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Nitrogen Transfers in a Rapidly Eroding Agroecosystem:Loess Plateau, China

Department of Geography and Inst. for Environ. Studies, Univ. of Toronto, Toronto, ON Canada M5S 1A1.

^* Corresponding author.

ABSTRACT

A study was initiated to assess the influence of high erosion rates and sediment hyperconcentrations on patterns of N loss in China's Loess Plateau region. The study was conducted in a 12.6-ha subbasin where the upper slopes are terraced and cultivated, and the lower slopes are grazed by sheep. Seventeen rainstorms were monitored over 2 yr at a flume located in the ephemeral channel at the basin outlet. Runoff events were characterized by large sediment concentrations (mean 508 kg m^–3), dilution of dissolved NO₃ + NO₂-N concentrations during periods of peak flow, and high sediment-associated Kjeldahl N concentrations near the end of events when larger-sized particles are deposited. Hyperconcentrated flow ensured that most entrained sediment and N was exported. Annual soil losses equaled 203.4 Mg ha^–1 and 176.9 Mg ha^–1 in 1987 and 1988. Total N losses equaled 76.4 kg ha^–1 and 44.1 kg ha^–1. In each year, one or two large events contributed the bulk of soil and N losses. About 98% of the N losses were sediment associated, with low-intensity events having the highest soluble proportion. A simple N mass balance shows that erosion losses of N are serious, and that most of the N removed from both cultivated and grazed areas is exported. Results indicate that N cycling in this semiarid area is strongly influenced not only by the amount of rainfall, but also by the timing and intensity of the storms.

(current address of senior author is: EVS Environ. Consultants, 195 Pemberton Ave., North Vancouver, BC Canada V7P 2R4)

Received for publication October 15, 1991.