An experimental study has been conducted in agricultural fields at the National Agricultural Research Center (Tsukuba, Japan; 36°010N, 140°070E, 25 m above sea level) from 1995 to 2002. Two flat and uniform 4-ha fields were used, which were surrounded by the similar type of cropped fields under different management conditions. The soil was a humic volcanic ash soil Andisol, which belongs to the Hydric Hapludands and is the major cultivated soil for upland crops in Japan. The field capacity and wilting point for the top 20-cm layer were estimated to be 0.44 m3 m—3 (-6kPa) and 0.275 m3m—3 (—1.5 MPa), respectively. The mean carbon and nitrogen contents of the top-soil (0-5 cm) were estimated to be 3.7% and 0.31%, respectively. Each field has been used with the cropping pattern (soybean (Glycine max L. Merr.)-rapeseed (Brassica napus L.)-forage corn (Zea mays L.)-wheat (Triticum spp.)) for every 2 year. We mainly used the data set from 1996 to 1998 for this particular analysis. The annual mean air temperature and annual total rainfall were 13.3°C (1996), 13.6°C (1997), and 14.9°C (1998), and 1,032 mm (1996), 972 mm (1997), and 1,536 mm (1998), respectively.
The CO2 flux over the field (ESFCO2 or SSFCO2) was measured by the eddy covariance method. The method has been widely used for the measurement of CO2, water vapor, and sensible heat fluxes over plant canopies (Leuning and Judd, 1996). We used an open-path eddy covariance system, the main components of which were a 3D sonic anemometer, a fast response infrared gas analyzer, and a data logger. The sonic anemometer was used for measuring the fluctuations of three components of wind speed, and virtual temperature, while the fluctuations in CO2 and water vapor concentrations were measured by the gas analyzer. Both sensors were of 10-Hz time resolution. A pair of sonic anemometer and gas analyzer-model DA600 (Kaijo Co., Japan) with path length of 20 cm and model E009A (Advanet Inc., Japan) with 20 cm-was used for 1995-1998 period. These sensors were installed at the central part of the field so that the height of the open path was at 1.5 m above the ground. The horizontal distance between the open paths of sonic anemometer and gas analyzer was 15 cm. Data were recorded to the data logger CR23X (Campbell Scientific Inc., USA), and CO2 flux as well as H2O and sensible heat fluxes were calculated from the covariance between vertical wind speed and each components for every 20 min. The CO2 flux derived from the covariance was corrected based on the method described in Webb et al. (1980). All CO2 flux data are defined as positive when they are coming from the surfaces toward the atmosphere. Owing to inherent limitations of the eddy covariance method, such data obtained under rainy conditions, and calm conditions (average wind speed at 20 min basis <0.3msec-1) were eliminated, and hourly average values were calculated for the analyses.
Surface temperatures of the field were measured remotely using three infrared thermometers (Model4000, Everest Inc.). One of them was installed at the height of 3 m, and the other two were attached to booms, the height of which was adjustable. Their height was fixed to 1.5 m during bare soil conditions, but adjusted so that plant leaves were targeted during cropped periods. All three sensors were set at nadir-looking angle during bare soil periods. The field of view of sensors was 15°. The emissivity was assumed to be 0.98, since the emissivity of the plants and soil was in the range of 0.95-0.99 (Olioso, 1995). Data from the infrared thermometers were acquired by the data logger at 1-sec rate and averaged for every 20min.
Spectral reflectance measurements were periodically obtained over the ecosystem using a hyperspectral radiometer and a hand-held radiometer. The hyperspectral one (FSFR1000, ASD) covered the wavelength range from 350 to 2,500 nm at 1 nm resolution. The hand-held radiometer (Opto-Research, Co. Ltd., Japan) was equipped with seven bands from the visible to short-wave infrared wavelength regions (560, 660, 830, 1,100, 1,200, 1,650, and 2,200 nm). Spectral observations were made from a height of 2 m above the canopy at the nadir-looking angle. The reflectance factor was calculated as a relative value to the reflectance of a BaSO4 standard panel, which was coated with Kodak Analytical Standard White Reflectance Coating (#6080) to about 1.0 mm thickness (Inoue et al., 1998). The white panel was calibrated using the commercial reference board, Spectralon (Labsphere, USA).
Air temperature and relative humidity were measured using a probe HMP45C (Campbell Scientific Inc.), which contains a platinum resistance temperature detector and a capacitive relative humidity sensor. Wind speed was measured using a three-cup anemometer model 03001 (Campbell Scientific Inc.). Incident and reflected solar radiations were measured by pyranometer CM3 (Kipp & Zonen, the Netherlands). Net radiation was measured by three sensors: Q6 (REBS Inc., USA), CG3, and NR-Lite (Kipp & Zonen). Photosynthetically active photon flux density was measured by a quantum sensor LI-190SB (Li-Cor Inc.). Soil heat flux was measured by two heat flux plates PHF-01 (REBS Inc.). Soil temperature was measured by an averaging thermocouple probe TCAV (Campbell Scientific Inc.), which was installed at depths of 5 and 10 cm below the soil surface. Volumetric soil water content was measured using TDR probes (CS615, Campbell Scientific Inc.) for the soil layers at the depths of 10, 20, and 30 cm at 20 min intervals.
Plant height, wet and dry biomass, and LAI were determined about every 10 days by destructive sampling. Sampled plants were divided into stems, green leaves, roots, and dead parts. The green leaf area was measured by an area-meter AAM8 (Hayashi-denkoh Co. Ltd.).
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