Intensified Aridity Hinders Soil Microbes From Improving Their Nitrogen Use Efficiency

Microbial nitrogen (N) use efficiency (NUE) is crucial for retaining N in soils and supplying N to plants. However, how soil microbial NUE in N-limited dryland responds to aridity remains poorly understood. Here we used ¹⁸O and ¹⁵N isotope labeling techniques to investigate the effects of climatic, edaphic, and biotic factors on microbial N metabolism along a 2200 km aridity gradient on the Tibetan Plateau. We found soil microbes could enhance their NUE to cope with N limitation, but this ability was hindered when aridity index (AI) < 0.12 (extremely dry conditions) where water limitation directly inhibited microbial growth. As water limitation weakened (AI > 0.12), microbes increased their NUE with decreasing aridity because they got more limited by N and temperature. The increase of microbial NUE increased microbial necromass N, contributing to the increase of soil total N. Our findings provide new insights into microbial N use strategies under water- and N-limited conditions and their vital role in N retention in ecosystems. This helps to deploy microbial potential in conserving N in soils for higher productivity and to better predict soil N processes under global changes.