic (ONM), 100% inorganic (NPK), and integrated nutrient management (INM) with 50% organic+50% inorganic inputs. The gas samples for GHGs (CO2, CH4, and N2O) were collected by static chamber method at about 15-day interval during 2012-13 growing season. The change in soil organic carbon (SOC) content was estimated in terms of the changes in SOC stock in the 0-15cm soil over the 9-year period covering 2004 to 2013. There was a net uptake of CH4 in all the treatments in both soybean and wheat crop seasons. The cumulative N2O and CO2 emissions were in the order of INM>ONM>NPK with significant difference between treatments (p<0.05) in both the crop seasons. The annual GWP, expressed in terms of CH4 and N2O emission, also followed the same trend and was estimated to be 1126, 1002, and 896kg CO2 eqha-1year-1 under INM, ONM, and NPK treatments, respectively. However, the change in SOC stock was significantly higher under ONM (1250kgha-1year-1) followed by INM (417kgha-1year-1) and least under NPK (198kgha-1year-1) treatment. The wheat equivalent yield was similar under ONM and INM treatments and was significantly lower under NPK treatment. Thus, the GWP per unit grain yield was lower under ONM followed by NPK and INM treatments and varied from 250, 261, and 307kg CO2 eqMg-1 grain yield under ONM, NPK, and INM treatments, respectively.

Changes of carbon stocks in alpine grassland soils from 2002 to 2011 on the Tibetan Plateau and their climatic causes

Based on field observations, remote sensing, and modeling, recent studies have reported inconsistent changes in soil organic carbon (SOC) stocks in grasslands of the Tibetan Plateau over the past few decades. However, direct evidence about the changes in SOC stocks in the plateau's grasslands coming from in situ, site-by-site, repeated surveys is rare. In this study, we carried out a repeated soil sampling to assess the changes in SOC stocks in the alpine grasslands across the Tibetan Plateau. Across all 41 sites in the alpine grasslands, SOC stocks exhibited a significant increase from 2002 to 2011 at an overall rate of 4.66gCm−2yr−1. Mesic and low-temperature-limited alpine meadows showed an average carbon gain of 25.8gCm−2yr−1, whereas the relatively dry alpine steppes exhibited a slight carbon loss of 11.9gCm−2yr−1. Spatially, the changes in SOC stocks were significantly related to the original SOC stocks across alpine steppes, and soils with low carbon tended to gain carbon. Moreover, the changes in SOC stocks were also associated with March–April precipitation in alpine meadows, and with mean annual precipitation (MAP) in alpine steppes, with drier sites generally gaining carbon. Overall, the alpine grasslands of the Tibetan Plateau significantly accumulated SOC over this 10-year period, but many more site surveys are needed to comprehensively access the changes in SOC stocks across alpine grasslands on the plateau; and management strategies enhancing the ability of C sequestration should differ between alpine meadows and steppes due to their contrasting climate conditions.

Soil organic carbon simulated with the AMG model in a high-organic-matter Mollisol

