Changes in δ15N in salt marsh sediments in a long-term fertilization study

Abstract

Nitrogen retention by salt marshes has been suggested as a means of mitigating the delivery of land-derived nitrogen loads to coastal waters. As land-derived nitrogen loads increase, it is unclear whether there is an upper limit to the amount of nitrogen retained by salt marshes. A long-term fertilization study in the Great Sippewissett Marsh on Cape Cod, USA, has been examining the changes to salt marsh vegetation and sediment processes as a result of increased nitrogen loading. To determine whether decadal-scale changes in nitrogen loading and sources are recorded in salt marsh sediments, we examined sediment δ 15 N and %N profiles from below low and high marsh vegetation in control and fertilized plots in Great Sippewissett Marsh. As expected, we found little change in δ 15 N values in control plots. Nitrogen burial, calculated using %N values in bulk sediments, was higher in fertilized plots, but did not increase over time. However, δ 15 N values in fertilized plots were higher than in control plots and increased over time, becoming heavier than the source fertilizer and continuing to increase linearly. The continuous increase in sediment δ 15 N values in fertilized plots over the fertilizer δ 15 N value suggests that denitrifying bacteria are responding to the increased nitrogen load and fractionating the available nitrogen. The nitrogen that remained unaccounted for by burial led us to conclude that 47 to 80% of the fertilizer nitrogen was denitrified.

Long‐term fertilization alters the relative importance of nitrate reduction pathways in salt marsh sediments

“…The Great Sippewissett Marsh is limited by N as shown by a comprehensive study on its N budget [ Valiela and Teal , ]. In the experimental plots receiving long‐term fertilization, enhanced primary production, nutrient retention, organic N content, N 2 production, and nitrous oxide production have been documented [ Valiela et al ., , ; Hamersley and Howes , ; Brin et al ., ; Kinney and Valiela , ; Uldahl , ; Ji et al ., ]. These previous studies showed that the various effects of long‐term fertilization were usually more pronounced in extra‐high‐fertilization (XF) plots than in high‐fertilization (HF) plots.…”

Functional gene pyrosequencing and network analysis: an approach to examine the response of denitrifying bacteria to increased nitrogen supply in salt marsh sediments

“…Fertilizers were added in the following doses: Control (C): no fertilizer added, low dose (LF): 0.85 g N m ‒2 wk ‒1 , high dose (HF): 2.52 g N m ‒2 wk ‒1 , and extra high dose (XF) 7.56 g N m ‒2 wk ‒1 . Figure modified from Kinney and Valiela (2013). …”

“…Early recognition of salt marshes' role as a sink for land-derived nitrogen led to the establishment of different experimental systems that examine how nutrient enrichment alters marsh macroecology and biogeochemistry (Valiela et al, 1975; Deegan et al, 2007). Multiple studies indicate that increasing anthropogenic N alters rates of N loss processes including denitrification, coupled nitrification-denitrification, and dissimilatory nitrate reduction to ammonia (DNRA) (Hamersley and Howes, 2005; Koop-Jakobsen and Giblin, 2010; Vieillard and Fulweiler, 2012; Kinney and Valiela, 2013). It is only within recent years, however, that genetic tools have been developed to examine, in detail, the microbial communities that underlie the biogeochemistry of marsh systems.…”

ElevatedCO2promotes long‐term nitrogen accumulation only in combination with nitrogen addition

“…Denitrification represents a major pathway of N loss in natural and constructed wetlands that ameliorates estuarine eutrophication (White & Howes, 1994;Hamersley & Howes, 2005;Reinhardt et al, 2006;Kinney & Valiela, 2013). However, denitrification can also lead to the production of N 2 O, a potent greenhouse gas with a global warming potential roughly 300 times that of CO 2 on a one hundred-year horizon (Smith, 1997;Wrage et al, 2001).…”