Fifteen-Year Change in Forest Floor Organic and Element Content and Cycling at the Turkey Lakes Watershed

To assess the long-term effects of atmospheric deposition on forest floor chemical composition, we took quantitative samplings of L-(Oi), F-(Oe), and H-(Oa) layers at an old-growth sugar maple-yellow birch stand on a till soil at the Turkey Lakes Watershed near Lake Superior, Ontario, Canada, in 1981 and 1996. We then assessed these samples for contents of organic matter (OM), total N, K, Ca, Mg, S, and Na, and exchangeable NH4 +, NO3 -, K+, Ca2+, Mg2+,${\rm SO}_{4}{}^{2-}$, and Na+. Over the 15-year period, total OM and element contents remained unchanged, with the exception of N, which increased significantly from 61.3 kmol/ha in 1981 to 78.4 kmol/ha in 1996. On an area basis, there were significant increases in exchangeable Ca2+(from 3.8 to 4.6 kmol/ha) and Na+(from 0.05 to 0.08 kmol/ha) and decreases in exchangeable${\rm NH}_{4}{}^{+}\text{-}{\rm N}$(from 1.41 to 0.95 kmol/ha) and${\rm SO}_{4}{}^{2-}\text{-}{\rm S}$(from 1.29 to 0.96 kmol/ha). There were no significant differences in average annual litterfall OM, N, Ca, Mg, S or Na inputs between 1980 and 1985 and between 1992 and 1997. Average annual wet-only${\rm SO}_{4}{}^{2-}\text{-}{\rm S}$deposition during 1981-86 was 0.30; during 1992-97, it was 0.21 kmol/ha. Annual wet-only${\rm NO}_{3}{}^{-}\text{-}{\rm N}$averaged 0.33 kmol/ha during 1981-86 and was similar during 1992-97. Throughfall was less rich in${\rm SO}_{4}{}^{2-}$and Ca2+, Mg2+, and Na+during 1992-97 than earlier. Throughfall NH4 +and NO3 -fluxes were unchanged. Efflux of cations from the forest floor reflected reduced throughput of${\rm SO}_{4}{}^{2-}$. Overall, the results suggest that in spite of atmospheric inputs, active biological processes-including litter input, fine-root turnover, and tree uptake-serve to impart stability to the mineral composition of mature sugar maple forest floor.