Nitrogen mineralization under summer fallow and continuous wheat in the semiarid Canadian prairie

The ability of soils to provide a portion of the N required by crops via N mineralization of organic matter is of economic and environmental importance. Over a 40-yr period (1967–2006), soil NO3-N and plant-N measurements were made under summer fallow and in systems cropped to spring wheat (Triticum aestivum L.), on a medium-textured Orthic Brown Chernozem (Aridic Haploboroll), at Swift Current, Saskatchewan. These values were used to estimate net N mineralization (Nmin). Each year, above-ground plant N was measured at harvest and soil NO3-N was measured before seeding, soon after harvest, and just prior to freeze-up in October. Also, in the first 18 yr of this study NO3-N and above-ground plant N were measured eight times between spring and fall in selected treatments; these data were used to make a more detailed estimate of Nmin. In a third experiment, conducted on the same soil at a nearby site in 1975, many small lysimeters were sampled six times between spring and harvest of spring wheat. We used this lysimeter study to assess the effect of N fertilizer rate and soilwater on net Nmin. Results from the more frequent sampling were more plausible than those from sampling at three different times per year. On average, net Nmin in the 20-mo summer fallow period was about 118 kg ha-1 (15 kg ha-1 between harvest and the first spring, 93 kg ha-1 between the first spring and second fall, and 10 kg ha-1 between the second fall and seeding). The average net Nmin under a wheat crop between spring and fall was between 53 and 63kg ha-1. Net Nmin increased with water, but excessive water appeared to reduce apparent net Nmin, probably due to leaching and denitrification losses of N, which were not assessed in our estimation of Nmin. Regression analysis was used to show a positive association between net Nmin and precipitation, between spring and fall, for most of the systems examined. There was evidence that tillage promotes N mineralization. At normal rates of N fertilizer (i.e., < 100 kg ha-1), fertilizer had no effect on Nmin. Net Nmin was directly proportional to fallow frequency, averaging 68, 83, and 90 kg ha-1 yr-1 for continuous wheat, fallow-wheat-wheat, and fallow-wheat rotations, respectively. Although our results may only be applicable to medium-textured soils of similar organic matter content in the Brown and Dark Brown Chernozemic soil zone, they provide data and information against which process-based models can be tested. They also provide useful first approximations of Nmin measured under field conditions where few long-term data currently exist. Key words: N mineralization, plant-N, fertilizer-N, crop rotation, irrigation, tillage