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Nitrogen (N) cycling in drylands under changing climate is not wellunderstood. Our understanding of N cycling over larger scales to date reliesheavily on the measurement of bulk soil N, and the information about internalsoil N transformations remains limited. The 15N natural abundance(δ15N) of ammonium and nitrate can serve as a proxy record for theN processes in soils. To better understand the patterns and mechanisms of Ncycling in drylands, we collected soils along a 3200 km transect at about100 km intervals in northern China, with mean annual precipitation (MAP)ranging from 36 to 436 mm. We analyzed N pools and δ15N ofammonium, dual isotopes (15N and 18O) of nitrate, and the microbialgene abundance associated with soil N transformations. We found that N statusand its driving factors were different above and below a MAP threshold of100 mm. In the arid zone with MAP below 100 mm, soil inorganic Naccumulated, with a large fraction being of atmospheric origin, and ammoniavolatilization was strong in soils with high pH. In addition, the abundanceof microbial genes associated with soil N transformations was low. In thesemiarid zone with MAP above 100 mm, soil inorganic N concentrations werelow and were controlled mainly by biological processes (e.g., plant uptake anddenitrification). The preference for soil ammonium over nitrate by thedominant plant species may enhance the possibility of soil nitrate lossesvia denitrification. Overall, our study suggests that a shift from abioticto biotic controls on soil N biogeochemistry under global climate changeswould greatly affect N losses, soil N availability, and other Ntransformation processes in these drylands in China

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