Examination of carbon and nitrogen cycling in a laboratory nutrient addition experiment with wetland soils from Piermont Marsh, New York

Background/Question/Methods

Coastal wetland systems receive significant inputs of nitrogen (N) and carbon (C) due to fertilizer runoff from agricultural land and sewer overflow from urban centers during storm events, altering the biogeochemical cycling within these ecosystems. Nutrient deposition on microbial communities may increase utilization of stored C pools via enhanced anaerobic activity in saturated wetland soils. However, both the extent to which fertilization alters C pools in anaerobic systems and the role microbial communities play in C transformation remains unclear.  We hypothesize that these additions of inorganic N and labile C stimulate the mineralization of soil C pools.  To test this hypothesis, soil cores were removed from Piermont Marsh (Piermont, NY) and incubated for at least two weeks under anaerobic conditions in two separate experiments.  At the onset of the incubation, conditions were established for C addition (acetate), N addition (ammonium and/or nitrate), C and N addition, no treatment, and sterilization.  Methane (CH₄) and carbon dioxide (CO₂) were measured in regular intervals to approximate the turnover of C.  Lastly, at the termination of the first experiment, water samples were incubated in selective anaerobic media to determine the concentration of iron, sulfate, and nitrate reducing microbial communities in each experimental unit.

Results/Conclusions

Both incubation experiments demonstrated that labile C (acetate) but not inorganic N additions stimulated the production of CH₄ and to a lesser degree CO₂.  The addition of acetate on wetland soils resulted in 12X increase in peak CH₄ production  on day 6 (0.45 mg C g^-1 dry soil day^-1) for experiment #1 and 50X increase in peak CH₄ production on day 8 (17.74 mg C g^-1 dry soil day^-1) for experiment #2 compared to the negative controls.  Further, the addition of acetate produced 2X greater CO₂ production for experiments #1 and #2 (average 10.42 and 4.00 mg C g^-1 dry soil  day^-1 respectively) for a four day period approximately one week after nutrient addition.  The high efflux of CO₂ and CH₄ observed compared with the level of acetate added indicates that nutrient additions stimulated C mineralization of native organic C.  The increased production of CH₄ and to a lesser degree CO₂ however can’t be assigned to a specific microbial metabolic group as no significant difference was observed in sulfate, iron, or nitrate reducer concentrations between treatments.  The measurements conducted in these experiments will help corroborate future experiments that we predict will demonstrate nutrient induced microbial priming in wetland soils.