Using soil enzymatic stoichiometry to understand ecosystem nutrient economies
Chronic nitrogen (N) deposition and ecosystem acidification has dramatically changed the availability of essential soil nutrients in many ecosystems. As N accumulates, other nutrients like phosphorus (P) may exert greater influence on important ecosystem processes. However, the impact of this change on potential shifts in nutrient limitation is unclear because most metrics are plant-centric and may poorly represent overall ecosystem nutrient economies. Soil microbial decomposers, due to the production of extracellular enzymes (EE), are the primary agents of organic matter turnover and nutrient cycling in soils. We reason that insights into ecosystem nutrient economics can be gained using a stoichiometry analysis of the activities of extracellular enzymes (EE) that microorganisms use to degrade organic matter because it integrates the needs for- and availability of nutrients relative to metabolic need for- and availability of plant derived C. Thus, we measured EE involved in C, N, and P cycling in mature glaciated and unglaciated temperate hardwood forests in Ohio with low ambient soil P pools. P availability was manipulated for six years either directly (adding phosphate) or indirectly (raising pH). To determine how the stoichiometry of microbial C and nutrient acquisition were influenced by P availability, we examined the ratios of C:N and C:P acquiring enzymes and compared them to plant-centric indices of nutrient limitations like tree growth, foliar N:P, etc.
Soil decomposers associated with unglaciated forests were more sensitive to P availability than those in glaciated forest soils. Increasing P availability has reduced the activity of P-acquiring enzymes by half and caused a 3-fold increase in the soil C:P acquiring enzyme activity ratio without altering the C:N enzyme activity ratio, suggesting that decomposers have shifted enzyme allocation from acquiring P to acquiring more C from plant cell walls, via increased b-glucosidase activity. This change in EE corresponded to a ~20-50% decrease in fine root biomass and lower foliar (and litter) N:P ratios because P translocation actually increased with elevated P despite no change in N translocation. Over five growing seasons, elevated P increased tree growth, but the response was greatest in arbuscular mycorrhizal (AM) trees (i.e. Acer spp.) and on unglaciated sites with the lowest available P. Overall, these results suggest that regional N deposition and acidification shifted Ohio hardwood forest nutrient economies towards P acquisition and away from N, with potential long-term consequences for forest composition and productivity.