Effects of Nitrogen and Phosphorus Addition on Soil Respiration in Northern Hardwood Forests

Abstract

Soil respiration - the CO2 efflux from the forest soil surface - is an important indicator of root and microbial activity and is sensitive to global changes such as climate warming, anthropogenic nitrogen deposition and elevated atmospheric CO2. I evaluated the response of total soil respiration (TSR) to changes in soil nutrient availability in temperate deciduous forests in New Hampshire. Low-level N (3 g/m2/year), P (1 g/m2/year) or N + P have been applied annually to thirteen northern hardwood stands of different age and site quality since 2011. My analysis of TSR for 2013, 2014, 2016, and 2017 confirmed the overall suppression effect of N addition across these stands (p< 0.001), but the responses varied considerably among stands. No consistent effects of P addition on TSR were detected, but a significant interaction between N and P and forest age was observed (P=0.04). No correlation was detected between the TSR response ratio ((treatment-control)/control) and either pre-treatment soil fertility indexes, or the response ratio of soil microbial respiration measured in the laboratory. Overall, the significant interaction of N and P and forest age suggests that the responses of TSR rates in northern hardwood forests depend on forest age as well as the nutrients applied (N, P, or N+P). Perhaps the variation in the response of TSR to nutrient additions among the stands might be attributed in part to differences in root respiration.