Long term trends in fertility of soils under continuous cultivation and cereal cropping in southern Queensland .VII. Dynamics of nitrogen mineralization potentials and microbial biomass

Abstract

The dynamics of nitrogen mineralization potential (N0) and mineralization rate constant (k) were studied in six major soils which had been used for cereal cropping for up to 20-70 years. In the top 0.1 m layer of virgin soils, N0 varied from 110 ± 22 mg kg-1 soil (Riverview) to 217 ± 55 mg kg-1 soil (Langlands-Logie), representing about 13% and 11%, respectively, of total N in these soils. Upon cultivation and cropping, N0 declined by 1 7 ± 0.5 mg kg-1 yr-1 (Riverview) to 4.8 ± 2.0 mg kg -1 yr -1 (Billa Billa). This represented < 20% of total N lost annually from the top layer (0-0.1 m depth) of these soils. The k values varied less than the N0 values, both within and among soils, and were also less affected by cultivation than N0. The mineralizable N in cultivated soil during cropping for periods up to 70 years can be estimated from N0 and k values, taking No as 5% of total N for soils of <40% clay and 15% of total N for soils of >40% clay and k as 0.066 week-1 at 40°C (0.027 week-1 and 0.054 week-1 at 25°C and 35°C, respectively). Organic C and N contained in the 'stabilized' microbial biomass (determined after 30 weeks' pre-incubation) accounted for 1.7-38% of total organic C and 2.0-5.1% of total N in the six soils studied. The microbial biomass C and N declined with cultivation in most soils, biomass N representing 10-23% of the total annual loss of N0. The microbial biomass, urease activity and total N, in addition to a number of other soil properties [e.g. light-fraction (<2 Mg m-3) C, sand-size C, CEC and ESP], were significantly correlated with N0 and k, thus indicating the existence of a myriad of environments for the activity, association and stability of microbial biomass and potentially mineralizable N in soil.