An assessment of carbon, nitrogen, and phosphorus storage and the carbon sequestration potential in Arcata’s constructed wetlands for wastewater treatment

A better understanding of the role that emergent macrophyte species play in nutrient storage and specifically carbon sequestration is needed to guide design and management of wetlands. In this 2009 study, Typha latifolia and Scirpus acutus were studied for biomass productivity, decomposition, and the potential for nutrient storage in a 0.8 ha (1.9 acre) constructed wetland for wastewater treatment in Arcata, California, USA. The constructed wetland, sampled at one point in time after 24 years of undisturbed growth demonstrated 300,000 kg/ha of biomass. Of the two dominant macrophyte species, Scirpus acutus demonstrated a more recalcitrant biomass that stores carbon and nitrogen. Mean productivity for Typha latifolia was 30,399 kg/ha and for Scirpus acutus was 42,264 kg/ha. Decay rates slowed through time for both Typha latifolia and Scirpus acutus. Decay rates were measured at days 58, 158, 238, 387, and 487 and for Typha latifolia were 0.003, 0.004, 0.001, 0.001, 0.001 d-1 and 0.0014, 0.0012, 0.0002, 0.0002, 0.0003 d-1 for Scirpus acutus. Due to the slow rates of decay, the wetland system shows accretion of 1.5 cm/yr in the peat layer and 1.4 cm/yr in the settled solids. The peat layer and settled solids contain 57% of the carbon mass, 79% of the nitrogen mass, and 34% of the phosphorus mass in the wetland system. Using the demonstrated productivity, decomposition, and nutrient concentration of the macrophytes, the treatment marsh sequesters 21,000 kg C/yr and has accumulated 120,000 kg C over 24 years. The carbon sequestered in the constructed wetlands for wastewater treatment plays a beneficial role in balancing greenhouse gas emissions from the treatment of municipal waste and for adding benefits to the City of Arcata facility.