Soil organic carbon (SOC) management requires a precise knowledge of how it is affected by soil use. Simulation models could help for this purpose. The AMG model is simple, requires information that is easily available, and uses few parameters. This model has neither been calibrated/adjusted nor validated for loamy soils with high SOC concentrations. We hypothesized that AMG would satisfactorily simulate SOC stock changes in soils with these characteristics. The aims of this work were: 1) to adjust and validate AMG for different tillage systems, nitrogen (N) fertilization levels and crop types for loamy-high-SOC Mollisols, and 2) to simulate future SOC changes under different production scenarios. We used SOC stocks (0-20 cm depth) from three long-term experiments (1976-2012) (tillage systems, crop rotations, and N fertilization) in the Southeastern Buenos Aires Province, Argentina (37º 45' S, 58º 18' W) on a complex of Mollisols. Data from two of those experiments was split into two groups to adjust unknown model parameters and for cross validation. Data from the third experiment was used for independent validation. The model was used to simulate SOC stock variation (30 yr) under different combinations of initial SOC stocks (SOCi, three levels) and crop rotations (six rotations regarding continuous cropping and crop-pasture rotations). Model performance was evaluated through statistical indicators based on observed-simulated value differences, and simple linear regression of observed on simulated values. Cross validation yielded promising indicators with the mean observed-simulated value differences close to 0 (&lt;em&gt;P &lt;/em&gt;&amp;gt; 0.05). Root mean square error (RMSE) and RMSE as percentage of the mean of observed values (RMSEp) were 6.0 Mg C ha&lt;sup&gt;-1&lt;/sup&gt; and 7.5%, respectively, which are acceptable. Simple linear regression of observed and simulated values was highly significant (&lt;em&gt;P &lt;/em&gt;&amp;lt; 0.01) with intercept and slope not different from zero and one (&lt;em&gt;P &lt;/em&gt;&amp;gt; 0.05), respectively, although R&lt;sup&gt;2&lt;/sup&gt; was low. Indicators of model performance by groups of treatments were, in general, acceptable and did not show clear trends associated with any management type. However, model performance was poorer under no tillage (NT) and N fertilization probably because of little observed data available for that treatment factor combination. Validation with independent data confirmed that AMG simulated SOC change satisfactorily. Future scenario simulations showed that when the SOCi stock was high (close to SOC saturation), even rotations with high intensification and carbon input produced a SOC stock decrease. Conversely, when the SOCi stock was low (35% loss of SOC with respect to saturation) all scenarios led to a SOC stock increase. However, AMG failed to acceptably simulate the expected effect of pastures in the rotation. The AMG model satisfactorily simulated SOC stock changes due to different management techniques of soils with a loamy surface texture and high original SOC stock. Therefore, the model could be used as a tool to help management planning with an admissible simulation error (RMSEp ~6%).

An exponential change decline function to estimate soil organic carbon stocks and their changes from topsoil measurements

Soil organic carbon (SOC) stocks and their changes are important indicators in ecosystem service assessments. Routine soil inventories are often limited to the topsoil, even though a non‐negligible fraction of SOC is known to be stored in deeper horizons. To assess SOC stocks and their changes in the upper metre of the soil profile, vertical extrapolation of topsoil SOC measurements is necessary. The commonly used exponential decline function is not valid, however, for soil types in which subsurface horizons with a larger SOC content (‘anomalies’) occur. Here, we propose an exponential change decline function to account for these profile anomalies. Therefore, we applied the exponential decline function to the difference between the recent (2008–11) and historical (1947–74) SOC contents in the topsoil and compared the results with those derived by the original method. We applied the exponential change decline function to 54 041 agricultural land units (7159 km2) in Flanders (Belgium) and were able to model specific profile characteristics such as spodic horizons, plaggic topsoil and peat substrates. For these particular land units, the exponential decline function underestimated SOC stocks; therefore, it compromised an in‐depth assessment of changes in SOC stocks over time. This study shows that the exponential change decline function is promising for certain soil types and will contribute to the more accurate assessment of ecosystem service indicators. In addition, we emphasize the need for more detailed descriptions of subsoil reference profiles, sampled by pedogenetic horizon rather than by fixed depth interval to optimize calibration of the decline functions.HighlightsWhen recent soil sampling is limited to the topsoil, extrapolation is needed to assess subsoil SOC stocks. We modified the exponential decline function to model SOC‐rich subsurface horizons with the integration of legacy data. An appropriate extrapolation approach is essential for in‐depth SOC assessments. Detailed subsoil data are needed to optimize the calibration of the decline functions.

Can Soil pH Be Used to Help Explain Soil Organic Carbon Stocks?

Topsoil (0–20 cm) data of 25 997 sites were analyzed across five land uses in the Jiangsu Province of China. The results showed that the residential/industrial land stored the highest amount of soil organic carbon (SOC) among all the land uses. The polynomial model can best describe the relationship between SOC density (SOCD) and pH, showing that SOCD increases over the pH range of 4.2–6.5, decreasing sharply in the range of 6.5–9.2. The reason may be that microbial activity tends to be maximal at intermediate soil pH levels (pH 6–7). The relationship between SOCD and pH was found to vary substantially with the land use. Soil pH can explain 4.3–63.8% of the variation in SOCD for all land uses, suggesting that this variable should be introduced in SOC models to better predict SOC stock changes in the future.

Impact of woody encroachment on soil organic carbon storage in the Lopé National Park, Gabon

AbstractThis study quantifies changes in soil organic carbon (SOC) stock as a result of woody encroachment on savannas. Changes in SOC stocks occur below 30 cm depth, indicating the subsoil as the principal compartment contributing to SOC sequestration, and suggesting the need to consider the entire profile (0–100 cm) to thoroughly assess the effect of woody encroachment on SOC stocks.

