•Errors in soil C pool analysis may propagate to models and broader conclusions.•Average mass loss from soil density fractionation (SDF) was 6%, C loss was 9%.•Potentially soluble C lost in SDF ...averaged 9% of the total C loss.•Particulate SDF losses were biased towards an underestimation of C in the POM pool.•Proportions of MAOM, free particulate, and soluble C did not differ seasonally.
To model global C budgets more accurately, we must better understand the dynamics and turnover of the many functional C pools that exist in soil. While soil density fractionation (SDF) is widely used to separate soil C pools based on the degree of stabilization by soil minerals, several studies have noted substantial losses of soil mass and soil C following SDF. As the source of these losses is unknown, they may lead to erroneous conclusions about SOM dynamics and inaccuracies in models of soil processes and global C cycling. For example, lab handling techniques such as air-drying soil could have an impact on soluble C losses. Alternatively, the observed C losses could represent pools of potentially soluble C that can vary by ecosystem and season, and thus might represent ephemeral, not well stabilized, and generally overlooked pools of soil C that are missing from SOM budgets and models. For example, in summer-dry climates such as the Pacific Northwest, soluble organic products of decomposition could accumulate over the summer when temperatures and microbial activities are high and precipitation is low, and subsequently leach during the fall and winter with precipitation. To address these divergent possibilities, soils were collected seasonally from a summer-dry forest in Oregon, and subsamples were subjected to 4 different laboratory handling procedures prior to fractionation: 1) air-drying and 2) oven-drying to simulate common laboratory drying techniques; 3) leaching to evaluate if potentially soluble C represented a pool that could be removed with simulated precipitation; and 4) immediate fractionation of fresh soil to determine if drying of soil caused artifacts in pools of potentially soluble soil C. Contrary to initial hypotheses, there were no seasonal trends to soluble DOC pools or total C loss during fractionation. Average mass loss during fractionation was 6% of initial dry weight and total C loss was 9% of total soil C. Soluble losses represented only 9% of total soil C loss. Particulate, or non-soluble mass loss was dominated by the high C:N free particulate organic matter, or LF. Thus, most C loss in this system was due to laboratory losses and that loss was biased towards an underestimation of LF matter in soil. DOC was a small enough component of loss that soil preparation prior to SDF was not significant. These results imply that in temperate soils, even in seasonally extreme ecosystems, relative proportions of mineral-associated organic matter and free particulate organic matter are seasonally robust, air drying of soil does not introduce error, and that immediate fractionation after field collection is not critical.
Dissolved organic carbon (DOC) flux is an important mechanism to convey soil carbon (C) from aboveground organic debris (litter) to deeper soil horizons and can influence the formation of stable soil ...organic C compounds. The magnitude of this flux depends on both infiltration and DOC production rates which are functions of the climatic, soil, topographic and ecological characteristics of a region. Aboveground litter quantity and quality was manipulated for 20 years in an old-growth Douglas fir forest under six treatments to study relationships among litter inputs, DOC production and flux, and soil C dynamics. DOC concentrations were measured at two depths using tension lysimeters, and a hydrologic model was created to quantify water and DOC flux through the soil profile. DOC concentrations ranged from 3.0–8.0 and 1.5–2.5 mg C/L among treatments at 30 and 100 cm below the soil surface, respectively. Aboveground detrital inputs did not have a consistent positive effect on soil solution DOC; the addition of coarse woody debris increased soil solution DOC by 58% 30 cm belowground, while doubling the mass of aboveground leaf litter decreased DOC concentrations by 30%. We suggest that high-quality leaf litter accelerated microbial processing, resulting in a “priming” effect that led to the lower concentrations. Annual DOC flux into groundwater was small (2.7–3.7 g C/m²/year) and accounts for < 0.1% of estimated litter C at the site. Therefore, direct DOC loss from surface litter to groundwater is relatively negligible to the soil C budget. However, DOC flux into the soil surface was much greater (73–210 g C/m²/year), equivalent to 1.4–2.4% of aboveground litter C. Therefore, DOC transport is an important source of C to shallow soil horizons.
Conserving soil carbon (C) and harnessing the potential for soil C sequestration requires an improved understanding of the processes through which organic material accumulates in soil. Currently, ...competing hypotheses exist regarding the dominant mechanisms that control soil C accumulation and transfers to mineral-associated pools. Long-standing hypotheses rely upon an assumed strong relationship between the quantity of organic inputs and soil C accumulation, while more recent hypotheses have shifted the focus towards the more complex controls of root activity, microbial processing and priming, and organo-mineral complexation. The Detrital Input and Removal Treatment (DIRT) experiment can test these competing hypotheses through field manipulations of detrital inputs. After 20 years of detrital manipulations in the wet, temperate forest of the H.J. Andrews Experimental Station, we found that with the termination of live root activity, the significant influx of dead root material and absence of soil priming by roots led to decreases in particulate organic matter (POM), but increases in stable mineral associated organic matter (MAOM). This suggests that soil mineral particles in undisturbed soils are not saturated with C in the presence of live roots and that pools of MAOM are sensitive to the balance between microbial-induced stabilization and microbial-induced priming and destabilization. Twenty years of aboveground litter removal did not change bulk soil C stocks or pools. Soil C stabilization did not increase in response to increases in high quality litter inputs, in contrast to recent theory, but in accordance with other empirical results. In contrast, increases in low quality wood litter led to a large increase in bulk soil C, with gains over 20 years confined to increases in POM. These findings offer insight into the pathways controlling soil C contents and provide potential explanations for the often-limited potential to increase mineral associated soil C in many vegetated soils and observed buffered responses of soil C stocks to disturbances such as drought, fire, and timber harvest.
Dissolved organic carbon (DOC) flux is an important mechanism to convey soil carbon (C) from aboveground organic debris (litter) to deeper soil horizons and can influence the formation of stable soil ...organic C compounds. The magnitude of this flux depends on both infiltration and DOC production rates which are functions of the climatic, soil, topographic and ecological characteristics of a region. Aboveground litter quantity and quality was manipulated for 20 years in an old-growth Douglas fir forest under six treatments to study relationships among litter inputs, DOC production and flux, and soil C dynamics. DOC concentrations were measured at two depths using tension lysimeters, and a hydrologic model was created to quantify water and DOC flux through the soil profile. DOC concentrations ranged from 3.0–8.0 and 1.5–2.5 mg C/L among treatments at 30 and 100 cm below the soil surface, respectively. Aboveground detrital inputs did not have a consistent positive effect on soil solution DOC; the addition of coarse woody debris increased soil solution DOC by 58% 30 cm belowground, while doubling the mass of aboveground leaf litter decreased DOC concentrations by 30%. We suggest that high-quality leaf litter accelerated microbial processing, resulting in a “priming” effect that led to the lower concentrations. Annual DOC flux into groundwater was small (2.7–3.7 g C/m
2
/year) and accounts for < 0.1% of estimated litter C at the site. Therefore, direct DOC loss from surface litter to groundwater is relatively negligible to the soil C budget. However, DOC flux into the soil surface was much greater (73–210 g C/m
2
/year), equivalent to 1.4–2.4% of aboveground litter C. Therefore, DOC transport is an important source of C to shallow soil horizons.
