Factors affecting N immobilisation/mineralisation kinetics for cellulose-, glucose- and straw-amended sandy soils

Abstract.

The kinetics of N immobilisation/mineralisation for cellulose-, glucose- and straw-amended sandy soils were investigated in a series of laboratory incubations. Three Scottish soils expected to exhibit a range of biological activity were used: a loamy sand, intensively cropped horticultural soil subject to large inputs of inorganic fertilisers and pesticides (Balmalcolm – pH 7.2, organic matter 3.3%); a sandy loam soil highly enriched with organic manures and used for organic vegetable production (Strathmiglo – pH 7.1, organic matter 7.3%); and a loamy sand soil of low fertility in a zero-grazing, low intensity organic ley-arable rotation (Aldrochty pH 6.0, organic matter 5.0%). Incubations of soils with 1,000 mg cellulose-C kg^–1 soil at 8°C, showed peak N immobilisation of 71±6, 92±6 and 65±15 mg N g^–1 added C for the Balmalcolm (after 34 days), Strathmiglo (after 34 days) and Aldrochty soils (after 63 days). The N remineralisation by the end of the incubation (>300 days) was 0, 50 and 22 mg N g^–1 cellulose-C in the Balmalcolm, Strathmiglo soil and Aldrochty soils, respectively. Only about 30% of the N immobilisation could be explained by soil microbial biomass N accumulation (much less than expected from model simulations). The C/N ratio of the extra microbial biomass was quite wide (19). Bacterial, protozoan and nematode biomass accounted for only 18%, 0.1% and 0.5% of the extra C immobilisation, respectively. These data suggest that fungal biomass growth and deposition of recalcitrant fungal metabolites are the main sinks for the N immobilised. With 1,000 mg glucose-C kg ^–1 added to the Balmalcolm soil, about 75 mg N g^–1 added C were immobilised after 6 days. Under less well aerated conditions at 15°C, immobilisation of only 10–20 mg N g^–1 added cellulose C took place in 2–4 weeks, but soluble organic C increased greatly. The N remineralised after 4–6 weeks.