Changes of soil organic carbon stocks and CO2 emissions at the early stages of urban turf grasses’ development

Urbanization coincides with remarkable expansion of turf grasses and their increasing role in environmental processes and functions, including carbon (C) sequestration. Soil organic carbon (SOC) stocks in turf grass soils are substantial, however, an intensive soil respiration is also likely. Therefore C sequestration in turf grasses remains uncertain, especially at the early stages after development, when C uptake and CO2 emissions are unbalanced. We analyzed changes in SOC stocks and CO2 emissions at the experimental turf grasses in Moscow megapolis during the three years period after establishment. An influence of the three contrast depths of organic layers (5, 10 and 20 cm) on soil and biomass C and on the ornamental functions of turf grasses was studied. Total CO2 emission from the turf grasses during the observation period exceeded C uptake in grass and root biomass by two to three times. Therefore the turf grasses at the early stages of development are important source of biogenic C. Although the C losses were substantial, CO2 emission decreased and C uptake in biomass increased by the end of the observation period. The highest ratio of sequestered and emitted C was obtained for the thick turf grass soil constructions with a 20 cm organic layer. The highest ornamental value, indicated by the projective cover and sprout density, was also obtained for the thick turf grasses, which is essential to consider for developing the best management practices and sustainable turf grass soil constructions.

Changes in soil carbon stock under the wheat-based cropping systems at Vojvodina province of Serbia

ABSTRACTThe aim of this study was to assess the changes in soil organic carbon (SOC) stock in relation to the carbon (C) input from nine wheat-based cropping systems and untilled grass. The SOC pool ranged from 32.1 to 49.4 Mg ha−1 at 0–20 cm and from 94 to 171 Mg ha−1 at 0–100 cm for the arable soil, while in untilled grassland, it was higher (54 and 185 Mg C ha−1, respectively). SOC stock was observed to be lower at the unfertilized 2-year rotation and higher at the 4-year rotation with manure and mineral fertilization. The study showed a winter wheat yield decrease of 176.8 kg ha−1 for a 1- Mg ha−1 SOC stock change in the 0–20-cm soil depth. The estimated C input for SOC stock maintenance was from 266 to 340 g C m−2 year−1 for winter wheat and rotations, respectively. Additional C input did not increase the SOC pool, suggesting that arable plots had a limited ability to increase SOC. These results provide guidance for the selection of management practices to improve C sequestration.

Soil organic carbon dynamics in typical durum wheat-based crop rotations of Southern Italy

Mediterranean agricultural areas are dominated by cropping systems based on winter cereals crops, summer irrigated crops, foragebased systems, and mixed succession with bare fallow. Soil organic carbon (SOC) is widely used to assess the environmental performance of these cropping systems, since it is strongly influenced by management practices and environmental conditions. This study evaluates the sustainability of representative intensive cropping systems of Southern Italy, in terms of SOC stock changes and CO2 emissions in the long-term perspective, using a process-based model (RothC10N) combined with a GIS-based spatialization procedure. On the basis of SOC modelling, results showed that crop management practices currently adopted by farmers did not guarantee SOC sequestration in all the rotations (–4.29 Mg C ha–1). The sustainability of cropping systems can be improved through management practices such as the retention of crop residues into the field and/or the rational use of irrigation for the summer crop (6.73 Mg C ha–1). This finding could help policy makers to provide suggestions for a more effective local implementation of agro-environmental measures.

Distributions of carbon in calcareous soils under different land uses in western Iran

Concentrations of Natural stable and unstable carbon in ecosystems have been used extensively to help to understand a wide range of soil processes and functions. This study was conducted to explore the effects of land use changes on different carbon fractions (F1, F2, F3 and F4), permanganate oxidizable carbon (POXC), soil organic carbon (SOC) and total organic carbon (TOC) associated with soils in calcareous soils of western Iran. Four popular land uses in the selected site including natural forest, range land, dryland farming and irrigated farming systems were employed as the basis of soil sampling. The results showed a strong relationship between land use conversion and SOC stocks changes. The greatest mean values for carbon content and the least mean values of CaCO3 in bulk topsoil (0–15 cm) in the forest land were observed. Dryland farming had the least both active and passive pools of C in comparison with the other land uses. The positive and significant correlations was observed between SOC, Total C and POXC contents and different C fractions. Taking C and POXC pools into account, a more definitive picture of the soil C is obtained than when only total C is measured. The influence of land use changes on overall soil carbon stocks could be helpful for making management decision for farmers and policy makers in the future, for enhancing the potential of C sequestration in western Iran.

