Changes in land use and climate are the main drivers of change in soil organic matter contents. We investigated the impact of the largest policy-induced land conversion to arable land, the Virgin Lands Campaign (VLC), from 1954 to 1963, of the massive cropland abandonment after 1990 and of climate change on soil organic carbon (SOC) stocks in steppes of Russia and Kazakhstan. We simulated carbon budgets from the pre-VLC period (1900) until 2100 using a dynamic vegetation model to assess the impacts of observed land-use change as well as future climate and land-use change scenarios. The simulations suggest for the entire VLC region (266 million hectares) that the historic cropland expansion resulted in emissions of 1.6⋅ 10¹⁵ g (= 1.6 Pg) carbon between 1950 and 1965 compared to 0.6 Pg in a scenario without the expansion. From 1990 to 2100, climate change alone is projected to cause emissions of about 1.8 (± 1.1) Pg carbon. Hypothetical recultivation of the cropland that has been abandoned after the fall of the Soviet Union until 2050 may cause emissions of 3.5 (± 0.9) Pg carbon until 2100, whereas the abandonment of all cropland until 2050 would lead to sequestration of 1.8 (± 1.2) Pg carbon. For the climate scenarios based on SRES (Special Report on Emission Scenarios) emission pathways, SOC declined only moderately for constant land use but substantially with further cropland expansion. The variation of SOC in response to the climate scenarios was smaller than that in response to the land-use scenarios. This suggests that the effects of land-use change on SOC dynamics may become as relevant as those of future climate change in the Eurasian steppes.

土地利用和气候的变化是土壤有机质含量变化的主要驱动因素。我们调查了 1954 年至 1963 年间最大的政策引发的土地转变为耕地的处女地运动 (VLC) 对 1990 年之后的大规模农田废弃以及气候变化对草原土壤有机碳 (SOC) 储量的影响俄罗斯和哈萨克斯坦。我们使用动态植被模型模拟了从 VLC 之前时期（1900 年）到 2100 年的碳预算，以评估观察到的土地利用变化以及未来气候和土地利用变化情景的影响。模拟表明，对于整个 VLC 地区（2.66 亿公顷），历史性的农田扩张导致排放量为 1.6·10 ¹⁵g (= 1.6 Pg) 碳在 1950 年和 1965 年之间，而在没有扩张的情况下为 0.6 Pg。从 1990 年到 2100 年，仅气候变化就预计会导致约 1.8 (± 1.1) Pg 碳的排放。假设在苏联解体后重新开垦到 2050 年的耕地可能导致 2100 年前 3.5 (± 0.9) Pg 碳的排放，而到 2050 年所有耕地的放弃将导致固碳 1.8 (± 1.2) Pg碳。对于基于 SRES（排放情景特别报告）排放路径的气候情景，SOC 仅在持续土地使用时适度下降，但随着农田的进一步扩张而大幅下降。SOC 响应气候情景的变化小于响应土地利用情景的 SOC。