Document: LYN-3-65-27

California grassland productivity and nutrient availability at the hillslope scale.

ALTHOUSE, L.*, F.CHAMRAN, J.P.SCHIMEL and V.LACAPRA

University of California at Santa Barbara, CA 93106 USA 1

Abstract:
Landscape position is a primary controller of nutrient availability and mobility. Between 1997 and 1999 we observed patterns in grassland productivity and nutrient availability related to terrain. Ridges had low NPP (above ground net primary productivity) compared to hollow positions. We evaluated the relationship between hillslope position, soil characteristics, and plant productivity, to predict nutrient transport and accumulation using nitrate as our model ion. Similar patterns of nutrient concentration were observed for NO3--N, Na+, Mg++, and Ca++ in 8 soil profiles instrumented with remote water samplers. In California's Mediterranean climate, N mineralization peaks during the moist, warm spring months, and declines during the long drought of summer and early fall. Ammonium production and consumption are 5 times higher in the hollow than the ridge positions, although the highest concentrations of NH4+ were observed at the ridge position in the spring (10 ug NH4+-N/g soil). Nitrate concentrations within the rooting zone (0-20 cm) were highest in the fall with over 20 ug NO3--N/g soil in the top 10 cm at the hollow position. Below the rooting zone, the hollow position (<200 cm) had the highest concentrations for all ions during the spring. Flushes of nutrient transport occur during periods of high rainfall at the beginning and end of the rainy season, before and after the annual grasses complete their brief life cycle. Nutrient transport is minimized at the height of plant growth and microbial activity during the spring. These patterns are exaggerated at different hillslope positions where hollows tend to accumulate and immobilize more nutrients than the ridge positions.

Keywords: California grassland, NPP, nutrient transport, nitrate, cations, hillslope

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This abstract is being presented at: 9:30 AM in session:
Oral Session #58: Landscape Ecology.