Long-term soil organic carbon dynamics in temperate cropland-grassland systems

Abstract

Increasing soil organic carbon (SOC) in agroecosystems enables to address simultaneously multiple goals such as climate change adaptation and mitigation as well as food security. As croplands are depleted in SOC, they offer a great potential to sequester atmospheric carbon (C). Nonetheless, croplands are still losing SOC under most of the current agricultural systems. Although many factors driving SOC dynamics have already been identified, their relative importance has not been quantified yet. Using one of the densest European soil monitoring networks with 250 sites established in western Switzerland, in the present study we (i) assessed long-term (over 30 years) SOC dynamics in croplands (CR), permanent grasslands (PG) and mountain pastures (MP), and (ii) prioritized the importance of land use, soil characteristics and sites conditions in driving SOC dynamics. The SOC levels in PG and MP were similar when clay content was accounted for, whereas CR were depleted in SOC by 3.9 mg C mg−1 clay as compared to PG. The majority (61 %) of CR had SOC:clay ratio below 1:10, but only 16 % of PG and MP sites reached this threshold. By contrast, soil organic matter stoichiometry (C:N:Porg ratios) was similar in CR and PG for comparable SOC content. The increase of C:Porg ratio with SOC content (dilution effect) and the high total P in CR and PG (legacy effect) indicate the possibility to sequester atmospheric C at reduced nutrient sequestration costs. SOC changes ranged from -0.61 to 1.32 mg g-1 soil yr−1 and were the highest in sites that experienced land-use changes. No PG were losing SOC, while CR sites exhibited both SOC gains and losses. Because of the predominance of the initial SOC content on SOC dynamics, land-use history must be accounted for when assessing the effect of management practices. The main manageable factors driving SOC dynamics were the time under temporary or permanent grasslands along with the soil total P. As PG already are rich in SOC and total P, organic amendments should be partly redirected to CR.