Thursday, 13 July 2006 - 5:00 PM

Chemical Signatures of Land Use History.

Daniel Markewitz, The Univ of Georgia, Athens, GA 30602-2152 and Daniel D. Richter Jr., Duke Univ, Box 90328, Durham, NC 27708-0328.

Signatures of long-continued land-use can be highly significant both chemically and morphologically. These changes include homogenized surface soils due to long-term cultivation, accelerated erosion, and altered soil fertility. These signatures of land-use history can be long lasting. As such, assessing the historical roots of chemical and morphological signatures and how these imprints might impact on our current interpretations of soil change is an important challenge for future soil studies. Here we use the long-studied Calhoun Experimental Forest in South Carolina, USA to identify specific chemical signatures that we interpret as resulting from previous land-use and demonstrate how these imprints affect present conclusions regarding soil change. The soils of the Calhoun Experimental Forest are acidic Ultisols. Surficial A and E horizons of these soils are relatively coarse (e.g., sandy loams) and if not severely eroded, can be relatively deep (>40 cm). Below the quartz-dominated coarse horizons, deep, clayey B horizons are acidic, composed mainly of kaolinitic clay minerals, quartz, and iron- and aluminum-oxy-hydroxides. On upland sites, vegetation prior to about 1800 was predominantly deciduous forest. The SE Piedmont of the USA was subdivided by royal and state grants in the mid-18th century, but in the first decades of the 19th century, the upland forests were extensively cleared, burned, and converted to agricultural fields. From about 1800 to the U.S. Civil War in the 1860s, agricultural fields were managed for cotton (Gossypium spp.), corn (Zea mays), tobacco (Nicotiniana tabacum), and other crops, generally with minimal amounts of fertilization. Following several years of cropping, farmers often abandoned their fields moving on to “fresh soil”. After the Civil War, agricultural fields in the Piedmont were more continuously cropped. As systems of sharecropping and tenant farming developed, fertilization and liming became more standard farm practices. Cotton production increased throughout the South until the 1920s when farms throughout the region began to be abandoned in large number. Pine forests expanded in area and in their volume of wood throughout much of the 20th century, due to regeneration in abandoned old fields and maturation. In the early 21st century, most old-cotton-fields in the Southern Piedmont are currently pine or mixed hardwood-pine forests, hayfields, and pastures. One of the more prominent signatures of land use history in the Southern Piedmont is soil erosion. Loss of topsoil throughout the region was dramatic. The interfluve landscape position of the Calhoun Forest, however, limited topsoil loss, although variation in coarse textured surface soil depth across the site is apparent and consistent with landscape position. The persistence of surface soil at this site had an important impact on soil accumulation of ~2 Mg-C ha-1 in the upper 0-15 cm over 35 yr after agricultural abandonment. This relatively slow rate of accumulation was attributed to the coarse textured surface soils that ensure ample aeration and decomposition with little protection of C through adsorption. As opposed to the observed increase in C, soil N was strongly depleted by ~800 kg ha-1 in the upper 0-60 cm of mineral soil. This translocation of N from soil to biomass, however, likely only resulted due to the large increase in N bioavailability wrought by agricultural fertilization. In the absence of N fertilization the rate of regrowth of the pine forest would likely have followed a different trajectory. The signatures of historical P and Ca fertilizer were also apparent in soil comparisons among pine forest, pasture, and hardwood land uses. Extractable P of 6-8 ėg g-1 in upper surface soil (0-30 cm) of pine and pasture are 3 to 4-fold greater compared to hardwood forest (~2 ėg g-1). Similarly, exchangeable Ca in 0-30 cm (~ 8 mmolc kg-1) under pine and pasture exceeded those in hardwood forest (~2 mmolc kg-1). In the case of Ca these concentration differences persist to depths >1 m. These historic signatures have influenced interpretations of soil change over the last four decades. Soil P dynamics in surface soils during this time indicate little change in Mehlich III extractable P, a readily bioavailable form, despite increases of ~80 kg ha-1 in forest biomass. Conversely, 1 M and concentrated HCl extractable pools of P have declined during this same period. Ironically, it is exactly these pools of acid extractable P that are generally absent in previously unfertilized soils in the region. In the absence of previous fertilization the P dynamics over these decades might well tell a different tale. Finally, exchangeable Ca at this site has declined in the 0-15 cm layer by an order of magnitude from ~4 to <0.5 mmolc kg-1. The acidification of soils at the Calhoun has been demonstrated to partly result from inputs of acidic deposition. These results might lead to claims of soil degradation under increasing atmospheric deposition if not for the recognition that the dynamics of Ca have been wholly altered by the historic use of the site. The impact of acidic deposition on soil of a native condition (i.e., low exchangeable Ca) is difficult to infer from the current observations. Clearly, in all these cases, how decade scale changes are to be interpreted is dependent on a clear understanding of the previous uses of the site and its remaining chemical and morphological signatures.

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