Four-dimensional evaluation of nitrogen burden from agricultural and using Pedometrics

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To establish reasonable method for the evaluation of nitrogen burden from agricultural land to the environment, four-dimensional evaluation of nitrous oxide (N_2O) flux was carried out in a 1 ha agricultural land in Hokkaido, taking spatial and temporal variability of N_2O flux into account. For the spatial analysis, a hundred gas and soil samples were collected hm the field in August, 1999, and N_2O flux and soil physical, chemical fad biological properties were determined. N_2O flux had very high variability and showed moderate spatial dependency with the range of about 50m, suggesting that at least 9 measurements (every 50m) are necessary for the reasonable evaluation of N_2O flux in this field. N_2O flux was generally high at sites with relatively low elevation and was positively correlated with indices representing soil organic matter. For the temporal analysis, N_2O flux was also measured at one site for 5 years, i.e.from 1995 until 1999. N_2O flux was relatively low in spring after fertilization, and relatively high in summer and autumn, accounting for about 20% and 80% of the total emission, respectively. The mechanism of N_2O emission in spring was considered to be nitrification, and that in summer and autumn was considered to be denitrification at the relatively reduced sites in soil aggregates when a part of nitrate was accumulated due to source-sink effect of soil aggregate. A mechanistic model was further established to evaluate N_2O emission based on the estimation of"reduced volume in soil aggregate", and showed high positive correlation with the log-transformed N_2O flux.