Fine root biomass and soil respiration reflect belowground carbon allocation in response to nutrient limitation in New Hampshire northern hardwoods

Background/Question/Methods

Theory suggests that greater effort should be allocated to roots and mycorrhizae for nutrient acquisition in settings where nutrient availability is low.  Fine root biomass gives one indication of plant allocation to acquisition of belowground resources.  Soil respiration is another indicator, since it reflects carbon allocation to the growth and activity of roots and mycorrhizae.  We measured root biomass once and soil respiration for two growing seasons in three sites in the White Mountains of New Hampshire, USA: Jeffers Brook is the most fertile, Hubbard Brook is intermediate, and Bartlett is the least fertile.  At each site, we had one mature stand (~100 years post-harvest) and one young stand (~30 years post-harvest).  We also measured litterfall, to account for soil respiration not coming from allocation to roots and mycorrhizae. We quantified net N mineralization and resin-available P as indicators of fertility. We predicted that fine root biomass and belowground carbon allocation (soil respiration corrected for litterfall decomposition) would be higher in the less fertile sites and that differences between old and young stands would also reflect differences in nutrient availability with stand age.

Results/Conclusions

Fine root biomass was not very different across sites (within 11%, p = 0.78), but at all sites, it was higher in old stands than young stands (37 to 42%, p < 0.005).  As predicted, soil respiration was highest at Bartlett, (1103 g C/m²/yr in the old stand), intermediate at Hubbard Brook, and lowest at Jeffers Brook, (669 g C/m²/yr in the young stand) (p = 0.06).  Consistent with fine root biomass, respiration was higher in old stands than young stands (by 2 – 17%; p = 0.11-0.34).  Belowground C allocation, calculated by subtracting annual litterfall from soil respiration and assuming no change in soil C storage, was highest at Bartlett, (943 g C/m²/yr in the old stand), intermediate at Hubbard, and lowest at Jeffers Brook (648 g C/m²/yr in the young stand) (p < 0.08).  Soil respiration and belowground C allocation were highest in stands where potential N mineralization was lowest.  These results support the claim that forests should allocate more carbon belowground in ecosystems with low nutrient availability.