Phosphorus limitation reduces microbial nitrogen use efficiency by increasing extracellular enzyme investments

Microbial nitrogen use efficiency (NUE), which reflects the proportion of nitrogen (N) taken up to be allocated to microbial biomass and growth, is central to our understanding of soil N cycling. However, the factors influencing microbial NUE remain unclear. Here, we explored the effects of climate factors, soil properties, and microbial variables on microbial NUE based on a survey of soils from 11 locations along a forest transect in eastern China. We found microbial NUE decreased with the ratio of acid phosphatase (AP) activity versus microbial growth rate. This suggested that increased microbial phosphorus acquisition decreased microbial NUE due to increasing investment in AP. However, microbial NUE increased with soil organic carbon content, because soil organic carbon is the source of material and energy for microbial growth and metabolism. Soil pH and mean annual temperature indirectly affected microbial NUE through their effects on the ratio of AP activity relative to microbial growth rate and soil organic carbon content, respectively. Our results improve our understanding and prediction of microbial NUE on a large spatial scale and emphasize the importance of phosphorus in affecting microbial metabolic efficiency.