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

为建立合理的农业用地对环境氮负荷的评价方法，在考虑N_2O通量时空变异的基础上，对北海道1 ha农业用地进行了N_2O通量的四维评价。在空间分析方面，于1999年8月在野外采集了100份气体和土壤样品，测定了N_2O通量和土壤的物理、化学和生物特性。N_2O通量具有非常高的变异性，在50m范围内表现出中等的空间依赖性，表明要合理评价该地区N_2O通量至少需要9次观测（每50m）。N_2O通量在海拔相对较低的站点总体较高，且与土壤有机质指标呈正相关。为了进行时间分析，还在一个站点测量了5年的N_2O通量，即1995 - 1999年。施肥后春季N_2O通量相对较低，夏季和秋季相对较高，分别占总排放的20%和80%左右。春季的N_2O排放机制为硝化作用，夏季和秋季的N_2O排放机制为土壤团聚体中相对减少的部位的反硝化作用，部分硝态氮因土壤团聚体的源汇效应而积累。建立了基于“土壤团聚体还原体积”估算N_2O排放的机理模型，该模型与对数变换N_2O通量呈高度正相关。

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.