PS 40-90 - Evaluating functional and taxonomic controls on ecosystem services in short rotation woody crop bioenergy plantations

Abstract: Short rotation woody crops such as Populus species and their hybrids are important components of the bioenergy plantation landscape in the US. These trees grow quickly both above- and belowground and have the potential to provide a range of ecosystem services including water quality mitigation from agricultural runoff and soil carbon storage and sequestration. In efforts to maximize these ecosystem services, we hypothesize that mixed clonal plantations will afford greater taxonomic and functional diversity. We established monocultures and mixed clonal plantings of six Populus clones across three taxa (P. deltoides, hybrids of P. deltoides × P. maximowiczii, and hybrids of P. deltoides × P. trichocarpa) and two contrasting nitrogen use efficiencies at two research sites containing three replicated blocks in central Mississippi. Experimental plots were established at the interface between row crop agriculture and small-order streams to evaluate the impact on water quality. We collected shallow groundwater for nutrient analysis approximately monthly and soil samples at the end of each growing season for elemental analysis in each experimental plot.

After two growing seasons, we observed greater mitigation of both nitrate (p = 0.040) and to a lesser degree ammonia (p = 0.074) in shallow groundwater attributed to functional differences, where plots planted with higher nitrogen use efficiency clones had greater reductions relative to agricultural wells (0.33 mg nitrate-N/L, 0.13 mg ammonia-N/L) compared to plots planted with low nitrogen use efficiency clones or plots planted with clonal mixtures of functional types. We observed greater orthophosphate mitigation (p = 0.036) attributed to taxonomic differences, with highest mitigation occurring in P. deltoides × P. maximowiczii plots. However, overall orthophosphate concentrations were extremely small and total phosphorus results are pending. We also observed 1-2% increases in soil carbon relative to agricultural soils in the first growing season, with the largest increases occurring in plots planted with trees with lower nitrogen use efficiency clones. Changes in soil carbon for year two are pending. In conclusion, we demonstrate small, but significant, improvements to ecosystem services within the first two growing seasons of short rotation woody crop bioenergy plantations. We anticipate greater improvements in ecosystem services and more diverse response across taxonomic and functional groups as root systems become more established.