Content of soil mineral nitrogen as influenced by sample extraction and preservation

While it has been demonstrated that soil mineral N (NO₃-N and NH₄-N) analysis is critical for determining N fertilizer rate for most field crops, it remains difficult to predict crop N requirement using soil mineral N content due to rapid changes during transportation, pretreatment and storage of soil samples. Proper handling and preserving of soil samples after collection were thus important for the accurate determination of soil mineral N. Analysis fresh soil samples for inorganic N has been recommended, but time constraints had usually require temporary storage of dried or frozen soil samples for later extraction and analysis of soil NO₃-N and NH₄-N. Meanwhile, 2 mol·L^-1 KCl and 0.01 mol·^L-1 CaCl₂ (both used as reference extraction regent for soil mineral N) have larger ionic concentrations. 2 mol·L(^-1 KCl has been proven to be beneficial for soil mineral N content stability, but 0.01 mol·L^-1 CaCl₂ was convenient for confection and has similar ionic strength in soil solutions. In the past several years, much effort has been devoted to the study of fertilizer N use efficiency. However, information on the fate of N in samples under different preservation methods and acquired for different extraction regent, as well as the effects of soil chemical and physical properties on soil mineral N concentration were lacking. This study investigated the impacts of soil sample extraction and preservation methods on mineral nitrogen content of three types of soil. The ability of two extraction regents (2 mol·L-1 KCl and 0.01 mol·L-1 CaCl2) was assessed. Five preservation methods were evaluated, including no preservation (samples were immediately extracted within 24 h of sample collection), air-drying, freezing at -19 oC for ten days, and extraction within 24 h and extract liquors frozen at -19 oC for 10 d and 70 d. Ten soil samples (with different fertilization treatments) were collected at 0~20 cm depth from three locations representing different soil types. The content of soil mineral nitrogen was determined using continuous flow auto-analyzer. The results showed that extractable NO3-N by 2 mol·L-1 KCl significantly (P < 0.01) correlated with that by 0.01 mol·L-1 CaCl2 for all the three soil samples. The contents of NO3-N in fresh soils (no preserved) were lower than in frozen soils and in air-dried soils. Compared with analysis of fresh samples, short time freezing of fresh soil extracts was a reasonable and relatively reliable method of determining soil inorganic N. However, NH4-N contents changed irregularly with different preservation methods.