Nitrogen Mineralization Modeling in Some Upland Soils in the Western Region of Thailand

A laboratory aerobic incubation experiment was conducted to assess the influence of plant residue, soil moisture content, and soil types on mineralization of organic N. The experimental design was a 2 x 3 x 5, factorial arrangement in a Randomized Complete Block design (RCBD) with three replications. The three factors were namely ;(1) two levels of plant residue (0 and 4 ton/rai of grinded cassava shoot), (2) three levels of soil moisture (PWP, ½ AWCA, and FC), and (3) Five upland soil series of the Western region of Thailand, consisting of three main soil textures (fine, medium, and coarse texture). Nitrogen mineralized from native and added organic matter was examined at specific time intervals for 1 year. The data was fitted to a logistic mathematical model describing the relation between mineral N versus moisture content (?), level of plant residue (PR), and time (t). The model was verified by predicting the amount of mineral N released under a given condition and the result was compared to the observed value under the same condition. Application of the model for field condition where soil moisture fluctuates was performed by differentiating the original model to obtain the relation between rate of mineral N production versus N (t), which was the implicit function of ?, PR, and t. Stepwise calculation of the cumulative mineral N with time was developed to predict the amount of N mineralized through time. The results revealed that addition of 4 ton/rai plant residue drastically increased mineral N by 3.6 folds. The effect of soil moisture content and mineralization of organic N confirmed the significance of water on microbial activities. A satisfactory result was obtained from the mathematical model verification. The b and R2 values were close to 1.0 and the t-test were non-significant. A rather high value of RMSE was obtained contributing to the cycles of microbial population fluctuation. Application of the model to the condition of fluctuating soil water content was performed and compared to the observation value at a given level of plant residue application and soil water content.