Soil Organic Carbon Stocks as Affected by Tillage Systems in a Double-Cropped Rice Field

Evaluating soil organic carbon stock changes induced by no-tillage based on fixed depth and equivalent soil mass approaches

Quantifying changes in soil organic carbon (SOC) stocks and other soil properties is essential for understanding how soils will respond to land management practices and global change. Although they are widely used, comparisons of SOC stocks at fixed depth (FD) intervals are subject to errors when changes in bulk density or soil organic matter occur. The equivalent soil mass (ESM) method has been recommended in lieu of FD for assessing changes in SOC stocks in mineral soils, but ESM remains underutilized for SOC stocks and has rarely been used for other soil properties. In this paper, we draw attention to the limitations of the FD method and demonstrate the advantages of the ESM approach. We provide illustrations to show that the FD approach is susceptible to errors not only for quantifying SOC stocks but also for soil mass-based properties such as SOC mass percent, C:N mass ratio, and δ13 C. We describe the ESM approach and show how it mitigates the FD method limitations. Using bulk density change simulations applied to an empirical dataset from bioenergy cropping systems, we show that the ESM method provides consistently lower errors than FD when quantifying changes in SOC stocks and other soil properties. To simplify the use of ESM, we detail how the method can be integrated into sampling schemes, and we provide an example R computer script that can perform ESM calculations on large datasets. We encourage future studies, whether temporal or comparative, to utilize sampling methods that are amenable to the ESM approach. Overall, we agree with previous recommendations that ESM should be the standard method for evaluating SOC stock changes in mineral soils, but we further suggest that ESM may also be preferred for comparisons of other soil properties including mass percentages, elemental mass ratios, and stable isotope composition.

The duration of soil organic carbon (SOC) sequestration in agricultural soils varies according to soil management, land-use history and soil and climate conditions. Despite several experiments have reported SOC sequestration with the adoption of no-tillage (NT) in Mediterranean dryland agroecosystems scarce information exists about the duration and magnitude of the sequestration process. For this reason, 20 years ago we established in northeast Spain a NT chronosequence experiment to evaluate SOC sequestration duration under Mediterranean dryland conditions. In July 2010 we sampled five chronosequence phases with different years under NT (i.e., 1, 4, 11, and 20 years) and a continuous conventional tillage (CT) field, in which management prevailed unchanged during decades. Soil samples were taken at four depths: 0–5, 5–10, 10–20 and 20–30 cm. The SOC stocks were calculated from the SOC concentration and soil bulk density. Furthermore, we applied the Century ecosystem model to the different stages of the chronosequence to better understand the factors controlling SOC sequestration with NT adoption. Differences in SOC stocks were only found in the upper 5 cm soil layer in which 4, 11 and 20 years under NT showed greater SOC stocks compared with 1 year under NT and the CT phase. Despite no significant differences were found in the total SOC stock (0–30 cm soil layer) there was a noteworthy difference of 5.7 Mg ha−1 between the phase with the longest NT duration and the phase under conventional tillage. The maximum annual SOC sequestration occurred after 5 years of NT adoption with almost 50% change in the annual rate of SOC sequestration. NT sequestered SOC over the 20 years following the change in management. However, more than 75% of the total SOC sequestered was gained during the first 11 years after NT adoption. The Century model predicted reasonably well SOC stocks over the NT chronosequence. In Mediterranean agroecosystems, despite the continuous use of NT has limited capacity for SOC sequestration, other environmental and agronomic benefits associated to this technique may justify the maintenance of NT over the long-term.

