Dissolved organic nitrogen in agricultural soils: Effects of sample preparation on measured values

There is increasing interest in the soil's dissolved organic nitrogen (DON) fraction. However, soil analytical procedures have to be such that the result is a true representation of the DON levels in the soil. Three separate studies investigated the effects of soil and extract preparation on measured soil mineral N (Nmin) and DON concentrations in agricultural soils, ranging in texture from sands to clays. In Study 1 (18 soils), storing soils fresh or freeze/thawing, extracting with 2 M KCl or distilled water and storing extracts fresh or frozen were all tested. Study 2 (60 soils) retested soil storage (fresh vs. frozen) and extractant (2 M KCI or distilled water). Study 3 compared water and 0.01 M CaCl(2) as extractants. Average Nmin values (KCl) were 13.2 and 4.5 mg kg(-1) dry soil in Studies 1 and 2. Average DON values (water) were 3.3 and 3.7 mg kg(-1) dry soil, respectively. Both 2 M KCl or 0.01 M CaCl(2) extracted more DON from soils than did water. We suggest water extracts give a more reliable estimate of true DON status. However, a practical difficulty was obtaining a clear extract due to deflocculation of clay in the water extract. Freezing soils or extracts before analysis affected measured DON values, with a statistically significant tendency for DON to decrease depending on the extractant used.