Differential responses of soil phosphorus fractions to varied nitrogen compound additions in a meadow steppe

Nitrogen (N) addition can greatly influence soil inorganic phosphorus (Pi) and organic phosphorus (Po) transformations. However, whether and how the N compound forms may differentially affect the soil P fractions remain unclear. Here, we investigated the responses of soil Pi (labile Pi, moderately-occluded Pi, and recalcitrant Pi) and Po fractions (labile Po and stable Po) to varying addition rates of three N compounds ((NH₄)₂SO₄, NH₄NO₃, and urea) in a meadow steppe in northern China. Our studies revealed that with increasing N addition rate, soil labile and moderately-occluded Pi increased, accompanied by decreases in soil recalcitrant Pi. This shift was attributed to N-induced soil acidification, which accelerated the conversion of recalcitrant Pi into labile and moderately-occluded Pi. Soil labile Po decreased with increasing rate of N addition, whilst soil stable Po was not affected. Regardless of the compound forms, N addition increased soil Olsen-P, suggesting a potential alleviation of P limitation in this grassland ecosystem. The effect of N addition on soil labile Pi was significantly greater with addition of urea than with addition of either (NH₄)₂SO₄ or NH₄NO₃, indicating that urea was more efficient in enhancing soil P availability. Addition of (NH₄)₂SO₄ imposed a more pronounced positive effect on soil moderately-occluded Pi than the addition of either NH₄NO₃ or urea, mainly due to the greater mobilization of recalcitrant Pi as a result of higher soil acidification strength of (NH₄)₂SO₄. These findings underscore the importance of considering the distinct effects of different N compounds when studying grassland soil P dynamics and availability in response to N addition.