Elucidating the effect and interaction mechanism of fulvic acid and nitrogen fertilizer application on phosphorus availability in a salt-affected soil

Abstract

The present study was performed to investigate the effects of the interaction of fulvic acid (FA) with different amounts of nitrogen fertilizer (N) on soil phosphorus availability and transformation in a salt-affected soil and to obtain the direct and indirect relationships between inorganic phosphorus (Pi) fractions and Olsen-P. An incubation experiment was conducted to evaluate the effects of nitrogen addition (215, 183, and 151 mg kg-1, referred as N100, N85, and N70), fulvic acid addition (30 and 60 mg kg-1, referred as F1 and F2), and their interactions on phosphorus availability (based on soil Olsen-P and inorganic P fractions). The results indicated that the application of N100 significantly reduced the release and availability of phosphorus (P). However, the phosphorus recovery (PR) as Olsen-P was significantly enhanced in the N100F1 and N100F2 treatments. The P kinetics was best described by the power function model. Moreover, labile Ca2-P and Ca8-P contents decreased as N application rate increased. Combined application of FA and fertilizer N increased the percentage of Ca2-P and decreased the percentage of Ca10-P at the end of the incubation period. According to the correlation analysis and path analysis, the direct effects of Ca2-P, Fe-P, and O-P on Olsen-P were significant after 112 days of incubation (P<0.05). The indirect effects of Ca2-P (R21Py1 = 0.57) and Fe-P (R24Py4 = 0.19) were essential contributors to the correlation between Ca8-P and Olsen-P. Additionally, the indirect effect of Ca8-P (R41Py1 = 0.53) contributed significantly to the correlation between Fe-P and Olsen-P. In conclusion, our results show that the availability of phosphorus can be improved by reducing the amount of nitrogen fertilizer applied in salt-affected soils. Integrating FA and inorganic N fertilizer enhances the release and availability of phosphorus, when excessive amounts of N fertilizers are applied to salt-affected soils.