Shinya Nakamura

The global optimization of parameters in process-based crop models is often considered computationally expensive. Gaussian process (GP) emulation is a widely used method for reducing the computational burden of the optimization process. Total above-ground biomass and cane dry weight of three Thai sugarcane cultivars (KK3, LK92-11 and 02-2-058) collected under rainfed and irrigated conditions were used to optimize cultivar-specific parameters in the Agricultural Production Systems sIMulator (APSIM)-Sugarcane crop model through a GP emulation. GP emulators were trained and validated to approximate APSIM-Sugarcane model and then used for optimizing the cultivar-specific parameters through the differential evolution algorithm. Resulting optimized parameters allowed to obtain simulations that quite well approximated the observed biomass and CDW (validation results between simulated and observed yields: R² 0.93–0.98; normalized root mean squared error: 5–22%; Willmott’s agreement index: 0.87–0.99). The best parametrization was obtained under the lowest water stressed conditions. Based on these results, we suggest that GP emulation can be efficiently implemented for the parameterization of computationally expensive simulators.

We measuredN2O emissions from a sugarcane field planted in spring 2011 in Okinawa,and examined theirvariations during the summer growing period (13 July to 4 August).Then we performed simulations with the DeNitrification and DeComposition(DNDC)and Agricultural Production Systems Simulator(APSIM)modelsto estimate N2O emissionsand compared the results with the observed data from the field experiment.Theresults showedthat (i) nitrification was the dominantprocessaffecting N2O emissionsand nitrifier denitrification occurred under high soil moisture conditions after rainfall;(ii) there was large spatial dispersion of N2O emissions even in the same field,and it was necessary to consider this dispersion to estimate total N2O emissions from the field; (iii) the emission factor of the N2O emissions might be larger than that used by the National GreenhouseGas Inventory of Japan;(iv) the DNDC model overestimated,and the APSIM model underestimated,the observed N2O emissions; and(v) simulation of N2O emissions associated withnitrificationby the DNDC model might be improved by modifyingcoefficients in the equationusedto calculate N2O production during nitrification.

We examined the influence of input soil hydraulic parameters on HYDRUS-1D simulations of evapotranspiration and volumetric water contents (VWCs) in the unsaturated zone of a sugarcane field on the island of Miyakojima, Japan. We first optimized the parameters for root water uptake and examined the influence of soil hydraulic parameters (water retention curve and hydraulic conductivity) on simulations of evapotranspiration. We then compared VWCs simulated using measured soil hydraulic parameters with those using pedotransfer estimates obtained with the ROSETTA software package. Our results confirm that it is important to always use soil hydraulic parameters based on measured data, if available, when simulating evapotranspiration and unsaturated water flow processes, rather than pedotransfer functions.

Nitrous oxide (N2O) emissions from Shimajiri-maji (calcaric dark red soil)from Okinawa, Japan, were measured in the laboratory by a closed-chamber incubation methodafter applying two different chemical fertilizers(normal fertilizer,NF;controlled-release fertilizer,CF). The total nitrogen content was 45% in both NF and CF. At higher temperatures (30°C) CF had better potential to reduce N2O emissions than atlower temperatures (20°C), withtotal N2O emissions,from highest to lowest,followingthe pattern of NFat 30°C>CFat 30°C>CFat 20°C>NFat 20°C.Under alternately saturated and drained conditions, the highest peaks of N2O fluxes tended to occur soon after drainagewith both CF and NF,and nitrification wasthe primary sourceof N2O emissions. Conversely, only slight N2O emissions were detected during prolonged saturated conditions,and it was assumed that denitrification was the secondary source of N2O. This suggests that replacing types of nitrogen fertilizers (replacing NF with CF) does not necessarily result in a direct reduction in N2O emissionsas some earlier studies have reported. It is therefore important to continuously monitor temperature and soil moisture content during fertilization and modify the fertilizer accordingly toachieve effective reduction of N2O emissions and minimize environmental impacts.