Equivalent Soil Mass Approach Research Articles

A simple soil mass correction for a more accurate determination of soil carbon stock changes

Agricultural soils can act as a sink for large quantities of soil organic carbon (SOC) but can also be sources of carbon to the atmosphere. The international standard for assessing SOC stock and measuring stock change stipulates fixed depth sampling to at least 30 cm. The tendency of bulk density (BD) to decrease with decreasing disturbance and increasing SOC concentration and the assumption of constant SOC and BD within this depth profile promotes error in the estimates of SOC stock. A hypothetical but realistic change in BD from 1.5 to 1.1 g cm−3 from successive fixed depth sampling to 30 cm underestimates SOC stock change by 17%. Significant effort has been made to evaluate and reduce this fixed depth error by using the equivalent soil mass (ESM) approach, but with limited adoption. We evaluate the error in SOC stock assessment and change generated from fixed depth measurements over time relative to the ESM approach and propose a correction that can be readily adopted under current sampling and analytical methods. Our approach provides a more accurate estimate of SOC stock accumulation or loss that will help incentivize management practice changes that reduce the environmental impacts of agriculture and further legitimize the accounting practices used by the emerging carbon market and organizations that have pledged to reduce their supply chain greenhouse gas (GHG) footprints.

Implication of land use shifting on land degradation and restoration potential of conservation agriculture in India's North-West Himalayan region

The soil organic matter is a crucial factor in determining soil characteristics and productivity; however various land management practices degrade or aggrade the soil health. The objective of this study was to look at the influence of land-use conversion on soil health by using the concept of stratification ratio (SR) of soil organic carbon (SOC) and total nitrogen (TN) and appraise SR as a predictor of SOC and TN stock and soil health for India's North-West Himalayan region. The research is oriented toward assessing the consequences of various land use regimes' impacts on SOC and TN depth distribution, storage, and stratification and, hence, identifying appropriate sustainable tillage techniques for the region. The research was accomplished in 2020–21 in the long-term experimental plot with four land uses, namely control [natural sal forest (Shorea robusta L.), conventional tillage (CT), reduced tillage (RT), and zero tillage (ZT)] in a rainfed system of north-western Indian Himalayas. A decrease of 79% in the mean weighted diameter of CT was observed after conversion from forest land to CT; however, the decrease was only 50% in the case of the adoption of ZT. Further, the surface soil (0–5 cm) SOC was significantly different from each other, with forest soil having the highest SOC (27.5 ± 0.21 g kg−1) and CT having the lowest SOC (11.0 ± 0.09 g kg−1). The stock of SOC and TN increased significantly with increment in soil depth, and among landuses, the highest SOM was observed with forest and the lowest with CT. Among the treatments, forest (56.56 ± 1.90 Mg ha−1) had significantly higher SOC storage than conservation agriculture (CA) (42.84 ± 0.27 Mg ha−1, ZT, and 41.41 ± 1.84 Mg ha−1, RT) and CT (41.33 ± 1.19 Mg ha−1) based on equivalent soil mass approach. For forest land use, except the surface layer (0–5:5–10), all the soil layers had SR >2, whereas, for ZT, the bottom two layers (0–5:20–25 and 0–5:25–30) had SR >2 and for RT, only the bottom layer (0–5:25–30) was having SR > 2. It was observed that the conversion of land use to CT reduced the SR of SOM drastically; however, by adopting CA, the SR had been restored to near normal in forest land use. The carbon pool index (CPI) was used to determine the effects of soil tillage and residue incorporation on soil quality improvement with respect to a sal forest. The CPI value increased significantly with an increase in soil depth for three land uses, and also, at each soil depth, the CPI followed the ZT > RT > CT trend. Thus, CA may be considered a viable alternative to CT for improving soil physicochemical parameters.

