98th ESA Annual Meeting (August 4 -- 9, 2013)

COS 82-3 - Shifts in forest soil enzyme stoichiometry due to season, pH, and phosphorus availability

Thursday, August 8, 2013: 8:40 AM
101G, Minneapolis Convention Center
Jared L. DeForest1, Kurt A. Smemo2, Michael N. Weintraub3, David J. Burke2, Sarah R. Carrino-Kyker2, Charlotte R. Hewins2 and Laurel A. Kluber4, (1)Department of Environmental and Plant Biology, Ohio University, Athens, OH, (2)The Holden Arboretum, Kirtland, OH, (3)Environmental Sciences, University of Toledo, Toledo, OH, (4)Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN
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.