Respiration of Louisiana Freshwater Floating Marsh Soils Amended with Ammonium, Phosphate, and Sulfate

Abstract

Wetland soils of the freshwater coastal deltaic regions of Louisiana have developed under decreasing influence from the Mississippi River, which has resulted in lower available nutrient conditions and sediment input relative to other coastal marshes. A laboratory soil respiration experiment was conducted to measure cumulative carbon dioxide (CO₂) and methane (CH₄) production in soils from a floating freshwater marsh in response to additions of added ammonium (N), phosphate (P), ammonium (N) + phosphate (P), and sulfate (S). CO₂ respiration was significantly greater over a 28-day period than controls following ammonium N, phosphorus, and sulfate addition at 10 mg L⁻¹. Nitrogen and phosphorus addition at 10 mg L⁻¹ also increased methane production. The lower sulfate amendment (10 mg L⁻¹) did not significantly increase CH₄ production. In contrast, the greatest sulfate treatment (100 m l⁻¹) significantly reduced total carbon (C) production by inhibiting CH₄ production. The fact that soil C/N (20.2) and C/P (355) ratios were both relatively low may partially explain why both N and P colimited microbial activity and respiration. While microbial activity of freshwater floating marsh soils was stimulated over the short term with increased ammonium, nitrogen, phosphorus, and sulfate exposure, it is unclear whether the increase would be the same over extended periods or would increase in plant productivity from nutrient additions compensate for any loss in soil carbon.

Keywords:

• Carbon dioxide
• methane
• Mississippi River
• nutrient enrichment
• respiration
• wetland

Acknowledgments

We thank J. P. Geaghan for statistical advice, L. E. Stanton and J. W. Gore for field assistance, and E. F. Peterson for laboratory assistance.