Vegetation patches modify the acquisition of nitrogen by plants and microorganisms in a degraded alpine steppe

Abstract

Nitrogen (N) is an essential nutrient for both plants and soil microbes, but it often has limited availability. Currently, little is known about the effects of different vegetation patch types on the partitioning of N between plants and soil microorganisms in grassland ecosystems. In the present study, we performed a 15N-labelling experiment (using 15N-NO3− and 15N-NH4+) to investigate N uptake by plant biomass and microbial biomass for five common vegetation patch types in a degraded alpine steppe on the Tibetan Plateau. The results showed that plants and soil microorganisms in all patches showed a clear preference for the uptake of NO3−. Plants in patches dominated by palatable species absorbed more N than plants in unpalatable species patches, while N uptake in the microbial biomass in unpalatable species patches was higher than that in palatable species patches. For the two soil depths, plants in Poa litwinowiana patches had the highest N uptake (NO3−: 13.32–51.28 mg m-2; NH4+: 0.35–1.36 mg m-2), whereas microbial biomass in Oxytropis glacialis patches had the highest N uptake (NO3−:846.97–1659.87 mg m-2; NH4+: 108.75–185.14 mg m-2) among the five vegetation patch types. For both forms of N, soil microorganisms acquired relatively more N than the plants in the five vegetation patch types (i.e., the ratio of microbial biomass N uptake to plant biomass N uptake was greater than 1). The N-absorbing capacity of plants decreased, whereas the capacity of soil microorganisms to take up N increased with the degradation of vegetation patches. Microorganisms that compete more strongly for N might reduce the uptake of nutrients by plants in degraded patches, which would not be conducive to the restoration of vegetation in N-limited alpine grasslands.