Interactive effects of nutrient and forest age on total belowground carbon allocation in northern hardwood forests in the White Mountains of New Hampshire

Background/Question/Methods Soil is the largest terrestrial reservoir of carbon, containing an estimated 1,550 Pg and almost three times more carbon than is contained in global vegetation (~500 Pg). More studies have been focused on vegetation because soil carbon stock is complicated to study, represents a high spatial variation, and changes very slowly. However, the effect of forest ecosystems by disturbances or by forest management on belowground carbon exchange is at least as important as aboveground effects on the global carbon budget. This study aimed to compare soil carbon dynamics after forest harvest associated with variation in site fertility and forest ages, the most sensitive factors of belowground carbon allocation, in northern hardwood forests in the White Mountains of New Hampshire. We calculated total belowground carbon allocation (TBCA, g C m^-2) as soil CO₂ efflux minus litterfall C.

Results/Conclusions TBCA of mature and young stands was 1016 ± 73 and 872 ± 4 in Bartlett, 910 ± 15 and 811 ± 27 in Hubbard Brook, and 696 ± 45 and 658 ± 41 in Jeffers Brook. Mature stands were significantly higher than young stands in Hubbard Brook (P < 0.05), and other mature stands were also higher than young stands. More fertile stands had higher TBCA, and Jeffers Brook was significantly lower than other more fertile stands (P < 0.05). Fine root biomass (< 5 mm) was markedly higher in mature stands than young in all sites, but there was no difference among nutrient gradients. Seasonal patterns of soil CO₂ efflux in all stands showed a high correlation with soil temperature at 10 cm depth, but no correlation with soil moisture at 5 cm depth. This result suggests that soil C allocation patterns in a northern hardwood forests after harvesting are dependent on site fertility and stand age. Some sites were not significantly different by nutrient and age gradient, therefore, further studies investigating factors affecting soil C dynamics and continuous measurements are necessary.