The sustainable land management program (SLMP) of Ethiopia aims to improve livelihoods and create resilient communities and landscape to climate change. Soil organic carbon (SOC) sequestration is one of the key co-benefits of the SLMP. The objective of this study was to estimate the spatial dynamics of SOC in 2010 and 2018 (before and after SLMP) and identify the SOC sequestration hotspots at landscape scale in four selected SLMP watersheds in the Ethiopian highlands. The specific objectives were to: 1) comparatively evaluate SOC sequestration estimation model building strategies using either a single watershed, a combined dataset from all watersheds, and leave-one-watershed-out using Random Forest (RF) model; 2) map SOC stock of 2010 and 2018 to estimate amount of SOC sequestration and potential; 3) evaluate the impacts of SLM practices on SOC in four SLMP watersheds. A total of 397 auger composite samples from the topsoil (0–20 cm depth) were collected in 2010, and the same number of samples were collected from the same locations in 2018. We used simple statistics to assess the SOC change between the two periods, and machine learning models to predict SOC stock spatially. The study showed that statistically significant variation (P < 0.05) of SOC was observed between the two years in two watersheds (Gafera and Adi Tsegora) whereas the differences were not significant in the other two watersheds (Yesir and Azugashuba). Comparative analysis of model-setups shows that a combined dataset from all the four watersheds to train and test RF outperform the other two strategies (a single watershed alone and a leave-one-watershed-out to train and test RF) during the testing dataset. Thus, this approach was used to predict SOC stock before (2010) and after (2018) land management interventions and to derive the SOC sequestration maps. We estimated the sequestrated, achievable and target level of SOC stock spatially in the four watersheds. We assessed the impact of SLM practices, specifically bunds, terraces, biological and various forms of tillage practices on SOC using partial dependency algorithms of prediction models. No tillage (NT) increased SOC in all watersheds. The combination of physical and biological interventions (“bunds + vegetations” or “terraces + vegetations”) resulted in the highest SOC stock, followed by the biological intervention. The achievable SOC stock analysis showed that further SOC stock sequestration of up to 13.7 Mg C ha--1 may be possible in the Adi Tsegora, 15.8 Mg C ha-1 in Gafera, 33.2 Mg C ha-1 in Azuga suba and 34.7 Mg C ha-1 in Yesir watersheds.

Enhancing soil organic carbon (SOC) stocks through fertilization and crop rotation will contribute to sustaining crop productivity and mitigating global warming.&amp;#160;Although it is known that cropping systems may affect SOC stocks by influencing the balance between C input and C decomposition, only few studies focused on the impact of different rice cropping systems on SOC stock changes in paddy soils.&amp;#160;In this study, we analyzed the differences in SOC stocks and their driving factors in the topsoil (0&amp;#8211;20 cm) with various fertilization measures in two rice-based cropping systems (i.e. rice-wheat rotation and double rice rotation systems) over the last four decades from seven long-term experiments in the Yangtze River catchment. The treatments include no fertilizer application (CK), application of chemical nitrogen, phosphorus and potassium fertilizers (NPK) and a combination of NPK and manure (NPKM). Results showed that during the last four decades, the topsoil SOC stock significantly increased by 8.6 t ha-1&amp;#160;on average under NPKM treatment in rice-wheat system and by 2.5&amp;#8211;6.4 t ha-1&amp;#160;on average under NPK and NPKM treatments in double rice system as compared with CK. A higher SOC sequestration rate and a longer SOC sequestration duration were found in NPKM treatment than that in NPK treatment in both cropping systems. The highest relative SOC stock percentage (SOC stock in fertilized treatments to CK) was observed under the NPKM treatment in both cropping systems, though no significant difference was found between these two cropping systems. However, the fertilization-induced relative increase of the SOC stock was 109.5% and 45.8% under the NPK and NPKM treatments, respectively in the rice-wheat system than that in the double rice system. This indicates that the rice-wheat system is more conducive for SOC sequestration. RF and SEM analyses revealed that the magnitude and influencing factors driving SOC sequestration varied between two systems. In the double rice system, continuous flooding weakens the influence of precipitation on SOC sequestration and highlights the importance of soil properties and C input. In contrast, soil properties, C input and climate factors all have important impacts on SOC sequestration in rice-wheat system. This study reveals that the rice-wheat system is more favorable for SOC sequestration despite its lower C input compared to the double rice system in China&amp;#8217;s paddies.

Afforestation changes soil organic carbon stocks on sloping land: The role of previous land cover and tree type

