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

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.