Mechanisms of soil persistent organic carbon loss in alpine ecosystems: Insights into microbial and calcium spatial footprint

Soil persistent organic carbon (OC) constitutes an ancient, previously unquantified global C sink. Yet, the mechanisms underlying the vulnerability of this pool and its functional components to global change remain unclear, especially in hydrothermal resource-restricted alpine ecosystems. Here, employing low-temperature ashing (mimics C natural oxidative processes) and ¹⁸O–H₂O incubation, we explored the differential mechanisms underlying the persistent OC loss caused by warming and nitrogen (N) deposition in alpine meadow versus alpine steppe. We observed a substantial decline in persistent OC of meadow soils, with N addition (−22%) exerting the largest decline, followed by warming (−8%). However, warming (−16%) negated the positive effects of N addition (+15%) on steppe persistent OC. Such dynamics in persistent OC were strongly linked to mineral-associated OC (MAOC) but not to particulate OC (POC). In meadow soils, N addition decreased MAOC by 23%, likely by reducing soil pH and suppressing both calcium (Ca) bridging and its physical protection via microaggregates. In steppe soils, the reduction in MAOC (−29%) was primarily due to warming-induced limitations in soil moisture, which, in turn, constrained the formation of microbial necromass C by suppressing microbial growth and lengthening turnover. Collectively, warming and N addition imposed spatial constraints on microbial and Ca footprint, which underlie the loss of MAOC in alpine steppe and meadow, respectively, and might affect soil C persistence more widespread.