SEASONAL FLUCTUATIONS IN NUTRIENT AVAILABILITY IN SOME NORTHERN CALIFORNIA ANNUAL RANGE SOILS

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Abstract
We studied the effects of climate and land-use history on seasonal variations in available soil nutrients at six northern California annual range sites. Available soil N, S, P, Ca, Mg, and K were measured monthly for 2 yr. Seasonal fluctuations in inorganic N were similar for both years over all sites; levels and variations in N were highest during winter and early spring and lowest during summer. Relative levels of available S, P, and Ca were also higher early in the year and declined later in the growing season, although the patterns were much less pronounced. Exchangeable Mg and K fluctuations were generally inconsistent and relatively smaller. Sheep grazing on unimproved pasture significantly (P < 0.05) increased average levels of available soil N, S, Ca, and Mg and decreased P availability. Seeding subclover (Trifolium subterraneum) and fertilizing with P and S increased available N and S levels. Converting chaparral brush to grassland decreased the availability of Mg and K.
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