N and P fertilization effects on soil respiration and microbial respiration in northern hardwood forests of New Hampshire

Background/Question/Methods

Anthropogenic N deposition has increased nitrogen availability in forests, which has the potential for altering belowground carbon cycle. In a previous study, we found that soil respiration and belowground carbon allocation increased with infertile stands having low N and Ca availability. However, it is not clear which component of soil respiration between root- and microbial- respiration contributes the soil respiration in different nutrient availability. Also, it is not clear how soil respiration is affected by P, another important nutrient in temperate forests. In this study, we fertilized N and P in 13 stands including three age classes and three different fertility sites. We measured soil respiration in the field and microbial respiration in organic horizons in the lab in all 13 stands. In five stands, we measured microbial respiration in trenches to remove root respiration from the soil respiration.

Results/Conclusions

Field measurements of soil respiration and microbial respiration were not different among control, N, P, and NP plots (a = 0.05). In the trenched plots in the field, the portion of microbial respiration to soil respiration was the highest in NP plots and the lowest in control and N plots. Microbial respiration in lab incubations Oa horizon material was the lowest in NP plots (p = 0.01). The field soil and microbial respiration was higher in sites with high N availability, but there was no relationship with soil P or Ca. The nutrient availability on the lab microbial respiration differed by horizon and by soil moisture condition; dry soils in Oe horizon were positively related to nutrient availability, while wet soils in Oa horizon was negatively related to nutrient availability. In this study, soil respiration is not affected by N and P availability over the short term. However, it is likely that fertilization effects may start from microbial respiration in organic horizon and N availability is the most important factor in this forest.