Analysis and modelling of the combined effect of plant nitrogen concentration after fertilization and development stage on rice radiation use efficiency

The equation describing aboveground biomass (AGB) accumulation as a function of intercepted photosynthetically active radiation and radiation use efficiency (RUE) is used in many crop growth models and is one of the conceptual bases of the methods for the estimation of dry matter productivity from satellite data. It has been observed that the increase in rice nitrogen (N) concentration following the N fertilization at the panicle initiation (PI) seems to temporarily increase the bio-mass accumulation rate. In terms of modelling, this could be translated in an increase in RUE. A growth chamber experiment has been carried out during 2006 in order to analyze these processes in a controlled environment. Measured variables were AGB and plant nitrogen concentration (PNC) (separately for blades and pseudoculms), leaf area index, chlorophylls concentration, stomatal conductance, and RUE. Two different N levels were applied: one able to maintain PNC at the critical threshold, the other able to assure PNC close to the luxury consumption (critical threshold and luxury consumption are concepts to be considered here according to the N dilution theory). The different behavior of the plants under the two N fertilization levels allowed deriving a model for simulating the RUE increase basing on PNC after the PI fertilization. The model developed has been compiled as a COM DLL (TOPFERT.DLL) and tested under the WARM simulation environment using measured data. The WARM accuracy improved with the use of the DLL (RRMSE = 15.6% compared to 25.8%; modelling efficiency = 0.97 compared to 0.92). The formalization of the acquired knowledge in a reusable and free of charge software component should favor the dissemination and validation activities of the model developed.