气候变化领域集成服务门户

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.