Studies on the Dynamics of "Recently" Clay-Fixed Nh₄⁺ Using ¹⁵n

The dynamics of fixation and release of NH 4 + in soils were studied using tracer N under field and laboratory conditions. Field data showed that release of fixed NH 4 + was relatively slow after an initial moderately fast release. Forty months of field weathering of Bainsville soil left 3.48 kg 15 N/ha in the 75-cm profile of the 13.5 kg 15 N/ha applied and most (76%) of this recovered 15 N was fixed NH 4 + –N. The relative quantitative importance of recently fixed NH 4 + in the various particle size fractions was not in the same order as the native fixed NH 4 + . The fine silt fraction (2–5 μm) fixed a larger amount (whole soil basis) than the fine clay fraction (< 0.2 μm). The coarse clay fraction (0.2–2 μm) fixed the most NH 4 + added as well as being the fraction containing the most native fixed NH 4 + . Sand size fractions were shown to contain native fixed NH 4 + and were capable of fixing a small amount of added NH 4 + . Measurements of recently fixed NH 4 + –N in various particle sizes covering four time intervals (up to 40 mo) of field weathering under fallow showed that the 0.2 to 2-μm fraction was quantitatively most important, the < 0.2-μm fraction most readily released recently fixed NH 4 + and the 2 to 5-μm fraction was the most stable with respect to recently fixed NH 4 + . Plant growth affected the relative stability of the recently fixed NH 4 + in the various particle size fractions. Laboratory studies of three Brookston soil samples revealed higher estimates of fixation of NH 4 + by direct tracer methods than by indirect extraction methods. This discrepancy was assumed to have been caused by some exchange of added NH 4 + with native fixed NH 4 + and by some NH 4 + being fixed on other sites. The relationship between mineralogical and related analyses to fixation of NH 4 + was discussed.