Location

Jurong Ecological Experimental Station (JREES) established by Nanjing University (NJU), classified as cropland.

Jurong is located in the middle and lower reaches of the Yangtze River plain, with a high topography from north to south and a low topography in the middle. The northern part of Jurong city is the middle section of the Ningzhen Mountain Range, the Baohua Mountains, the southeastern part is the north-south Mao Mountains, and the central part is a plain, with a low altitude of 15 m on average. The rivers in Jurong city are from the Yangtze River system, the Qinhuai River system and the Taihu Lake system, and the reservoirs include Beishan Reservoir, Lunshan Reservoir, Jurong Reservoir, Chishan Lake Reservoir, Ersheng Reservoir and Maoshan Reservoir. The land cover is mainly agricultural type (paddy field and dry land), and the woodland is distributed in the mountains in the north and southeast, with relatively well-preserved northern subtropical vegetation.

The eddy flux tower was established at the Jurong Ecological Experimental Station (JREES) established by Nanjing University (NJU) since April 2015. The JREES is located at the Houbai State Farm (HBSF) (31°48′24.59″N, 119°13′2.15″E).

This site is relatively flat and homogeneous, and provided sufficient upwind fetch for adequately measuring mass and energy fluxes using the eddy covariance system. 70% of the flux footprints came from areas within 104 ± 10 m and 191 ±20 m around the eddy covariance tower during daytime and nighttime, respectively. The source location with maximum contribution to the measured fluxes were typically 50 m around the eddy covariance tower.

Climatic

The climate here is characterized as northern subtropical semihumid monsoon climate, which is characterized by a strong monsoon, four distinct seasons, a mild climate, abundant rainfall, and a long frost-free period.

According to the long-term climate data from the Jurong Meteorological Observatory (about 17 km away from JREES), the average annual temperature and ≥10 °C annual accumulated temperature are 15.5 and 4952.7 °C, respectively. The annual extreme maximum temperature is 39.6°C and the annual extreme minimum temperature is -14.8°C. The mean annual precipitation is 1099.1 mm, of which 255.2 mm (23% of the year) in spring, 526.5 mm (48% of the year) in summer, 184.5 mm (17% of the year) in autumn and 132.9 mm (12% of the year) in winter. The average annual sunshine hours is 1999.3 h, of which 512.4 h (26% of the year) in spring, 572.7 h (29% of the year) in summer, 500.4 h (5% of the year) in autumn and 414.4 h (21% of the year) in winter. The average annual relative humidity and wind speed are 78.6% and 2.8 m s-1, respectively.

Vegetation and soil

The land cover is mainly agricultural land (paddy field and dry land), and the woodland is distributed in the mountains in the north and southeast, with well-preserved northern subtropical vegetation.

Rice (Oryza sativa L.) was mainly planted in seedbeds and transplanted by mechanical means. The seeds used in the seedbeds were 60 kg ha-1 and the transplanted plants were spaced 25×16 cm apart. The rice was harvested by combine harvesters and the straw was returned to the field in full quantity. In addition, a rice-duck agroecological model was developed during the growing season.

The soil type of the experimental site is typical rice soil, and the soli texture is clay loam, consisting of 32.6–36.9% clay, 37.7 41.7% silt, and 25.4–27.1% sand, and with an average bulk density of 1.41 kg m−3 in the soil profile (0–100 cm).

The soil surface (0-30 cm) of the experimental site contained 16.73%, 61.10%, and 22.17% sand, flour, and clay grains, respectively; 15.76 g/100g of gravel; 51.7 cm3/100cm3 of soil porosity; 6.31 of soil pH (pH value for measuring soil H2O). Soil organic matter content was 1.99 g/100g; root abundance was 5.39, indicating that most plant roots were distributed within 0-30 cm of soil; soil total nitrogen, total phosphorus and total potassium contents were 0.12 g/100g, 0.05 g/100g and 1.71 g/100g, respectively; soil alkaline nitrogen, effective phosphorus and effective potassium contents were 78.29 mg/kg, 6.49 mg/kg and 66.36 mg/kg, respectively.

Fertilizer application at the experimental site consisted of agricultural compost and returned rice stubble before transplanting; chemical fertilizer was applied in three applications, namely, base fertilizer: urea (N content 45%) 150 kg ha-1, calcium superphosphate (effective P2O5 content 15%) 450 kg ha-1, and potassium chloride (K2O content 60%) 225 kg ha-1 at the time of transplanting; tiller fertilizer: urea 60 kg ha-1 at the tiller stage; and tassel fertilizer: urea 90 kg ha-1 at the tasseling stage.

Observation system

Fluxes of CH4, CO2, latent heat (LE), and sensible heat (H) fluxes over the plant canopy were measured continuously with an EC system. The EC system mainly consists of following components: (1) an ultrasonic anemometer (WindMaster Pro, Gill Instruments Limited, Hampshire, UK) to measure three-dimensional wind speed and sonic temperature; (2) an open-path CH4 analyzer (LI-7700, LI-COR Inc., Lincoln, NE, USA) to measure CH4 concentrations, and (3) an open-path CO2/H2O analyzer (LI-7500A, LI-COR Inc., Lincoln, NE, USA) to measure the concentrations of CO2 and water vapor. All the three EC sensors were installed on a galvanized steel pipe tripod mast, perpendicular to the prevailing wind direction, with a sensor separation of about 20 cm and the ultrasonic anemometer 3.5 m above the ground. To minimize flow distortions, the CH4 analyzer and the ultrasonic anemometer were adjusted 20 cm (south) and 10 cm (west), respectively, the CO2/H2O analyzer was installed with horizontal separation of 20 cm (north) and 10 cm (east) from the ultrasonic anemometer, and the head was tilted about 15° from vertical to minimize the amount of precipitation that accumulated on the window.

The height of the flux tower, the ultrasonic anemometer and the infrared analyzer were 3.5 m above the ground. Relative humidity (RH) and ambient air temperature (TA) were recorded using two humidity and temperature probes (HMP155, Vaisala Group, Helsinki, Finland) at heights of 0.5 m and 2.6 m above the ground, respectively. Rainfall (P) was measured with a tipping bucket rain gauge (Texas Electronics TR-525M, Texas Instruments, Dallas, USA). Photosynthetic photon flux density (PPFD) was measured at 2.25 m above the ground with a quantum sensor (LI-190SL, LI-COR Inc., Lincoln, NE, USA). Net radiation (Rn) was measured with a four-component radiometer (CNR4, Kipp & Zonen, Delft, Netherlands). Soil heat flux (SHF) was measured with heat flux plates (HFP01, Hukseflux Thermal Sensors, Delft, Netherlands) buried 0.05 m below the soil surface at three different replicate locations. Volumetric soil water content (SWC) were measured with soil probes (Stevens Hydra Probe II, Stevens Water Monitoring Systems, Portland, USA) at depths of 0.05 m (SWC5cm) and 0.20 m (SWC20cm) under the soil surface. Two soil temperature (TS) sensors (LI-7900-180, LI-COR Inc., Lincoln, NE, USA) were buried 0.05 m (TS5cm) and 0.20 m (TS20cm) below the soil surface for measuring soil temperatures.

Fluxes were sampled at 10Hz by a data logger (Sutron 9201B, Sutron Corp., Sterling, VA, USA), averaged 30 minutes.

Principal Investigator

Weimin Ju     E-mail: juweimin@nju.edu.cn