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

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 15 N natural abundance (δ 15 N) 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 δ 15 N of ammonium, dual isotopes ( 15 N and 18 O) 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 bi-otic 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.

Higher capability of C3 than C4 plants to use nitrogen inferred from nitrogen stable isotopes along an aridity gradient

“…Species that prefer nitrate relative to ammonium generally have lower δ 15 N values than species that prefer ammonium (Houlton et al 2007). In our previous study along the same climatic gradient, we found that δ 15 N values of ammonium consistently increased, while δ 15 N values of nitrate increased first and then decreased with increasing aridity (see Liu et al 2017). These results suggest a shift of dominant inorganic N sources for plants with aridity, resulting in a positive relationship between aridity and plant δ 15 N values and a decoupled relationship between plant and soil bulk δ 15 N values.…”

The fitness of chemotrophs increases when their catabolic by‐products are consumed by other species

“…Although the intensity of the competition between abiotic and microbial reactions depends on environmental factors (e.g. temperature, coexisting chemical substances, water availability and pH) (Melton et al ; Liu et al ), we have assumed that microbial reactions occur much faster than non‐biological reactions and so we have neglected the latter.…”

Mineral N stock and nitrate accumulation in the 50 to 200 m profile on the Loess Plateau

“…The KCl extract was analyzed immediately for NH4-N and NO3-N concentrations using a Lachat Flow Injection Analyzer (AutoAnalyzer3-AA3, Seal Analytical, Mequon, WI) (Kachurina et al, 2000). In order to identify the sources of accumulated nitrate, the isotope compositions of nitrate (δ 15 N and δ 18 O) were analyzed based on the isotopic analysis of the produced N2O from NO3-N (Liu et al, 2017). The value is expressed as:…”