Ammonium production during microbial nitrate removal in soil microcosms from a developing marsh estuary

Abstract

Rapid development and urbanization in the South Carolina (SC) coastal plain may introduce significant nutrients to adjacent tidal creeks and salt marsh estuaries, and threaten estuarine water quality. Microbial denitrification in estuarine soils plays an important role in removing excessive nitrate ( NO 3 − ) in coastal waters. Relative contributions of denitrification and ammonium production during nitrate reduction via dissimilatory nitrate reduction to ammonium (DNRA) and soil mineralization determine whether N is lost from the system or retained as ammonium ( NH 4 + ) . The objectives of this study were to compare background, short-term and long-term potential denitrification ( NO 3 − and glucose added) rates, and NH 4 + production during microbial NO 3 − conversion in a developing marsh estuary, SC (USA). Denitrification rates were measured using the acetylene block technique in an undeveloped fresh water site (T1W), an undeveloped Spartina marsh (Grave's Dock, GD), and a Spartina marsh at a golf course resort (Chechessee marsh, C3M). Background denitrification with no added NO 3 − was primarily controlled by NO 3 − concentration in soils and surface water. Adding glucose did not enhance either short-term or long-term potential denitrification rates in GD marsh soils. NH 4 + production during microbial NO 3 − removal was significant at both marsh sites, and N-mass balance based on N 2O and NH 4 + production suggested a significant contribution of NH 4 + from sources other than DNRA. DNRA was estimated to account for approximately 16.3% and 0% of total added NO 3 − removal in GD surface (0–10 cm) and subsurface (30–40 cm) soils, and 1.9 and 23.2% in C3M surface and subsurface soils. Excessive NH 4 + generation from processes other than DNRA may be attributed to NO 3 − stimulated mineralization, and this stimulation was estimated to enhance soil ammonification by 0.5∼4 times compared to background NH 4 + generation with no NO 3 − added. Our results suggest that although the marsh soils displayed high potential of NO 3 − removal via denitrification, the NH 4 + produced via a combination of DNRA and NO 3 − enhanced mineralization may allow NH 4 + to accumulate and be transported to coastal waters.