Multi-year soil microbial and extracellular phosphorus enzyme response to lime and phosphate addition in temperate hardwood forests

While reported short-term microbial responses to nutrient enrichment experiments are common, we provide results from a six-year data set in response to field manipulations of soil phosphorus (P) availability. We hypothesized that long-term shifts in P economics should result in the persistent suppression of P-acquiring extracellular enzymes when compared with ambient soils, as opposed to acclimation by the microbial community. P availability was experimentally increased, either directly by addition of phosphate fertilizer or indirectly by raising soil pH with lime in relatively P rich (i.e., glaciated) and P poor (i.e., unglaciated) forest soils. Soil chemistry, phosphatase enzymes (phosphomonoesterase, PM and phosphodiesterase, PD), and microbial structure (PLFA analysis) were measured annually for six years. Since the start of the experiment, the microbial communities have become more distinct across treatments. PD enzyme activity was more responsive to the treatments than PM, especially in the glaciated elevated P treatment where ambient P availability was greatest. The treatments dramatically suppressed P-acquiring enzyme activity with no indication of acclimation. These results suggest that the initial conditions and ambient nutrient supply of an ecosystem will have a strong influence on its response sensitivity to continuous nutrient enrichment.