Microbial community responses to phosphorus and nitrogen inputs in the organic soil horizons of two contrasting temperate beech forests

Abstract

Due to increasing nitrogen (N) deposition from the atmosphere, temperate forests are progressively becoming phosphorus (P) limited. Trees take up P mainly from the soil solution, which soil microorganisms can replenish by mineralizing organic P through enzymatic hydrolysis (e.g. using phosphatases). We investigated how bacterial (including phosphatase harboring bacteria) and fungal communities in organic horizons with contrasting nutrient status respond within months to changes in soil N and P concentrations. A field experiment with water-soluble N (NH4NO3) and P (KH2PO4) additions was conducted at a high-P and a low-P beech (Fagus sylvatica L.) forest site in Germany. Bacterial (16S rRNA), fungal (ITS), and alkaline (phoD) and acid (acpA) phosphatase harboring community structure was investigated by molecular fingerprinting. Bacterial and fungal community structure was mainly related to available P (resin P) in the high-P site but related to more stable P pools (sequentially extracted NaOH/EDTA P) and total P in the low-P site. The increased importance of strategies to access more stable P forms in the low-P site was reflected by a higher relative abundance of some dominant alkaline phosphatase harboring taxa. In the high-P site, increased available P following P addition was the main influencing factor on community structure. By contrast, in the low-P site predominantly N addition induced differences in the microbial communities, which was linked to decreased P concentrations in several soil pools. Increasing N deposition might thus pose greater impacts on microbial communities in low-P compared to high-P sites, by further increasing the already high competition for P. Our findings illustrate that the soil P status in the organic horizon can shape the structure of microbial communities and their roles in the P cycle.