SOIL-NITROGEN MINERALIZATION DURING LABORATORY INCUBATION - DYNAMICS AND MODEL-FITTING

Soil nitrogen mineralization kinetics were studied for eight treatment s of two soils in an aerobic long-term (30 wk) incubation experiment. Soil mineral-N (NH4+ and NO3-) in the leachates was measured every wee k during the first 9 wk and every 2 or 3 wk thereafter. The NH4+ perce ntage of the mineral-N ranged between 85 and 99% for all treatments in the first week of incubation and remained high (>80%) in several trea tments until the end of wk 4. Starting at wk 7, NH4+ concentrations we re negligible in all treatments. The net N mineralization rate was 15- 24 mg N kg(-1) wk(-1) during the first 4-6 wk and 2-5 mg N kg(-1) wk f rom wk 8 until the end of the incubation. Four models, (i) a one-compo nent, first-order exponential model (the single model), (ii) a two-com ponent, first-order exponential model (the double model), (iii) a one- component, first-order exponential model including a constant term (th e special model), and (iv) a hyperbolic model, were at to the cumulati ve mineral-N vs time data using a non-linear regression procedure. The goodness of fit of the four models depended on the duration of incuba tion. With 30 wk data, the double and special models were significantl y better than the other two models; with the first 15 wk data, the fou r models had essentially the same goodness of fit for seven out of eig ht treatments. The values of the regression parameters derived from ea ch model also depended on the incubation duration. Results from this s tudy show that the pool size and mineralization rate parameters in the different models are merely mathematically-defined quantities obtaine d from the kinetic analysis of the net N mineralization and do not rep resent any rigorously-defined pool sizes of potentially-mineralizable N and their mineralization rate constants in the soils. Copyright (C) 1996 Elsevier Science Ltd