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

The kinetics of nitrogen immobilization/mineralization 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: aloamy sand, intensively cropped horticultural soil subject to large inputs of inorganic fertilizers 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 1000 mg cellulose-C per kg soil at 8 °C, showed peak N immobilization of 71 6, 92 6 and 65 15 mg N per g added C for the Balmalcolm (after 34 d), Strathmiglo (after 34 d) and Aldrochty soils (after 63 d). The N remineralization by the end of the incubation (>300 d) was 0, 50 and 22 mg N per g cellulose-C in the Balmalcolm, Strathmiglo soil and Aldrochty soils, respectively. Only about 30% of the N immobilization could be explained by soil microbial biomass N accumulation (much less than expected from model simulations). The C/N ratio of theextra microbial biomass was quite wide (19). Bacterial, protozoan and nematode biomass accounted for only 18%, 0.1% and 0.5% of the extra C immobilization, respectively. These data suggest that fungal biomass growth and deposition of recalcitrant fungal metabolites are the main sinks for the N immobilized. With 1000 mg glucose-C per kg added to the Balmalcolm soil, about 75 mg N per g added C were immobilized after 6 d. Under less well aerated conditions at 15 °C, immobilization of only 10-20 mg N per gadded cellulose-C took place in 2-4 weeks, but soluble organic C increased greatly. The N remineralized after 4-6 weeks.