Abstract
Abstract. Soils are globally significant sources and sinks of atmospheric CO2. Increasing the resolution of soil carbon turnover estimates is important for predicting the response of soil carbon cycling to environmental change. We show that soil carbon turnover times can be more finely resolved using a dual isotope label like the one provided by elevated CO2 experiments that use fossil CO2. We modeled each soil physical fraction as two pools with different turnover times using the atmospheric 14C bomb spike in combination with the label in 14C and 13C provided by an elevated CO2 experiment in a California annual grassland. In sandstone and serpentine soils, the light fraction carbon was 21–54% fast cycling with 2–9 yr turnover, and 36–79% slow cycling with turnover slower than 100 yr. This validates model treatment of the light fraction as active and intermediate cycling carbon. The dense, mineral-associated fraction also had a very dynamic component, consisting of ∼7% fast-cycling carbon and ∼93% very slow cycling carbon. Similarly, half the microbial biomass carbon in the sandstone soil was more than 5 yr old, and 40% of the carbon respired by microbes had been fixed more than 5 yr ago. Resolving each density fraction into two pools revealed that only a small component of total soil carbon is responsible for most CO2 efflux from these soils. In the sandstone soil, 11% of soil carbon contributes more than 90% of the annual CO2 efflux. The fact that soil physical fractions, designed to isolate organic material of roughly homogeneous physico-chemical state, contain material of dramatically different turnover times is consistent with recent observations of rapid isotope incorporation into seemingly stable fractions and with emerging evidence for hot spots or micro-site variation of decomposition within the soil matrix. Predictions of soil carbon storage using a turnover time estimated with the assumption of a single pool per density fraction would greatly overestimate the near-term response to changes in productivity or decomposition rates. Therefore, these results suggest a slower initial change in soil carbon storage due to environmental change than has been assumed by simpler (one-pool) mass balance calculations.
Highlights
Soil organic matter (SOM) is a complex mixture of substances having turnover times ranging from days to millennia (Trumbore and Czimczik, 2008)
We investigated two soil physical fractions, the mineral-associated, dense fraction and the light fraction, and determined turnover times when modeled as two pools in each fraction
Compared to the dense fraction (DF), the light fraction (LF) was more carbon-rich, had a wider C / N ratio, and, in the elevated CO2 plots, had incorporated more of the fossil fuel isotopic signature (Table 2). These results are consistent with the LF consisting of plant material in an early stage of decomposition that is on average cycling more quickly than is the DF
Summary
Soil organic matter (SOM) is a complex mixture of substances having turnover times ranging from days to millennia (Trumbore and Czimczik, 2008). Models with a limited number of pools or SOM categories do a good job of predicting steady-state carbon stocks, they may not be adequate for predicting responses to perturbation or climate change (Davidson et al, 2000; Derrien and Amelung, 2011). Increasing the resolution of soil carbon turnover estimates is important for understanding biogeochemical cycling of carbon, nitrogen, and other SOM-related elements, as well as predicting the response to environmental change (Schimel et al, 1994; Trumbore, 2000; Torn et al, 2009). Torn et al.: A dual isotope approach to isolate soil carbon pools
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