Prediction and verification of nitrogen decomposition patterns of various manure types in paddy fields by phosphate buffer extraction

We studied three aspects of animal manure: first, whether the nitrogen (N) decomposition pattern of animal manure could be estimated by a decomposition model (Hayami, 1985); second, the relationship between the ratio of organic N extracted from manure using four different methods and the coefficients of the decomposition model; third, the effect on yield and N uptake in paddy fields by applying the amount of compost calculated with our model to predict the N decomposition pattern. To build our model, we examined N decomposition patterns of 13 different manure types (five from cattle, three from swine, and two from poultry, two from both cattle and swine, and one from cattle, swine and poultry; some manures included supplementary ingredients such as chaff, straw, saw dust, and wood chips) in paddy fields by burying unwoven cloth bags filled with manure in the field. Our results were the following: 1. After two months of burial (at about 1000℃ of cumulative air temperature), decomposition rates were highest in poultry manure, followed by swine and cattle manure. In addition, the decomposition rates of all manure tended to decrease when the cumulative air temperature exceeded about 1000℃, the inflection point. 2. We created a decomposition model where coefficients of the model were determined from decomposition patterns of the manure. D=eT^r (where D is N-decomposition rate (%); e is acceleration coefficient of N decomposition; T is cumulative air temperature×10^<-3>; and r is difficulty level coefficient of N decomposition). Decomposition patterns of N for all manure could be explained by the model with a determination coefficient of 0.93 or higher. In addition, since the inflection point was at cumulative air temperature of about 1000℃, where T=1, at that point, D and e became equal. Therefore, we suggest that the acceleration coefficient is equivalent to easily decomposable organic N at the inflection point. 3. The relationship between the acceleration coefficient and the difficulty level coefficient was well explained by the following equation: y=2.1x^<-0.77>. 4. The ratio of organic N extracted from the manures varied depending on the extraction method and the solutions used: 6-42% for extraction with phosphate buffer, 3-31% for extraction with diluted sulfuric acid, 18-48% for extraction with hot water, and 32-65% for extraction with an acidic detergent. 5. The correlation coefficient between the ratio of extracted N and the acceleration coefficient was highest (r=0.97) for extraction with a phosphate buffer: the ratio of the extracted N to the acceleration coefficient was about 1:1. Therefore, we suggest that extracting organic N with a phosphate buffer is a preferable method to estimate the ratio of easily decomposable N in paddy field soils. 6. Measured N decomposition patterns of the three different manures (cattle, swine, and poultry) were well explained by our predicted models. Root mean square errors between the measured N decomposition rate and the predicted rate were 2.4-2.7. 7. We applied our model to calculate the amount of N supply during the time between three manure applications and the full heading date of rice. We then calculated the amount of manure needed to equal the amount of N supply from chemical fertilizer. When the corresponding manure amount was added to the field, the rice yield and N uptake were slightly less than those of chemical fertilizer plots.