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Does the Acid Hydrolysis–Incubation Method Measure Meaningful Soil Organic Carbon Pools?

^a Natural Resource Ecology Lab., Colorado State Univ., Fort Collins, CO 80523-1499
^b Dep. of Biology, Bradley Univ., Peoria, IL 61625

View larger version (23K): [in a new window]   Fig. 1. Nonhydrolyzable soil C as a function of total soil C for each of four different methods of acid hydrolysis: (a) standard method, 6 M HCl for 16 h at 116°C (data from Campbell et al., 1967; Collins et al., 2000; Follett et al., 1997; Haile-Mariam et al., 2000; Jenkinson and Rayner, 1977; Martel and Paul, 1974a, 1974b; Martel and LaSalle, 1977; McLauchlan and Hobbie, 2004; Morris et al., 2006, unpublished data; Paul et al., 1997, 2001a, 2001b); (b) 6 M HCl for 18 h at 96°C (Plante et al., 2006); (c) 6 M HCl for 18 h at 100°C (Tan et al., 2004); and (d) 2 M H2SO4 for 4 h (Vlassak et al., 1969).

View larger version (26K): [in a new window]   Fig. 2. Proportion of nonhydrolyzable C as a function of depth for (a) cultivated conventional till, (b) cultivated no-till, (c) forest, and (d) grassland samples. Median values are represented by the solid line in each box, mean values by the dotted line, 25th and 75th percentiles by the upper and lower bounds of the box, 10th and 90th percentiles by the upper and lower whiskers, and fifth and 95th percentiles by dots above and below boxes, where available. The number of data points within each depth–management combination is indicated in parentheses above the x axis. Data obtained with other methods were normalized to those obtained with 6 M HCl at 116°C for 16 h using data from Fig. 1.

View larger version (22K): [in a new window]   Fig. 3. Nonhydrolyzable soil C as a function of total soil C for samples from four different land-use types: (a) cultivated conventional till, (b) cultivated no-till, (c) native forest, and (d) native grassland (data from Campbell et al., 1967; Collins et al., 2000; Follett et al., 1997; Haile-Mariam et al., 2000; Jenkinson and Rayner, 1977; Martel and LaSalle, 1977; Martel and Paul, 1974a, 1974b; McLauchlan and Hobbie, 2004; Morris et al., 2006, unpublished data, Paul et al., 1997, 2001b; Plante et al., 2006).

View larger version (24K): [in a new window]   Fig. 4. Proportion of nonhydrolyzable C (%NHC) in silt- and clay-sized fractions isolated from 53- to 250-µm microaggregates (data from Plante et al., 2006).

View larger version (17K): [in a new window]   Fig. 5. Carbon dioxide evolution rate curves (µg C g soil–1 d–1) for three depths (0–20, 25–50, and 50–100 cm) from a Michigan corn-belt soil (from Collins et al., 2000).

View larger version (22K): [in a new window]   Fig. 6. Size of the slow soil C pool (Cs) determined using the hydrolysis–incubation method as a function of mean residence time (MRT) determined using 13C content of soil for Corn Belt and Northern Prairie soils (from Collins et al., 2000).

View larger version (14K): [in a new window]   Fig. 7. Mean residence time (MRT) of total soil organic C by 14C-dating vs. 13C after a C3C4 plant switch to continuous corn for soils from three depths (adopted from Paul et al., 2001b).

View larger version (25K): [in a new window]   Fig. 8. CO2 fluxes generated using DAYCENT modeling parameterized with hydrolysis–incubation data (solid line) compared with CO2 fluxes measured in the field (dots represent means with standard error) for the conventional tillage plots at the Kellogg Biological Station–Long Term Ecological Research site (from Paul et al., 1999).