Nutrient losses in runoff from grassland and shrubland habitats in southern New Mexico: II. Field plots

Losses of dissolved nutrients (N, P, K, Ca, Mg, Na, Cl, and SO4) in runoff were measured on grassland and shrubland plots in the Chihuahuan desert of southern New Mexico. Runoff began at a lower threshold of rainfall in shrublands than in grasslands, and the runoff coefficient averaged 18.6% in shrubland plots over a 7-year period. In contrast, grassland plots lost 5.0 to 6.3% of incident precipitation in runoff during a 5.5-year period. Nutrient losses from shrubland plots were greater than from grassland plots, with nitrogen losses averaging 0.33 kg ha(-1) yr(-1) vs 0.15 kg ha(-1) yr(-1), respectively, during a 3-year period. The greater nutrient losses in shrublands were due to higher runoff, rather than higher nutrient concentrations in runoff. In spite of these nutrient losses in runoff, all plots showed net accumulations of most elements due to inputs from atmospheric deposition. Therefore, loss of soil nutrients by hillslope runoff cannot, by itself, account for the depletion of soil fertility associated with desertification in the Chihuahuan desert.