Ion exchange resin–soil mixtures as a tool in net nitrogen mineralisation studies

Mixed-bed ion exchange resins (IER) were mixed with intact soil aggregates and incubated at 60% water filled pore space in closed polyethylene bags for 12 weeks. To test IER effects on N losses, nitrification and net N mineralisation, an arable soil and a grassland soil, differing in organic matter content, were chosen and two crop residues (wheat straw, sugar-beet leaves) with different C-to-N ratios were added to the arable soil. It was proposed that IER might exert an influence on N cycling similar to that of plant roots. Nitrification was inhibited by adsorption of NH4 in the +IER treatments. Net N mineralisation was greater in the grassland soil than in the arable soil which had less soil organic matter. Without incorporation of additional organic substrates, net N mineralisation was not affected by IER in both soils. Straw addition to the arable soil caused immediate N immobilisation in the −IER treatment, whereas N mineralisation continued in the +IER treatment. Incorporation of sugar-beet leaves into the arable soil highly increased net N mineralisation and microbial biomass N in the −IER treatment. In the +IER treatment, the enhancement of both N mineralisation and microbial biomass N was less pronounced. Thus, IER mixed into soil samples can exert either a stimulating (wheat straw) or dampening (sugar-beet leaves) effect on N mineralisation. Soil–IER mixtures can prevent losses and re-immobilisation of mineralised N and mimic nutrient exchange properties of plant roots. It is concluded that in incubation experiments they can better reflect conditions in the vicinity of roots than incubations without IER or with incorporation of IER in confined resin bags as long as water and aeration conditions are not largely changed. Soil–IER mixtures may also be a useful tool for studying root-induced changes in net N mineralisation.