Response of forest soil respiration to nutrient addition depends on site fertility

Flux of CO₂ from the forest soil surface ((Formula presented.)) reflects the activity of roots and microbes responding to plant and soil properties that are influenced by global changes such as nitrogen deposition and increasing temperature and atmospheric CO₂. We added low levels of N (3 g/m²-year), P (1 g/m²-year) or N + P to thirteen northern hardwood stands of different age and soil N cycling and measured soil respiration, microbial respiration and fine root turnover. We hypothesized that soil respiration would decline in response to nutrient addition, but that this response would vary depending on forest age and N cycling rate. Soil respiration was significantly higher in successional (<40-year-old) than mature stands (>90-year-old). Overall, no significant treatment effects or age x treatment interactions were observed. However, on an individual stand basis, significantly lower soil respiration was observed in nutrient addition plots at four of the most infertile sites. Over half of the variation in the response ratio (fertilized-control/control) of soil respiration to fertilization was explained by using pre-treatment N cycling rate as a predictor: i.e., the greatest reduction in soil respiration on N and N + P fertilized plots occurred on the sites with lowest pre-treatment soil N mineralization and litterfall N flux. Nutrient additions did not significantly affect either fine root turnover (minirhizotrons) or microbial respiration (laboratory incubations). Perhaps responses of fine root biomass or rhizosphere C flux influenced the  response of soil respiration to increasing soil fertility.