Impact of potassium on plant uptake of non-exchangeable NH4 +-N

Abstract

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. 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. 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.