Nitrogen cycling involving non-exchangeable ammonium in a gray luvisol

 There is considerable interest in determining the bio-availability of non-exchangeable NH4+ (NEA) because it constitutes nearly 10% of the total N in the top 1 m of soil. NEA is NH4+ present in the soil that is not extractable with neutral K salt solutions, e.g. 1M KCl. This study was conducted in 1994 and 1995 to quantify the amount of NEA released by a gray luvisolic clay loam soil in Alberta, Canada, during the growing season. Replicated plots under: (1) continuous bromegrass, (2) continuous barley, and (3) barley since 1991 but previously under a continuous forage legume, were sampled four times through the growing season to a depth of 80 cm. The first sampling was done before seeding of barley. Cropping systems and their interaction with time of sampling had no effect on NEA. The NEA-N pool in the soil decreased significantly during grain-filling and maturing of barley, and had returned to approximately its initial level by the following spring. The maximum amount of NEA-N released was 15 g m–2 at barley grain-filling in 1994, and 10 g m–2 3 weeks after barley had matured in 1995. Approximately 24% of the released NEA accumulated as 1M-KCl-extractable NH4+ in 1994, 20–30% was absorbed by the barley crop, and most of the remainder was probably assimilated by soil micro-organisms and immobilized in soil organic matter. The release of significant amounts of NEA during crop growth has implications for N cycling and measurements of mineralization/immobilization turnover rates.