[Effects of storage temperature and time on the contents of different nitrogen forms in fresh soil samples.]

Soil nitrogen forms and contents are of great importance in ecological studies. The storage methods of soil samples have great effects on the accuracy of determination of nitrogen contents. We aimed to select a reasonable storage method for soil samples with forest soil of Castanopsis faberi fore-st at Wanmulin Nature Reserve in Jian'ou City as an example. The contents of soil ammonium, nitrate, total nitrogen, soluble organic nitrogen, amino acid nitrogen and microbial biomass nitrogen were measured in soil samples under the storage conditions of different temperature (at 25, 4 and -20 ℃) and different times (0, 7 and 30 days). The nitrogen contents during the process of cultivating under the room temperature after being frozen were also measured. The results showed that the contents of all nitrogen forms except for amino acid nitrogen were increased in the soil samples that stored at the room temperature for seven days. There were no significant differences between the contents of all the tested nitrogen forms in the refrigerated or frozen samples and the fresh soil samples. The changes of nitrogen content in soil samples at refrigerated and frozen storage were more stable than those at room temperature storage. The low temperature storage could stimulate soil mineralization. Hence, after stored for 30 days, contents of all the tested nitrogen forms in the refri-gerated and frozen storage soil samples were significantly higher than those in the fresh samples except for the soluble organic nitrogen, whereas there was no significant difference between the refrigerated and frozen storage methods. Therefore, fresh samples should be promptly processed when taken back to the laboratory. If the samples needed to storage, it should not be stored more than half a month. If the samples need a longer storage time, it must be placed in lower temperature (at -40 or -80 ℃). Pre-incubation treatment was required when the low temperature storage soil sample was subjected to an experiment. In the process of pre-incubation, the contents of all the tested nitrogen form in soil samples gradually approached the level of fresh soil sample with the increases of incubation time except for that of nitrate which decreased firstly and then increased rapidly. After incubation for about one week, the nitrogen content of soil sample returned to the level that was close to that of the fresh soil. In combination with studies previously reported, soil samples collected from field and air dried samples needed a pre-incubation for 5-14 days, and the pre-incubation time for the cold storage sample should not be less than one week.