THE INFLUENCE OF SOIL MOISTURE ON UREA HYDROLYSIS AND MICROBIAL RESPIRATION IN JACK PINE HUMUS

Abstract

The influence of soil moisture on urea hydrolysis and microbial respiration in mixed L and F horizons of a Brunisolic soil under a boreal forest jack pine (Pinus banksiana Lamb.) stand was determined in an incubation study at 13 °C. Respiration rates in untreated soil were similar over a wide range of moisture contents below 300% (45% water-holding capacity). Significant decreases in respiration rates occurred only below 60% moisture content. Initial microbial response to urea increased as soil moisture rose from 20 to 300%. There was sufficient urease in pine humus to hydrolyze rapidly the equivalent of 200 kg urea-N∙ha⁻¹, when moisture was not limiting. During the drying of initially moist soil (340% H₂O) to 240% H₂O or lower, urea hydrolysis was retarded significantly and microbial respiration reduced by an average of 25% over that observed in a constantly moist soil.