Soil samples were collected at intervals of 1, 2, 4, 8, and 16 weeks after the incorporation of the organic materials. For the submerged experiment, a 3-cm diameter cylindrical tube was used to sample the soil at 0-5 cm depth. A composite sample of 3 subsamples was collected per pot per sampling time with the same pot used to take samples throughout the experiment. For the aerobic experiment, a whole pot was collected to represent each treatment at each collection time.
The fresh soil samples were analyzed for mineral N ammonium nitrate and, for the submerged experiment only, microbial biomass N MBN. Mineral N was extracted by 1N KCl 1:5 soil:extractant and N determined by flow injection analyzer, while MBN was determined by fumigation-extraction method details in Vityakon et al. 2000.