A slight increase in soil pH benefits soil organic carbon and nitrogen storage in a semi-arid grassland

Grassland soil organic carbon and nitrogen storage (SOC storage and SNC storage) have been regarded as indicators to evaluate impacts of global climate change on ecosystem functions due to their significant impact on atmospheric carbon (C) concentration. The variations in vegetation and soil properties in diverse vegetation patch types may change soil C and nitrogen (N) sequestration capacity. However, the quantities of SOC and SNC stored and the mechanisms behind the variation in this storage under diverse vegetation patches in grassland ecosystems are still unclear. Here we conducted a field experiment to measure the variations in vegetation composition, soil properties and SOC storage and SNC storage among five common vegetation patch types in the Eastern Eurasian steppe. Then we investigated the link between vegetation variation and the SOC and SNC storage. The results showed that (1) the combined effects of competitive ability and physiological stress drove the unimodal relationship between species diversity and soil pH. (2) Biomass production did not reach its maximum in patch types with the greatest plant diversity due to saline-alkaline stress, but the live vegetation N storage reached its maximum in these highly diverse patches due to complementary resource utilization effects. (3) Although biomass production of patches with the greatest biodiversity did not reach a maximum due to a slight increase in soil pH, the largest SOC and SNC storage values were found in the highly diverse patches. Our study implies that in natural grassland, high levels of species diversity may accelerate the decomposition rate, resulting in more recalcitrant organic C and N are released into the soil. At the same time, our observation that reductions in the area of the originally dominant patch type due to the expansion of other species during grassland degradation suggests that comprehensive measurements of SOC and SNC storage in different vegetation patches should be undertaken for accurate evaluation of the C and N sequestering capacity of grasslands. Our results can also help policy makers determine how to achieve sustainable development of grasslands based on C and N sequestration.