COS 65-6 - Phytoremediation of Legacy Phosphorus: Forage Cover Species Dynamically Changes Legacy P Uptake and Loss as Leachate

Background/Question/Methods

Phosphorus addition is strongly linked to eutrophication, the nutrient enrichment of water bodies resulting in algal blooms. Eutrophication can damage fish habitat, impair recreational waterways, and reduce waterfront property values. Despite this, management designed to reduce P-induced eutrophication has consistently failed to improve water quality. This lag in effective management is the influence of ‘legacy’ P, phosphorus leftover from long-term nutrient addition. After decades of fertilizer application, for example, soils can saturate with soluble phosphate [PO₄]^-3. This ‘legacy’ phosphate can leach into the water table and pollute waterways for decades, even absent of new P additions.

Research suggests phytoremediation may reduce P-loss while gradually depleting legacy P. Plants may decrease P-loss as leachate by reducing soil water loss. However, plants may also increase P-loss by increasing the solubility of phosphorus. The impact of plant cover on leachate is unexplored and unincorporated in many watershed models – a source of uncertainty for predicting eutrophication risk. This is of particular importance in modeling ranchland systems, where pasture is traditionally considered a homogeneous land cover type lacking spatial nutrient flux information. We evaluated differences in plant P-uptake and leachate P-loss across several forage grass species proposed for phytoremediation of a heavily-fertilized ranch system.

Results/Conclusions

We found that forage species plays a significant role in both uptake of phosphorus and reducing the amount of P lost through soil leachate. Across all species we examined, soil beneath Stargrass (Cynodon plectostachyus) lost less phosphorus as leachate compared to all other common forage species examined (p < 0.05). In addition, Stargrass also extracted more phosphorus from the soil compared to other species and demonstrated accordingly higher tissue P concentrations (p < 0.05) and total aboveground P-biomass (p < 0.01) compared to other forage grasses.

Our findings also suggest that while certain species may function as suitable extractors of phosphorus, reducing legacy P stores, some good-candidate extractors also increase leachate P loss (p < 0.05). This discrepancy is a novel observation, and important for optimizing bioremediation aimed at managing polluted water bodies. There appears to be a potential tradeoff between the long-term reduction of legacy P and some short-term risks, such as increasing leachate, that drive eutrophication. Current models of P-leaching do not consider the potential dynamics of the variable species-level plant influences on leaching rates and plant P-uptake. This research suggests forage crop identity can variably affect these fluxes and should be included in watershed modeling.