Abstract：
Converting natural vegetation for agriculture has resulted in the loss of approximately 5% of the current global terrestrial soil organic carbon (SOC) stock to the atmosphere. Increasing the agricultural area under grassland may reverse some of these losses, but the effectiveness of such a strategy is limited by how quickly SOC recovers after conversion from cropland. Using soil data and extensive land-use histories gathered during the national German agricultural soil inventory, this study aims to answer three questions regarding agricultural land-use change (LUC): (i) how do SOC stocks change with depth following LUC; (ii) how long does it take to reach SOC equilibrium after LUC; and (iii) what is the legacy effect of historic LUC on present day SOC dynamics? By using a novel approach that substitutes space for time and accounts for differences in site properties using propensity score balancing, we determined that sites that were converted from cropland to grassland reached a SOC equilibrium level 47.3% (95% confidence interval (CI): 43.4% to 49.5%) above permanent cropland levels 83 years (95% CI: 79 to 90 years) after conversion. Meanwhile, sites converted from grassland to cropland reached a SOC equilibrium level -33.6% (95% CI: -34.1% to -33.5%) below permanent grassland levels after 180 years (95% CI: 151 to 223 years). We estimate that, over the past century, today\'s German agricultural soils (16.6 million ha) have gained about 40 million Mg C. Furthermore, croplands with historic LUC from grassland are losing SOC by -0.26 Mg ha-1 year-1 (10% of agricultural land) while grasslands historically converted from cropland are gaining SOC by 0.27 Mg ha-1 year-1 (18% of agricultural land). This study shows that even long-standing temperate agricultural sites likely have ongoing SOC change as a result of historical LUC.
摘要：
将自然植被转化为农业导致目前全球陆地土壤有机碳（SOC）存量的约5％损失到大气中。增加草地下的农业面积可能会扭转其中的一些损失，但是这种策略的有效性受到从农田转化后SOC恢复的速度的限制。利用德国国家农业土壤清查期间收集的土壤数据和广泛的土地利用历史，这项研究旨在回答有关农业土地利用变化（LUC）的三个问题：（i）LUC之后SOC存量如何随深度变化；（ii）LUC之后达到SOC平衡需要多长时间；（iii）历史LUC对当今SOC动态的传统影响是什么？我们确定，从农田转换为草地的地点在转换后83年（95％CI：79至90年）达到了高于永久农田水平47.3％（95％置信区间（CI）：43.4％至49.5％）的SOC平衡水平。同时,180年后（95％CI：151至223年），从草地转变为农田的地点达到了低于永久草地水平的SOC平衡水平-33.6％（95％CI：-34.1％至-33.5％）。我们估计,在过去的一个世纪里，今天的德国农业土壤(1660万公顷)增加了约4000万MgC。此外，具有草地历史LUC的农田SOC减少了-0.26Mgha-1year-1（占农业用地的10％），而历史上从农田转化的草地SOC增加了0.27Mgha-1year-1（占农业用地的18％）。这项研究表明，由于历史上的LUC，即使是长期存在的温带农业地点也可能会发生持续的SOC变化。