Carbon stocks differ among land-uses in agroforestry systems in western Canada

Agroforestry systems (AFS) are increasingly being recognized as sustainable land-use systems that can increase carbon (C) sequestration and improve food security. However, the relative contributions of various C pools to the overall C stock of AFS and their dependence on factors such as tree age, species diversity and density is still unclear in many regions. Using an equivalent soil mass approach, we investigated the effect of three different AFS (hedgerow, shelterbelt, and silvopasture) and their component land-use types (treed areas and adjacent cropland/grassland) on various C pools across 36 sites in central Alberta, Canada. On average, the treed area within AFS stored 178.5 t ha−1 more C relative to adjacent cropland or grassland, although the total C stock did not differ among the three AFS types. Most of the C in the treed area of AFS was stored in the mineral soil (61%), overstory trees (28%), and the LFH (litter, partially decomposed litter and humus, 10%) layer, while midstory trees, shrubs, and understory vegetation together accounted for the remaining 1% of the C storage. The C stock size in the treed area was closely associated with tree stand characteristics of the three AFS, with the C pool size in the LFH positively related to tree age. Our results highlight that the amount of C stored in the LFH layer is substantial and should be considered when assessing C storage within AFS and suggest that investing in AFS can be an effective strategy for meeting part of Canada's C reduction goals to mitigate climate change.

The effects of management practices on soil organic carbon stocks of oil palm plantations in Sumatra, Indonesia

The rapid increase in global production of and demand for palm oil has resulted in large-scale expansion of oil palm monoculture in the world's tropical regions, particularly in Indonesia. This expansion has led to the conversion of carbon-rich land-use types to oil palm plantations with a range of negative environmental impacts, including loss of carbon from aboveground biomass and soil. Sequestration of soil organic carbon (SOC) in existing oil palm plantations is an important strategy to limit carbon losses. The aim of this study was to investigate SOC stocks of oil palm plantations under different management systems. Soil samples were collected from three different management systems (best management practices (BMP), current management practices typical of large plantations (CMP) and smallholder management practices (SHMP)) in north Sumatra, Indonesia. Plantations were divided into four management zones that were sampled separately with four replicate profiles in the weeded circle, frond stack, harvesting path and interrow zones. All the soil samples were collected from five (0–5, 5–15, 15–30, 30–50 and 50–70 cm) soil depths. Soil samples were analysed for concentration of SOC, soil texture, soil bulk density and pH. Calculations of SOC stocks in the soils were undertaken according to the fixed-depth and equivalent soil mass approaches. Results showed that SOC stocks of plantations under BMP (68 t ha−1) were 31% and 18% higher than under CMP (57 t ha−1) and SHMP (46 t ha−1) respectively. In the BMP system, soils under the interrow zone that received enriched mulch and frond stack positions stored significantly more SOC than the harvesting path of the BMP system (77, 73 and 57 t ha−1 respectively). BMP also had a 33% higher fresh fruit bunch yield compared to the SHMP system. This study shows that residue incorporation or retention as a part of BMP could be an effective strategy for increasing SOC stocks of oil palm plantations and confirms that these management practices could improve yields from SHMP systems.

Assessing changes in soil carbon stocks after land use conversion from forest land to agricultural land in Japan

In the context of their role in global climate change, changes in soil carbon stocks resulting from land use conversions from forest to agricultural lands (cropland and grassland) and subsequent agricultural activities were investigated with data from a total of 46 soil sampling sites across Japan (28 cropland and 18 grassland sites). Collection of soil sample pairs from forest land and adjacent agricultural land converted from forest land allowed derivation of the soil carbon change factor (SCCF; ratio of soil carbon stock in the agricultural land to that in the forest land at the time of soil sampling). Depth-based SCCFs for a fixed depth of 0–30 cm were 23% greater than mass-based SCCFs determined by the equivalent soil mass approach, in which soil carbon stocks in all replicated samples from forest land and agricultural land were calculated for different depths but equal masses of the soil mineral fraction. The overestimation of SCCFs by the depth-based approach was more pronounced in Andosols, particularly in croplands on Andosols, because soil bulk densities in forest lands were much lower than those in croplands on Andosols. The mass-based SCCFs for croplands (upland fields and orchards) and grasslands (pasture land and grazing land) after deforestation were 0.741 ± 0.259 (a 25.9 ± 25.9% decrease in soil carbon stocks after an elapsed time of 2–85 years) and 0.858 ± 0.212 (a 14.2 ± 21.2% decrease after an elapsed time of 26–81 years), respectively. Therefore, conversions from forest land to cropland or grassland resulted in a significant carbon loss from most soils in Japan. Soil carbon stocks in cropland converted from forest land declined as the elapsed time after deforestation increased although the uncertainty was large. Of the environmental variables, soil type (Andosols versus non-Andosols) and tillage frequency during cultivation had little impact on mass-based SCCFs; however, the SCCFs decreased with higher mean annual air temperature at the sites.

