Microbial response on changing C:P stoichiometry in steppe soils of Northern Kazakhstan

The stoichiometric ratio of carbon (C): phosphorus (P) acquisition is strongly correlated with soil available C:P ratio. However how the stoichiometric relationship between acquiring C and P through microbial metabolism affects bioavailable P is poorly understood in semi-arid agricultural ecosystems. Our objective was to investigate the underlying mechanisms of the P availability in typical P-limited steppe soil from Kazakhstan in response to mineral nutrient (Na2HPO4) with and without Dactylis glomerata L. leaves addition in a 38-day incubation experiment. Four bioavailable P fractions content (CaCl2-P, Citrate-P, Enzyme-P, and HCl-P) were improved. Sole application of P fertilizer decreased the maximal velocity (Vmax) of P acquisition enzyme (phosphomonoesterase) but increased microbial C limitation, resulting in increasing the ratio of C to P acquisition but decreasing the ratio of available dissolved organic C: Olsen-P. In contrast, plant residues returning (the application of sole D. glomerata leaves and the combined application of D.glomerata and mineral P) increased Vmax of C (β-1, 4-glucosidase, β-D-cellobiosidase, β-1, 4-xylosidase) and P acquisition enzymes, however decreasing microbial C and P limitation through improving microbial metabolism. Furthermore, the spearman correlation and piecewiseSEM analysis suggested that microbial C limitation and EEAC:P had a negative effect on P availability, illustrating that the decreasing of microbial C limitation can improve soil bioavailable P. The decomposition of organic residues eliminated microbial P limitation and increased P availability by allocating C and P acquisition enzymes to balance the stoichiometric ratio of microbial C and P demand.