Articles | Volume 14, issue 4
Research article
03 Mar 2017
Research article |  | 03 Mar 2017

Abiotic versus biotic controls on soil nitrogen cycling in drylands along a 3200 km transect

Dongwei Liu, Weixing Zhu, Xiaobo Wang, Yuepeng Pan, Chao Wang, Dan Xi, Edith Bai, Yuesi Wang, Xingguo Han, and Yunting Fang

Abstract. Nitrogen (N) cycling in drylands under changing climate is not well understood. Our understanding of N cycling over larger scales to date relies heavily on the measurement of bulk soil N, and the information about internal soil N transformations remains limited. The 15N natural abundance (δ15N) of ammonium and nitrate can serve as a proxy record for the N processes in soils. To better understand the patterns and mechanisms of N cycling in drylands, we collected soils along a 3200 km transect at about 100 km intervals in northern China, with mean annual precipitation (MAP) ranging from 36 to 436 mm. We analyzed N pools and δ15N of ammonium, dual isotopes (15N and 18O) of nitrate, and the microbial gene abundance associated with soil N transformations. We found that N status and its driving factors were different above and below a MAP threshold of 100 mm. In the arid zone with MAP below 100 mm, soil inorganic N accumulated, with a large fraction being of atmospheric origin, and ammonia volatilization was strong in soils with high pH. In addition, the abundance of microbial genes associated with soil N transformations was low. In the semiarid zone with MAP above 100 mm, soil inorganic N concentrations were low and were controlled mainly by biological processes (e.g., plant uptake and denitrification). The preference for soil ammonium over nitrate by the dominant plant species may enhance the possibility of soil nitrate losses via denitrification. Overall, our study suggests that a shift from abiotic to biotic controls on soil N biogeochemistry under global climate changes would greatly affect N losses, soil N availability, and other N transformation processes in these drylands in China.

Short summary
The use of 15N natural abundance of soil ammonium and nitrate demonstrates a clear shifting contribution from abiotic to biotic controls on N cycling along a 3200 km dryland transect in northern China, with a threshold at mean annual precipitation of 100 mm. Abiotic factors were the main driver below threshold, shown by the accumulation of atmospheric N and NH3 losses. In the area above threshold, soil N cycling was controlled mainly by biological factors, e.g., plant uptake and denitrification.
Final-revised paper