Changes in phosphorus fractions in response to long-term nitrogen fertilization in loess plateau of China

Phosphorus (P) fractions and their availability in soils can be affected by long-term nitrogen (N) fertilizer application, which can subsequently affect plant growth and productivity. The extent of P fixation at different N fertilization rates is still unclear and this study highlighted an appropriate N fertilization rate showing minimum P fixation and higher P content uptake by the plants. Paradox changes of P into various organic (Po) and inorganic (Pi) fractions and their contribution in P content uptake and yield was evaluated under long-term (12 year) application of different N rates (0, 80, 160, 240, and 320 kg N ha- 1, hereafter, N0, N80, N160, N240, and N320, respectively) in winter wheat (Triticum aestivum L.). Phosphorus fractions were measured using Tiessen and Moir fractionation scheme modified from Hedley method. N rates significantly decreased available (especially NaHCO3-Pi) and residual fractions of P, when compared with N0. The NaHCO3-Pi showed a significant positive relationship with total plant P content and grain yield. Grain yield was maximum in N160 treatment with no significant difference between N160 and higher N levels. Increase in N rates from N0 to N320 little increased organic P fractions; NaHCO3-Po increased from 64.0 to 69.7 mg kg- 1, HCl-Po increased from 60.5 to 87.1 mg kg- 1, but NaOH-Po decreased from 46.5 to 31.2 mg kg- 1. All Pi fractions increased, and maximum contents were observed for HClD- -Pi fraction ranging between 189-265 mg kg- 1 from N0 to N320 treatments. HClD-Pi fraction was highest contributor (39%) in total P, which is transformable into available P with time. A decrease in residual P by 7.4% was observed in N320 compared with N0 treatment. Residual P fraction was positively correlated with grain yield, biomass, total P content uptake, and with the moderate available fractions (NaOH-Pi, NaOH-Po, and HClD-Pi). It can be concluded that excessive N fertilization does not always result in increased crop yield but can increase P fixation (HClD-Pi form) and decreases residual P contents. Therefore, optimum N fertilization should be applied keeping in view the crops requirement. This study advocates that N application at 160 kg N ha- 1 can maintain higher wheat yield (5844 kg ha- 1) in Loess Plateau, with no significant increases in P fixation and loss of residual P.

Automated summary

The study found that increasing nitrogen fertilizer application rates can reduce the available forms of phosphorus in the soil and enhance phosphorus uptake by plants, but excessive nitrogen fertilization can lead to increased phosphorus fixation.