Long-Term Soil Carbon and Nitrogen Analysis at the Calhoun LTSE from 1962 to 2017

Abstract

Soil carbon and nitrogen are critical to global element cycling, yet there is much to learn about how human-induced land use changes are dramatically altering these dynamics. This study examines 60-year changes in soil organic carbon (SOC) and soil organic nitrogen (SON) in the Calhoun Long-Term Soil-Ecosystem Experiment (LTSE) located in the Piedmont of South Carolina, USA. The site has transitioned from historical cotton cultivation to secondary loblolly pine forests and provides a unique opportunity to assess SOC and SON trends across 55 years (1962–2017) and four soil depths (0-60 cm). We hypothesize that SOC and SON initially decline during early forest development due to insufficient organic matter inputs and high N demand for forest growth, followed by gradual C and N accumulations, with trends differing greatly by soil depth. Using segmented regression analysis, the study identified breakpoint years where accumulation was initiated. Results revealed significant depth-dependent patterns: surficial soils (0-15 cm) exhibited a rapid decline followed by earlier recovery, whereas deeper layers (15-60 cm) showed later and slower or negligible reccumulation after initial declines. Clearcutting in 2007 had no significant effect on SOC or SON across intact and clearcut plots, suggesting logging debris offset the lack of fresh organic matter inputs at the beginning of reforestation. Overall, this study highlights the complexities of SOC and SON dynamics in forested landscapes, particularly in response to historical land-use legacies and forest management practices.