Desorption of mineral-bound phosphorus across different cropping systems and agronomic strategies to promote efficient input use

Phosphorus (P) is a limiting nutrient to production in various agricultural ecosystems and many soils contain large amounts of legacy P that is strongly adsorbed on iron (Fe) and aluminum oxides and therefore poorly available to plants. Here we investigate how much mineral-bound P is mobilized in different cropping systems and the influence of agronomic practices on this process. Root ingrowth cores filled with a mix of sand and P-saturated goethite were incubated in the topsoil of: 1) sugar beet in Sweden under sufficient or deficient P additions, 2) perennial Kernza (Thinopyrum intermedium) in Sweden as monoculture or intercropped with Alfalfa with organic management, contrasted to annual winter wheat with inorganic fertilization, and 3) soybean grown in a P-fixing soil in Kenya with or without fertilizer and biochar addition. After one growing season, desorption was evaluated based on the amount of P that remained in the goethite and its ratio with Fe. Possible relationships between the rate of P desorption and abundance of arbuscular mycorrhizal fungi (AMF) were explored by profiling phospholipid fatty acid (PLFA) and neutral lipid fatty acid (NLFA) biomarkers in the sand-goethite mixture collected from the cores.

Significant P desorption was found in the rhizosphere of sugar beets under deficient P supply but not in bulk sand-goethite mixture. No P desorption was observed in Kernza as monocrop or intercropped with alfalfa, nor winter wheat. Soybeans had a strong ability to desorb P under all nutrient regimes but significantly more when no fertilizer was applied. In the soybean experiment a significant positive relationship was found between P removal rates and abundance of AMF.

These results indicate that strategies for downregulating P application rates and promoting crop-AMF association can be a viable avenue to mobilize legacy P and help make more efficient use of nutrient inputs. At the same time, cropping systems and soil conditions have an overriding influence on the potential P desorption and must therefore be carefully considered during implementation.