The role of microbes in the increase of organic phosphorus availability in the rhizosheath of cover crops

Abstract

Background and aims: The characterisation of plant-available phosphorus (P) pools and the assessment of the microbial community in the rhizosheath of cover crops can improve our understanding of plant–microbe interactions and P availability. Methods: Mustard (Sinapis alba), phacelia (Phacelia tanacetifolia) and buckwheat (Fagopyrum esculentum) were grown as cover crops before soybean (Glycine max) in an on-farm experiment on a soil low in available P in southwest Germany. The cycling of P through the cover crop biomass and the enzyme-availability of organic P (Porg) pools in the cover crop rhizosheath were characterised. The soil microbial community (PLFA), activity (acid and alkaline phosphomonoesterase, as well as phosphodiesterase), and microbial P were assessed. The abundance of 16S-rRNA and phoD, coding for alkaline phosphomonoesterase in bacteria, were quantified using real-time qPCR. Results: Mustard contained the greatest amount of P in its large biomass. In the rhizosheath of all cover crops, the concentration of enzyme-labile Porg was higher than that in the control bulk soil, along with substantial increases of microbial abundance and activity. There were little differences among cover crop species, few changes in the bulk soil and only a limited carryover effect to soybean, except for fungi. Conclusions: Turnover of microbial biomass, especially saprotrophic fungi, increased by rhizodeposition of cover crop roots; this was likely responsible for the observed increases in enzyme-available Porg. Microbial function was correlated linearly with microbial biomass, and the data of enzyme activity and phoD did not suggest a difference of their specific activity between bulk and rhizosheath soil.