Converting temperate long‐term arable land into semi‐natural grassland: decadal‐scale changes in topsoil C, N, 13C and 15N contents.

Summary: Converting arable land to permanent grassland remains an efficient option for increased carbon (C) storage in agricultural land. We quantified changes in C and nitrogen (N) in topsoil from the Sandmarken experiment (initiated in 1894 in Denmark) before and after its conversion to semi‐natural grassland in 1998. Because of different fertilizer application in the arable phase, the grass was established on soils with different initial fertility. Archived soils sampled during 1942–2012 from plots subjected to different treatments in the arable phase were analysed for C, N, 13C and 15N. With crop yields, topsoil C contents and the C‐TOOL model, we estimated mean C inputs in the arable phase of 0.4, 1.4 and 1.7 Mg C ha−1 year−1 for unmanured, mineral‐fertilized and animal‐manured plots, respectively, and C inputs in the grassland phase of 3.2–3.8 Mg C ha−1 year−1. In the arable phase, topsoil showed mean losses of 0.10 Mg C and 0.012 Mg N ha−1 year−1, whereas δ13C increased by 0.002‰ and δ15N by 0.013‰. Grassland establishment reverted losses of C and N to gains of 0.29 Mg C and 0.017 Mg N ha−1 year−1; δ13C now decreased by 0.065‰ and δ15N by 0.074‰. Fertilizer history did not affect these changes markedly. Converting this low‐yielding sandy soil from arable to grassland use provided an overall annual gain of 0.39 Mg C and 0.029 Mg N ha−1 in the topsoil. Changes in δ13C and δ15N indicated a reduced rate of C turnover and a less leaky N cycle under grassland. Highlights: Assessed change in C and N storage with change in land use from arable to semi‐natural grasslandOverall soil C sequestration was 0.39 Mg C ha−1 year−1 because C loss was avoided from continued arable useSoil δ13C and δ15N decreased when arable land was converted into grasslandIn the grassland phase, the modelled C input was not affected by fertilizer history [ABSTRACT FROM AUTHOR]