Fine roots dynamics and trace gas fluxes in two lowland tropical forest soils
Fine root dynamics have the potential to contribute significantly to ecosystem-scalebiogeochemical cycling, including the production and emission of greenhouse gases.This is particularly true in tropical forests which are often characterized as having largefine root biomass and rapid rates of root production and decomposition. We examinedpatterns in fine root dynamics on two soil types in a lowland moist Amazonian forest,and determined the effect of root decay on rates of C and N trace gas fluxes. Rootproduction averaged 229 ( 35) and 153 ( 27) gm2 yr1 for years 1 and 2 of the study,respectively, and did not vary significantly with soil texture. Root decay was sensitive tosoil texture with faster rates in the clay soil (k50.96 year1) than in the sandy loam soil(k50.61 year1), leading to greater standing stocks of dead roots in the sandy loam.Rates of nitrous oxide (N2O) emissions were significantly greater in the clay soil(13 1ngNcm2 h1) than in the sandy loam (1.4 0.2 ngNcm2 h1). Root mortalityand decay following trenching doubled rates of N2O emissions in the clay and tripledthem in sandy loam over a 1-year period. Trenching also increased nitric oxide fluxes,which were greater in the sandy loam than in the clay. We used trenching (clay only) anda mass balance approach to estimate the root contribution to soil respiration. In clay soilroot respiration was 264–380 gCm2 yr1, accounting for 24% to 35% of the total soil CO2efflux. Estimates were similar using both approaches. In sandy loam, root respirationrates were slightly higher and more variable (521 206 gCm2 yr1) and contributed 35%of the total soil respiration. Our results show that soil heterotrophs strongly dominatesoil respiration in this forest, regardless of soil texture. Our results also suggest that fineroot mortality and decomposition associated with disturbance and land-use change cancontribute significantly to increased rates of nitrogen trace gas emissions.