Effects of nitrogen application rate on phosphorus transformation in an Alfisol: Results from phosphate-oxygen isotope ratios

Abstract

The effects of N application on P nutrient uptake by plants are well known. However, soil P transformation under the influence of N application and the underlying mechanisms are not yet clearly understood. In this study, we investigated the effect of N application on P uptake by crops and the soil P transformation process using the phosphate oxygen isotope technique combined with the sequential extraction method. The results showed that the rapeseed yield initially increased drastically with N application, but a yield leveling off effect was observed at the highest N level. After applying P fertilizer with a low δ18OP value (original δ18OP value of 12.9‰), we observed that P in P fertilizer was transformed primarily into the NaOH-Pi pool. The lowest N rate (75 kg ha−1) and the highest N rate (225 kg ha−1) promoted exchange between the NaOH-Pi pool and the NaHCO3-Pi pool. The moderate N rate (150 kg ha−1) possibly promoted the transformation of the NaOH-Po pool to the NaOH-Pi pool. In the highest N treatment, the δ18OP value of the 1 M HCl-Pi pool significantly increased (19.0‰) and changed toward equilibrium, but the pool size was decreased, suggesting that the highest N rate possibly enhanced the transformation of the otherwise stable P pool. Overall, these results suggested that the specific N application rate could promote the turnover and transformation of soil P, which partly accounts for the enhanced soil P bioavailability and the increased crop yield and possibly leads to a high risk of P losses into the environment.