Soil Microbial Biomass, Activity, and Community Structure As Affected By Mineral P Fertilization in Two Grassland Soils.

Soil microorganisms response to mineral phosphorus (P) fertilization in forage stands is poorly understood. We assessed the effects of P fertilization on soil microbial biomass, activity, and community structure in timothy swards on a Kamouraska clay at Lévis (Qc), Canada and a Labarre clay loam at Normandin (Qc), Canada. Three P rates (0, 20, and 40 kg P ha^-1) replicated three times have been applied in the spring of each year at the two sites since 2010. Soil samples (0-10 cm) were collected in June, August, and October during the growing season of 2013 to document  soil microbial biomass C, N, and P (SMB-C, SMB-N, SMB-P), dehydrogenase and alkaline phosphomonoesterase activities, and phospholipids fatty acid markers of microbial community structure. All soil microbial parameters significantly varied with sampling time. Phosphorus fertilization increased SMB-P on the three sampling dates, but never affected SMB-C, SMB-N, dehydrogenase and alkaline phosphomonoesterase activities, or  the microbial community structure. One year of data suggests that four years of P fertilization has limited effects on the composition and function of the soil microbial community of timothy swards in eastern Canada.