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

It is controversial whether no tillage (NT) should be regarded an effective agro-ecosystem management to increase soil organic carbon (SOC). Some studies suggest NT leads to a notable accumulation of SOC stock across the whole sampling profile over the long-term, whereas other studies argue that the effect of duration on SOC stock increase is limited. Similar controversy is common with respect to the role of climatic conditions in SOC changes. In addition, despite the gradual recognition of the importance of equivalent soil mass (ESM) compared to fixed depth (FD) approach, in terms of SOC stock evaluation, the ESM approach mainly focuses on the direct impact of bulk density but ignores the potential indirect effect on SOC concentration. To further understand the SOC sequestration mechanisms under NT, responses of SOC stock after the adoption of NT were investigated based on FD and ESM approaches. The results showed that NT significantly changed the SOC stock distribution in different soil depths according to both FD and ESM, but no significant changes were observed when soil sampling deeper than 50 cm. Notably, the results illustrated that the use of FD overestimated the SOC stock in the soil surface due to the increased bulk density under NT, but underestimated the SOC stock in 30–70 cm soil layers compared to ESM. Consequently, the overestimate of SOC based on FD may not be as high as previously assumed across the sampling depth. Mean annual temperature and mean annual rainfall did not lead to any significant changes in SOC stock of the overall soil profile or different layers, indicating the limited impacts of climatic conditions on carbon sequestration after the adoption of NT. Long-term NT resulted in a significant accumulation in SOC stock in the top 5 cm of the surface soil, but SOC stock changes with time in soil layers deeper than 5 cm was not significant in both FD and ESM approaches. The overall changes in SOC stock as time increased were not significant across the whole profile based on FD, but was weakly significant based on ESM. The results illustrate that using the overall SOC stock change of the whole sampling soil profile deeper than 30 cm masks the beneficial change in SOC stock in the 0–5 cm soil over long-term adoption of NT. The biased interpretation of depth factor is the main reason that fuels the debate over whether long-term NT is beneficial for the accumulation of SOC stock, and hence must be properly considered. Overall, the data illustrate that the depth factor is not only important in terms of assessing total SOC stock changes under NT, but also critical when evaluating the effectiveness of FD and ESM, as well as the impact of experimental duration on SOC increase. The depth factor should thus be comprehensively considered in SOC stock assessment and more field experiments sampling to deeper depth are urgently needed to clarify whether NT is beneficial for SOC sequestration and climate mitigation.

Soils' dirty little secret: Depth-based comparisons can be inadequate for quantifying changes in soil organic carbon and other mineral soil properties.

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.

Sampling probes affect bulk density and soil organic carbon measurements

AbstractSoil sampling equipment can be a major source of bias in soil organic carbon (SOC) stock estimations. The objective of this study was to evaluate the impact of sampling probes on soil bulk density (BD) and SOC stocks calculated using fixed depth (FD) and equivalent soil mass (ESM) methods. Soil samples were collected to 30 cm using three probes with different diameters and divided into 0–10‐, 10–20‐, and 20–30‐cm layers. The probe with smallest diameter measured higher BD at 0–10 cm in 42% of fields and was significantly different when averaged across fields, while no consistent differences were observed at lower depths. This study shows that sampling probes with different diameters may introduce biases in BD and SOC measurements at individual or combined soil layers when calculated using the FD approach. The ESM approach reduced the differences in mean SOC stocks calculated using different probes.

Land use effects on organic carbon storage in soils of Bavaria: The importance of soil types

Numerous studies have reported substantial changes of soil organic carbon (SOC) stocks after converting forests into agricultural land and vice versa. However, some studies suggested that agricultural soils might contain similar amounts of SOC as forest soils. Losses of SOC induced by cultivation might be overestimated due to shallow soil sampling and application of inaccurate pedotransfer functions. We investigated the impact of different land uses on total SOC storage down to the subsoil on the basis of 270 soil profiles in southeast Germany under similar climatic and pedogenic conditions using an equivalent soil mass (ESM) approach. Land use effects on SOC storage were strongly affected by soil class, which comprised soil types with similar pedogenesis. Both slightly lower (<20%) and even higher SOC stocks were found under cropland compared with forest land for different soil classes. A comparison of different soil classes under grassland and forest land also showed no considerable differences of SOC stocks. Soil cultivation may not generally be associated with a strong decline of SOC, as tillage probably promotes the formation of organo-mineral associations and a relocation of SOC with depth may decrease its decomposition. This finding should be taken into consideration when estimating and managing the emission and sequestration of C in soils. We assume that many studies based on topsoils alone may have underestimated agricultural SOC stocks, particularly when an ESM approach is used. Our results highlight the need for soil type-specific evaluations in terms of interpreting the effects of land use management on SOC stocks.

