Plant carbohydrate limitation on nitrate reduction in wetland microcosms.

Although nitrate limitation in most natural wetlands results in pseudo-first-order reductions, large site-to-site variations in apparent denitrification rates cannot be easily explained by water quality (e.g., pH, Temp, DOC) or plant productivity. Our microcosm results show increasing nitrate removal efficiencies at higher ratios of total applied plant carbon to nitrate reduced, suggesting that denitrification rates may be limited by the rates of supply of both electron donor or acceptor, described by an applied carbon to nitrate (C(App): N(Red)) ratio. However, the observed first-order rate constants varied more strongly (r2 = 0.77, p <0.0001) with the acid-soluble carbohydrates to nitrate (CH2O(App): N(Red)) ratio than the total C(App): N(Red) ratio. Although observed rate constants for bulrush (Scirpus sp.) were significantly lower (0.01 <p<0.06) than for other plant sources (Hydrocotyle sp., Lemna sp., Typha sp.), there were no significant differences in the plant-specific rate constants when compared at the same CH2O(App): N(Red) ratio. In full-scale wetlands, this suggests that either high plant productivity or low NO3- loading (high CH2O(App): N(Red)) may contribute to a high effective denitrification rate. In contrast, low productivity or a high NO3- loading (low CH2O(App): N(Red)) would promote a lower denitrification rate. This co-limitation between plant carbon and nitrate may confound first-order rate comparisons in full scale denitrification wetlands since highly N-loaded systems may become carbon limited, requiring higher order reaction kinetics to better describe performance variations. In addition to hydraulic residence time. temperature and nitrate removal data, extending these results to compare large wetlands will require estimation of the CH2O(App): N(Red) ratio from an inventory of plant species, productivity estimates and carbon quality.