QUANTIFYING BULK AND MOBILE CARBON AND NITROGEN IN THE DOMINANT LANDSCAPES OF THE UPPER SAN PEDRO RIVER BASIN, ARIZONA

Abstract

This study is designed to quantify the presence and transport potential of carbon (C) and nitrogen (N) from the dominant landscapes and vegetation types to the stream/riparian ecosystem along the San Pedro River in southeastern AZ. Previous biogeochemical studies of desert streams have focused largely on nutrient cycling within the riparian corridor, with significantly less research on linkages between uplands and the stream system. Surface water inputs during monsoon storms account for 65% of the water balance, and may also be an important component of nutrient inputs to semi-arid streams. Quantifying the bulk C and N of shrub lands and grasslands in the upland, terrace, and riparian forest soils and litters, and conducting experiments to measure potentially-mobile fractions of these pools allows for estimation of potential loads from subwatersheds to the San Pedro River of southern Arizona. Organic matter amounts and anion chemistry from first and second order ephemeral flows were statistically similar to amounts measured in laboratory leach tests of upland soils. Upland and terrace slopes displayed similar bulk C and N, while terraces displayed higher extractable C and N. Higher potential inputs from terraces may be offset by the decreased flood potential in the near-stream environment. Higher C and Nin shrubland runoff, coupled with woody encroachment in the uplands may increase C and N inputs in subsequent years.