Nutrient-enhanced decomposition of plant biomass in a freshwater wetland

Abstract

a b s t r a c tWe studied soil decomposition in a Panicum hemitomon (Schultes)-dominated freshwater marsh located in southeastern Louisiana that was unambiguously changed by secondarily-treated municipal wastewater effluent. We used four approaches to evaluate how belowground biomass decomposition rates vary under different nutrient regimes in this marsh. The results of laboratory experiments demonstrated how nutrient enrichment enhanced the loss of soil or plant organic matter by 50%, and increased gas production. An experiment demonstrated that nitrogen, not phosphorus, limited decomposition. Cellulose decomposition at the field site was higher in the flowfield of the introduced secondarily treated sewage water, and the quality of the substrate (% N or % P) was directly related to the decomposition rates. We therefore rejected the null hypothesis that nutrient enrichment had no effect on the decomposition rates of these organic soils. In response to nutrient enrichment, plants respond through biomechanical or structural adaptations that alter the labile characteristics of plant tissue. These adaptations eventually change litter type and quality (where the marsh survives) as the % N content of plant tissue rises and is followed by even higher decomposition rates of the litter produced, creating a positive feedback loop. Marsh fragmentation will increase as a result. The assumptions and conditions underlying the use of unconstrained wastewater flow within natural wetlands, rather than controlled treatment within the confines of constructed wetlands, are revealed in the loss of previously sequestered carbon, habitat, public use, and other societal benefits.

The belowground intersection of nutrients and buoyancy in a freshwater marsh

“…Aerial imagery from the first photographs taken in the 1930s to before effluent was first added, in November 2006, showed no clear differences between any marsh within the study area (including reference marshes); all marshes appeared without open water areas within them (Bodker et al 2015;see …”

“…There was no significant area of open water then, and it was a continuous vegetative cover of herbaceous perennials, esp. P. hemitomon (Bodker et al 2015). P. hemitomon was once 25% of the areal coverage of freshwater marsh vegetation in coastal Louisiana (Chabreck 1972) and is a keystone species forming a homogenous plant community of roots and rhizomes that may or may not be attached to the soil beneath.…”

“…These are seaward of a Pleistocene terrace edge and overlie the mineral remnants of a former embayment of the Gulf of Mexico. Bottom sediment matter was dated as 1100 years ± 100 years old (uncalibrated; Bodker et al 2015).…”

“…The conversion can happen through (1) the direct loss of soil organic material by decomposition in situ (Boar et al 1989;Bodker et al 2015), and (2) as an enhanced susceptibility to scouring and lift off through chronic and episodic high energy hydrologic events (Cypert 1972). Gas formation, when trapped, lowers the soil bulk density and increases buoyancy (Hogg and Weins 1988a, b).…”

“…What is needed is to test the relative strength of these two contrasting hypotheses for marsh loss (top-down or bottom-up). This was done when a stable and homogenous expanse of Panicum hemitomon marsh converted to open water after the introduction of partially-treated sewage into it creating four major consequences (Bodker et al 2015): (1) Nutrient-rich water elevated marsh soil decomposition, (2) inserted cellulose strips decomposed faster closest to the partially-treated sewage introduction, (3) the decomposition of belowground soil organic matter was more rapid with the addition of the partially-treated water, and (4) decomposition rates increased in proportion to increasing tissue nitrogen: phosphorus content. Nitrogen, not phosphorus, appears to be the major factor enhancing decomposition.…”