Bioenergy crops are expected to provide biomass to replace fossil resources and reduce greenhouse gas emissions. In this context, changes in soil organic carbon (SOC) stocks are of primary importance. The aim of this study was to measure changes in SOC stocks in bioenergy cropping systems comparing perennial (Miscanthus × giganteus and switchgrass), semi‐perennial (fescue and alfalfa), and annual (sorghum and triticale) crops, all established after arable crops. The soil was sampled at the start of the experiment and 5 or 6 years later. SOC stocks were calculated at equivalent soil mass, and δ13C measurements were used to calculate changes in new and old SOC stocks. Crop residues found in soil at the time of SOC measurements represented 3.5–7.2 t C ha−1 under perennial crops vs. 0.1–0.6 t C ha−1 for the other crops. During the 5‐year period, SOC concentrations under perennial crops increased in the surface layer (0–5 cm) and slightly declined in the lower layers. Changes in δ13C showed that C inputs were mainly located in the 0–18 cm layer. In contrast, SOC concentrations increased over time under semi‐perennial crops throughout the old ploughed layer (ca. 0–33 cm). SOC stocks in the old ploughed layer increased significantly over time under semi‐perennials with a mean increase of 0.93 ± 0.28 t C ha−1 yr−1, whereas no change occurred under perennial or annual crops. New SOC accumulation was higher for semi‐perennial than for perennial crops (1.50 vs. 0.58 t C ha−1 yr−1, respectively), indicating that the SOC change was due to a variation in C input rather than a change in mineralization rate. Nitrogen fertilization rate had no significant effect on SOC stocks. This study highlights the interest of comparing SOC changes over time for various cropping systems.

How much can we increase biomass yield by promoting land conversion from annual to perennial crops? Will increased biomass extraction for biorefineries reduce soil organic carbon (SOC) and total nitrogen (TN) stock? Which cropping system is more stable for biomass production over time? To our knowledge, no study has concurrently investigated the effects of land conversion from annual to perennial crops on biomass yield, yield stability, and changes in SOC and TN stock, which limits the understanding and application of sustainable agroecosystems producing biomass for biorefineries. Based on five-year continuous observations in central Jutland Denmark, our results showed that perennial crops significantly increased biomass yield by 19% and yield stability by 88% compared to annual crops. Perennial crops significantly increased SOC content by 4% and SOC stock by 11% at 0–100 cm depth across the five years. The opposite responses of SOC content and stock under annual and perennial crops led to even more significant differences between the crop types. Perennial crops had no effect on soil TN content and increased soil TN stock to one meter depth by 22%, whereas continuous annual crops had no effect on it. Neither annual nor perennial crops had effects on SOC and TN stock when estimated based on equivalent soil mass because the soil density increased under perennial crops. Our results showed that changes in SOC and TN stock between annual and perennial crops varied with the specific calculating methods (fixed depth/equivalent mass), thus the selected methods should be clearly defined in the future research. Increases in SOC content at one meter depth were positively correlated with biomass yield and yield stability, suggesting a win-win strategy for climate mitigation and food security. Altogether, our results highlight the potential to redesign the current cropping system for sustainable intensification by selecting proper perennial crops for green biorefineries.

Refining benchmarks for soil organic carbon in Australia’s temperate forests

&amp;lt;p&amp;gt;Bioenergy crops are expected to provide biomass to replace fossil resources and reduce greenhouse gas emissions. In this context, their effect on soil carbon sequestration is of primary importance. There is a wide range of candidate crops including perennial C4 crops or annual crops but their impact on soil organic carbon (SOC) stocks remain very uncertain as shown by the wild variability in published experimental results.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;In this study, we measured the changes in SOC stocks under perennial (miscanthus and switchgrass), semi-perennial (fescue and alfalfa) and annual (triticale and sorghum or maize) bioenergy crops managed with two N fertilisation rates. The experiment called &amp;amp;#8220;Biomass &amp;amp; Environment&amp;amp;#8221; is located in northern France on a deep loamy soil (Haplic Luvisol) and was set up in 2006. The soil was sampled at the start of the experiment, in 2011-2012 and again in 2018 (0-60 cm, 5 layers). SOC stocks were calculated at equivalent soil mass and &amp;amp;#948;&amp;lt;sup&amp;gt;13&amp;lt;/sup&amp;gt;C was systematically measured and used to calculate changes in new and old SOC stocks. In 2018, the SOC distribution in different soil particle-size fractions was also characterized for some treatments.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;After 12 years, there was a large increase in SOC concentration (+7.6 g kg&amp;lt;sup&amp;gt;-1&amp;lt;/sup&amp;gt; on average) under perennial crops in the surface layer (&amp;amp;#8776; 0-5 cm) but a slight decrease in deeper layers. Changes in &amp;amp;#948;&amp;lt;sup&amp;gt;13&amp;lt;/sup&amp;gt;C also showed that more than half of the new SOC accumulated in the surface layer. In addition, the additional SOC storage in the first layer was found in coarse organic fractions (50-200 and 200-2000 &amp;amp;#956;m) but also in the more stabilised 0-50 &amp;amp;#956;m fraction. SOC concentration under semi-perennial crops increased in the two first layers (&amp;amp;#8776; 0-20 cm), from 10.2 g kg&amp;lt;sup&amp;gt;-1&amp;lt;/sup&amp;gt; in 2006 to 11.6 g kg&amp;lt;sup&amp;gt;-1&amp;lt;/sup&amp;gt; in 2018 on average and slightly decreased below. Under annual crops, a decrease in SOC concentration was observed in all layers and particularly in the third layer (&amp;amp;#8776; 20-33 cm). There was no significant effect of the N fertilisation. Over the old ploughed layer (&amp;amp;#8776; 0-33 cm), SOC stocks increased between 2006 and 2018 under perennial and semi-perennial bioenergy crops (by 3 and 2 t C ha&amp;lt;sup&amp;gt;-1&amp;lt;/sup&amp;gt; on average respectively) and decreased by 7 t C ha&amp;lt;sup&amp;gt;-1&amp;lt;/sup&amp;gt; on average under annual crops.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;This study show that different bioenergy crops can have contrasted impacts on SOC stocks but also on SOC distribution in the soil profile.&amp;lt;/p&amp;gt;

