Phosphorus (P) limits plant growth in many soils. Its availability is strongly affected by microorganisms that mineralize organic P via extracellular enzymes and solubilize bound inorganic P.
Material and Methods
We studied how processes of microbial P mobilization and community composition are affected by nitrogen (N) and P addition in six grassland soils located in South Africa, USA, and England fertilized in a full factorial design (control, +N, +P, and +NP). We screened for P‑solubilizing bacteria (PSB), determined their relative abundance, and identified PSB based on 16S rRNA. In addition, we analyzed phosphatase activity, microbial biomass carbon (MBC), microbial community composition and soil chemical properties.
Across all soils, the abundance of PSB was 44 % and 72 % lower in the NP fertilized treatment than in the control in the topsoils and deeper soil, respectively. The abundance of PSB ranged between 0.3 and 56 % of all colony forming units across all soils and treatments. The relative change in the abundance of PSB correlated negatively with the relative change in dissolved N (DN) in the topsoils. In contrast, phosphatase activity was positively correlated with DN and was significantly higher in the N fertilized than in the P fertilized treatments. In a multiple linear model, the abundance of PSB could be explained by soil C/P ratio, DN, and the bacterial community. MBC and microbial community composition did not differ significantly between the treatments. However, the community composition of PSB changed significantly from Ctrl and P to N and NP fertilized plots. Most PSB belonged to Pseudomonas sp., Acinetobacter sp., Paraburkholderia sp., Burkholderia sp., and Enterobacterales.
Taken together, this is the first study showing that combined NP fertilization leads to strong decreases in the abundance of PSB in grassland soils on different continents and that N fertilization significantly changed the community composition of PSB.