Shifts in forest soil enzyme stoichiometry due to season, pH, and phosphorus availability

Background/Question/Methods

Human activity has increased the atmospheric input of nitrogen (N) and acidic compounds to most temperate forests. As N accumulates, other nutrients like phosphorus (P) may have a greater influence on limiting ecosystem processes such as decomposition and perhaps N cycling.  Our goal was to address the question: what is the influence of P availability and soil acidity on ecosystem C and N cycling?  We addressed this question by measuring the activities of five microbial extracellular enzymes (EE) involved in C, N, and P cycling in temperate hardwood forests of glaciated and unglaciated regions of eastern Ohio where soil pH and P availability has been manipulated for three years (treatments: control, elevated pH, elevated P, and elevated pH + P). EE are the primary agents of organic matter turnover and nutrient cycling in soils, and are therefore important mediators of ecosystem function. We measured EE activities at 5 separate times over one year (November, February, May, July, and September), to represent phenological changes in nutrient demand.  To determine how the stoichiometry of microbial C and nutrient acquisition were influenced by elevated P and pH we examined the ratios of C:N and C:P acquiring enzymes.

Results/Conclusions

We observed a strong relationship (r² = 0.87) between C:N and C:P acquiring enzymes among sites and treatments when averaged across seasons.  The unglaciated sites were P>N>C limited, whereas the glaciated sites were C>P>N limited.  For the unglaciated sites, increasing P availability decreased P limitation, but increased C limitation.  Elevating pH or pH+P increased C limitation without increasing N limitation.  For the glaciated sites, elevating pH+P increased C limitation, whereas the other treatments had minimal influence. Additionally, stoichiometric relationships and treatment effects shifted by season.  For example, the slope of C:N vs. C:P acquiring enzymes varied from 2 to 5, seasonally, with the yearly average of 4.3, suggesting that seasonal biological P demand is important.  July was the most P-limited month for both sites.  However, elevated pH+P greatly enhanced P limitation for September and November for the unglaciated sites, but had limited influence on the glaciated sites.  Our results suggest season and available P both had a strong influence on enzyme stoichiometry, but the influence of soil pH was minimal. Moreover, our findings emphasize the importance of interactions between N and P in controlling decomposition processes and that nutrient limitation and/or co-limitation are scale-dependent conditions that might be phenologically driven.