The DiStefano and Gholz (1986) resin core for measuring nitrogen mineralization in terrestrial soils was modified for use in wetlands where bidirectional exchange of surface and ground water and dynamic hydrologic conditions make in situ measurements difficult. The modified resin core for wetlands includes three ion-exchange resin bags placed both on top and below the enclosed surficial (0-5 cm) soil in a PVC core tube. This design was tested in the alluvial, piedmont floodplain of Difficult Run, Virginia. Three replicate resin cores were incubated monthly for 12 consecutive months at five locations along a hydrogeomorphic gradient from levee to backswamp and included the range in elevation, hydrology, vegetation, soil texture and organic content found in piedmont floodplains.
Results/Conclusions
The percents of total resin-trapped extractable ammonium, nitrate, and soluble reactive P found in the middle resin bags were low, indicating that the ion-exchange resin in the inner and outer bags was capable of trapping nearly all inorganic nutrients advecting out of the incubated soil and advecting into the resin core from above and below over the duration of the incubations. Net ammonification, nitrification, and P mineralization rates (umol m-2 d-1) all significantly varied among the five sites, whereas net N mineralization rates did not differ along the floodplain hydrogeomorphic gradient. All fluxes significantly varied over the year, with significant site by time interactions. Net P mineralization rates were positively correlated with soil moisture content, soil TP, TC, and TN, the mass of sediment, TC, and TN deposited in the month prior to the incubation, and negatively correlated with the amount of extractable SRP at the beginning of the incubation. Net ammonification rates were negatively correlated with soil pH, whereas net nitrification and N mineralization rates were uncorrelated with potential explanatory variables. The modified resin core method was compared to concurrent and adjacent closed polyethylene bag incubations. The modified resin cores generated statistically greater net P mineralization and statistically similar net N mineralization, ammonification, and nitrification flux estimates compared to the bag method, although mean net ammonification and N mineralization rates were greater in the modified resin cores while the bag method indicated net ammonia immobilization. The modified resin core design successfully measured in situ rates of soil nutrient mineralization in dynamically flooded floodplain wetlands, identified that fluxes responded to hydrogeomorphic variation, and proved to be a useful method for investigating biogeochemical fluxes and their controls in wet ecosystems.
