Grassland management and integration during crop rotation impact soil carbon changes and grass-crop production.

To investigate the impact of integrating ley grassland in crop rotations on soil C changes at depths of 0–30 and 0–60 cm, field data regarding C input, soil C and crop production were collected during 2005–2016 from a long-term experimental site in Lusignan, southwestern France. Root biomass C was sampled annually, and the stable C isotope signature (δ13C) was determined to quantify the amount of C input from the root biomass. The results showed that integrating ley grassland in crop rotations increased the C input in the 0–30 and 0–60 cm soil layers. Carbon stocks also increased in the 0–30 cm soil layer but not always in the 0–60 cm layer. However, during the first full rotation (2005–2013), treatments involving 3-year and 6-year grasslands showed 37.5% and 62.5% losses of their crop production, respectively. In crop rotations, integrating ley grassland together with high-nitrogen (N) or low-N fertilisation did not impact the C input, changes in soil C in either the 0–30 or 0–60 cm soil layer, or even crop production, while grass production decreased by 22.2–66.6% under low-N fertilisation. Compared with cropland, permanent grassland did not show a greater C input, whereas the latter showed a C stock increase of 0.6–1.4 Mg C ha−1 yr−1. The analysis of 2005–2013 crop rotation data suggested that both above- and belowground C inputs should be considered as a single total C input. An estimated baseline C input rate of 1.9 Mg C ha−1 yr−1 was suggested to maintain soil C stocks for some of the crop rotations at the site. Considering the balance of food production and soil C stocks, crop rotation in conjunction with 3 years of high-N-fertilised grassland is recommended. The Lusignan national long-term observatory experiment in the Nouvelle-Aquitaine region of France showing integration of grassland in crop rotations and the target plots in this study in red frames (a) and its experimental design (b) [Display omitted] • Integrating ley grassland into crop rotations increased the C stock only in topsoil. • Nitrogen application rates impacted either C input or soil C changes in ley grassland. • Permanent grassland had similar C inputs as cropland but greater soil C increases. • Above- and belowground C inputs had similar importance to soil C stocks. • A C input of 1.9 Mg C ha−1 yr−1 was proposed to maintain soil C stocks for crop rotations. [ABSTRACT FROM AUTHOR]