Nutrient availability and soil organic matter decomposition response to prescribed burns in mid-atlantic brackish tidal marshes

Prescribed fire in tidal marshes has been shown to generate short-term increases in plant-available nutrients, but the long-term implications of fire on nutrient availability and organic matter decomposition have not been well established. Two manipulative experiments were conducted over 1 yr within long-term annual burn and no-burn blocks at the Blackwater National Wildlife Refuge in Cambridge, MD to study the effects of canopy removal and ash deposition on nutrient availability and organic matter decomposition rates following burning. Ash deposition had no significant effects on study variables. At no-burn sites, porewater NH ₄ ⁺ and PO ₄ ³⁺ were significantly lower in July in sites with canopy removal (0.15 mg NH ₄ ⁺ L ^-1 and 0.04 mg PO ₄ ³⁺ L ^-1) compared to sites with a canopy (0.73 mg NH ₄ ⁺ L ^-1 and 0.08 mg PO ₄ ³⁺ L ^-1). Similar results were found through a canopy replacement treatment at annual burn sites. Decreased porewater NH ₄ ⁺ and PO ₄ ³⁺ corresponded to increased total biomass nutrient content. No-burn sites with canopy removal treatments showed significantly lower organic matter decomposition rates than did treatments with a canopy in July (66.5 vs. 74.1 % cotton tensile strength loss), corresponding to decreases in porewater NH ₄ ⁺ and PO ₄ ³⁺. Plant ash provided a fertilizer pulse of 0.22 g N m ^-2 and 0.16 g P m ^-2, which were amounts of N and P likely too small to increase plant productivity when deposited during late winter/early spring. Prescribed fire appears to affect nutrient availability and organic matter decomposition in these marsh soils primarily through the mechanism of increased biomass production due to canopy removal.