Comparison of Factors Affecting Soil Nitrate Nitrogen and Ammonium Nitrogen Extraction

Extraction of soil nitrate nitrogen (NO3 −-N) and ammonium nitrogen (NH4 +-N) by chemical reagents and their determinations by continuous flow analysis were used to ascertain factors affecting analysis of soil mineral N. In this study, six factors affecting extraction of soil NO3 −-N and NH4 +-N were investigated in 10 soils sampled from five arable fields in autumn and spring in northwestern China, with three replications for each soil sample. The six factors were air drying, sieve size (1, 3, and 5 mm), extracting solution [0.01 mol L−1 calcium chloride (CaCl2), 1 mol L−1 potassium chloride (KCl), and 0.5 mol L−1 potassium sulfate (K2SO4)] and concentration (0.5, 1, and 2 mol L−1 KCl), solution-to-soil ratio (5:1, 10:1, and 20:1), shaking time (30, 60, and 120 min), storage time (2, 4, and 6 weeks), and storage temperature (−18 oC, 4 oC, and 25 oC) of extracted solution. The recovery of soil NO3 −-N and NH4 +-N was also measured to compare the differences of three extracting reagents (CaCl2, KCl, and K2SO4) for NO3 −-N and NH4 +-N extraction. Air drying decreased NO3 −-N but increased NH4 +-N concentration in soil. Soil passed through a 3-mm sieve and shaken for 60 min yielded greater NO3 −-N and NH4 +-N concentrations compared to other treatments. The concentrations of extracted NO3 −-N and NH4 +-N in soil were significantly (P < 0.05) affected by extracting reagents. KCl was found to be most suitable for NO3 −-N and NH4 +-N extraction, as it had better recovery for soil mineral N extraction, which averaged 113.3% for NO3 −-N and 94.9% for NH4 +-N. K2SO4 was not found suitable for NO3 −-N extraction in soil, with an average recovery as high as 137.0%, and the average recovery of CaCl2 was only 57.3% for NH4 +-N. For KCl, the concentration of extracting solution played an important role, and 0.5 mol L−1 KCl could fully extract NO3 −-N. A ratio of 10:1 of solution to soil was adequate for NO3 −-N extraction, whereas the NH4 +-N concentration was almost doubled when the solution-to-soil ratio was increased from 5:1 to 20:1. Storage of extracted solution at −18 °C, 4 °C, and 25 °C had no significant effect (P < 0.05) on NO3 −-N concentration, whereas the NH4 +-N concentration varied greatly with storage temperature. Storing the extracted solution at −18 oC obtained significantly (P < 0.05) similar results with that determined immediately for both NO3 −-N and NH4 +-N concentrations. Compared with the immediate extraction, the averaged NO3 −-N concentration significantly (P < 0.05) increased after storing 2, 4, and 6 weeks, respectively, whereas NH4 +-N varied in the two seasons. In conclusion, using fresh soil passed through a 3-mm sieve and extracted by 0.5 mol L−1 KCl at a solution-to-soil ratio of 10:1 was suitable for extracting NO3 −-N, whereas the concentration of extracted NH4 +-N varied with KCl concentration and increased with increasing solution-to-soil ratio. The findings also suggest that shaking for 60 min and immediate determination or storage of soil extract at −18 oC could improve the reliability of NO3 −-N and NH4 +-N results.