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Nitrogen Changes in Forest Litter after Fertilization with Calcium-Urea

Texas A&M Univ., 1380 A&M Circle, El Paso, TX 79927

G. R. Gobran and G. I. Ågren

Dep. of Ecology and Environmental Research, Swedish Univ. of Agricultural Sciences, Uppsala, Sweden

*Corresponding author.

ABSTRACT

Urea used to fertilize acid forest soils can be lost by NH₃ volatilization. The objective of this research was to examine, in the laboratory, the impact of CaCl₂, applied with urea, on NH₃ losses from (i) forest floor litter (FFL) alone, (ii) FFL with CaCO₃, and (iii) wood ash (WA) amended FFL. Ammonia loss, urease activity, and N availability were measured as functions of the CaCl₂ treatments. Low NH₃ loss occurred during the first 10 d after application of urea; however, NH₃ loss increased in the 10 d after a second urea application. Application of CaCO₃ or WA to the FFL prior to urea fertilization caused slightly faster NH₃ losses than observed from FFL alone. Reuse of FFL with CaCO₃ (after extensive leaching) also resulted in very rapid NH₃ loss. When CaCl₂ was added with urea, however, at a molar ratio of 0.5 or 1.0, there were negligible NH₃ losses from either unamended FFL or FFL with CaCO₃. Low urease activity was detected in unamended FFL after urea application (16 °C). Higher urease activity was measured in FFL after treatment with a 0.25 molar ratio of CaCl₂/urea (1:4 Ca/urea), but NH₃ losses in the presence of CaCl₂ were extremely low or nondetectable. Urease activity in all WA treatments was high. Based on the amount of KCl-extractable N (NH₄, urea) and volatilized NH₃, it appeared that N was rapidly immobilized at 0.5 and 1.0 Ca/urea in both unamended FFL and FFL amended with CaCO₃. Recovery of urea-N was complete in all of the WA treatments and in the urea and 0.25 molar ratio of CaCl₂ + urea in FFL and FFL + CaCO₃. Extractable N and volatile NH₃ from the 0.5 and 1.0 soluble Ca/urea treatments (FFL and FFL with CaCO₃) accounted for less than one-half of the N applied. It was hypothesized that CaCl₂ at molar ratios > 0.5 with respect to urea stimulated biological immobilization of NH₄.

Contribution from the Swedish Univ. of Agricultural Sciences. Research supported by the National Swedish Environmental Protection Board.

Received for publication October 11, 1988.