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DIVISION S-3—SOIL BIOLOGY & BIOCHEMISTRY

Predicting Soil Nitrogen Mineralization Dynamics with a Modified Double Exponential Model

^a School of Resource Management, the Univ. of Melbourne, Parkville 3052, Victoria, Australia
^b CSIRO Land and Water, GPO Box 1666, Canberra 2601, ACT, Australia
^c Present address: NR&M, 80 Meiers Rd, Indooroopilly, Brisbane, Qld. 4068, Australia

^* Corresponding author (weijin.wang{at}nrm.qld.gov.au).

The double exponential model that separates mineralizable soil organic N into active (N_a) and slow (N_s) pools has been widely used to describe net N mineralization dynamics. However, the biological meanings of the model parameters and their relationships to soil properties and environmental conditions remain to be elucidated. In the present study, 18 soils were incubated at 35°C and 55% water-holding capacity (WHC) for 41 wk and two soils at 16 factorial combinations of temperature (5, 15, 25, or 35°C) and moisture (8, 11, 15, or 19%) for 29 wk. Although the model closely fitted the net N mineralization data, the model parameters often appeared to lack biological meanings and could vary with incubation time, temperature, and soil moisture in unpredictable manners. Thus, the conventional double exponential model was modified by (i) using defined mineralization rate constants for N_a and N_s under standard temperature (35°C) and moisture (approximately 55% WHC); and (ii) using soil-specific and fixed N_a and N_s values to estimate temperature- and moisture-dependent rate constants under non-standard conditions. This technique basically eliminated the time effect on the estimates of pool sizes and resulted in the rate constants with a consistent Q₁₀ response to temperature and linear response to moisture changes. Predictions of soil net N mineralization dynamics under field conditions using the parameters estimated in laboratory agreed closely with the measured data. Multiple regression analysis indicated that the size of N_a is correlated with the initial water-soluble organic N and microbial biomass in soil, whereas N_s represents the combined effects of all the factors regulating long-term net N mineralization.

Abbreviations: DOC, dissolved organic C • DON, dissolved organic N • k_a, mineralization rate constant for N_a • k_s, mineralization rate constant for N_s • LFC, light fraction organic C • LFN, light fraction organic N • MBC, microbial biomass C • MBN, microbial biomass N • N_a, active organic N pool • N_s, slow organic N pool • TON, total organic N • WHC, water-holding capacity

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