AN INCUBATION STUDY OF NITROGEN ADDED AS UREA TO SEVERAL MANITOBA SOILS WITH PARTICULAR REFERENCE TO RETENTION OF NITROGEN

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Abstract
The objective of this study was to determine the magnitude of biologically immobilized and/or chemically fixed N (N retained) resulting from N addition to soils under laboratory conditions. Urea (200 μg N/g of soil) tagged with 21.0 atom % 15N was added to 11 Manitoba soils. The relationship between microbial immobilization and/or chemical fixation of added N and soil characteristics was determined using forward stepwise and multiple linear regression analyses. Three to 32 ppm of added N was immediately retained by the soils after urea was added. Biological immobilization and/or chemical fixation was virtually completed within the first 4 wk of incubation with only one soil showing continuous increase in the retention of applied N during the remainder of the experiment. Amounts of urea N retained in soils during the 12-wk incubation period ranged from 16 to 91 ppm. The initial retention of N was positively correlated with sand + silt fraction and negatively correlated the pH of the soil, the R2 value being 0.86**. Correlations between, 15N retained and organic matter (OM) content for 4 and 8 wk of incubation were positive (R2 = 0.40* and 0.53*, respectively). After the 12 wk of incubation, the retention of added N by soils was positively correlated with OM and negatively correlated with soil pH (R2 = 0.74**). The retention of N with pooled data of 4, 8 and 12 wk of incubation was positively correlated with OM and negatively correlated with both pH and CEC of the soils (R2 = 0.73**). Mineralization of soil N and retention of added N for 4, 8 and 12 wk of incubation were found to be positively correlated, the r2 values being 0.42*, 0.52* and 0.42*, respectively. Gaseous losses of applied N appeared to be appreciable in only one soil. Ten of the eleven soils showed no or little N interchange, suggesting that a significant proportion of the applied urea N appeared to be truly immobilized and/or fixed in soils.
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