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

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, C-13 and N-15. 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 delta C-13 increased by 0.002 parts per thousand and delta N-15 by 0.013 parts per thousand. Grassland establishment reverted losses of C and N to gains of 0.29 Mg C and 0.017 Mg N ha(-1) year(-1); delta C-13 now decreased by 0.065 parts per thousand and delta N-15 by 0.074 parts per thousand. 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 delta C-13 and delta N-15 indicated a reduced rate of C turnover and a less leaky N cycle under grassland.