Microbial activity, nutrient dynamics and litter decomposition in a Canadian Rocky Mountain pine forest as affected by N and P fertilizers

To study the specific effects of N and P fertilizers on soil microbial processes under field conditions, a pine forest in southern Alberta was fertilized with ammonium nitrate and urea (0 and 188 kg N ha(-1), respectively) singly and in combination with triple super phosphate (0 and 94 kg P ha(-1), respectively). Microbial respiration (BR), substrate induced respiration (SIR), metabolic quotient (qCO(2)) and rates of microbially mediated key processes were monitored in the forest floor FH material during the growing periods of spring 1990 to fall 1992. A transient increase in soil NH4+ availability was detected following N addition but significant increases in PO43- availability were detected throughout the study as a result of P enhancement. Microbial variables were unaffected by N addition, whereas soil organic matter and SIR was increased with P fertilization. Microbial BR and qCO(2) were reduced with P fertilization suggesting more efficient utilization of C. Increased net mineralization of P in the P-fertilized plots was found during the second and third summers after fertilization, following immobilization of P during the first year. In contrast, NH4+-N mineralization in the N-fertilized plots was significantly increased only during the first year. Rates of nitrification were unaffected by either N or P addition. Decomposition of pine litter was enhanced with N addition only during the first year and P had no effect on decomposition, Addition of N and P at these rates does not appear to impose a major ecological stress to the soil ecosystem. (C) 2002 Elsevier Science B.V. All rights reserved.