Observation items and determination methods
Rainfall at the study site was measured with standard rain gauges placed in the center of the National first-class high-quality red plum apricot Demonstration area, which was about 50 m from the Shanghuang Eco-experiment weather station, as a part of Guyuan Eco-experiment weather station under Institute of soil and water conservation of Chinese Academy of Sciences. The study also included the determination of the soil moisture content, plant root distributions, and other plant growth parameters.
The experimental plots were located in the 23-year-old red plum apricot forest planted in the bench terrace in 1996 and 1-year-old red plum apricot forest planted in the bench terrace in 2018. The sampling pits (soil profile) was dug in red plum apricot forest at the experimental site for investigating soil profile and sampling purposes, whose dimensions were 1m2 × 4 m depth on the red plum apricot forest in April, 13, 2018. The undisturbed soil samples were collected for 3 times at the depth of 0 to 5, 20 to 25, 40 to 45, 80 to 85, 120 to 125, 160 to 165, 200 to 205, 240 to 245 and 395 to 400 cm with cutting rings (a 5 cm in high, 5 cm in inner diameter and 100 cm3 in cubage). At the same time,the disturbed soil of about 100g at each depth was collected for determination of soil structure at the State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau.
Cutting ring was used to measure the bulk density, total porosity, capillary porosity, saturation moisture content. The core samples (undisturbed soil sample) collected were used with cutting rings to measure the soil bulk density, capillary porosity and noncapillary porosity. The bulk density was determined by oven-drying the cores at 105-110℃, and the total porosity was calculated as 1-bulk density/soil particles density, assuming that the density of soil particles was 2.65g/cm3. Noncapillary porosity was the difference between total porosity and capillary porosity. Soil particles were measured with master sizer 2000 laser particle analyzer and grain size was graded on the USA standard. Soil water contents at different soil suctions ( 0.1, 0.2, 0.4, 0.6, 0.8, 1.0, 2.0, 4.0, 6.0 bar, 1 bar = 1.0×105 Pa ) were measured by a HITACHI centrifuge, made by Instrument Co., Jappan. Because Huangmian soil had been contracted when measuring with a centrifuge, the researchers measured the shrink amount of soil samples in the cutting ring by vernier callipers at different soil suctions and then calculated the volumetric soil water content.
Select 1-year-old and 23-year-old red plum apricot tree with average height and canopy as the sample of study. Two holes with 5.3 cm in diameter were made by holesaw in the place about 40cm cm apart from the 1-year-old red plum apricot tree, and two 4-m long aluminum access pipes were placed in the holes with an interval of 1 m between them. Another two holes with 5.3 cm in diameter were made by holesaw in the middle of the radius of red plum apricot tree canopy, about 2 m away from the tree base (centre) to the exterior margin of the canopy in the 23-year-old red plum apricot tree planted in the bench terrace in 1996. The interspaces between access pipes and soil were filled with some fine earth in case water might flow through the interspaces. A neutron probe, CNC503A (DR), made by Beijing Nuclear Instrument Co., China, was used for long-term monitoring of the field soil water content because of its high precision in situ (Wang and others 2000; Evett and others 2012; Guo and Shao, 2013). Before measuring the volumetric soil water content (VSWC), the neutron probe was calibrated for the soil in the study area by using standard methods (Hauser 1984). The calibration equation for this soil at the site is y = 55.76x + 1.89, where y is VSWC, and x is the ratio of the neutron count in the soil to the standard count. The measuring depth ranged from 0 to 400 cm in the period from April to October, 2018. Measurements were made with 15-day intervals in time and 20 cm intervals in depth. Measurements were made every 15 days to a depth of 4 m in increments of 20 cm starting at the 5 cm depth. When measuring soil water content at different soil depth, first put the probe into the aluminum access pipes and change the measuring line of the neutron probe to confirm the weather or not the soil depth equal planned depth of determination according to the display device of soil depth .Second, press the start button and then read and record the numbers of soil water content at different soil depth on the display screen of the neutron probe. The soil water content obtained for each measuring depth was taken to be representative for the soil layer that included the measuring point ± 10 cm depth, apart from that for the 5 cm depth, which was taken to represent the 0 to 10 cm soil. The measurements were also made before and after each rain event in the red plum apricot forest.
Height, diameter at the base and size of the canopy of the 1-year-old red plum apricot tree growing on the plots were investigated and measured, and estimate the maximal infiltration depth and Soil Water Resources Use Limit by Plant. the relationship between the colour of leaf or the size of fruit and the soil water was investigated and estimate the suitable amount of leaf and vimen when the soil water resources in the maximal infiltration depth is approach to or smaller than Soil Water Resources Use Limit by Plant in 23-year-old red plum apricot tree. The measurements of red plum apricot tree growth were carried out in the time period from mid-April to October, and the measurements of precipitation and soil water were carried out from January to December in 2018 to 2020.