Mineral-fixed ammonium in clay- and silt-size fractions of soils incubated with ¹⁵N-ammonium sulphate for five years

Summary

Four soils with 6, 12, 23, and 47% of clay were incubated for 5 years with ¹⁵N-labeled (NH_{4 2}SO₄ and hemicellulose. The incubations took place at 20°C and 55% water-holding capacity. Samples of whole soils, and clay- (<2 μm) and silt-(2–20 μm) size fractions (isolated by ultrasonic dispersion and gravity sedimentation) were analysed for labeled and native mineral-fixed ammonium. Mineral-fixed ammonium in non-incubated soil samples accounted for 3.4%–8.3% of the total N and showed a close positive correlation with the soil clay content (r ² = 0.997). After 5 years of incubation, the content of mineral-fixed ammonium in the clay fraction was 255–430 μg N g⁻¹, corresponding to 71%–82% of the mineral-fixed ammonium in whole soils. Values for silt were 72–166 μg N g⁻¹ (14%–33% of whole soil content). In the soils with 6% and 12% clay, less than 1 % of the labeled clay N was present as mineral-fixed ammonium. In the soil with 23% clay, 3% of the labeled N in the clay was mineral-fixed ammonium. Labeled mineral-fixed ammonium was not detected in the silt fractions. For whole soils, and clay and silt fractions, the proportion of native N present as mineral-fixed ammonium varied between 3% and 6%. In contrast, the proportion of labeled N found as mineral-fixed ammonium in the soil with 4701o clay was 23%, 38% and 31% for clay, silt, and whole-soil samples, respectively. Corresponding values for native mineral-fixed ammonium were 12%, 16%, and 10%. Consequently, studies based on soil particle-size fractions and addressing the N turnover in clay-rich soils should consider the pool of mineral-fixed ammonium, especially when comparing results from different size fractions with those from fractions isolated from soils of a widely different textural composition.