Abstract Shifting cultivation systems of Southeast Asia are rapidly intensifying, especially through shortening of the fallow periods. It is typically assumed that intensification will result in a depletion of soil organic carbon (SOC) stocks, but existing estimates of carbon stocks in these systems are variable, and there is little certainty about the carbon outcomes of intensification. We investigated the effects of intensification on SOC stocks of a shifting cultivation system in northern Laos. Volume-specific soil samples were collected from 20 sites representing: i) various rotation intensities (fallow periods of 3–4 years and 7–10 years), ii) various stages of the rotation cycle (fallows and active fields) and iii) reference plots (old regrowth of 25–30 years). Samples were analyzed for SOC, soil texture, pH, Total Nitrogen and permanganate oxidizable carbon (POXC) – an active carbon fraction that has been suggested as an easily measured early indicator of land use induced changes in soil quality and SOC. There were no significant differences between SOC concentrations or stocks of any of the sites under shifting cultivation and the reference sites. However, the SOC stock under fallows in the intensive rotation category was significantly larger than the SOC stock under fallows in the extensive rotation category. This is likely because inputs of dead root biomass from the slashed vegetation provides an important input to the SOC pool. Fallow sites under intensive rotation had significantly higher contents of POXC in the topsoil than the active fields, which suggests that POXC captures the immediate effect of the decreased input of litter to the topsoil during the cultivation period. We conclude that in this study there is no evidence that intensification of shifting cultivation leads to a decline in total soil carbon stock, or to a decline in the more active carbon fraction measured by POXC. Therefore, narratives of shifting cultivation leading to a decline in soil carbon stocks need to be revisited, and land use policies related to the system should not uncritically be based on this incorrect assumption.

中文翻译：

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尽管轮作集约化，土壤有机碳储量保持不变

摘要 东南亚的轮耕制度正在迅速加强，特别是通过缩短休耕期。通常假设集约化将导致土壤有机碳 (SOC) 储量枯竭，但这些系统中碳储量的现有估计是可变的，并且集约化的碳结果几乎没有确定性。我们调查了集约化对老挝北部轮垦系统 SOC 储量的影响。从 20 个地点收集特定体积的土壤样品，代表：i) 各种轮作强度（休耕期为 3-4 年和 7-10 年），ii) 轮作周期的各个阶段（休耕和活动田地）和 iii) 参考地块（25-30 年的旧再生）。分析样品的 SOC、土壤质地、pH、总氮和高锰酸盐可氧化碳 (POXC) – 一种活性碳部分，已被建议作为土地利用引起的土壤质量和 SOC 变化的易于测量的早期指标。任何轮作种植地点和参考地点的 SOC 浓度或储量之间没有显着差异。然而，密集轮作类别中休耕的 SOC 储量明显大于广泛轮作类别中休耕的 SOC 储量。这可能是因为来自砍伐植被的死根生物量输入为 SOC 库提供了重要输入。密集轮作的休耕地表土中 POXC 含量显着高于活动田，这表明 POXC 捕获了栽培期间凋落物输入表土减少的直接影响。我们得出的结论是，在本研究中，没有证据表明轮垦的集约化会导致土壤总碳储量下降，或导致 POXC 测量的活性碳含量下降。因此，需要重新审视轮垦导致土壤碳储量下降的说法，与该系统相关的土地利用政策不应不加批判地基于这种错误的假设。