Higher carbon sequestration on Swedish dairy farms compared with other farm types as revealed by national soil inventories

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

Soil organic carbon (SOC) stock (in metric tons of carbon per hectare) is calculated from SOC concentration (in grams per kilogram) and soil bulk density (ρb; in grams per cubic centimeter). Temporal changes in SOC stock are used to calculate terrestrial carbon sequestration rates used in global climate change models. The inherent variability in soil properties like SOC and ρb means that larger sample sizes may be needed to accurately determine SOC stocks. Our objective was to calculate the minimum sample size required to detect changes in ρb, SOC, and SOC stock for two land uses. Surface soils (0-5 cm) from two reclaimed mine soils and two managed hay fields in northern West Virginia were intensively sampled (60-74 samples each). Mean SOC and SOC stock values were larger in the hay fields (40 g/kg, 29 Mg ha−1) than in the mine soils (20 g/kg, 20 Mg ha−1), but ρb was larger in reclaimed mine soils (1.4 g cm−3) than in hay field soils (1.2 g cm−3). The ρb variance was larger in mine soils than that in hay field soils, but field variances for a given land use were similar (0.09 and 0.11 [g cm−3]2 in mine soils; 0.02 and 0.03 [g cm−3]2 in hay field soils). The variances in SOC concentration and SOC stock were not related to land use and were not similar within a land use. As a result, the minimum number of samples required to detect a change in ρb, SOC, and SOC stock was a site-specific property and cannot be assumed a priori.

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.

<p>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.</p><p>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 “Biomass & Environment” 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 δ<sup>13</sup>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.</p><p>After 12 years, there was a large increase in SOC concentration (+7.6 g kg<sup>-1</sup> on average) under perennial crops in the surface layer (≈ 0-5 cm) but a slight decrease in deeper layers. Changes in δ<sup>13</sup>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 μm) but also in the more stabilised 0-50 μm fraction. SOC concentration under semi-perennial crops increased in the two first layers (≈ 0-20 cm), from 10.2 g kg<sup>-1</sup> in 2006 to 11.6 g kg<sup>-1</sup> 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 (≈ 20-33 cm). There was no significant effect of the N fertilisation. Over the old ploughed layer (≈ 0-33 cm), SOC stocks increased between 2006 and 2018 under perennial and semi-perennial bioenergy crops (by 3 and 2 t C ha<sup>-1</sup> on average respectively) and decreased by 7 t C ha<sup>-1</sup> on average under annual crops.</p><p>This study show that different bioenergy crops can have contrasted impacts on SOC stocks but also on SOC distribution in the soil profile.</p>

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

ABSTRACTThis study presents a method for estimating the soil organic carbon (SOC) stock changes in mineral agricultural soils developed for the Finnish GHG inventory. SOC stock changes in mineral cropland and grassland soils from 1990 to 2013 were calculated by combining agricultural statistics and national conversion factors to estimate the organic inputs to soil, along with the Yasso07 soil carbon model. The effects of selected key assumptions on the simulation results were studied. The method yielded SOC change estimates closer to the observed SOC change than were the results of the previously used Tier 1 method. The SOC stocks of croplands in 1-m soil profiles were slightly decreasing in most regions of the country. At the national level, the decrease was on average 0.05 Mg C ha–1 year–1 (0.01%). Selection of climate data (annual vs. long-term mean) and the initialization procedure had large impacts on the simulated SOC stock and change results, whereas the simulations at regional and subnational levels provided similar results. The method was found to be suitable for the GHG inventory and preferable to the Tier 1 method. The modular structure of the system allows for continuous improvements when more information and data are gathered.

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

Estimating temporal and spatial changes in soil organic carbon stocks and its controlling factors in moraine landscapes in Denmark

