Soil erosion and significance for carbon fluxes in a mountainous Mediterranean-climate watershed.

In topographically complex terrains, downslope movement of soil organic carbon (OC) can influence local carbon balance. The primary purpose of the present analysis is to compare the magnitude of OC displacement by erosion with ecosystem metabolism in such a complex terrain. Does erosion matter in this ecosystem carbon balance? We have used the Revised Universal Soil Loss Equation (RUSLE) erosion model to estimate lateral fluxes of OC in a watershed in northwestern Mexico. The watershed (4900 km2) has an average slope of 10 degrees +/- 9 degrees (mean +/- SD); 45% is >10 degrees, and 3% is >30 degrees. Land cover is primarily shrublands (69%) and agricultural lands (22%). Estimated bulk soil erosion averages 1350 Mg x km(-2) x yr(-1). We estimate that there is insignificant erosion on slopes < 2 degrees and that 20% of the area can be considered depositional. Estimated OC erosion rates are 10 Mg x km(-2) x yr(-1) for areas steeper than 2 degrees. Over the entire area, erosion is approximately 50% higher on shrublands than on agricultural lands, but within slope classes, erosion rates are more rapid on agricultural areas. For the whole system, estimated OC erosion is approximately 2% of net primary production (NPP), increasing in high-slope areas to approximately 3% of NPP. Deposition of eroded OC in low-slope areas is approximately 10% of low-slope NPP. Soil OC movement from erosional slopes to alluvial fans alters the mosaic of OC metabolism and storage across the landscape.

Direct measurement of denitrification in a Prosopis (Mesquite) dominated Sonoran Desert ecosystem.

Denitrification was directly measured using the acetylene inhibition technique in a Sonoran Desert ecosystem dominated by Prosopis glandulosa. Soil under Prosopis and from the unvegetated area between Prosopis was wetted with 50 mm of water and denitrification measured for 48 hours. The mean denitrification rate under Prosopis was 11.6 g N ha-1h-1 compared to only 0.2 g N ha-1h-1 away from Prosopis. The denitrification response to wetting was rapid and rates peaked about 24 h after water application.The much higher denitrification under Prosopis probably results from high available organic C under Prosopis, but other soil chemical and physical changes effected by Prosopis may influence denitrification rates. About 0.5 kg N ha-1 of Prosopis cover may be lost from this ecosystem by denitrification after infrequent major rainfalls.