Can δ2H of Organic Matter in Soils be Used for Understanding Organic Matter Sources and Cycling?

Abstract:

Recent research has suggested that root/belowground organic matter (OM) is a more likely and important source of soil OM than is shoot/aboveground OM. Carbon and nitrogen isotopes have traditionally been used to study OM cycling in soils, even though above- and below- ground OM pools are not always isotopically distinct. Stable isotope ratios of hydrogen (δ²H) in roots and soil OM have recently been shown to provide an additional tracer for soil OM cycling (Ruppenthal et al., 2015). The δ²H values of roots are not influenced significantly by evaporative transpiration and therefore are more closely correlated to climate and precipitation δ²H values as compared to shoot OM. We found that the δ²H values of roots from near the surface were more negative by 8-40‰ relative to the δ²H values of fresh leaf tissues and surface litter. In soils, we combined physical density separation and isotopic (δ²H, δ¹³C, δ¹⁵N) analyses to understand how root-derived OM contributes to the OM stored in soils. The δ²H and δ¹³C of soil OM from 1) free, 2) occluded, and/or 3) mineral associated soil fractions in upper soil horizons (up to 1 m) will be presented and compared from a suite of soils collected from a CZO (Critical Zone Observatory) climosequence in the southern Sierra Nevada range (USA). Whether or not the δ²H of these soil fractions relates to climate and precipitation patterns will also be discussed.