Microbial respiration, biomass, metabolic quotient and litter decomposition in a lodgepole pine forest floor amended with nitrogen and phosphorous fertilizers

In order to elucidate the mechanisms by which N and P fertilizers affect below ground microbial processes, ammonium nitrate (AN) and urea (U) (0, 188, 300 kg N ha−1) singly and in combination with triple super phosphate (TSP), (0, 94, 150 kg P ha−1) were added to pine F/H material under laboratory conditions and certain microbial variables were monitored over 120 d. Ammonium nitrate significantly suppressed basal and substrate induced respiration (SIR) when added singly or in combination with TSP at all sampling times. A temporary (d 10) increase in qCO2 values were produced by AN, but significantly lower values were detected after 40 d of incubation. Litter decomposition was suppressed by AN after 40 d. Although transient enhancements in basal respiration and SIR were caused by U, significant suppressions of these measurements were detected in the later phases of the experiment. Urea caused significant elevations in qCO2 values and enhanced rates of litter decomposition only at d 20 and 40. Inhibitory effects of TSP on microbial respiration and SIR were observed only when added singly and no effects on litter decomposition were found. The observed changes in microbial indices as a result of fertilizer addition could not be directly related to changes in soil pH. These laboratory results suggest that the present recommended rates of fertilizers suppress microbial activity.