Effects of land-use history on soil macro- and trace elements in the Piedmont of Southeastern North America.

摘要

Land use histories affect the rate and pattern of soil nutrients at local, regional, and global scales. However, few studies have focused on land-use change of soil trace elements (B, Mn, Zn, Cu and Fe). In this dissertation, the main focus was long-term biogeochemical cycling patterns and spatial heterogeneity of soil trace elements in response to land use change. With a statistically rigorous and spatially explicit design, we conducted studies in and around the USDA Forest Service's Calhoun Experiment Forest in SC, on soils that support uncultivated hardwood forests, cultivated fields, and old-field pine forests. Our first study indicated that spatial heterogeneity is greatly reduced in many soil properties by agricultural practices, but that old-field forest development on previously cultivated soils re-structured heterogeneity of soil properties within a few decades. We document cases in which land use alters both soil properties' central tendencies and their heterogeneity (C, N, CN, Ca, K, Fe, Mn and Zn), and one case in which changes are apparent in central tendency but less so in their heterogeneity (Db). In our second study, samples of the upper 0.6-m mineral soil archived in 1962 and 1997 revealed three cycling patterns of soil trace elements: (1) Extractable B and Mn were significantly depleted because tree uptake of B and Mn from mineral-soil greatly outpaced resupplies from atmospheric deposition, mineral weathering, and deep-root uptake. (2) Extractable Zn and Cu changed little during forest growth, indicating that nutrient resupplies kept pace with accumulations by the aggrading forest. (3) Oxalate-extractable Fe increased substantially during forest growth, by about 10-fold more than accumulations in tree biomass. This study indicated that forest Fe cycling is qualitatively different from that of other macro- and micro-nutrients. Thirdly, our results revealed that long-term cultivation substantially increased the crystallinity of soil iron oxides as indexed by oxalate and dithionite-citrate extractions, and that old-field pine forests diminished crystallinity indices of Fe oxides, presumably altering reactivity and function of Fe oxides in the ecosystem. Overall, this dissertation observes a wide range of responses to land use changes among the ecosystem's trace elements and macro-nutrients and helps illustrate the dynamics of soil systems on time scales of decades to centuries.