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

Abstract

The literature was reviewed and analyzed to determine the feasibility of using a combination of acid hydrolysis and CO 2 -C release during long-term incubation to determine soil organic carbon (SOC) pool sizes and mean residence times (MRTs). Analysis of 1100 data points showed the SOC remaining after hydrolysis with 6 M HCl ranged from 30 to 80% of the total SOC depending on soil type, depth, texture, and management. Nonhydrolyzable carbon (NHC) in conventional till soils represented 48% of SOC; no-till averaged 56%, forest 55%, and grassland 56%. Carbon dates showed an average of 1200 yr greater MRT for the NHC fraction than total SOC. Longterm incubation, involving measurement of CO 2 evolution and curve fitting, measured active and slow pools. Active-pool C comprised 2 to 8% of the SOC with MRTs of days to months; the slow pool comprised 45 to 65% of the SOC and had MRTs of 10 to 80 yr. Comparison of field 14 C and 13 C data with hydrolysis-incubation data showed a high correlation between independent techniques across soil types and experiments. There were large differences in MRTs depending on the length of the experiment. Insertion of hydrolysisincubation derived estimates of active (C a ), slow (C s ), and resistant pools (C r ) into the DAYCENT model provided estimates of daily field CO 2 evolution rates. These were well correlated with field CO 2 measurements. Although not without some interpretation problems, acid hydrolysis-laboratory incubation is useful for determining SOC pools and fluxes especially when used in combination with associated measurements. SOIL ORGANIC MATTER (SOM) is a complex mixture of plant-and microbiologically-derived compounds (Stevenson, 1994). Fractionation schemes have been employed to (i) isolate meaningful pools that provide information on C and nutrient cycling; (ii) test hypotheses on soil formation and ecosystem functioning; (iii) relate SOM characteristics to soil management and global change; and (iv) aid in developing models describing SOM dynamics. Observed differences in MRTs from months to centuries to millennia for different SOM fractions prompted the development of models with multiple soil C pools with differing MRTs (Jenkinson and Rayner, 1977;Van Veen and Paul, 1981;Parton et al., 1987). To date, no single biological, physical, or chemical fractionation technique has been developed that adequately describes the continuum of SOC that exists in nature. Mismatches between measurements of 14 C-labeled plant materials using three size-density fractions and model outputs have been identified (Magid et al., 1996). Motavalli et al. (1994) and Smith et al. (2002) were not successful in their attempts to equate the active pool to microbial biomass plus soluble C fraction or the light fraction. Similar problems were found when attempts were made to equate the size of the slow pool in models with the particulate organic matter fraction (Metherell et al., 1995). However, more recent work investigating free and aggregate-associated SOC has produced more promising ...

How to link soil C pools with CO<sub>2</sub> fluxes?

“…duration of incubation period, sampling frequency and timing) may strongly bias all parameters (Scharnagl et al, 2010). The results linking pools and fluxes obtained by this approach are therefore poorly comparable with other studies, because the fitted pool sizes and the rates strongly depend on incubation duration (Paul et al, 2006;Scharnagl et al, 2010). Moreover, other experimental conditions (soil amount, incubation conditions, CO 2 sampling strategy, .…”

“…This approach (frequently termed biological approach or biological CO 2 fractionation) is based on the kinetics of CO 2 efflux from soil (without C input) and is typically used to evaluate the results of incubation (Kätterer et al, 1998;Paul et al, 2001Paul et al, , 2006 Pendall and King, 2007;Cabaneiro et al, 2008), field (Taneva et al, 2006) or modeling studies (Scharnagl et al, 2010). The principle is also based on first-order kinetics (Parton et al, 1987), but of the CO 2 efflux from soil and not of the C pools as in the previous LTBF approach.…”

“…The common example for this fact is the discordance between δ 13 C of microbial biomass and δ 13 C of CO 2 efflux from soil after C 3 -C 4 vegetation change (Werth et al, 2006). Due to this discordance between MRTs of pools and fluxes (Collins et al, 2000;Paul et al, 2006;Taneva et al, 2006), the calculation of the contribution of SOM pools to CO 2 fluxes based only on the MRT of C pools will underestimate the fluxes. Therefore, in this review focused on the approaches linking C pools with CO 2 fluxes, I don't describe the approaches allowing estimation of MRT and turnover time of pools (Trumbore, 2006;Derrien and Amelung, 2011).…”

“…without interactions; no priming effects) to the CO 2 efflux with different rates. Accordingly, the total C mineralized to CO 2 (CO 2 (t)) until time t can be calculated as:If only one pool (i = 1) contributes to the CO 2 efflux, then the fitted parameters P (0) and k correspond to the pool size and its decomposition rates (Paul et al, 2006). The size and the rate determine what this pool contributes to the total CO 2 efflux from soil.…”

Temperature and vegetation effects on soil organic carbon quality along a forested mean annual temperature gradient in North America

“…In this widely used method, SOC quality is kinetically defined through measurements of CO 2 release from incubated soils (Paul & Clark, 1996;Townsend et al, 1997;Paul et al, 2001Paul et al, , 2006. Lab incubation experiments provide insights about microbial utilization of SOC and hence SOC quality without the confounding influences of root respiration and climatic variation that are encountered in the field (Collins et al, 2000).…”