Biochemical quality and accumulation of soil organic matter in an age sequence of Cunninghamia lanceolata plantations in southern China

Biochemical protection is an important mechanism for maintaining the long-term stability of the soil carbon (C) pool. The labile and recalcitrant pools of soil organic matter (SOM) play different roles in regulating C and N dynamics; however, few studies have characterized the capacity of soil C sequestration while considering the biochemical quality of SOM. The aim of the present study was to assess the changes in the soil organic carbon (SOC) and nitrogen (N) pools during a traditional rotation period (25 years) of a Chinese fir (Cunninghamia lanceolata) plantation with an emphasis on SOM biochemical quality. Three different forest stand development stages—young (6 years old), middle-aged (16 years old) and mature (25 years old)—were selected for soil sampling to a depth of 100 cm. Total C and total N of the soil was analysed to determine the changes in the SOC and N stocks among the three development stages using an equivalent soil mass (ESM) approach. Bulk soils were fractionated into labile and recalcitrant fractions using the acid hydrolysis method to identify the quality of SOM. The mineral soil organic carbon pool at a 1-m depth slightly decreased from the young stand to the middle-aged stand and rapidly increased by 28 % to reach a maximum in the mature stand. SOC accumulation in the surface soil predominated the changes in total SOC stocks in all three stands. The increased N was reflected in the entire depth, and the highest soil N accumulation was in the mature stand. The recalcitrant C concentration and SOC were positively correlated. The non-hydrolysable C proportion was lower in the middle-aged stand versus the young stand (8.69 % loss), while the labile C percentage was higher (13.89 % gain). In the mature stand, the recalcitrant C index increased to 39.84 %. The recalcitrant index of C decreased with an increasing soil depth, whereas the recalcitrant index of N dramatically increased. These results highlighted the significant effect of the stand age and the soil depth on the storage and biochemical availability of SOM in Chinese fir plantations of southern China. The recalcitrant index of C changed with the change in SOC concentration, indicating that biochemical protection mechanism plays an important role in soil C sequestration. In addition, more attention should be paid to subsoil C protection in the management of Chinese fir plantations because of low biochemical stability.

Soil types influence predictions of soil carbon stock recovery in tropical secondary forests

Tropical forests are major sinks of terrestrial carbon (C) both above- and below-ground. As a consequence their destruction and degradation is considered the second largest anthropogenic source of carbon dioxide to the atmosphere. Also contributing to the changing dynamics of the global carbon cycle is the widespread and significant expansion of secondary forest. Secondary forests that colonise abandoned agricultural lands can potentially recover above-ground C stocks to historical levels in a few decades. However, the dynamics of below-ground C stored as soil C stocks are unaccounted for in several tropical regions. Similarly, although parent materials are known to differ in chemical and physical properties, little is known about the relationships of soil C stocks with environmental predictors and whether they interact with soil types during natural forest regeneration. We investigated whether soil organic carbon (SOC) stocks change with secondary forest age in two contrasting soil types (derived from either basalt or granite). Soil and vegetation parameters were analysed to determine the best predictors of SOC stock changes in secondary forests. SOC stocks from 24 secondary forests (up to 69years since pasture abandonment) were compared with those from active pastures and mature forests. We found that clay-rich soils (originating from basalt parent material) store higher amounts of SOC, although these stocks remain unchanged as secondary forests matured. In contrast, SOC stocks in granite soils tend to be lower in young secondary forests and increase rapidly to levels comparable to mature forests. Moreover, our analysis indicated that soil pH and woody plant diversity are strong candidates as predictors of SOC stock variations, yet it appears this is within the context of soil type. Our results support the contention that models predicting SOC stocks during forest succession should not rely only on secondary forest age. Instead, predictions of SOC stocks can be improved with the inclusion of basic information on vegetation cover and soil type (especially soil texture).