Biodegradation of Soluble Organic Matter as Affected by Land-Use and Soil Depth

Determining the biodegradability of soluble organic matter (OM) is important in understanding its role in biogeochemical cycles. We evaluated C and N biodegradation for two frequently studied fractions of soluble OM, water (0.01 mol L-1 CaCl2) and salt (0.5 mol L-1 K2SO4) extractable organic matter (WEOM and SEOM, respectively). Soil samples were collected from topsoil (0-20 cm) and subsoil (60-80 cm) at four sites across a long-term established land-use sequence. The biodegradation dynamics of WEOM and SEOM were determined during a 90-d laboratory incubation using a two-pool model. Compared with SEOM, the amount of C and N of WEOM further varied with land-use and soil depth. The proportion of biodegradable C and N was considerably larger for WEOM than SEOM, consistent with greater delta C-13 (close to soil organic matter [SOM]), C/N ratio, and proportion of aromatic compounds (determined by specific ultraviolet absorbance at 254 nm [SUVA(254)]) for SEOM. For both C and N, the turnover of the slowly biodegradable pool explained the lower biodegradability of SEOM. Depletion in C-13 along with an increase in SUVA(254) demonstrated a preferential consumption of labile compounds that were mainly mineralized during the initial 16 d of the incubation. The proportionally lower biodegraded C than N for both fractions was due to the longer half-life of the slowly biodegradable C. Both soil depth (only for WEOM) and land-use affected the proportion of the biodegradable pools (fast vs. slow) of C and N. In addition, land-use strongly influenced the turnover rate of the fast biodegradable pool. There was no evidence that soluble organic C and N from subsoil are less biodegradable than those of topsoil.