Effect of Organic and Inorganic Mulches on Soil Moisture, Soil Temperature, Stomatal Conductance and Leaf Temperature Changes of Pistachio (Pistaciavera L.) Trees in Arid and Semi-arid Climate

Mulching is a useful practice with the potential of conserving moisture, reducing evaporation, modifying soil temperature, and improving aeration as well as releasing nutrients in the soil profile. Mulching involves the use of organic materials (e.g. crop residues, straw, grasses, and farmyard manure) or inorganic and synthetic materials (e.g. polyethylene sheets, and gravels). Application of mulch can notably as a soil management method influence agricultural crop production despite the limited amount of water available in arid and semi-arid areas.
Field studies were conducted in a randomized complete block design with five replications for two years in 2014 and 2015 at Feyzabad city, Iran (34° 40´ N, 58° 25´ E). The aim of the study was to investigate the effects of different mulching treatments on physiological traits of pistachio (Pistaciavera L.) in relation to soil temperature and moisture variations. The treatments were: M1 (whitepolyethylene film mulch covered with two centimeter of soil with width cut), M2 (white polyethylene film mulch covered with two centimeter of soil with circular cut), M3 (green polyethylene film mulch which was as a single layer pulled on water strip so that irrigation water passes under the coverage), M4 (woodchip mulch), M5 (barley residue mulch), and CK (control or no mulching). The measurement (soil moisture and temperature, stomatal conductance and leaf temperature) was carried out periodically on the central trees (4 trees per treatment) every 12, 24 and 36 days after irrigation between 12-15 pm from early-May to late September. The data obtained from the experiment were subjected to an analysis of variance (ANOVA) by using SAS 9.1 software. Difference between means was compared using least significant difference test (LSD) at 5% level (p ≤ 0.05).
The results showed that the mean soil moisture percentage in all mulches especially plastic mulches was higher compared to the control over two years of experiment. During the two years of experiment, treatments of M1, M2 and M3 with 35.2, 35 and 38.9 °C and treatments of M4, M5 and CK with 28.8, 29.6 and 32.8 °C indicated the highest and lowest average soil temperature at 20-30 cm soil depth, respectively. Also, similar results were observed at 50-60 cm soil layer. M3 and CK treatments with 47.7 and 41.1 mmol/m-2s-1 had the highest and lowest mean stomatal conductance, respectively. The mean values of leaf temperature during the growing period under M1, M2, M3, M4 and M5 treatments were 2.2, 1.9, 2.4, 1.7 and 1 °C in 2014, and 1.8, 2.2, 2.2, 1.5 and 1.1 °C in 2015 lower than CK (control) treatment, respectively. According to correlation results significant difference was not observed between stomatal conductance and soil moisture, but the relationship of this parameter with soil temperature and relationship between leaf temperature with soil moisture and soil temperature were significant. Thus, this results demonstrate that stomatal conductance and soil temperature weresignificantly affected by soil temperature, so that an increase in soil temperature to 40 °C, canbe improved these traits if other soil and climatic factors werenot limiting. The results also showed that M3 mulch (plastic mulch on soil surface) despite the higher soil water storage due to providing of soil high temperatures, can be leads to moisture stress in heavy cropping year.
The results of thisstudy showed that organic and inorganic mulches improved the soil moisture content throughout the two growing seasons, thus reduced negative effects of water shortage. Mulch treatments indicated smaller fluctuations in soil temperature compared to CK (control) treatment, possibly due to insulating against heat radiation. The results of our study suggested that increase in soil temperature in M3 treatment, despite higher water storage, had limited impacts on improving stomatal conductance and leaf temperature from early July in the second year. Thus, it seems that high soil temperature in this treatment in ON year that tree's water and nutrients requirement is higher than OFF year acts as a limiting factor and restricts absorption of water and nutrients, resulting in the decrease of physiological activity of trees. Generally, it is conclude that white polyethylene films covered with soil especially M2 and organic mulches due to providing optimum soil temperature, preserving soil moisture and lower cost can be better management options to be applied in pistachio orchards especially in arid and semiarid areas, where traditional irrigation methods with high evaporation level are used.