The replacement of native vegetation by pastures or tree plantations is increasing worldwide. Contradictory effects of these land use transitions on the direction of changes in soil organic carbon (SOC) stocks, quality, and vertical distribution have been reported, which could be explained by the characteristics of the new or prior vegetation, time since vegetation replacement, and environmental conditions. We used a series of paired-field experiments and a literature synthesis to evaluate how these factors affect SOC contents in transitions between tree- and grass-dominated (grazed) ecosystems in South America. Both our field and literature approaches showed that SOC changes (0-20cm of depth) were independent of the initial native vegetation (forest, grassland, or savanna) but strongly dependent on the characteristics of the new vegetation (tree plantations or pastures), its age, and precipitation. Pasture establishment increased SOC contents across all our precipitation gradient and C gains were greater as pastures aged. In contrast, tree plantations increased SOC stocks in arid sites but decreased them in humid ones. However, SOC losses in humid sites were counterbalanced by the effect of plantation age, as plantations increased their SOC stocks as plantations aged. A multiple regression model including age and precipitation explained more than 50% (p<0.01) of SOC changes observed after sowing pastures or planting trees. The only clear shift observed in the vertical distribution of SOC occurred when pastures replaced native forests, with SOC gains in the surface soil but losses at greater depths. The changes in SOC stocks occurred mainly in the silt+clay soil size fraction (MAOM), while SOC stocks in labile (POM) fraction remained relatively constant. Our results can be considered in designing strategies to increase SOC storage and soil fertility and highlight the importance of precipitation, soil depth, and age in determining SOC changes across a range of environments and land-use transitions.

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.

SummaryAssessments of changes in soil organic carbon (SOC) stocks depend heavily on reliable values of SOC content obtained by automated high‐temperature C analysers. However, historical as well as current research often relies on indirect SOC estimates such as loss‐on‐ignition (LOI). In this study, we revisit the conversion of LOI to SOC using soil from two long‐term agricultural field experiments and one arable field with different contents of SOC, clay and particles <20 μm (Fines20). Clay‐, silt‐ and sand‐sized fractions were isolated from the arable soil. Samples were analysed for texture, LOI (500°C for 4 hours) and SOC by dry combustion. For a topsoil with 2 g C and 30 g clay 100 g−1 soil, converting LOI to SOC by the conventional factor 0.58 overestimated the SOC stock by 45 Mg C ha−1. The error increased with increasing contents of clay and Fines20. Converting LOI to SOC by a regression model underestimated the SOC stock by 5 Mg C ha−1 at small clay and Fines20 contents and overestimated the SOC stock by 8 Mg C ha−1 at large contents. This was due to losses of structural water from clay minerals. The best model to convert LOI to SOC incorporated clay content. Evaluating this model against an independent dataset gave a root mean square error and mean error of 0.295 and 0.125 g C 100 g−1, respectively. To avoid misleading accounts of SOC stocks in agricultural soils, we recommend re‐analysis of archived soil samples for SOC using high‐temperature dry combustion methods. Where archived samples are not available, accounting for clay content improves conversion of LOI to SOC considerably. The use of the conventional conversion factor 0.58 is antiquated and provides misleading estimates of SOC stocks.Highlights Assessment of SOC contents is often based on less accurate methods such as LOI.Reliable accounts of changes in SOC stocks remain high on the agenda (4‰ initiative).Conversion of LOI to SOC is considerably improved by accounting for clay content.Converting LOI to SOC by the conventional factor 0.58 leads to grossly overestimated SOC stocks.

Afforestation of degraded land significantly influences soil organic carbon (SOC) and total nitrogen (STN) sequestration. The interaction effects of plantation age and climate gradient on SOC and STN changes following afforestation are not well understood. In this study, five sites were selected along a precipitation gradient (410–600 mm yr−1) in the Loess Plateau. The SOC and STN stocks at a depth of 0–200 cm were measured in cropland and black locust (Robinia pseudoacacia L.) forests with different plantation ages, that is, young forest (&lt;15 years), middle‐aged forest (15–25 years), and old forest (&gt;25 years). The SOC and STN stocks in the 0‐ to 200‐cm profiles of young forest, middle‐aged forest, and cropland increased significantly with mean annual precipitation (p &lt; .05), whereas the increasing trend of the SOC stocks of old forest was not significant, indicating an age‐dependent change in the SOC and STN stocks across the precipitation gradient. The SOC stock change (∆SOC) following afforestation increased with mean annual precipitation in young forest, but it had a decreasing trend in middle‐aged and old forests. The STN stock changes (∆STN) in the three forests were negative at most sites, and they all decreased along the precipitation gradient. There were significant positive correlations between ∆SOC and ∆STN (p &lt; .01), and 1‐g STN stock accumulation was accompanied by 8.40‐, 6.10‐, and 10.48‐g SOC accumulation for young forest, middle‐aged forest, and old forest, respectively. The different patterns of SOC and STN stock changes should be incorporated into soil C and N modelling and estimation.

National and sub-national assessments of soil organic carbon stocks and changes: The GEFSOC modelling system

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