Effect of Urea Fertilization on Nitrification in Forest Soils of the Pacific Northwest
1974
Heilman, Paul
Nitrate levels in untreated samples of upper mineral soil from 14 Douglas-fir (Pseudotsuga menziesii) and western hemlock (Tsuga heterophylla) sites incubated at 20C for 21 days varied from 0 to 45 ppm N. No relationship of nitrification to soil pH was evident but N and organic C content appeared to be associated with nitrate production; for nitrate versus total N, r = 0.85, and versus organic C, r = 0.75, while nitrate versus C:N ratio, r = −0.38. Little nitrate was produced in soils with C:N ratios above 27. Treatment of the soils with urea at approximately 400 ppm N increased nitrate levels in similarly incubated samples by an average of 70 ppm N. The increases varied considerably with two soils still containing less than 4 ppm N as nitrate and four with over 100 ppm nitrate N (one exceeded 300 ppm N). N fertilization in the field markedly increased capability of the majority of soils tested to form nitrate following subsequent urea treatment. Low temperature (2.5 and 5C) restricted nitrification whether or not soils were treated with urea. Generally, rate of nitrification in urea-treated soils increased rapidly with increase in temperature above 5C. The patterns of increase in nitrate with increase in temperature and time suggest that low nitrification capability in some soils results primarily from low population of nitrifying organisms and that nitrate levels following urea treatment are linked to growth of the population of nitrifying organisms in response to a large supply of ammoniacal N.
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