Microbial biomass accumulates P but not N in response to N and P fertilization in northern hardwood forests
Stoichiometry of the soil microbial biomass could contribute to the regulation by soil microbial activities of the availability and recycling of major nutrients like nitrogen (N) and phosphorus (P). The influence of microbial stoichiometry should depend on how constrained biomass element ratios are relative to their environment. We asked whether microbial biomass N:P ratios are responsive to elevated nutrients by testing effects of added N, P, and N+P, as part of a forest ecosystem experiment examining nutrient limitation in 13 northern hardwood forest stands (replicated stands of three ages) in central New Hampshire. Plots in each stand had been fertilized with either 30 kg N ha-1 yr-1, 10 kg P ha-1yr-1, the same amounts of N+P, or nothing (control) for 4 years. We analysed microbial biomass N and P in Oe, Oa, and surface mineral horizons in the middle of the growing season.
Microbial biomass N:P decreased in response to added N and P, with the most marked effect of N+P, in the Oe soil horizon (p<0.03). This rather unexpectedly uniform response originated from an increase in microbial biomass P in response to added P and N+P, and to a non-significant decline in microbial biomass N in response to added N. Biomass N:P did not respond to fertilization in Oa or mineral soils, despite a significant increase in microbial biomass P in response to added P in the Oa, and to N and P in mineral soil. Hence, it appears that microbial biomass acts as a sink for excess P but not N. Our results suggest that plasticity in microbial biomass P could reduce excess available P in these forest soils.