Evaluation of Some Indices of Potentially Mineralizable Nitrogen in Soil

A series of soil N mineralization indices were evaluated using 153 samples chosen from arable fields representing a wide range of soil types, management practices, and climatic zones. These indices were compared against potentially mineralizable N (N 0 ) determined by aerobic incubation at 25°C for 24 wk. Three different pools of mineralizable N were recognized: Pool I, the mineralization flush on rewetting in the first 2 wk; Pool II, gross N mineralization in the next 22 wk; and Pool III, the potentially mineralizable N, predicted from the fitted curve, that did not mineralize during the incubation period. Pool I was highly correlated with CaCl 2 –N, KCl‐NH 4 , and KCl‐NO 3 , which extract soil mineral N. Pool III was significantly correlated with ultraviolet absorbance of NaHCO 3 extract at 205 and 260 nm (NaHCO 3 –205 and −260), Illinois soil N test, NaOH direct‐distillation N, and hot KCl‐NH 4 , which mostly extract hydrolyzable organic N. All indices except the mineral N based methods, phosphate‐borate buffer method, and microbial biomass C were significantly related to N 0 , which includes both Pools II and III. The NaHCO 3 –260, NaOH direct‐distillation N, and Illinois soil N test had the highest correlations with N 0 ( r 2 = 0.74, 0.61, and 0. 51, respectively). Total organic C and N represent long‐term changes in N 0 and were almost as effective in predicting N 0 as the other indices ( r 2 = 0.60 and 0.67, respectively); however, they would be expected to be less sensitive to short‐term changes in N 0 due to changes in soil management practices and history.