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

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Abstract
The dynamics of nitrogen mineralization potential (NO) 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, NO 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, NO 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 NO values, both within and among soils, and were also less affected by cultivation than NO. The mineralizable N in cultivated soil during cropping for periods up to 70 years can be estimated from NO and k values, taking NO as 5% of total N for soils of 40% clay and k as 0.066 week-1 at 40.degree. C (0.027 week-1 and 0.054 week-1 at 25.degree. C and 35.degree. C, respectively). Organic C and N contained in the ''stabilized'' microbial biomass (determined after 30 weeks'' pre-incubation) accounted for 1.7-3.8% 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 NO. 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, and sand-size C, CEC and ESP], were significantly correlated with NO 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.