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Reliable techniques for predicting N mineralisation dynamics in soil are required for sustainable management of land resources. The concept of potentially mineralisable N (N ₀) and its determination procedures were re-appraised in this study. Leaching soil before incubation removed considerable amounts of soluble organic N. But the leached soils had higher rates of net N mineralisation than the unleached controls during a subsequent 2-week incubation, suggesting that using the total amount of leached- (organic + inorganic) N for calculating N ₀ may not be warranted. N ₀ and the mineralisation rate constant k estimated with the conventional model-fitting technique varied significantly with soil, incubation time, temperature and moisture, without consistent trends. To obtain an N ₀ value that is unequivocally indicative of soil N mineralisation capacity, the incubation should be conducted under standardised temperature (35°C) and moisture (55–65% water holding capacity or –30 to –10 kPa) conditions, and a standard mineralisation rate constant (k̄=0.054 week–¹) should be used for all soils. Assuming that N ₀ for a soil does not change with environmental conditions, its value was then fixed as a constant when fitting the first-order kinetic model to estimate k under non-standard conditions. This technique eliminated the confounding relationship between N ₀ and k, which is inherent in the conventional curve-fitting procedure, and thus basically precluded the effect of incubation time on N ₀ estimation. The resultant N ₀ represents not only the quantity, but also the quality of substrates and their interaction with the soil matrix. The k values showed close and consistent relationships with temperature and moisture.
