DataSheet1_Defining Quantitative Targets for Topsoil Organic Carbon Stock Increase in European Croplands: Case Studies With Exogenous Organic Matter Inputs.docx

The EU Mission Board for Soil Health and Food proposed a series of quantitative targets for European soils to become healthier. Among them, current soil organic carbon (SOC) concentration losses in croplands (0.5% yr−1 on average at 20 cm depth) should be reversed to an increase of 0.1–0.4% yr−1 by 2030. Quantitative targets are used by policy makers to incentivize the implementation of agricultural practices that increase SOC stocks. However, there are different approaches to calculate them. In this paper, we analyzed the effect of exogenous organic matter (EOM) inputs on the evolution of SOC stocks, with a particular focus on the new European targets and the different approaches to calculate them. First, we illustrated through two case-study experiments the different targets set when the SOC stock increase is calculated considering as reference: 1) the SOC stock level at the onset of the experiment and 2) the SOC stock trend in a baseline, i.e., a control treatment without EOM addition. Then, we used 11 long-term experiments (LTEs) with EOM addition in European croplands to estimate the amount of carbon (C) input needed to reach the 0.1 and 0.4% SOC stock increase targets proposed by the Mission Board for Soil Health and Food, calculated with two different approaches. We found that, to reach a 0.1 and 0.4% increase target relative to the onset of the experiment, 2.51 and 2.61 Mg C ha−1 yr−1 of additional C input were necessary, respectively. Reaching a 0.1 and 0.4% increase target relative to the baseline required 1.38 and 1.77 Mg C ha−1 yr−1 of additional input, respectively. Depending on the calculation method used, the estimated amounts of additional C input required to reach each quantitative target were significantly different from each other. Furthermore, the quality of C input as represented by the C retention rate of the additional organic material (EOM and crop residue), had a significant effect on the variation of SOC stocks. Our work highlights the necessity to take into consideration the additional C input required to increase SOC stocks, especially for soils with decreasing SOC stocks, when targets are set independently of the baseline.

欧盟土壤健康与粮食使命委员会（EU Mission Board for Soil Health and Food）提出了一系列旨在提升欧洲土壤健康水平的量化目标。其中，当前农田土壤有机碳（soil organic carbon, SOC）浓度在20厘米土层下平均以每年0.5%的速率流失，到2030年需将这一趋势扭转至每年提升0.1%~0.4%。政策制定者依托此类量化目标，激励推广可提升土壤有机碳储量的农业生产实践。然而，针对此类目标的测算存在多种不同方法。

本研究分析了外源有机物料（exogenous organic matter, EOM）输入对土壤有机碳储量动态的影响，重点围绕欧盟最新提出的土壤健康目标及其测算方法展开。首先，本研究通过两组案例试验，阐明了以两种不同基准测算土壤有机碳储量增幅时对应的目标差异：一是以试验起始时的土壤有机碳储量水平为基准，二是以基线（即不添加外源有机物料的对照处理）的土壤有机碳储量变化趋势为基准。随后，本研究利用欧洲农田中11组添加外源有机物料的长期定位试验（long-term experiments, LTEs），针对该使命委员会提出的0.1%和0.4%土壤有机碳储量增幅目标，通过两种不同测算方法分别估算了所需的碳输入量。

研究结果显示，若以试验起始值为基准达成0.1%和0.4%的增幅目标，分别需要额外输入2.51和2.61 Mg C ha⁻¹ yr⁻¹的碳；若以基线为基准达成上述增幅目标，则分别需要额外输入1.38和1.77 Mg C ha⁻¹ yr⁻¹的碳。不同测算方法下，达成各量化目标所需的额外碳输入量存在显著差异。此外，以额外有机物料（外源有机物料与作物残体）的碳留存率表征的碳输入质量，对土壤有机碳储量的变化存在显著影响。本研究强调，在脱离基线设定土壤有机碳储量提升目标时，尤其针对当前土壤有机碳储量呈下降趋势的农田，必须充分考虑达成目标所需的额外碳输入量。