The Quantity and Mineralization Rate Constant of the Active Pool of Soil Organic Nitrogen Under Corn-Soybean-Wheat Rotation in Southern Ontario.

The soil organic nitrogen (N) greatly contributes to the soil N supplying of agricultural soils in humid environment.  However, soil organic N is not a homogenized pool. Sharifi et al. (2007) defined three distinct soil mineralizable N pools using a long-term aerobic incubation study:  labile (Pool I), intermediate (Pool II) and stable (Pool III). Pool I is composed of the labile organic N pool and is the amount of N released within the first two weeks of the aerobic incubation. The objectives of this study were to (i) determine the quantity and mineralization rate constant (k) for Pool I, and (ii) to explore the relationship of Pool I parameters with field-based indices of soil N supply (yield, relative yield and plant N uptake) in a selection of grain corn (Zea mays) sites in Southern Ontario. Soil samples from 13 experimental corn trials that were under a corn-soybean-wheat rotation were incubated at 25^oC and 55% water filled pore space for 24 weeks and leached at days 0, 2, 4,7, 14, 28, 42, 56, 70, 84, 112, 140 and 168.  The quantity of N mineralized and k of Pool I was then related to field-based indices of soil N supply. The labile organic N values ranged from 11-103 mg N kg^-1 with a mean of 42 mg N kg^-1 and contributed up to 50% of the total N mineralized over the course of incubation.  The mineralization rate varied from 0.09-0.81 mg N wk^-1 with a mean of 0.35 mg N wk^-1 and was approximately 50 times larger than the mineralization rate of Pool II.   The mineralization rate was significantly correlated with yield, relative yield and plant N uptake (r=0.27, 0.31 and 0.42) but the N mineralized during this time period was not (r=0.28, 0.13 and 0.20). These findings suggest that Pool I contributes a significant portion to the N supplying capacity of agricultural soils, however, the quality of this pool may be a better indicator of N availability under field conditions.