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Plant Uptake, Microbial Immobilization, and Residual Soil Fertilizer of Urea-Nitrogen in a Grass-Legume Association¹

ABSTRACT

A field experiment throughout three growing seasons was conducted to evaluate the fate of urea-N in a grass-legume association. The urea N removed by the forage accounted for 25.0, 2.7, and 1.4% of the total applied N in 1980, 1981 and 1982, respectively. The legume forage contained significantly less fertilizer N than the grasses and weeds. About 59% of the initially applied N was accounted for in the 0 to 40-cm topsoil layer at the end of the first cropping season, and a large proportion of residual fertilizer N (about 86.5%) was present as organic forms. Of the processes following the spring application of urea, microbial immobilization was largely prevailing and limited downward movement, plant uptake and losses. Soil biomass C and N, determined by the chloroform fumigation-incubation method, showed the highest values 29–59 d after the application of urea. Microbial biomass represented a considerable fraction of residual fertilizer N in soil accounting for 10.3, 11.9, and 19.3% of the total 59, 535, and 822 d after the application, respectively.

¹ Contribution from the Institute for Soil Chemistry, C.N.R., Via Corridoni 78, 56100 Pisa, Italy.

² Soil Scientist, Research Associate and Graduate Student, Institute for Soil Chemistry, C.N.R., Via Corridoni, 78, 56100 Pisa, (Italy).

Received for publication January 4, 1984. Accepted for publication October 17, 1984.