Improved sampling methods document decline in soil organic carbon stocks and concentrations of permanganate oxidizable carbon after transition from swidden to oil palm cultivation

Oil palm plantations are spreading rapidly throughout Southeast Asia and in some countries, they are promoted as carbon sinks compared to the swidden cultivation systems that they often replace. However, little is known about the impacts of this land use change on soil organic carbon (SOC) stocks or soil quality. This study uses resampling of archived soil samples to investigate the sensitivity of permanganate oxidizable carbon (Pox-C) concentration to a change in land use from swidden cultivation to small-scale oil palm plantation on an Ultisol in Sarawak, Malaysia. Furthermore, the results of two different methods of calculating SOC stocks are compared – namely the fixed depth approach and the equivalent soil mass approach, which is sensitive to changes in soil bulk density. Results show that using a method that is sensitive to changes in bulk density is important as the soil bulk density increases upon establishment of oil palm. Thus, topsoil carbon stocks significantly decreased 3–8 years after oil palm establishment as measured by the equivalent soil mass approach, but only marginally and insignificantly decreased according to the fixed depth approach. After 15 years of oil palm, carbon stocks were 40% lower according to the fixed depth approach but 50% lower when using the equivalent soil mass approach. Importantly, the resampling of geo-referenced soil gives more consistent data, and lends credibility to the observation of large reductions in SOC stocks. The concentration of Pox-C in the 0–10cm layer declines exponentially as oil palm plantations age and can serve as an indicator of change in the soil ecosystem brought about by the investigated land use transition. Pox-C is not more sensitive to this change than standard SOC analyses, but it may serve as an inexpensive, fast and field-suitable means of estimating the SOC status of different land use systems.

Organic Carbon Dynamics in Different Soil Types After Conversion of Forest to Agriculture

AbstractState‐of‐the‐art predictive models of soil organic carbon (SOC) dynamics associated with land use changes are unable to reflect the diversity of tropical soil types as the knowledge of contrasting site‐specific factors in mediating the response of the SOC pool is sparse. This paper examines the influence of soil type and management on SOC dynamics following the conversion of forests to annual cropping in Ghana. Soil from primary forests and from areas with short (2–7 years) and long (20 years) histories of maize cultivation was sampled from a Vertisol dominated by smectite and Ultisol dominated by kaolinite. Wet sieving was used to separate soil fractions below and above 250 µm. SOC concentrations and δ13C signatures of SOC in soil fractions and bulk soil were determined. SOC stocks were calculated by the commonly used fixed depth approach and by the equivalent soil mass approach. After 20 years of cultivation of the Vertisol, the total SOC content was 40 per cent lower than under forest, and about 95 per cent of the forest‐derived SOC had been lost. After 20 years of cultivation of the Ultisol, total SOC content was only about 20 per cent lower than under forest and merely 30 per cent of the forest‐derived SOC had been lost. Both soil types were managed as they would typically be in small scale farming systems, thus the higher SOC losses and the substantial loss of forest‐derived SOC from the Vertisol question the conventional concept of smectite having a higher SOC‐stabilizing potential than kaolinite under field conditions. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Effects of tillage on contents of organic carbon, nitrogen, water-stable aggregates and light fraction for four different long-term trials

Soil management may affect C and N dynamics in soils, but the underlying processes are not well understood. Our objective was to quantify the impact of different tillage treatments on the amount and distribution of free and occluded light fractions (fLF and oLF, respectively), on the water-stable macro-aggregate (>250μm) contents, and on organic carbon (Corg) storage. Four long-term tillage trials were carried out on loess soils in Germany with sugar beet followed by two years of winter wheat as crop rotations. The different tillage treatments trialled were regular conventional tillage (CT, to 30cm), mulch tillage (MT, to 10cm) and no-tillage (NT). Soils were sampled in 0–5cm, 5–25cm and 25–40cm depth after 18–25years of the different tillage treatments. These four long-term tillage trials on plots differing in soil texture and climatic conditions revealed consistent results between them. Average crop yields of sugar beet and winter wheat from 2004 to 2010 were higher under CT and MT than under NT. The NT and MT treatments produced significantly higher Corg contents than the CT treatment in 0–5cm soil depth. The Corg stocks in the sampled profile, based on the equivalent soil mass approach (CT: 0–40, MT: 0–38, NT: 0–36cm), were significantly higher for the MT treatment than for the CT and NT treatments. The fLF, oLF, and macro-aggregate contents were significantly higher for the NT and MT treatments than for the CT treatment in the top 5cm, whereas in 5–25cm depth, the oLF contents were significantly higher for the CT and MT treatments. The correlation of the macro-aggregate content against the fLF and oLF contents suggested that the macro-aggregate content is not directly influenced by the different tillage treatments but by the contents of available biomass, presumably due to the higher biomass input via higher crop yields under CT and MT and the vertical distribution of the residue input by mulching and plowing. Stepwise multiple linear regression analysis suggested that the Corg content was the most important factor influencing the macro-aggregate content in the soils of the four long-term trials, whereas the contents of fLF and silt were negatively related to the macro-aggregate content.

