Soil microbial carbon and phosphorus as influenced by phosphorus fertilization and tillage in a maize-soybean rotation in south-western Quebec

Abstract

The management of the soil microbial P pool could improve system sustainability. The long-term impact of inorganic P inputs (0, 40 and 80 kg P2O5 ha-1) and tillage (conventional and ridge tillage) on soil microbial biomass P (SMB-P) was defined in the soybean phase of a 10-yr-old maize-soybean rotation, on a Gleysolic clay-loam. Soil microbial biomass C (SMB-C) and SMB-P were determined four times in the growing season. Yearly applications of 40 kg P2O5 ha-1 increased soil organic carbon level, partly explaining the increase in SMB-P measured at this rate. Results suggest that P-mediated modification of soil microbial community structure also contributed to increase SMB-P at this P rate. The increase of application of P rate (80 kg P2O5 ha-1) produced the largest soybean yield, but generally decreased SMB-P. Our results and those of others suggest that balanced soil fertility (corresponding to fertilizer recommendations in our case) promotes soil microbial development. The use of ridge tillage did not increase the soil organic carbon level, but did increase SMB-P. The SMB-P pool was large (equivalent to 24.4kg P2O5 ha-1) in the 0- to 20-cm soil layer, but unrelated to yield. Improving the ability of crops to access this pool of soil P would increase the value of its management. Key words: Conventional tillage, conservation tillage, P fertilization, soil microbial biomass C, P, and C to P ratio