Phosphorus release from shoots of Phleum pretense L. after repeated freeze-thaw cycles and harvests

Abstract

Vegetated buffer strips (VBS) are a common best management practice used in agricultural landscapes because of their possible effectiveness to reduce loadings of sediments and nutrients, including phosphorus (P), to adjacent surface waters. In cold climates, buffer vegetation is prone to numerous freeze-thaw cycles (FTCs). The freezing may cause plant cells to lyse and release P, which would reduce buffer effectiveness. Greenhouse experiments were conducted to investigate if P fertilization of soils growing timothy grass (Phleum pretense L.) and the number of FTCs affect the type and concentrations of P released from the above-ground biomass. Timothy grass was grown in a greenhouse with and without P fertilizer application. Above-ground biomass was harvested twice: after 65 days of growth and again 37 days later, and these samples were subjected to zero, three, or six FTCs. Water extractions of the above-ground biomass were analysed for water extractable P (WEP) and alkali P with an oxidative agent (P-OH). Above-ground biomass was additionally analysed for nitric acid digested P (P-HNO3). Results from both harvests showed no significant difference in WEP or P-OH concentrations between soil fertilizer treatments, but there was a significant difference in WEP concentrations with increased FTCs. Combined data from both harvests showed that after six FTCs, 65–100% of P-OH was released as WEP, and 19–55% of P-HNO3 was released as WEP. These results indicate that timothy grass undergoing multiple FTCs can become a source of highly bioavailable P and that, as previous work has showed, harvesting vegetation is a potentially useful management practice in order to reduce P losses to surface waters.