Effectiveness of Biomass Harvesting from Stormwater Detention Areas in Reducing Phosphorus Discharges

Abstract:

Stormwater Detention Areas (SDAs) in agricultural landscapes are considered to be the most important Best Management Practice by state agencies in Florida. Two main processes responsible for Phosphorus (P) retention in SDAs are soil adsorption and plant uptake. Long term pumping of agricultural drainage may saturate the SDA’s soil with P which can put these systems at the risk of becoming a source of P. Given that these systems already occupy part of the farmland and are costly to build, interventions are needed to sustain SDAs as a sink of P. Soil and vegetation P content at two SDAs (SDA1 and 2) in south Florida was quantified in addition to inflow and outflow P loads. Analyses showed that soil was saturated with limited to no P adsorption capacity remaining. Negative Soil Phosphorus Storage Capacity (SPSC) indicated that soil was at a risk of P release. Given these conditions, the only avenue to remove P from SDAs without any potentially undesirable ecological impacts, was biomass harvesting. At SDA1, results showed that harvesting the aboveground biomass would result in 19% extra P retention if the current vegetation (Para grass, Brachiaria mutica) is harvested. Given that aboveground tissue P content of Para grass is very low, replacing it with another native grass (Maidencane, Panicum hemitomon) and harvesting it annually could retain most of the incoming P load. A similar analysis showed that at SDA2, almost 40% additional P could be retained by harvesting aboveground biomass of the dominant vegetation (Torpedo Grass, Panicum repens and Smartweed, Polygonum hydropiperoides). A spatial analysis in conjunction with SPSC values and aboveground plant P indicated that biomass harvesting can transform both the SDAs from a source to sink in 2 to 3 years. A fifty year net present value analysis showed that overall it is an economically feasible strategy with an average annual benefit of $3,223 and $34,825 for SDA1 and 2, respectively. Harvesting aboveground biomass has the potential to become a part of “payment for environmental services” program and is one of the less intensive methods to sustain SDAs as a net sink of P in the long-term without causing detrimental effects to the downstream ecology.