Clay‐Fixed Labeled Ammonium as a Source of Available Nitrogen

Abstract

AbstractIn soils low in organic matter but high in clay mineral content, the fixation and release of NH4 by soil clay may have a significant impact on available N, in addition to the organic N. The objective of this work was to evaluate the contribution that nonexchangeable NH4, newly fixed by clay minerals, may have in KCl‐extractable N in soil. The source of clay‐fixed NH4 was beidellite clay equilibrated with (15NH4)2SO4 and then washed with either KCl or CaCl2 solutions. Two calcareous soils were mixed with differently treated beidellite at a 2:1 ratio, mixed with sand, and incubated for 56 d at 30°C and 60% of water‐holding capacity. Periodically, NH+4 and NO−3 extractable in 1 M KCl and their 15N abundance were determined. Controls for each beidellite treatment were incubated without soil. Concentrations of KCl‐extractable N were considerably higher when beidellite was saturated with Ca than when it was saturated with K throughout the experiment. They increased with time in the presence of soil and remained unchanged in the controls. The recovery of beidellite‐15NH4 as KCl‐extractable N in soil showed a continuous release of clay‐fixed NH4 from K‐beidellite, from 4.8 and 6.5% of added NH4 at 1 d to 10.2 and 12.5% at 56 d in the two soils. The recovery of N from Ca‐beidellite was approximately 30% of added N already at 1 d, but the change with time was inconsistent in the two soils. The difference in the release of N from Ca‐saturated beidellite compared with that of K‐saturated beidellite was attributed to incomplete collapse of the mineral lattice in the Ca‐saturated clay, which enabled renewal of exchangeable NH4 and rendered the interlayer nonexchangeable NH4 more accessible to exchange with K or to biological activity. In K‐saturated clay, the interlayer nonexchangeable NH4 was trapped and, consequently, the release was slow.