Soil sampling and analysis

Shelterbelts in the TDRS had been planted for 1, 4, 7, and 10 years and are drip irrigated with a salinity of 4.04 g/L, because this salinity is most common and is used to irrigate the largest area near the TDRS. In July 2014, monitoring took place adjacent to sets of five drip-emitters. Soil samples 30 cm from the emitters were collected daily before and after irrigation for a 15-day irrigation period for T. ramosissima and H. ammodendron, and a 10-day irrigation period for C. arborescens, respectively. All samples were collected to a 200-cm depth at 9:00 am at 20-cm increments. Soil moisture-salinity dynamics in one irrigation cycle were averaged for four shelterbelt ages, and moisture-salinity distribution for each differently aged shelterbelt was averaged from daily data.

In addition, for a shelterbelt planted in 2003, sets of five emitters, each irrigated with groundwater having salinities of 2.82, 4.82, 10.00, 18.36, and 25.90 g/L (Fig 1), were monitored to a depth of 200 cm with increments of 20 cm during one irrigation cycle. Sampling plots were 30 cm from the emitters and perpendicular to the plant rows.

All samples were divided into two portions for moisture analysis and electrical conductivity (EC) measurement. Soil moisture was determined by mass loss after drying at 105°C for 12 h. The daily soil water storage (W_c) in one irrigation cycle was calculated by [33]:

where W_c (mm) is soil moisture storage; V_s (cm³/cm³) is soil water content; and H (mm) is the thickness of the soil layer.

Another portion of the samples was air-dried and sieved through a 1-mm sieve. Soluble salt estimates were then based on extracts of one part soil to five parts water by weight, and the EC was determined using a conductivity meter. All values were averaged from five duplicates.

Data for the 1, 4, 7, and 10-year shelterbelts irrigated with 4.04 g/L salinity groundwater were used to analyze the daily dynamics of soil moisture and salinity. Average values for the shelterbelts planted for different years were used to analyze the effect of shelterbelt age on soil moisture-salinity distribution. Average values for shelterbelts irrigated with different salinity were used to analyze the effect of irrigation salinity on soil moisture and salinity distribution.