Location

Ansai Loess Plateau temperate shrub-grass flux observation site (36°51′30″N, 109°19′23″E, and elevation 1260 m) is located in the middle of Loess Plateau in Ansai Shaanxi province, which belongs to Ansai national ecological experiment station and Chinese Ecosystem Research Network (CERN). The observation site of flux is in a north facing hillside of the station with a slope from 18°-20°, and the observation area is 120m×80m.

Climatic

Ansai station is in the semi-arid temperate climate zone with an annual mean temperature of 8.8℃，the maximum and minimum monthly temperature are -6.9℃ (Jaunary) and 22.6℃ (July), respectively. The annual mean precipitation amount is 540 mm, and the precipitation mainly concentrated during July-September.

Vegetation and soil

The vegetation type of this area belongs to transition zone of temperate deciduous broad-leaved forest to temperate grassland. The original forests at this site were clearance cut and destroyed since 19th century，which also caused the serious water soil and loss. The flux observation area is dominated by natural secondary grassland, occupied by Bothriochloa ischaemum, Stipa bungeana, Artemisia gmelinii and Artemisia giraldii. Meanwhile, some natural secondary shrub species such as Hippopha Rhamnoides were also observed. The soil type is dominated by loessial soil and silt loam, with highly calcareous (pH 8.4), thick soil layer (50–80 m), and low organic carbon.

Observation system

(1) Flux observation system：An eddy covariance system, observed water and carbon fluxes, was mounted at 4 m on a tower. It consisted of a LI-7500 open-path CO₂/H₂O analyzer (LI-7500, Li-cor , USA) and a three-dimensional sonic anemometer (CSAT3, Campbell , USA). In addition, soil heat flux was measured by two HFP01 (Campbell, USA) heat flux plate, and were mounted at heights of 0.05m below the ground.

(2) Meteorological observation: Net radiation and quantum sensor of photosynthetically active radiation were mounted at height of 1.5m, through a four-component radiometer (Model CNR-1, Kipp & Zonen Inc., The Netherlands) and a photosynthetically active radiatiometer (Model LI190SB, Licor Inc.), respectively. Air temperature and relative humidity sensors (Model HMP45C, Campbell Scientific Inc., USA) and wind speed/direction sensors (A100R, Vector Inc.) were mounted at heights of 1, 2, 3, 5m, above the ground. A wind speed and direction sensor was also mounted at height of 6m above the ground. Precipitation was monitored with a rain gauge (Model 52203, RM Young Inc., USA).

(3) Soil temperatures were made at depths of 2, 5, 20, 50 and 100 cm with thermocouples (109SS, Campbell Scientific Inc., USA), meanwhile soil volumetric water contents were recorded at depths of 5, 20 and 50 cm using three TDR probes (Model CS616-L, Campbell Scientific Inc., USA).

(4) The flux data were sampled at 10 Hz with 30 min averages collected by CR3000 dataloggers (Model CR3000, Campbell Scientific Inc., USA), and  meteorological data were sampled at 1 Hz with 30 min averages collected by CR1000 dataloggers (Model CR1000, Campbell Scientific Inc., USA).

Principal Investigator

 Yunming Chen

E-mail: ymchen@ms.iswc.ac.cn