Nitrogen and phosphorus dynamics during runoff events over a transition from grassland to shrubland in the South-Western United States.

Abstract

During the last 150 years, land degradation across the semi-arid grasslands of the south-western United States has been associated with an increase in runoff and erosion. Concurrent with this increase in runoff and erosion is a loss of nitrogen (N) and phosphorus (P), which are plant-essential nutrients. This study investigates the runoff-driven redistribution and loss of dissolved and particulate-bound N and P that occurs during natural runoff events over a trajectory of degradation, from grassland to degraded shrubland, in central New Mexico. Runoff-driven nutrient dynamics were monitored at four stages over a transition from grassland to shrubland, for naturally occurring rainfall events over 10 × 30 m bounded runoff plots. Results show that particulate-bound forms of N and P are responsible for most of N and P lost from the plots due to erosion occurring during runoff events. Results suggest that for high-magnitude rainfall events, the output of N and P from the plots may greatly exceed the amount input into the plots, particularly over shrub-dominated plots where erosion rates are higher. As these results only become apparent when monitoring these processes over larger hillslope plots, it is important to recognize that processes of nutrient cycling related to the islands of fertility hypothesis may have previously been overstated when observed only at smaller spatial scales. Thus, the progressive degradation of semi-arid grassland ecosystems across the south-western United States and other semi-arid ecosystems worldwide has the potential to affect N and P cycling significantly through an increase in nutrient redistribution and loss in runoff.