Understanding the process of land use/cover changes (LUCC) can provide experience on the enhancement of soil organic carbon (SOC) stocks and carbon sequestration potential for different areas. This study is uniquely to divide different ecological functional areas, and originally combine the machine learning method and soil carbon pool dataset for regional comparative analysis, to compare and quantitatively analyze the drivers of LUCC and the changes in SOC stocks effected by LUCC over 30 years. The results show that topography and climate changes are the main drivers affecting LUCC in four natural areas, while soil factors and population changes do not cause significant effects. The total SOC stocks in Qinghai was increased by 71.18 Tg C and 107.19 Tg C in 0–30 cm and 0–300 cm layers, respectively, and the highest SOC stocks within 0–300 cm were in Pastoral area. Desert and Gobi area had the lowest SOC stocks in both 0–30 cm and 0–300 cm layers. SOC stocks increased in both 0–30 cm and 0–300 cm layers only in Sanjiangyuan Natural Reserve, while the Desert and Gobi area showed a decrease in both over 30 years. This study emphasizes the significant impact of grassland changes on SOC stocks, indicating the importance of considering these changes in land management and ecological protection policies. The initial and original SOC stocks of pre-LUCC may influence the SOC stocks in post-LUCC. The response of SOC stocks changes to LUCC was varies in different areas. The heterogeneity of different ecological functional areas is affected by multiple factors and SOC stocks will become more complex among these areas in the future. These findings contribute to the development of ecological protection policies and the enhancement of regional land management strategies.

Organic farming (OF) enhances top soil organic carbon (SOC) stocks in croplands compared with conventional farming (CF), which can contribute to sequester C. As farming system differences in the amount of C inputs to soil (e.g. fertilization and crop residues) are not enough to explain such increase, shifts in crop residue traits important for soil C losses such as litter decomposition may also play a role. To assess whether crop residue (leaf and root) traits determined SOC sequestration responses to OF, we coupled a global meta‐analysis with field measurements across a European‐wide network of sites. In the meta‐analysis, we related crop species averages of leaf N, leaf‐dry matter content, fine‐root C and N, with SOC stocks and sequestration responses in OF vs. CF. Across six European sites, we measured the management‐induced changes in SOC stocks and leaf litter traits after long‐term ecological intensive (e.g. OF) vs. CF comparisons. Our global meta‐analysis showed that the positive OF‐effects on soil respiration, SOC stocks, and SOC sequestration rates were significant even in organic farms with low manure application rates. Although fertilization intensity was the main driver of OF‐effects on SOC, leaf and root N concentrations also played a significant role. Across the six European sites, changes towards higher leaf litter N in CF also promoted lower SOC stocks. Our results highlight that crop species displaying traits indicative of resource‐acquisitive strategies (e.g. high leaf and root N) increase the difference in SOC between OF and CF. Indeed, changes towards higher crop residue decomposability was related with decreased SOC stocks under CF across European sites. Synthesis and applications. Our study emphasizes that, with management, changes in crop residue traits contribute to the positive effects of organic farming (OF) on soil carbon sequestration. These results provide a clear message to land managers: the choice of crop species, and more importantly their functional traits (e.g. leave and root nitrogen), should be considered in addition to management practices and climate, when evaluating the potential of OF for climate change mitigation.

