Moist- and dry-season nitrogen transport in Sierra Nevada soils

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 NO3--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 NH4+-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 NH4+-N adsorption, but only the soil of andesitic origin appeared to sorb NO3--N. Andesitic soils of the Sierra Nevada may thus serve as a temporary sink for NO3--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 MH4+-N to greater depths in both soils of forested cover (approximate to 40 compared with 20 cm from spring to late summer-early fall), and for NO3--N (approximate to 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.