Drylands cover approximately 41% of the Earth's land surface and play an essential role in providing ecosystem services and regulating carbon (C) and nitrogen (N) cycling. The precipitation regimes in drylands are predicted to change during the 21^st century, and more extreme climatic regimes will make dryland ecosystems more vulnerable to enhanced drought in some regions and intensive rain in others.

To better understand the responses and associated mechanisms of Chinese grassland ecosystems to global changes, Prof. HAN Xingguo led a research campaign in July and August 2012, in northern China.

The research was carried out along a 3,200 km transect across Gansu Province and Inner Mongolia, covering a longitude from 87.4^oE to 120.5^oE and a latitude from 39.9^oN to 50.1^oN.

From west to east along the transect, the mean annual precipitation (MAP) increases from 36 mm to 436 mm. This natural gradient of precipitation provides an ideal system for identifying the response of soil N dynamics to water availability. 

Nitrogen isotope technology provides tracing, integration and indication functions, and facilitates in understanding N status of ecosystems.

In a previous study, a hump-shaped pattern of ¹⁵N natural abundance (δ¹⁵N) in bulk soil N along this precipitation gradient was reported, with a threshold at an aridity index of 0.32, corresponding to a mean annual precipitation (MAP) of approximately 250 mm.

It was suggested that soil microbes and plants are main control factors for N cycling in the areas below and above the threshold, respectively. This threshold is also the empirical threshold defining the arid and semi-arid areas in China.

Under the guidance of Prof. FANG Yunting and ZHU Weixing from Institute of Applied Ecology, Chinese Academy of Sciences, Dr. LIU Dongwei and her colleagues analyzed the concentrations and N isotopes of soil ammonium and nitrate collected from the grassland transect.

This research found that in the areas with MAP less than 100 mm soil nitrate was accumulated, with highest concentrations of up to 1400 mg kg-1. The ¹⁵N natural abundance of soil nitrate increased in the areas with MAP less than 100 mm but decreased in the areas with MAP more than 100 mm.

After further consideration of the ¹⁸O natural abundance of soil nitrate, this study demonstrated that MAP of 100 mm is another threshold for the N cycling. Specifically, in the areas with MAP less than 100 mm, the abiotic factor was the main controller on N cycling, plant cover was sparse, and microbial activity was limited.

Nitrogen input, mostly in the form of atmospheric deposition, largely accumulated. The higher pH associated with a lower MAP is likely a dominant driver of NH₃ volatilization.

While in the areas with MAP more than 100 mm, the main controller on N cycling was biotic factor, i.e. plant N uptake and microbial denitrification. The preference for soil ammonium over nitrate by the dominant plant species may enhance the possibility of soil nitrate losses via denitrification.

The new threshold found by this research has a great implication for understanding the effects of precipitation change on ecosystem structure and function, and provides scientific basis for further classification of arid regions and for effective management of dryland ecosystems.  

The results have been published in Biogeosciences titled "Abiotic Versus Biotic Controls on Soil Nitrogen cycling in Drylands along a 3200 km Transect".