Watershed Buffering of Legacy Phosphorus Pressure at a Regional Scale: A Comparison Across Space and Time

Phosphorus (P) plays a crucial role in both agricultural production and water quality. There has been growing recognition of the importance of legacy P (surplus P that has accumulated in watersheds over time) for understanding contemporary water quality outcomes; however, little is known about how different watersheds respond to cumulative pressures from surplus P. The buffering capacity concept describes the ability of watersheds to attenuate P loading to surface waters by retaining P inputs over time. To explore the role of various watershed characteristics in buffering capacity, we used historic P data to calculate indices describing long- and short-term buffering for 16 large watersheds in southern Quebec, Canada, across a 30-year time span (1981-2011). We examined the correlation between these buffering capacity indicators and a set of key geochemical, hydrological, landscape and socio-ecological variables that we hypothesized could influence P buffering dynamics. Both short- and long-term buffering metrics were most strongly correlated with hydrological characteristics. Riverine TP flux across the watersheds was most strongly correlated with long-term buffering, which could represent a dominant influence of legacy P on contemporary riverine P flux. However, short- and long-term watershed buffering indices were not correlated with each other, suggesting distinctly different timescales and mechanisms of buffering. Combining estimates of long-term P accumulation along with biophysical characteristics of the watershed (including hydrology) explained a much greater share of the variation in riverine TP flux (R-2=0.69) than biophysical characteristics alone (R-2=0.36). Our findings reinforce the need to consider P buffering capacity and legacy P accumulation to help guide decision making around regional water quality targets across human-dominated landscapes.