COMPARISON OF LABORATORY AND MODELING SIMULATION METHODS FOR ESTIMATING SOIL CARBON POOLS IN TROPICAL FOREST SOILS

Availability of methods to determine kinetically-defined soil carbon pools may assist in better understanding soil organic matter turnover in tropical soils and its relationship with soil mineral fractions and nutrient cycling. Our objective was to compare three methods of estimating soil C pools for the top mineral soil horizon of 13 tropical forest soils with a wide range of clay content and differing soil mineralogies. The methods were: (i) regression analysis of CO2-C release from a 341 day incubation of unamended soils; (ii) results of C analysis procedures including determinations of soluble C, microbial biomass C and light fraction C; and (iii) CENTURY model simulations of equilibrium values of soil C pools. Soil mineralogy did not have a significant effect on CO2-C release, expressed as a proportion of total organic C. during incubation. However, allophanic soils had significantly higher total organic C, soluble C and light fraction C than soils of smectitic, kaolinitic or oxidic mineralogies. Clay and sand contents significantly correlated with cumulative proportional CO2-C release. The active C pool, as determined by the CENTURY equilibrium method and measurements of soluble plus microbial biomass C, were less than the active C pool estimated by the incubation-regression method. Measured light fraction e was smaller than estimates from the CENTURY equilibrium method and incubation-regression estimates. Total organic C, soluble plus microbial biomass C and light fraction C had the highest correlations with cumulative incubation CO2-C release. Of the CENTURY model C pool estimates, only the slow C pool estimate correlated with incubation CO2-C release. The use of C analyses as soil C pool estimates for model simulations of the long-term incubation resulted in an underestimation of actual incubation C02-C release. This underestimate was caused by a smaller slow pool estimated by light fraction analysis. In addition, structural and metabolic C pools were not measured and they have a large short-term effect on CO2-C release. Use of CENTURY equilibrium estimates, including estimates of structural and metabolic C, resulted in simulated CO2-C release comparable to actual CO2-C release patterns. However, the use of the CENTURY equilibrium method may be limited by the difficulty of obtaining adequate soil, plant and climatic information to run model simulations and by the validity of CENTURY model assumptions for factors controlling soil C pools under tropical climatic conditions.