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Harvard Forest Symposium Abstract 2015

  • Title: What makes stable soil carbon? Insights from DIRT
  • Primary Author: Kate Lajtha (Oregon State University)
  • Abstract:

    Soil ecologists have long argued that models of global carbon dynamics may not be accurately capturing soil organic matter dynamics and the response of soil carbon pools to changes in temperature, precipitation, or land use. Because soils contain more than three times as much as carbon as the atmosphere, and four and a half times more carbon than the world’s biota small changes in soil organic matter stability could result in a large effect on atmospheric carbon pools.

    Soil carbon has been shown to be sensitive to soil texture and mineralogy, as well as to disturbances such as tilling or forest harvest, and most global models try to capture these dynamics. However, most models also assume a strong positive relationship between plant detrital inputs and carbon sequestration in soils, even though several studies have suggested that this relationship might be complex and non-linear. Research in agricultural systems has shown that soils have finite capacities to sequester C, and might “saturate”, or achieve maximum equilibrium levels under different combinations of soil texture, mineralogy, and climate. Similarly, increases in labile carbon inputs to soil can cause disproportionate increases in microbial respiration rates, known as priming, causing a paradoxical decrease in soil carbon pools with increases in high quality litter inputs.

    The Detritus Input and Removal Treatment (DIRT) experiment was designed to assess how rates and sources of plant litter inputs control accumulation and dynamics of organic matter in soils over decadal time scales. The Harvard Forest plots are now 25 years old, representing the oldest of the DIRT sites still active, and will be resampled in 5 years. Treatments include: Control, Double Litter (DL), No Litter (NL), No Roots (NR), and No Inputs (NI). After 20 years of manipulation, doubling litter inputs did not increase surface soil C content, in contrast to predictions. Similarly, doubling litter inputs did not increase either light or heavy density fraction pools of carbon, or any measures of labile soil carbon. However, the activities of two key enzymes (β-glucosidase, phosphomonoesterase) increased 30% with litter additions, suggesting that litter additions stimulated microbial activity, and likely caused significant priming of old soil carbon by labile carbon inputs. Although some DIRT sites (in PA and OR) also show signs of priming of old soil organic matter by fresh C inputs, not all sites do, suggesting key site differences that we have not yet discovered.

    Exclusion of either aboveground litter or root inputs resulted in sharp declines in O-horizon carbon content, but smaller decreases in total mineral soil carbon. Above-ground leaf litter exclusion resulted in a 19% decline in total profile mineral soil carbon whereas root exclusion resulted in a 9% decline, indicating the importance of aboveground inputs to long-term carbon pools. This was unexpected, as many studies – and many models – suggest that root inputs are more important to soil carbon sequestration than are above-ground inputs, as above-ground inputs are more efficiently respired than stabilized. We concluded that soil carbon pools in forests may not respond linearly or immediately to aboveground or belowground litter inputs, and thus efforts to sequester carbon by managing productivity and associated litter inputs will not likely result in increased carbon storage over short time frames.

    Our research in the next few years will be to synthesize existing DIRT data cross-network using a model developed by Ben Sulman that suggests that soil texture might explain when old soil C might undergo priming by labile C inputs from roots and litter. We also hope to expand the DIRT network by working with the extensive NutNet network of grassland sites and 3 more LTER sites, and to establish additional non-US DIRT sites in a broad range of forested sites.

  • Research Category: Large Experiments and Permanent Plot Studies; Soil Carbon and Nitrogen Dynamics