Study on the Methods of Sample Collection and Keeping in Forest Soil NO3-N Determination

Based on the research of red soil sample that developed from granite, the results are as follows: （1） Because the content of NO3-N in soil, especially the top soil, is infected seriously by the precipitation, soil sample should be collected on the day that avoid special weather such as rain. It is better to collect on the day without raining for 3 to 5 days or 1 week after rainstorm; （2） Three keeping ways of soil sample are taken in the test, that were wind drying, cooling and freezing. Error analysis showed that the average relative error of wind drying, cooling and freezing way were -8.16%, -5.18% and 0.185% respectively. The best keeping way is freezing（in temperature of -6～-7 ℃）. （3） Through three group contrastive experiments,it was found that the best vibrating time of sample testing was 30 minutes.