Strong root respiration response to nitrogen and phosphorus addition in nitrogen-limited temperate forests.

Nutrient availability is one of the key regulator of the global forest carbon balance. The use of fossil fuels and fertilizers has increased the amount of biologically reactive nitrogen (N) in recent decades and N fertilization also changes the availability of other nutrients such as phosphorous (P) and potassium (K). The increased soil nutrient availability is known to stimulate forest growth, but we currently lack comprehensive understanding of the response of soil respiration and its three components (roots, microbes, and ectomycorrhizal (ECM) fungi) to the increased soil N, P and K availability. We conducted a 4-year field fertilization experiment with N, P and K addition in an N-limited temperate forest and separated ECM fungi respiration (Rm), root respiration (Rr) and heterotrophic microbial respiration (Rh) from total soil respiration. Our results showed that Rr increased with N and P addition while Rh and Rm did not respond to nutrient addition. Rm, Rr and Rh varied substantially from year to year, but their responses to nutrient addition did not fluctuate in different years. Our results indicate that in N-limited forest ecosystems, Rh and Rm may not respond substantially to future changes in nutrient addition and that inter-annual variation in climate may be the determinant of soil CO2 efflux in response to global changes.