A field-scale study of carbon, water, and energy flow in irrigated rice

Rice is a primary food source for half the world's population, but little is known of how changes in the physical environment affect biomass accumulation and crop yield at the field-scale. Destructive biomass sampling can be used to determine biomass accumulation, but spatial and temporal variability make it difficult to relate changes in the physical environment to changes in plant biomass production. Carbon dioxide exchange rate (CER) has been used in several crops to estimate biomass accumulation since the ratio of biomass to carbon content is relatively constant, but few studies using CER have addressed field-scale carbon, water, and energy balance of an irrigated rice crop. The objective of this project was to study the interaction between carbon assimilation by the crop and the physical environment. A tower-based micrometeorological sampling system measured concentration differences of CO2, water vapor, and heat between up and downward moving eddies to evaluate the carbon, water, and energy balance of a rice crop over two years. Environmental parameters such as intercepted photosynthetic photon flux density (Qi), net radiation, and temperature were also measured. Data were analyzed to compare biomass accumulation, estimated from physical samples, with that estimated from measured CER. In addition, half-hourly averages of CER were compared to Qi and evaporation (E) to evaluate radiation use efficiency (CER/Qi) and water use efficiency (CER/E) at different physiological stages of rice growth. Finally, the energy balance of the rice crop was evaluated to determine how energy flux densities were partitioned into water/soil surface and vegetative components. We found strong correlation between biomass accumulation and cumulative CER for both years of the study. Changes in CER as well as radiation use efficiency corresponded to changes in leaf area index and anthesis. We also found the majority of energy flux out of the rice crop was in the form of latent heat flux from the vegetative surface. The results suggested conditional sampling was a viable method for evaluating biomass accumulation in rice and can be used to evaluate dynamic changes in carbon, water, and energy balance at the field scale.