Options for increasing the biological cycling of phosphorus in low-input and organic agricultural systems

Abstract

In all systems, phosphorus (P), particularly phosphate, is inexorably drawn to sorption surfaces or precipitated, and is removed from the readily plant-available P pools. Mobilising this less-available P is a significant challenge in organic and low-input agro-ecosystems where readily available P sources may not be applied regularly. Although good-quality inputs, with low C : P ratio, and rotations may be effective in managing organic P cycles in pasture and forestry systems, extensive cropping systems will remain problematic. Two conceptual models are proposed to use organic matter additions to transfer this recalcitrant P into the biocycle, one involving highly labile organic matter and the other involving less-labile organic additions. Current literature provides some evidence for the efficacy of both models, but further work is necessary to clarify which is optimal for agro-ecosystems relying on either low inputs of P or forms of P that are allowed in organic agricultural systems, such as manures or rock phosphate. The microbial biomass is integral to P biocycling in organic systems, but further work is necessary to clarify the role of fungal and bacterial decomposers in decomposition processes. Identifying P-efficient cultivars with low translocation from roots to shoots to provide an 'organic' P source for following crops, essentially optimal rotation sequences for mobilising P using plants, is an avenue of promising research potential. Enzyme activation of recalcitrant organic P sources is unlikely to be effective if applied to bulk soils; however, further research on enzyme activities in the rhizosphere holds promise. We strongly recommend that further research be directed toward understanding and limiting reactions of inorganic P with soil colloids and minerals, either through chemical means (pH change, competitive sorption) or introduction and manipulation of organic materials. The research issues identified above overlap considerably with conventional agricultural concerns and benefits will accrue to both conventional and organic producers where further research is promoted.