Plant materials and site description
The research area is located in the southern part of the Loess Plateau, belonging to the temperate continental monsoon climate. The experimental areas of the study was located in Zhangjiagou village (34°19 '36 "N, 107°37' 16" E) and Liujiagou village (34°19 '41 "N, 107°38' 40" E) in Qishan county, Shaanxi Province. The annual average sunshine hours are 2185h, the average temperature is 11.6℃, the extreme maximum temperature is 41.4℃, the extreme minimum temperature is -20.6℃, and the frost-free period is 198 days. The mean annual precipitation is 679mm, which can basically meet the needs of crop growth. However, due to the influence of monsoon climate, the annual seasonal distribution is not uniform. The precipitation in June to September accounts for about 70% of the annual precipitation. In the test area, the soil was mainly made of loess mother material, with organic matter content of 2.45%, total nitrogen content of 0.116% and total phosphorus content of 0.098%(IUSS 2015).
Soybean is the major crop in the researched area. The crop under tree canopies selected in this research was soybean (Qindou 8). Xiangling, a local walnut variety that is most widely planted in the area, was selected. The walnut trees were planted in 2016 (1a) at 3m×6m row spacing. The width of soybean alleys was 4.8m, while that of walnut tree strips was 1.2m. Soybean was planted on June 6, 2017 at 30cm*30cm row spacing. The planting density of monocropped soybean was the same as that of its intercropped counterpart. An experimental plot used to measure dynamic changes in root length density of tree seedlings in different planting years was established in 2007, which underwent the same arrangements as in the experimental field described above.
Photosynthetic and respiration rate sampling
On three consecutive sunny days, the net photosynthesis rate (Pn), stomatal conductance (Gs), respiratory rate (Tr) and intercellular CO2 concentration on the second fully expanded leaf of soybean, which were selected during 9:00-11:00 a.m., were measured with the portable photosynthesis system (Li-6400; LI-COR Inc., Lincoln, NE, USA).
Soil sampling
Soil Moisture Content 20 sampling points distributed in an s-shaped form were established in the sample plot, where the soil auger was used to obtain samples at the depth of 0-20, 20-40, 40-60cm. 100g of soil was weighed and place it in the oven at 105±2℃to dry the constant weight (W). Samples were taken during the branching, flowering and pod bearing periods, respectively.
See formula 1 in the supplementary files.
Collection and measurement of photosynthetic data
Root sampling
Root samples were taken with a soil auger. The total sampling depth was 60cm, with 10cm for each layer. Three sampling points distributed in a triangle form, far away from fringe areas, were established in the sample plot. Samples were taken during the branching, flowering and pod bearing periods, respectively.
Soybean fine roots were prepared through soaking, washing and separating procedures. Fine roots with a diameter less than 2mm were selected. WinRHIZO root analysis system was applied to obtain the morphological indicators of fine roots, such as root length density and fine root diameter.
After the fixed sample plots were established, a minirhizotron system (BTC-100 Borescope Root Ecology Monitoring System) was employed to carry out in-situ observation of seasonal growth dynamics of roots. Root imaging data were collected every 10 days over an entire growth cycle. Root length parameter was obtained using the Winrhizo-tron-MF to calculate root length density, which was followed by a calculation of fine root length for the observation period.
Pianka(1973) niche overlap formula (symmetric α method) is adopted to calculate the competition intensity index among walnut soybeans, and the formula is shown as follows:
See formula 2 in the supplementary files.
Aih = Ahi, and Aih≤1, r is depth of soil layer, and r≤3, Pij and Phj are respectively the proportion of species i ( length density of soybean) or species h ( length density of soybean) using the j ( length density of soybean and walnut) resource bit in its utilization of all resource bits. Pianka method is insensitive to the number of individuals of a population or its quantitative characteristics of a population in a community, but it can objectively reflect the extent of niche overlap between populations and the changes of niche overlap relation between populations, and is used to measure the degree of competition of Shared resources among populations under the condition of resource shortage.
The data of the test were analyzed by SAS 8.2 software for variance analysis, and the figures were made by OriginPro9.0 software. On the basis of data analysis, the soil layers discussed were set as 4 soil layers of 0-20, 20-40 and 40-60 cm.