Accelerated Rates of Nitrogen Cycling and N2O Production in Salt Marsh Sediments due to Long-Term Fertilization

Abstract:

Intensified sedimentary production of nitrous oxide (N₂O), one of the most potent greenhouse gases, is one of the many possible environmental consequences of elevated nitrogen (N) loading into estuarine ecosystems. This study investigates the response to over 40 years of fertilization of nitrogen removal processes in the sediments of the Great Sippewissett Marsh in Falmouth, MA. Sediment slurries were incubated (1.5 hr) with trace amounts (< 10% of ambient concentration) of ¹⁵NH₄⁺ + ¹⁴NO₃^- or ¹⁵NO₃^- + ¹⁴NH₄⁺. An additional parallel incubation with ¹⁵NH₄⁺ + ¹⁴NO₃^- and 1 mM of allylthiourea (ATU) was included to measure rates of anaerobic ammonia oxidation (anammox). Well-homogenized slurries filled about 10% of the volume in the gas-tight incubation vials, and the rest of the volume was replaced with an O₂/He (20%/80%) mixture. The production of ²⁹N₂, ⁴⁴N₂O and ⁴⁵N₂O were determined using isotope ratio mass spectrometry.

The rate of total N₂O production in fertilized sediments (0.89 nmol hr^-1 g^-1 wet weight) was 30-fold higher than in unfertilized sediments. The ratio of N₂O to N₂ production was also significantly higher in fertilized sediments (2.9%) than in unfertilized sediments (1.2%). This highlights the disproportionally large effect of long-term fertilization on N₂O production in salt marsh sediments. The reduced oxygen level and higher ammonium concentrations in situ probably contributed to the significant rise in N₂O production as a result of long-term fertilization. When detected, anammox and coupled nitrification-denitrification accounted for 10% and 14% of the total N₂ production in fertilized sediments (30.5 nmol hr^-1 g^-1 wet weight), respectively, whereas neither was detected in unfertilized sediments. Thus these experiments indicate that N loading has important effects on multiple N cycle processes that result in N loss and N₂O production.