Aims and background

Release of ‘non-exchangeable’ NH₄ ⁺-N from interlayers of 2:1 clay minerals is postulated to depend not only on soil solution NH₄ ⁺-N concentration but also on the concentration of K⁺ and Ca²⁺. Concentrations of all three cations are altered in rhizosphere compared to soil solution at larger distance from the root surface.

Methods

Non-exchangeable NH₄ ⁺-N pool was labelled with ¹⁵ N. Treatments including application of K⁺, Ca²⁺ and K⁺ + Ca²⁺ were established. In a compartment system approach we analysed changes in soil solution concentrations of ¹⁵NH₄ ⁺-N, ¹⁵NO₃ ⁻-N, K⁺ and Ca²⁺ in situ at different distances from the root surface over time and related them to the release of non-exchangeable ¹⁵NH₄ ⁺-N and uptake of ¹⁵ N by plants.

Results and conclusions

The ¹⁵ N enrichment in plant tissue was significantly lower in treatments with K⁺ application compared to those without. This was in line with smaller depletion of non-exchangeable ¹⁵NH₄ ⁺-N in the rhizosphere for these treatments and also with lower ¹⁵ N abundance in soil solution NO₃ ⁻-N fraction. Hence, K⁺ application hampered the release of NH₄ ⁺ from the interlayers. A promoting effect of increasing Ca²⁺ concentrations on release of non-exchangeable NH₄ ⁺-N could not be evaluated since the Ca²⁺ concentration in soil solution was largely controlled by small amounts of carbonate contained in the substrate and thus the addition of Ca²⁺ did not result in a relevant increase of soil solution Ca²⁺ concentration as originally intended.

The use of ¹⁵ N to follow the fate of non-exchangeable NH₄ ⁺-N proved very useful as it provides a higher sensitivity for all measured fractions compared to total N. However, as soil N fractions equilibrate with each other labelling one fraction exclusively is not possible.