Nitrogen fertilization and the rhizosphere effect on nitrogen cycling: A meta-analysis

The mechanism by which nutrient availability modifies rhizosphere nitrogen (N) cycling is not well understood. Two meta-analyses were thus conducted to reveal how rhizosphere processes related to N cycling and the rhizosphere effect (RE) on N cycling would change with N fertilization. Empirical evidence has shown that the RE on ammonium (NH4+) and microbial biomass N changed from positive to negative after N fertilization. The RE on total N, and protease and urease activities was positive, and not usually affected by N fertilization. In response to N fertilization, the RE on nitrate (NO3−) not ammonia oxidizing bacteria (AOB) was more negative, and the RE on ammonia oxidizing archaea (AOA) shifted from insignificant to positive. Evidence of the RE on nosZ gene down- or up-regulated by N fertilization was lacking. Interestingly, NH4+ in the rhizosphere remained unaffected by N fertilization. In the rhizosphere where AOB (110.7 %) and AOA (13.54 %) often responded to N fertilization by an increase, nitrate exhibited a large increase (115.11 %). Nitrogen fertilization increased denitrification genes of not only nirS (24.36 %) and nirK (78.25 %) but also nosZ (64.05 %) in the rhizosphere. Overall, our work provides a window to see how N fertilization modifies REs to affect the N status and cycling in the rhizosphere. Findings from our meta-analyses highlight that the altered RE on NH4+ by N fertilization may have a minor impact on NH4+ availability in the rhizosphere. Nitrate enrichment in the rhizosphere after N fertilization is unlikely to be the result of a positive RE on NO3−. When N is fertilized, rhizosphere variations in total N, AOB, protease and urease activities, and nosZ gene may not result from altered RE on these parameters.