Inconsistent autotrophic respiration but consistent heterotrophic respiration responses to 5-years nitrogen addition under natural and planted Pinus tabulaeformis forests in northern China

Abstract

Increased atmospheric N deposition is known to have significant effects on soil respiration (Rs) in natural and planted forest ecosystems. Responses of Rs to such N enrichment has been widely investigated in forest ecosystems, both in natural and planted forests. As natural and planted forests differ in many aspects (e.g. species composition, forest management and succession stage, soil properties etc.), the responses of Rs in natural and planted forests to N addition may be different. However, so far, few studies have made a direct comparison between natural versus planted forests. To fill this gap, we have examined how autotrophic (Ra), heterotrophic (Rh) and total Rs respond to experimental N addition in a natural and a planted Pinus tabulaeformis forests in northern China. Three levels of N addition (CK = 0, LN = 50 and HN = 100 kg N ha−1 yr.−1) were applied monthly over five growing seasons. Soil respiration and its components, soil properties, microbial biomass (MBC), enzymes activity and fine root biomass were measured. After 5-years of N addition, our results showed that: (1) for all three treatments, mean annual Rs of the planted forest were significantly higher than that of the natural forest; (2) in the natural forest, mean annual Rh was reduced by 16.8 and 28.3% under LN and HN treatments, respectively, whereas in the planted forest, mean annual Rh was reduced by 14.4 and 18.3% under LN and HN treatments, respectively. (3) mean annual Ra was increased by 47.6 and 59.5% under LN and HN, respectively, in the natural forest. In contrast, in the planted forest, LN and HN both enhanced Ra to the same rate. In both natural and planted forests, the inhibition of Rh was largely associated with the decreased microbial biomass C (MBC) and reduced activity of the cellulose degrading enzyme. However, inconsistent patterns of Ra to N addition of different intensities in the natural vs. planted forests might be due to root ammonium toxicity under high N-availability scenarios. We demonstrated that the natural and planted forests may differ in their Rs responses to N addition, depending on different responses of Ra. Our results may have potential implications for forest management and predicting forest ecosystem carbon balance under future N scenarios.