A comparison of repeated soil inventory and carbon flux budget to detect soil carbon stock changes after conversion from cropland to grasslands

Assessment of soil carbon (C) stock changes over time is typically based on the application of two methods, namely (i) repeated soil inventory and (ii) determination of the ecosystem C budget or net biome productivity (NBP) by continuous measurement of CO2 exchange in combination with quantification of other C imports and exports. Here, we applied both methods in parallel to determine C stock changes of two temperate grassland fields previously converted from long-term cropland. The grasslands differed in management intensity with either intensive management (high fertilization, frequent cutting) or extensive management (no fertilization, less frequent cutting). Soil organic C stocks (0–45 cm depth) were quantified at the beginning (2001) and the end (2006) of a 5 year observational period using the equivalent soil mass approach. For the same period and in both fields, NBP was quantified from net CO2 fluxes monitored using eddy covariance systems, and measured C import by organic fertilizer and C export by harvest. Both NBP and repeated soil inventories revealed a consistent and significant difference between management systems of 170 ± 48 and 253 ± 182 g C m−2 a−1, respectively. For both fields, the inventory method showed a tendency towards higher C loss/smaller C gain than NBP. In the extensive field, a significant C loss was observed by the inventory but not by the NBP approach. Thus neither flux measurements nor repeated soil sampling may be suitable for tracking absolute changes in SOC, but both give similar answers with respect to relative changes.

Soil carbon storage and stratification under different tillage systems in a semi-arid region

Changes in the agricultural management can potentially increase the accumulation rate of soil organic carbon (SOC), thereby sequestering CO2 from the atmosphere. In a long-term experiment (1992–2008) we examined the effects of various tillage intensities: no-tillage (NT), minimum tillage with chisel plow (MT), and conventional tillage with mouldboard plow (CT), on the topsoil profile distribution (0–30cm) of SOC, on a semi-arid loamy soil from Central Spain. The crop sequence established was cheap pea (Cicer arietinun L.) cv. Inmaculada/barley (Hordeum vulgare L.) cv. Volley. Soil organic carbon in the various tillage treatments was expressed on a content bases and the equivalent soil mass approach. Measurements made at the end of 17 years showed that in the 0–30cm depth, stocks of SOC had increased under NT compared with MT and CT. Most dramatic changes occurred within the 0–5cm layer where plots under NT had 5.8 and 7.6Mgha−1 more SOC than under MT or CT respectively. No-tillage plots, however, exhibited strong vertical gradients of SOC with concentrations decreasing from 0–5 to 20–30cm. Stratification ratios of SOC in 1992 showed no significant differences between tillage systems. On the contrary, from 1993 onwards all stratification ratios were significantly higher in NT than in the other two tillage systems. In addition, since 2003 stratification ratios of SOC obtained under NT were systematically >2 and more than 2-fold those obtained under MT and CT. Stratification ratios >2 are uncommon under degraded conditions and could suggest that NT management system may have the most benefits to soil quality in semi arid regions with low native soil organic matter.

