Soil carbon and nitrogen dynamics along replicated chronosequences of abandoned agricultural lands in southeastern Ontario

Abstract

Widespread abandonment of agricultural land has occurred in northeastern North America over the past two centuries. Soil carbon often increases as sites naturally regenerate towards perennial grasslands or forests. Understanding the large-scale controls on the potential and rate of soil carbon sequestration is necessary in order to evaluate the significance of this sink to the global carbon cycle. Furthermore, we need to understand the key roles soil microorganisms play in regulating ecosystem processes through their control over soil carbon and nitrogen dynamics. Such studies are rare at the century long time scale of temperate forest succession. Additionally, research has taken place primarily on productive agricultural soils, while abandonment is more common on marginal agricultural soils. We characterized patterns of total and labile soil carbon and nitrogen and microbial dynamics in mature forest and adjacent agricultural field sites, and in replicated chronosequences of forest successional sites on marginal soils of southeastern Ontario, Canada. Total soil carbon was significantly depleted in the top 10 cm of current agricultural fields as compared to forest sites and increased at a rate of 10 g C m-2 yr-1 across our 100-year chronosequences. There was no difference in carbon loss or accumulation over time in three soil types differing in texture and parent material, suggesting that time since abandonment is more important than soil type in determining carbon accumulation within this climatic region. In contrast, free-light fraction carbon did not increase over time and thus most carbon accumulated in pools with slower turnover times. Soil microbial biomass carbon and nitrogen increased significantly following abandonment and our results strongly suggest that microbial growth during all phases of succession was limited by carbon supply. In contrast, net nitrogen mineralization and nitrification rates during mid-summer did not change consistently over the first 100 years following agricultural abandonment. Therefore, inorganic nitrogen supply rates into the plant available pool were similar across the entire successional sequence. Together, the results of these two studies demonstrate the potential for carbon sequestration in abandoned agricultural soils across this climatic region and highlight the importance of plant-soil interactions for understanding carbon cycling during ecosystem development.