Influence of Non-nitrogenous Soil Amendments on Soil CO2 Efflux and Fine Root Production in an N-Saturated Northern Hardwood Forest

Non-nitrogenous mineral nutrients may be an important constraint on forest productivity and belowground processes in many ecosystems. We measured responses of soil CO2 efflux (FCO2), fine root production, and root-free incubation soil respiration to experimental additions of non-nitrogenous mineral nutrients (phosphorus (P) + potassium (K) fertilizer, dolomitic lime, and P + K plus lime) over 2 years in a sugar-maple-dominated forest in central Ontario; this region receives some of the highest anthropogenic nitrogen (N) inputs in North America, and evidence exists for co-limitation by P, magnesium (Mg), and calcium (Ca) of the growth of dominant trees. Soil amendments, in particular P + K fertilization, reduced FCO2, fine root production and microbial respiration, with decreases in FCO2 of 28-51% in fertilized compared to control plots. Partial regression analyses indicated that soil available P had a negative effect on FCO2, fine root production, and microbial respiration, but detected no significant effects of N, Ca, or Mg. Path analysis further suggested that available P reduced both fine root production and microbial respiration, and that these effects were largely responsible for reduced FCO2. There was also a residual direct negative relationship between available P and FCO2, which may represent reduced metabolic activity of roots. The study indicates that P is a critical nutrient dominating belowground processes in an N-saturated forest ecosystem, and suggests that additions of P may enhance C sink strength in managed forests in part through reductions in soil CO2 efflux.