Tuesday, November 14, 2006 - 1:45 PM

Evaluation of Some Indices of Potentially Available Organic Nitrogen in Soil.

Mehdi Sharifi1, Bernie Zebarth1, David Burton2, and Cynthia Grant3. (1) Potato Research Centre, #850 Lincoln Rd., P.O.Box 20280, Fredericton, NB E3B4Z7, Canada, (2) Nova Scotia Agric. College, Truro, NS B2N 5E3, Canada, (3) Agriculture and Agri-Food Canada, Agriculture & AgriFood Canada, Box 1000A RR 3, Brandon, MB R7A 5Y3, CANADA

A series of indices of potentialy available soil organic N 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 (N0) determined by aerobic incubation at 25oC for 24 wk, and using a first-order kinetics model. Three different pools of mineralizable N were recognized: (I) the mineralization flush upon rewetting in the first 2 wk, (II) gross N mineralization in the next 22 wk, and (III) the potentially mineralizable N predicted from the fitted curve that did not mineralize during the incubation period. These pools represented an average of 1.3, 4.4 and 1.3 % of total organic N, respectively. Pool-I was highly correlated with CaCl2-N, KCl-NH4 and KCl-NO3, which extract soil mineral N. Pool-III was significantly correlated with UV absorbance of NaHCO3 extract at 205 and 260 nm (NaHCO3-205 and -260), Illinois Soil N Test, NaOH direct-distillation N and hot KCl-NH4, which mostly extract hydrolysable organic N. All indices except mineral N based methods, phosphate-borate buffer method and microbial biomass C were significantly related to N0, which includes both pool-II and III. The NaHCO3-260, NaOH direct-distillation N and Illinois Soil N Test had the highest correlations with N0 (r2 = 0.74, 0.61 and 0. 51, respectively). Total organic C and N represent long term changes in N0 and were almost as effective in predicting N0 as the other indices (r2 = 0.60 and 0.67, respectively), however, they would be expected to be less sensitive to short term changes in N0 due to changes in soil management practices and history. The NaHCO3-260 method is accurate, simple, rapid, and inexpensive and may offer a practical option for routine soil N mineralization testing in arable soils.