Comment on “Determining soil carbon stock changes: Simple bulk density corrections fail” [Agric. Ecosyst. Environ. 134 (2009) 251–256

Lee et al. (2009) showed the equivalent soil mass (ESM) approach for correcting bulk density changes when estimating soil carbon stock or density. The ESM approach from Ellert and Bethany (1995) attempts to correct for differences in bulk density from different sampling dates by calculating the mass of soil C in an equivalent soil mass per unit area. This is done by first designating the mass of the heaviest soil layer as the equivalent mass. The C density from subsequent sampling is then calculated by estimating the thickness of the deepest soil layer required to attain the equivalent mass. Lee et al. (2009) showed various calculations of the ESM approach. In this comment, we should like to point out that the material coordinate system, which is simpler andmore general, is better for handling this issue. In fact, Gifford and Roderick (2003) have proposed the use of the mass (or material) coordinate system for soil C accounting. The material coordinate or Lagrange system was proposed by Smiles and Rosenthal (1968) for calculating water flux in swelling soils. It has been applied in calculatingwater flows in swelling soils (McGarry and Malafant, 1987; Ringrose-Voase et al., 2000).For C accounting, we based the C density on the mass of the soil mineral material. First, we calculate the mineral mass of each sampling

Carbon and nitrogen storage by deep-rooted tall fescue ( Lolium arundinaceum) in the surface and subsurface soil of a fine sandy loam in eastern Canada

Compared to annual crops, the use of deep-rooted vegetation, such as perennial grasses, in agricultural systems can increase soil organic C (SOC) storage deep within the soil profile. However, this potential increase could be adversely influenced by physical limitations in the soil profile. Tall fescue ( Lolium arundinaceum Schreb.) was grown for 7 years after a long-term tillage experiment on a fine sandy loam (Orthic Podzol) in Prince Edward Island which had naturally compacted subsoil. Soil samples from the 0–60 cm soil depth (divided into five depth increments) were collected periodically from 2000 to 2007 to evaluate the potential for tall fescue to increase SOC and total N (TN) storage and depth distribution in the soil profile, especially in subsurface soil layers. Light fraction C (LF-C) and particulate organic matter C (POM-C) were also evaluated as indicators of SOC change in the soil profile. Annual tall fescue biomass yield (>7 Mg dry matter ha −1) was above regional norms, but rooting depth (70–90 cm) was relatively shallow. Comparison of the 2000 and 2007 data showed that tall fescue increased SOC and TN in the 0–10 and 40–60 cm soil depths. Over the 0–60 cm soil depth, SOC content increased by 23% and TN by 34%. Use of the equivalent soil mass approach, to adjust for soil bulk density increases over the 7-year period, reduced the SOC gain to 17% (increase of 12.8 Mg C ha −1). This SOC increase corresponded with standard decomposition calculations based on plant biomass inputs. Both LF-C and POM-C indicated that tall fescue increased SOC down the soil profile. This research demonstrates the important role that plant roots which play in SOC storage deep within the soil profile, even when soil profile conditions may adversely influence rooting depth.

Carbon and nitrogen stocks in a Brazilian clayey Oxisol: 13-year effects of integrated crop–livestock management systems

Integrated crop–livestock management systems (ICLS) have been increasingly recommended in Brazilian agroecosystems. However, knowledge of their effect on soil organic carbon (SOC) and total nitrogen (TN) concentrations and stocks is still limited. The study was undertaken to evaluate the effects of ICLS under two tillage and fertilization regimes on SOC and TN concentrations and stocks in the 0–30 cm soil layer, in comparison with continuous crops or pasture. The following soil management systems were studied: continuous pasture; continuous crop; 4 years’ crop followed by 4 years’ pasture and vice-versa. The adjacent native Cerrado area was used as a control. Under the rotation and continuous crop systems there were two levels of soil tillage (conventional and no-tillage) and fertility (maintenance and corrective fertility). The stock calculations were done using the equivalent soil mass approach. The land use systems had a significant effect on the concentrations of SOC and TN in the soil, but no effect was observed for the soil tillage and fertilizer regimes. For these two latter, some significant discrepancies appeared in the distribution of SOC and TN concentrations in the 0–30 cm layer. Carbon storage was 60.87 Mg ha −1 under Cerrado, and ranged from 52.21 Mg ha −1 under the ICLS rotation to 59.89 Mg ha −1 with continuous cropping. The decrease in SOC stocks was approximately 8.5 and 7.5 Mg ha −1, or 14 and 12%, for continuous pasture and ICLS respectively. No-tillage for 10 years after the conversion of conventional tillage to no-tillage under the continuous crop system, and 13 years of conventional tillage in continuous cropping did not result in significant changes in SOC stocks. The SOC and TN stocks in surface layers, using the equivalent soil mass approach rather than the equivalent depth, stress the differences induced by the calculation method. As soil compaction is the principal feature of variability of stocks determinations, the thickness should be avoid in these types of studies.