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Comparison of Labile Soil Organic Matter Fractionation Techniques

^a Environmental Studies Program, Dartmouth College, 6182 Steele Hall, Hanover, NH 03755
^b Dep. of Ecology, Evolution, and Behavior, Univ. of Minnesota, 1987 Upper Buford Circle, St. Paul, MN 55108

View larger version (24K): [in a new window]   Fig. 1. Model II regressions between total soil organic carbon (SOCT) and five different methods for measuring labile soil organic carbon (SOCL): (a) light fraction C (P < 0.0005), (b) microbial C (P < 0.00001), (c) hydrolyzable C (P < 0.00001), (d) incubation labile C (P < 0.0005), and (e) Day 12 labile C (P < 0.005). All measurements were made on samples from the 0- to 10-cm soil depth.

View larger version (34K): [in a new window]   Fig. 2. Model II regressions between different techniques to measure labile soil C pools: (a) microbial C and light fraction C (P < 0.005), (b) microbial C and hydrolyzable C (P < 0.00001), (c) light fraction C and hydrolyzable C (P < 0.001), (d) Day 12 labile C and light fraction C (P < 0.001), (e) Day 12 labile C and hydrolyzable C (P < 0.0005), and (f) Day 12 labile C and microbial C (P < 0.005). All measurements were made on samples from the 0- to 10-cm soil depth.

View larger version (33K): [in a new window]   Fig. 3. Model II regressions between incubation labile carbon (Cl) and other techniques for measuring labile soil organic C pool sizes: (a) light fraction C (P < 0.2), (b) hydrolyzable C (P < 0.001), (c) microbial biomass C (P < 0.1), and (d) Day 12 labile C (P < 0.00001). The Cl is determined with a two-pool model using constrained nonlinear curve fitting to respiration data from a 360-d laboratory incubation. All measurements were made on samples from the 0- to 10-cm soil depth.

View larger version (17K): [in a new window]   Fig. 4. Cumulative CO2–C respired during the course of a long-term laboratory incubation of soils at 25°C. An equation is fit to the data Ct = Cl (1 – e–klt) + krt, where Ct is the known cumulative amount of C respired at sampling period t, Cl is the size of the labile C pool, kl is the decay constant for the labile pool, and kr is the mineralization rate for the recalcitrant pool, which is set at 3 µg g–1 soil d–1. For this sample, Cl = 1544 kg ha–1, kl = 0.012 d–1.

View larger version (17K): [in a new window]   Fig. 5. Relationship between the size of the labile carbon pool (Cl) and the decay constant for the labile pool (kl). These two parameters are simultaneously determined by fitting the curve Ct = Cl (1 – e–klt) + krt, where Ct is the known cumulative amount of C respired at sampling period t, and kr is the mineralization rate for the recalcitrant pool which is set at 3 µg g–1 soil d–1 (r2 = 0.55, P < 0.0001, y = 0.025 – 0.0000065x).

View larger version (21K): [in a new window]   Fig. 6. Relationship between the size of the labile C pool (Cl) determined without constraining kr and Cl determined with kr set at 3 µg g–1 soil d–1 (P < 0.9).

View larger version (17K): [in a new window]   Fig. 7. Model II regressions between an index of soil aggregate size, geometric mean diameter (GMD), compared with two different soil C measurements: (a) hydrolyzable C (P < 0.01) and (b) total SOC (P < 0.05). Higher values of GMD indicate larger aggregate size. All measurements were made on samples from the 0- to 10-cm soil depth.