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Harvard Forest Symposium Abstract 2007

  • Title: Changes in water extractable soil carbon and iron in the chronic nitrogen fertilization plots
  • Primary Author: Bryan Dail (University of Maine)
  • Abstract:

    Studies of acid deposition to forest soils have shown multiple consequences for soil chemical and biological properties. These consequences include pH decrease and the mobilization of Fe, Al, and heavy metals which can lead to plant and microbial decline, a decrease of base saturation, and a decrease in organic matter processing/mineralization. The forest floor (litter and O horizons) typically have a much larger capacity to buffer incoming acidity and can consume protons through cation exchange, and through protonation of organic matter. An additional acidity-consuming process is reductive formation of Fe2+ and Mn2+ which can then leach from the soil. In an O horizon, some of this iron is likely to be organic in nature (from decomposing plant remains) or result from mixing from underlying mineral horizons. O horizons may neutralize much of the precipitation-borne acid, and thus protect underlying mineral soils unless these processes are exceeded by chronic or concentrated inputs. Chronic acid deposition may outstrip both the inherent cation exchange capacity and, by way of causing a reduction in plant and microbial production, reduce replenishment of the buffering capacity of the forest floor matter. Indeed, plant decline is evident in a red pine plantation at the Harvard Forest Chronic N deposition site which, along with a mixed hardwood site, have received 50 and 150 kg N/ha/yr since 1990. A signal of acid inputs exceeding the soil buffering capacity would be changes in the export of dissolved organic matter and soil metals key to buffering the acidity.

    We measured water extractable C and Fe from O-horizon and mineral soils obtained from the Harvard Forest Chronic N deposition plots and note significant changes in these potentially acid modifiable soil fractions. Extractable C appears to be declining in the treated red pine plantation O-horizon but on the increase in the underlying mineral soils (Fig 1). However, very little difference was apparent in the forest floor and soils under a mixed hardwood forest. Differences in extractable C from the pine plantation soils were significant only at the highest fertilization rate (~150 kg N/ha/yr) and may indicate a mortality-related depletion of litter inputs to the forest floor, but an increase in mobility of existing DOC through the soil mineral horizons. Extractable iron was more complex in regard to horizons and treatments differences under both forest types. Our expectations were that a decrease in pH would lead to an increase in soluble Fe. However, O horizon soils under pine showed a significant decrease in soluble Fe consistent with increased N treatment and decreased pH (pH data not shown). The reverse was the case with N treatment and soluble Fe in the hardwood forest. Mineral soils from both forest types showed declines in the extractable Fe fraction consistent with a depletion of Fe available to buffer acid inputs. The decoupling of the soluble Fe fraction in the hardwood O horizon (increased with N inputs) and mineral horizons (decreased with N inputs)may be a result of different proximate sources of iron in these two horizons. Since primary production in the hardwood forest has not declined as in the pine plantation, past pH-driven increases of soluble Fe in the rooting zone of mineral soils may have led to an increase in plant available Fe forms which is now cascading to the forest floor as Fe is either leached from the canopy or as a result of Fe enriched litter inputs.

  • Research Category: Soil Carbon and Nitrogen Dynamics

  • Figures:
  • Dail_Figure 1.pdf
    Dail_Figure 2.pdf