2.3 Environmental monitoring
The soil volumetric moisture content, air temperature, soil temperature and rainfall were measured by a data logger system (CR1000 data logger; Campbell Scientific, Inc., USA). The volumetric moisture content in the surface layer (0–15 cm depth) was recorded every 30 min in three replicates for each plot using time-domain reflectometer (TDR) probes (CS616; Campbell Scientific, Inc., USA). The moisture probes were installed near the polyvinyl chloride (PVC) columns (see below). Air temperature was recorded every 30 min, and soil temperature (5 cm depth) was recorded every 30 min in duplicate for each plot, using thermistor probes (Model 108; Campbell Scientific, Inc., USA). Rainfall was also recorded every 30 min using a TE525MM device (Campbell Scientific, Inc., USA). All sensors were connected to a data logger system (CR1000 data logger; Campbell Scientific, Inc., USA). The soil moisture sensors were calibrated in each treatment plot in each year by comparing measured field soil moisture (as mentioned below) and recorded soil moisture through sensors.