Cover crops, compost, and conversion to grassland to increase soil C and N stock in intensive agrosystems

Abstract: Organic fertilization or conversion to grassland may increase soil organic carbon (SOC) and soil total nitrogen (STN). However, responses of net SOC and STN accumulation are sometimes inconsistent and little is known about temporal patterns when those strategies are stopped. We (a) assessed the effects of rye (Secale cereale L.) and hairy vetch (Vicia villosa Roth) cover crops (CCs) on SOC and STN during a 4-year “enriching” period (EP), and a following 2-year “depleting” period (DP); (b) compared these strategies with compost application and conversion to permanent fescue (Festuca arundinacea Schreb.); and (c) determined the responses of maize (Zea mays L.), soybean (Glycine max L. Merr.), and sunflower (Helianthus annuus L.) yields. Crop yield increased only in maize with compost and hairy vetch, which downsizes the role of these strategies to support productivity. SOC and STN increased with 4-year compost (+ 10.2 Mg C ha−1; + 0.5 Mg N ha−1), rye (+ 8.8 Mg C ha−1; + 0.6 Mg N ha−1), and hairy vetch (+ 6.9 Mg C ha−1; + 0.6 Mg N ha−1). Afterwards, SOC stock loss during 2-year DP tended to be higher than annual C input where there were CCs, and accounted for about 70% of annual C input where there was compost. High SOC loss highlights the weak effect of CCs for long-term SOC stabilization. Conversely, STN increased even during DP, which indicates a more lasting effect. Green manuring with CCs may be relevant for enhancing SOC and STN, although the beneficial effects are short-lived. Conversion to grassland remains the reference strategy. Graphic abstract: [Figure not available: see fulltext.]