Differential effects of sugar maple, red oak, and hemlock tannins on carbon and nitrogen cycling in temperate forest soils |
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Authors: | Jennifer M Talbot Adrien C Finzi |
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Institution: | (1) Department of Biology, Boston University, 5 Cummington Street, Boston, MA 02215, USA |
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Abstract: | Tannins are abundant secondary chemicals in leaf litter that are hypothesized to slow the rate of soil-N cycling by binding
protein into recalcitrant polyphenol–protein complexes (PPCs). We studied the effects of tannins purified from sugar maple,
red oak, and eastern hemlock leaf litter on microbial activity and N cycling in soils from northern hardwood–conifer forests
of the northeastern US. To create ecologically relevant conditions, we applied tannins to soil at a concentration (up to 2 mg g−1 soil) typical of mineral soil horizons. Sugar maple tannins increased microbial respiration significantly more than red oak
or hemlock tannins. The addition of sugar maple tannins also decreased gross N mineralization by 130% and, depending upon
the rate of application, decreased net rates of N mineralization by 50–290%. At low concentrations, the decrease in mineralization
appeared to be driven by greater microbial-N immobilization, while at higher concentrations the decrease in mineralization
was consistent with the formation of recalcitrant PPCs. Low concentrations of red oak and hemlock tannins stimulated microbial
respiration only slightly, and did not significantly affect fluxes of inorganic N in the soil. When applied to soils containing
elevated levels of protein, red oak and hemlock tannins decreased N mineralization without affecting rates of microbial respiration,
suggesting that PPC formation decreased substrate availability for microbial immobilization. Our results indicate that tannins
from all three species form recalcitrant PPCs, but that the degree of PPC formation and its attendant effect on soil-N cycling
depends on tannin concentration and the pool size of available protein in the soil. |
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Keywords: | Tannin Nitrogen cycle Carbon cycle Polyphenol– protein complex |
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