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 fo r 12 weeks. To test IER effects on N losses, nitrification and net N minera lisation, 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 propose d that IER might exert an influence on N cycling similar to that of plant r oots. Nitrification was inhibited by adsorption of NH4 in the + IER treatme nts. Net N mineralisation was greater in the grassland soil than in the ara ble soil which had less soil organic matter. Without incorporation of addit ional organic substrates, net N mineralisation was not affected by IER in b oth soils. Straw addition to the arable soil caused immediate N immobilisat ion in the -IER treatment, whereas N mineralisation continued in the +IER t reatment. Incorporation of sugar-beet leaves into the arable soil highly in creased net N mineralisation and microbial biomass N in the -IER treatment. In the +IER treatment, the enhancement of both N mineralisation and microb ial biomass N was less pronounced. Thus, IER mixed into soil samples can ex ert either a stimulating (wheat straw) or dampening (sugar-beet leaves) eff ect on N mineralisation. Soil-IER mixtures can prevent losses and re-immobi lisation of mineralised N and mimic nutrient exchange properties of plant r oots. It is concluded that in incubation experiments they can better reflec t 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 c onditions are not largely changed. Soil-IER mixtures may also be a useful t ool for studying root-induced changes in net N mineralisation: (C) 2000 Els evier Science Ltd. All rights reserved.