Secondary forest-to-rubber (Hevea brasiliensis) plantation conversion is an important recent land-use change in the montane regions of mainland Southeast Asia. This land-use conversion caused a reduction of soil organic carbon (SOC) stocks by on average 19% down to 1.2m over 46years. Due to the mountainous topography of the region, most rubber plantations include narrow terraces parallel to contours. Manual terrace construction involves cutting of the soil from the upper slope and piling up the removed soil on the soil surface downslope. Soil redistribution by terrace construction may affect SOC dynamics through exposure of the subsurface soil at the terrace inner sides (cut section) and soil burial at the terrace outer edges (fill section).Our study, conducted in southern Yunnan province of China, aimed to quantify SOC stock changes induced by terrace construction. In three rubber plantations aged 5, 29 and 44years, we systematically sampled the terraces according to soil redistribution zones, and the original sloping areas in between the terraces were used as reference.At the cut section of the terrace, topsoil removal caused a depletion of SOC stocks in the youngest plantation followed by SOC stock recovery in the two oldest plantations. The recovery of SOC stocks at the cut section in the two oldest plantations was attributed to the capacity of the exposed subsurface soil to store new organic carbon inputs from roots and litter, and to sedimentation of eroded topsoil materials from the upper slope. At the fill section of the terrace, soil deposition resulted in higher total SOC stocks compared to the reference position in all plantations. This was due to the deposition of redistributed soil material on top of the original soil surface combined with the partial preservation of carbon in the buried soil. Overall, the increase of SOC in the exposed subsurface soil at the cut sections, and the partial preservation of SOC in the buried soil at the fill sections resulted in higher SOC stocks down to 1.2m at the terraces compared to the reference positions in the two oldest plantations. Our results imply that terracing may alleviate SOC losses caused by the conversion of secondary forest to terraced rubber plantation.

Reliable determination of the soil organic carbon stock (SOCS) and its time trend at field scale is a key condition to value soil organic carbon (SOC) sequestration as a negative emission technology (NET) at farm level. Limiting the stock estimation to 30 cm depth is acceptable on the range of some decades (Balesdent et al., 2018). The carbon stock, however, is not directly estimated from the SOC content. SOC content must be multiplied by the bulk density (BD) of the corresponding layer. BD determination is time consuming and tedious to determine, and changes with time due to soil swelling with water, soil tillage, and changes in SOC. Therefore, the changes in SOCS must be monitored on an equivalent soil mass (ESM) basis, by referring to the sampled soil mass of the previous sampling rather than to a constant depth layer. Corrections of the mass, simplification of the soil mass determination overcoming the BD determination issue, as well as a simplified one-layer method have been proposed (Wendt and Hauser, 2013). However, this simplified ESM method requires the sampling and analysis of at least two layers for sampled mass correction. Moreover, the field volume percentage of the coarse (&gt; 2 mm) fraction must be determined and removed from the sampled layer volume, which is not well documented. On the other hand, and to our best knowledge, private companies providing SOCS certificates sample the soils at constant depth using mechanical gauges that do not allow to control the quality of the extracted core. Finally, the errors associated with these different technical options needs to be clarified.This study was performed using samples collected in 60 fields from different farms of the Swiss Leman-Lake region. It aimed at providing a full reliable methodology to determine SOCS at field scale, while solving the remaining issues, namely to determine the errors associated to the different parameters estimated and to simplify the ESM one-layer method to decrease the sampling and analytical costs. The minimum detectable change was determine (i) for sampling performed using the mechanical gauges at constant depth, (ii) for the ESM one-layer method as described in (Wendt and Hauser, 2013), (iii) the additional error introduced by coarse fraction estimation and gauge diameter and (iv) a simplification of the one-layer ESM method taking into account local average properties of the soil below the 0-30 cm sampled layer.

Soil organic carbon content and stock change after half a century of intensive cultivation in a chernozem area