The potential for an increase in the intensity of forest management for biomass fuel over the coming decades makes it important to determine the plant-available stocks of nutrients throughout the northeastern United States. If forests are to continue regenerating, the large amounts of N, P, Ca, Mg, and K removed from the ecosystem by forest harvests must be replaced by a combination of atmospheric inputs, the depletion of plant-available nutrients stocks in the rooting zone, weathering of primary minerals, and fertilizer inputs.
We use a regional dataset from 42 quantitative soil pits in which exchangeable, organic, and mineral nutrient contents were measured in 14 northern hardwood stands across the White Mountain region of New Hampshire. We conduct a simple rotation analysis to determine the number of harvest rotations (under three intensity regimes) required to exhaust the available soil stocks of N, P, Ca, Mg, and K in the soil across the region. Atmospheric deposition was included as a source of available nutrients but we ignored baseline and harvest-induced nutrient leaching, and generally overestimated stocks of available nutrients (e.g. the entire soil organic matter pool), so our results likely overestimate the number of rotations possible without biotically accelerated weathering.
Results/Conclusions
At current N deposition rates, all modeled rotations (30-year whole-tree harvest, and 100-year stem-only and whole-tree harvests) would be sustainable in terms of N at all sites in the region. However, calcium limitation was encountered after only 1-4 rotations under the 100-year whole-tree harvest scenario, and after 2-10 rotations in the 100-year stem-only scenario. Under the 30-year whole-tree harvest scenario, several sites encountered P limitation before Ca limitation, in as little as 1 rotation. However, the potential ability of ectomycorrhizal fungi to weather the primary mineral apatite in the B or even C horizons may offer an escape from Ca and P limitation, though likely at a cost to aboveground production. Regenerating forests in the region already show some signs of P limitation and increased apatite weathering rates. Of the mineral-derived nutrients, Mg and K seem to be the least susceptible to biotic depletion of the exchangeable and organic pools, but could ultimately become limiting because they are not found in minerals as easily weathered as apatite. All stands were located on granitic till, but variations soil depth and in parent material mineralogy drive a fivefold difference in available Ca, and a tenfold difference in weatherable Ca.