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Moist- and Dry-Season Nitrogen Transport in Sierra Nevada Soils

Dep. of Environmental and Resource Sciences, Univ. of Nevada, 1000 Valley Rd., Reno, NV 89512

S. W. Tyler

Desert Research Inst., Dandini Ln., Reno, NV

R. R. Blank

USDA-ARS, Reno, NV

* Corresponding author ( wilymalr{at}ers.unr.edu).

ABSTRACT

Characterization of ion mobility is a first step in the understanding of nutrient flux relationships in alpine forested watersheds. Our purpose was to: (i) investigate the transport dynamics of inorganic N in Sierra Nevada soils by applying unspiked source water as a control, and a 1 mmol_c L^-1 NH₄NO₃ spiked solution to two soil types (granitic and andesitic) under forest and meadow conditions via artificial rainfall during moist (spring) and dry (late summer-early fall) seasons; and (ii) evaluate the presence of macropore or preferential matrix flow from characteristic profile distributions of the two N forms. Peak concentrations in surface flow discharge (runoff) of both N forms from control treatments occurred early for both soil types and were highest from the meadow areas (1.6–0.4 mg NO^-₃-N L^-1 meadow to forest for granitic soil and 0.3–0.1 mg L^-1 meadow to forest for andesitic soil; 0.75–0.37 mg NH⁺₄-N L^-1 meadow to forest for granitic soil and 0.25–0.18 mg L^-1 meadow to forest for andesitic soil). Both soils exhibited NH⁺₄-N adsorption, but only the soil of andesitic origin appeared to sorb NO^-₃-N. Andesitic soils of the Sierra Nevada may thus serve as a temporary sink for NO^-₃-N deposition, whereas meadow areas of both soil types appear to serve as a source. Higher initial soil moisture for the spiked treatments generally resulted in the transport of NH⁺₄-N to greater depths in both soils of forested cover ( 40 compared with 20 cm from spring to late summer-early fall), and for NO^-₃-N ( 50 compared with 40 cm) as well. Wet-season (spring) mobility must therefore be considered an important groundwater nutrient transport mechanism in Sierra Nevada watershed soils.

A contribution of the State Water Resource Inst., Reno, NV, the Hydrologic Sciences Program, and the Nevada Agric. Exp. Stn., Univ. of Nevada, Reno.

Received for publication July 15, 1996.