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Soil respiration is the largest single efflux in the global carbon cycle and varies in complex ways with climate, vegetation, and soils. The suppressive effect of nitrogen (N) addition on soil respiration is well documented, but the extent to which it may be moderated by stand age or the availability of soil phosphorus (P) is not well understood. We quantified the response of soil respiration to manipulation of soil N and P availability in a full-factorial N x P fertilization experiment spanning 10 years in 13 northern hardwood forests in the White Mountains of New Hampshire, USA. We analyzed data for 2011 alone, to account for potential treatment effects unique to the first year of fertilization, and for three 3-year periods; data from each 3-year period was divided into spring, summer, and fall. Nitrogen addition consistently suppressed soil respiration by up to 14% relative to controls (p £ 0.01 for the main effect of N in 5 of 10 analysis periods). This response was tempered when P was also added, reducing the suppressive effect of N addition from 24 to 1% in one of the ten analysis periods (summer 2012–2014, p = 0.01 for the interaction of N and P). This interaction effect is consistent with observations of reduced foliar N and available soil N following P addition. Mid-successional stands (26–41 years old at the time of the first nutrient addition) consistently had the lowest rates of soil respiration across stand age classes (1.4–6.6 lmol CO2 m-2 s-1), and young stands had the highest (2.5–8.5 lmol CO2 m-2 s-1). In addition to these important effects of treatment and stand age, we observed an unexpected increase in soil respiration, which doubled in 10 years and was not explained by soil temperature patterns, nutrient additions, or increased in fine-root biomass.