Drylands contain a third of the organic carbon stored in global soils; however, the long-term dynamics of soil organic carbon in drylands remain poorly understood relative to dynamics of the vegetation carbon pool. We examined long-term patterns in soil organic matter (SOM) against both climate and prescribed fire in a Chihuahuan Desert grassland in central New Mexico, USA. SOM concentration was estimated by loss-on-ignition from soils at 0–20 cm depth each spring and fall for 25 years (1989–2014) in unburned desert grassland and from 2003 to 2014 following a prescribed fire. SOM concentration did not have a long-term trend but fluctuated seasonally at both burned and unburned sites, ranging from a minimum of 0.9% to a maximum of 3.3%. SOM concentration declined nonlinearly in wet seasons and peaked in dry seasons. These long-term results contrast not only with the positive relationships between aboveground net primary production and precipitation for this region, but also with previous reports that wetter sites have more SOM across drylands globally, suggesting that space is not a good substitute for time in predicting dryland SOM dynamics. We suggest that declines in SOM in wet periods are caused by increased soil respiration, runoff, leaching, and/or soil erosion. In addition to tracking natural variability in climate, SOM concentration also decreased 14% following prescribed fire, a response that magnified over time and has persisted for nearly a decade due to the slow recovery of primary production. Our results document the surprisingly dynamic nature of soil organic matter and its high sensitivity to climate and fire in a dry grassland ecosystem characteristic of the southwestern USA.

中文翻译：

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荒漠草原土壤有机质对气候和火灾的敏感性

旱地含有储存在全球土壤中的有机碳的三分之一；然而，相对于植被碳库的动态，旱地土壤有机碳的长期动态仍然知之甚少。我们研究了美国新墨西哥州中部奇瓦瓦沙漠草原的土壤有机质 (SOM) 与气候和规定火灾的长期模式。SOM 浓度是通过每年春季和秋季 25 年（1989-2014 年）在未燃烧的沙漠草原和 2003 年至 2014 年在规定的火灾后从 0-20 厘米深度土壤的燃烧损失估算的。SOM 浓度没有长期趋势，但在已燃烧和未燃烧地点均存在季节性波动，范围从最低 0.9% 到最高 3.3%。SOM 浓度在雨季呈非线性下降，在旱季达到峰值。这些长期结果不仅与该地区地上净初级生产力和降水之间的正相关关系形成对比，而且与之前的报告形成对比，即全球干旱地区的湿润地点具有更多的 SOM，这表明空间不能很好地替代时间进行预测旱地 SOM 动态。我们认为湿润期 SOM 的下降是由土壤呼吸、径流、淋溶和/或土壤侵蚀增加引起的。除了跟踪气候的自然变化外，SOM 浓度在规定的火灾后也下降了 14%，由于初级生产恢复缓慢，这种反应随着时间的推移而放大并持续了近十年。