High-rate nitrogen loading accelerates organic phosphorus loss through enzymatic and non-enzymatic processes in a semi-arid grassland

Abstract

Semi-arid grasslands suffer from increased nitrogen (N) loading, which affects phosphorus (P) cycling. Phosphorus is essential but limited for all living organisms. Organic P (Po) is crucial for P supply in grasslands response to N loading, but its transformation process remains unclear. To elucidate P cycling, we evaluated P fractions, phosphatase activities, and their coding genes (pho genes phoD, phoX, and phoC) at both DNA and RNA levels in response to a nitrogen loading gradient. The results showed that the total P and Po contents significantly decreased with increasing N loading rates. In contrast, plant and litter biomass P exhibited an opposite trend but were insufficient to offset soil P loss. Unexpectedly, neither the phosphatase activities nor microbial biomass increased with increasing N loading rates, although the DNA abundance of pho genes responded positively to N enrichment. A partial correlation network indicated that N loading rates decreased Po levels through phosphatase activities, which were significantly associated with soil pH and the RNA abundance of pho genes. Moreover, non-enzymatic mineralization may be enhanced in Po cycling in semi-arid grasslands undergoing high N loading, especially following the initial stage of enzymatic mineralization. Although the activity of phosphatases did not continue to rise with increased N loading, the strategy for addressing N-induced P limitation through overconsumption of Po via non-enzymatic processes would accelerate the degradation of the ecosystem in the future.