Effects of plant carbon quality on microbial nitrate reduction in wetlands.

摘要

The relative abundance of both aquatic and terrestrial vascular plants, and non-vascular plants such as algae, determines the bioavailable carbon supply for denitrification in wetlands. In flow-through microcosms, normalizing dried plant matter additions to a carbohydrate basis explained differences in plant-specific nitrate reduction. Treatments with higher carbohydrate content, Cattail (Typha latifolia) and Marsh Pennywort (Hydrocotyle umbellata), required lower dry matter additions to achieve equal nitrate reduction than low carbohydrate Bulrush (Scirpus acutus) or Duckweed (Lemna minor) treatments. However, since losses of soluble carbohydrates and higher carbon to nitrogen ratios in leached treatments (p 0.05) show rapid carbohydrate depletion, lignin content may exert greater control on denitrification as wetland litter ages.; Field validation of carbon quality effects on denitrification was carried out at the San Joaquin Marsh in Irvine, CA (USA). Soil amendments of crop residues were applied to the marsh in late spring, providing carbon at the time when the highest denitrification rates were needed to maintain effluent nitrate below 1 mg/l. Barnyard grass (Echinochloa crusgalli) and wheat straw (sp. Ttriticeae) amendments increased denitrification rates (p 0.001) in microcosm studies. However, the lack of emergent reeds in the open water design of the marsh promoted large algal populations, Chlorophyll- a accounting for up to 20% of the observed inorganic nitrogen removal and 30% of the organic nitrogen export at the marsh.; Despite the influence of direct algal competition for nitrate removal at the San Joaquin Marsh, the typically large annual and site-to-site variations in apparent zero order denitrification rate reported for many wetlands may reflect a plant carbohydrate co-limitation with nitrate. In the microcosms, first-order rate constants varied with the ratio of applied carbohydrates to nitrate reduced (p 0.0001). This co-limitation means that for the vast majority of wetlands, the larger differences in productivity between emergent, floating and submersed aquatic plants and the algae may dominate more subtle differences in carbon quality and its effects upon denitrification. Since the productivity of emergent reeds may be two to five times higher than other wetland species, their presence will strongly affect the applied carbon to nitrate ratio and consequently, the performance of denitrification wetlands.