SEASONAL VARIATION OF INTERLAYER AMMONIUM IN THE SOIL OF A PEACH ORCHARD

The results indicate that the reserve of interlayer ammonium is subject to seasonal fluctuations which depend on nitrogen uptake by the trees and on the mineralization of organic nitrogen. The amount of fixed nitrogen increased when plant uptake and mineralization of organic nitrogen were low (winter). Decreases in interlayer ammonium were found during periods when plant uptake of nitrogen was rapid, but did not correspond to an increase in mineralization of organic nitrogen. During the summer (mid-July) no reduction in ammonium reserves was found since mineralization of organic nitrogen was very intense. Successively, between the end of July and the beginning of September, decreases were found in interlayer ammonium and in the extent of mineralization of organic nitrogen. The largest amounts of nitrogen fertilizer applications to the soil (200 kg N ha^-1) limited considerably the release of the interlayer ammonium reserves, preventing its incorporation to the nitrogen cycle of the soil. The results obtained clearly indicate that the interlayer ammonium reserves participate actively in the nitrogen cycle in the soil and that the interlayer ammonium is very sensitive to the extent of mineralization of organic nitrogen. The discrete quantities of ammonium both fixed and released during the year indicate that this reserve is

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