CLAY-FIXED LABELED AMMONIUM AS A SOURCE OF AVAILABLE NITROGEN

In soils low in organic matter but high in clay mineral content, the f ixation 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 wo rk was to evaluate the contribution that nonexchangeable NH4, newly fi xed by clay minerals, may have in KCl-extractable N in soil. The sourc e of clay-fixed NH4 was beidellite clay equilibrated with ((NH4)2SO4)- N-15 and then washed with either KCl or CaCl2 solutions. Two calcareou s soils were mixed with differently treated beidellite at a 2:1 ratio, mixed with sand, and incubated for 56 d at 30-degrees-C and 60% of wa ter-holding capacity. Periodically, NH4+ and NO3- extractable in 1 M K Cl and their N-15 abundance were determined. Controls for each beidell ite treatment were incubated without soil. Concentrations of KCl-extra ctable N were considerably higher when beidellite was saturated with C a than when it was saturated with K throughout the experiment. They in creased with time in the presence of soil and remained unchanged in th e controls. The recovery of beidellite-(NH4)-N-15 as KCl-extractable N in soil showed a continuous release of clay-fixed NH4 from K-beidelli te, 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 inconsist ent in the two soils. The difference in the release of N from Ca-satur ated beidellite compared with that of K-saturated beidellite was attri buted to incomplete collapse of the mineral lattice in the Ca-saturate d clay, which enabled renewal of exchangeable NH4 and rendered the int erlayer nonexchangeable NH4 more accessible to exchange with K or to b iological activity. In K-saturated clay, the interlayer nonexchangeabl e NH4 was trapped and, consequently, the release was slow.