Impact of grazing on soil δ15N of mountainous grassland ecosystems over the northern Tianshan Mountains, China

The total N and δ15N of rhizosphere soil 0-10 and 10-20, 20-30 cm of Stipa capillata, Festuca ovina, Carex turkestanica and Seriphidium borotalense in the mountainous grassland ecosystems over the northern Tianshan Mountains were measured. δ15N of dominant plants' rhizosphere soil in grazing and fencing grassland was compared to reveal the impacts of different usage patterns on δ15N of rhizosphere soil in different plants and different soil layer. The results indicated that the range of soil δ15N of mountainous grassland ecosystems over the northern Tianshan Mountains were from 2.25‰ to 16.03‰. Grazing significantly decreased δ15N in 0-10 cm soil. The δ15N in 0-30 cm soil of grazing grassland (7.14‰±0.67‰) increased by 2.8% than that of fencing grassland (6.95‰±0.34‰) although there was no significant difference between them. The δ15N in soil significantly increased with the increasing of soil depth in both grazing and fencing grassland and it had significantly negative correlation with soil total N. The impacts of grazing on δ15N in rhizosphere soil of different plants were different. The δ15N in the 0-30 cm rhizosphere soils of Stipa capillata and Festuca ovina in the grazing grassland were 17% and 53% higher than that in fenced grassland respectively. However, the δ15N in the 0-30 cm rhizosphere soils of Carex turkestanica and Seriphidium borotalense were lower than that in fenced grassland. The measurement of δ15N supported an aggregative indicator of N cyclic process in a typical area for the ecosystem and showed that grazing changed the N cycle of grassland ecosystem.