Both soil quality degradation and climate change mitigation issues emphasize the need to increase, or at least stabilize, the topsoil organic carbon content (wt%) in arable land. This on-farm study aimed at measuring the impact of agricultural practices on changes in soil organic carbon (SOC) content over 10 years. A total of 120 fields belonging to 120 farms representative of the cropping systems and soil properties in Western Switzerland (Lake Geneva region) was randomly selected. The field 0–20 cm topsoil was sampled at a 10-years interval, and the corresponding cropping practices were gathered using farmer’s interviews and the mandatory records of yearly practices at field level in Swiss-farms. Only 1) organic matter inputs and 2) cover-crop intensity were significantly correlated to SOC increase while 3) the soil tillage intensity and 4) the soil saturation in carbon expressed as a SOC to clay content ratio were correlated to SOC decrease. Among others, temporary meadows were not correlated to changes in SOC content mainly due to increased tillage and decreased cover-crops between meadows. Organic farming did not correlate either with SOC changes due to the large tillage intensity applied for weed control. The observed SOC content changes ranged from −56‰ to +74‰ and were well explained by a linear regression model with additive effect of the four identified SOC change factors. The additivity of these factors means that farmers can emphasize the methods of their choice when regenerating their soils. This study advocates that strict no-till is not required at low carbon saturation level (small SOC:Clay ratio). However, as carbon saturation increases, conservation tillage and then no-till practices become necessary to further increase SOC contents. These findings are in accordance with previous studies showing that since 2015 SOC is increasing at more than +4‰ on average in the region and provide practical insights to further manage the transition of farming systems towards soil regeneration.

<p>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 (> 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.</p> <p>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.</p>

<p>Restoring soil quality has become a priority, with special focus on increasing the soil organic carbon (SOC) content which is the major factor of topsoil quality. Therefore, conservation agriculture (CA) techniques receive growing interest. Among the options considered to increase the SOC of cultivated land, the proportion of temporary grassland (TC) in the rotation is often cited.</p> <p>We monitored the topsoil SOC content of 120 fields from 120 farms in the Swiss Leman Lake region and analysed the relationships between the percentage of TG in the rotation, the other cropping practices, and the observed changes of the SOC content over 10 years.</p> <p>The cropping practices showing an impact on the annual rate of change (SOC-ARC) in the SOC content were shown to be the Soil Tillage Intensity Rating (STIR), the percentage of bare soil in the year, the cover-crop frequency and diversity, and the farm manure application converted in humified organic matter (Dupla et al., 2022). SOC content was decreasing with increasing STIR and percentage of bare soil, increasing with cover-crop frequency and diversity, and manure application.</p> <p>The SOC:clay ratio of the monitored fields ranged from 3% to 14% and was on average low compared to the structure vulnerability thresholds quantified in the studies of (Johannes et al., 2017; Prout et al., 2020). The percentage of TG in the rotation ranged from 0% to 70% of the 10-year period, 75 fields had no TG (TG-) and 45 had (TG+). Though the SOC:clay ratios of the TG- fields was significantly smaller than those of the TG fields, the SOC:clay was not increased with the percentage of TG in the TG+. On the whole data set, the percentage of TG showed no correlation with SOC-ARC, but it was positively correlated when considering only the TG+ fields. The impacts of the farm manure, cover-crop diversity, soil cover and STIR on SOC-ARC were increased with increasing TG percentage.</p> <p>The livestock units per ha were significantly higher for the TG+ fields but were not correlated with the percentage of TG. The farm manure input was not correlated to the TG percentage, but when slurry and digestate were excluded, a positive correlation was observed. The STIR decreased and the percentage of soil cover increased with increasing TG percentage. However, when considering only the main crops in the rotation, the percentage of bare soil and the STIR increased with TG percentage, while the number of species in the cover crops decreased, showing the higher the TG percentage, the more detrimental to the soil were the practices of the farmers in the main crops of the rotation. The number of species in the cover crops was decreasing with increasing TG percentage.</p> <p>This study revealed that increasing the proportion of temporary grassland is not a guarantee of SOC increase compared to rotations without TG. More detrimental practices may offset the potential benefit of TG, though TG has a synergistic effect with CA practices. Therefore, temporary grasslands cannot be a keyword guaranteeing soil regeneration but can contribute to it provided that the CA agronomic factors are properly applied.</p> <p> </p> <p>Dupla, X., Lemaître, T., Grand, S., Gondret, K., Charles, R., Verrecchia, E., Boivin, P., 2022. On-Farm Relationships Between Agricultural Practices and Annual Changes in Organic Carbon Content at a Regional Scale. Frontiers in Environmental Science 10, 13.</p> <p>Johannes, A., Matter, A., Schulin, R., Weisskopf, P., Baveye, P.C., Boivin, P., 2017. Optimal organic carbon values for soil structure quality of arable soils. Does clay content matter? Geoderma 302, 14–21. https://doi.org/10.1016/j.geoderma.2017.04.021</p> <p>Prout, J.M., Shepherd, K.D., McGrath, S.P., Kirk, G.J.D., Haefele, S.M., 2020. What is a good level of soil organic matter? An index based on organic carbon to clay ratio. European Journal of Soil Science n/a. https://doi.org/10.1111/ejss.13012</p>