Long-term land use affects phosphorus speciation and the composition of phosphorus cycling genes in agricultural soils

Citation

Liu, J., Cade-Menun, B.J., Yang, J., Hu, Y., Liu, C.W., Tremblay, J., LaForge, K., Schellenberg, M., Hamel, C., Bainard, L.D. (2018). Long-term land use affects phosphorus speciation and the composition of phosphorus cycling genes in agricultural soils, 9(JUL), http://dx.doi.org/10.3389/fmicb.2018.01643

Plain language summary

Phosphorus (P) is an essential element for all plants. Understanding changes in P cycling in soils from different land uses is important for sustainable management, to have optimal crop growth but minimal loss of P from land to water, where it can cause algal blooms. This study used soils from Southwest Saskatchewan with four long-term land uses: annual cropland, tame (crested wheat) pastures, native prairie pastures and road side ditches. Four separate locations were used, each with the four land uses near to one another. In these soils, P cycling was studied with a combination of advanced chemical techniques to identify P forms, measurements of enzyme activities to determine P dynamics, and advanced microbiology techniques to identify the microbial community. The different land uses altered the chemical forms of P in the soils, from differences in plant and fertilizer inputs. This in turn changed the soil microbial community and the enzyme types and activities to cycle P. The most disturbed soils, in the annual cropland and roadside ditches, had the highest microbial diversity and enzyme activities to cycle P. The least disturbed sites were the tame and native pastures. These had the lowest microbial diversity, and contained P forms not seen at the other sites.

Abstract

Agriculturally-driven land transformation is increasing globally. Improving phosphorus (P) use efficiency to sustain optimum productivity in diverse ecosystems, based on knowledge of soil P dynamics, is also globally important in light of potential shortages of rock phosphate to manufacture P fertilizer. We investigated P chemical speciation and P cycling with solution P nuclear magnetic resonance, P K-edge X-ray absorption near-edge structure spectroscopy, phosphatase activity assays, and shotgun metagenomics in soil samples from long-term agricultural fields containing four different land-use types (native and tame grasslands, annual croplands, and roadside ditches). Across these land use types, native and tame grasslands showed high accumulation of organic P, principally orthophosphate monoesters, and high acid phosphomonoesterase activity but the lowest abundance of P cycling genes. The proportion of inositol hexaphosphates (IHP), especially the neo-IHP stereoisomer that likely originates from microbes rather than plants, was significantly increased in native grasslands than croplands. Annual croplands had the largest variances of soil P composition, and the highest potential capacity for P cycling processes based on the abundance of genes coding for P cycling processes. In contrast, roadside soils had the highest soil Olsen-P concentrations, lowest organic P, and highest tricalcium phosphate concentrations, which were likely facilitated by the neutral pH and high exchangeable Ca of these soils. Redundancy analysis demonstrated that IHP by NMR, potential phosphatase activity, Olsen-P, and pH were important P chemistry predictors of the P cycling bacterial community and functional gene composition. Combining chemical and metagenomics results provides important insights into soil P processes and dynamics in different land-use ecosystems. 31