Spatial effects of nitrogen deposition on soil organic carbon stocks in patchy degraded saline-alkaline grassland

Abstract

Soil organic carbon (SOC) represents the largest carbon sink and plays a critical role in mitigating global warming. Elevated nitrogen (N) deposition has substantial effects on soil carbon sequestration in grassland ecosystems. However, almost no study has examined the effects of N deposition in widely degraded saline-alkaline grasslands. This is a critical knowledge gap because saline-alkaline degradation is one of the key global threats to carbon sequestration services provided by grasslands. Here, we conducted a 3-year N addition experiment in a patchy degraded saline-alkaline grassland to examine the effects of simulated N deposition on SOC stocks, along with spatial effects for five types of saline-alkaline degraded patches (multi-species-mixture patches; Leymus chinensis patches; Puccinellia tenuiflora patches; Chloris virgata patches and Artemisia anethifolia patches). N addition had no overall effect on SOC stocks in the degraded saline-alkaline grassland, but showed pronounced spatial effects depending on the saline-alkaline characteristics of degraded patches. Specifically, SOC decreased in multi-species-mixture patches with low saline-alkaline stress (soil 7 < pH < 8), but no change was observed in other types of degraded patches with high saline-alkaline stress (soil pH > 9). Although N addition promoted plant growth and increased aboveground biomass in all degraded patches, the resulting aboveground carbon input did not further contribute to soil carbon sequestration. Rather than aboveground biomass, soil pH was the critical controlling factor for soil carbon in the saline-alkaline grassland. However, N addition did not alter soil pH in all degraded patches. In multi-species-mixture patches with lightly saline-alkaline soils, N addition reduced species richness and increased microbial C:N ratio, which could accelerate soil carbon decomposition, thereby decreasing SOC stocks. Our findings suggest that increased global N deposition would not restore natural soil carbon in degraded saline-alkaline grassland, and instead, may further deteriorate soil carbon sequestration in lightly degraded patches with high plant diversity and low saline-alkaline stress.