The Influence of Long-Term Fertilization with and without N and P on P Forms and Cycling in Continuous Wheat Cropping Systems in a Semiarid Agroecosystem.

Understanding factors that drive soil phosphorus (P) cycling and control plant P availability will produce more P-efficient cropping systems. This in turn will reduce P fertilization and the build-up of soil legacy P. This study used long-term plots from a continuous wheat cropping system in Saskatchewan Canada to determine the effects of nitrogen (N) and P fertilization on soil phosphatase activity, N and P availability, P forms, and soil microbial communities and their functional genes. The +N+P and –N+P plots were established in 1967, while subplots of +N-P and –N-P were established in 1995. Fresh soil samples collected at anthesis were used for enzyme assays (acid phosphatase, alkaline phosphatase, and diesterase), ³¹P-NMR analysis, and for DNA extraction and metagenomic analysis. Anion exchange membranes were used to monitor the flux in nitrate and phosphate availability in the soil, and air-dried subsamples were used for total P (PT), organic P (PO), soil test P (Olsen P), and P fractionation (modified Chang & Jackson). Long-term N fertilization significantly (P<0.05) decreased pH from 5.6 to 4.4, which in turn affected P speciation and the soil microbial community, while plots without N had poor plant growth regardless of N fertilization. The concentration of PT was greater in the +P plots regardless of N fertilization. There were no differences in the concentration of total PO across treatments, but PO as a percentage of PT was greater in treatments receiving N than those without, as was loss on ignition and phosphatase activity. Fertilization with N influenced OP in P fractionation, with PO accumulating in the reductant- and acid-soluble fractions without N, and accumulating in the NaOH-extractable fraction with N, regardless of P treatment. In contrast, P fertilization increased inorganic P in NH₄F- and NaOH-extractable fractions, regardless of N, which was also observed in preliminary ³¹P-NMR results.