Mechanisms of soil nitrogen conservation in a degraded karst ecosystem of Southwest China

Distinctive geological and hydrological characteristics are widely believed to promote a leaky nitrogen (N) cycle in karst regions. This paradigm has long shaped our understanding of the low soil N recovery and pronounced N limitation observed in degraded karst ecosystems; Paradoxically, however, previous studies have shown that these systems exhibit a low potential for N loss. Thus, an unknow mechanism may contribute to soil N conservation in degraded karst ecosystems. Here, we quantified hydrologic and gaseous N losses in karst grasslands and grass-shrublands of Southwest China through a comprehensive field monitoring campaign. An in-situ 15 N tracing experiment, combined with measurements of soil gross N transformation rates, was conducted to elucidate the underlying mechanisms of N conservation. Our results confirmed consistently low N losses, with annual fluxes of 0.5 kg N ha −1 y –1 for nitrous oxide, 0.6 kg N ha −1 y –1 for ammonia, and leaching losses of 0.5 and 0.3 kg N ha −1 y –1 for ammonium and nitrate, respectively. Less than 1 % of the experimentally applied 15 N was lost via these pathways. We identified three complementary mechanisms governing N conservation: (i) elevated microbial N assimilation driven by strong microbial N demand; (ii) substantial abiotic N adsorption promoted by high clay and exchangeable calcium content; and (iii) efficient plant N uptake induced by N limitation. Collectively, these processes enhance ammonium retention and utilization while reducing nitrate production and availability, thereby minimizing N loss via nitrification, denitrification, and leaching. This study provides the first systematic, ecosystem-scale evidence that degraded karst systems can possess a high internal N retention capacity. These findings necessitate a rethink of the prevailing leaky N paradigm and provide new insights into how N cycling controls ecological succession in these vulnerable regions.