Effect of grape leaves and pomegranate peel on soil structural stability and water repellency in different salinity levels

Introduction Crop production in arid and semi arid regions especially in saline soils always has many problems. Soil low organic matter content is one of the limiting factors in arid condition. Incorporation of plant residues is a good strategy for increasing the soil organic carbon and consequently for improving soil physical quality (34). However, some studies have shown that addition of organic matter in to the soil can increase soil water repellency (17, 18). This study was conducted to investigate the effect of grape leaves and pomegranate peels on the soil structural stability and soil water repellency in different salinity levels.
Materials and Methods The arable soil was collected from the soil surface layer (0–20 cm) of Bastam Agricultural Research field in Semnan province and passed through a 4 mm sieve. A greenhouse experiment was conducted with three treatments including plant residues type (Grape leaves and pomegranate peels), amount of plant residues (0, 2 and 5 g 100 g-1 soil ) and salinity (1.5, 7 and 15 dS m-1). Plant residues were collected from grape and pomegranate gardens and after drying, milled and passed through a 1 mm sieve. Plant residues were mixed with soil and salinity treatment was applied with calcium chloride salt. Pots were filled with mixture of saline soil and plant residues and incubated in the greenhouse for 50 days. The soil organic carbon, soil hot water and diluted acid carbohydrate, soil microbial basal respiration, water dispersible clay and soil water repellency were measured at the end of the experiment.
Results and Discussion Pomegranate peels increased the soil organic carbon content and hot water and diluted acid carbohydrates more than the grape leaves (Table 5) due to greater C:N ratio and lower microbial decomposability (37). Soil microbial basal respiration was 15.5% lower in pomegranate peel treatments than grape leaves and (Table 5). Water dispersible clay decreased by increasing the amount of plant residues (Table 8). Soil organic carbon increased by the amount of plant residues. Soil organic carbon is an important factor in stability of soil aggregate and consequently decreases the soil water dispersible clay (7). Strong negative correlation between soil organic carbon and soil water dispersible clay can confirm these results. In addition, the soil carbohydrates are known as an important factor in stability of aggregate especially for macroaggregates (40).
Salinity increment from 1.5 to 15 dS m-1 caused a reduction in water dispersible clay from 45.1 to 31.2 g kg-1 soil (Table 9). Calcium as a divalent cation is an important factor in soil structural stability and probably decreased the soil water dispersible clay (7).
Soil repellency index was greater than 1.95 in all treatments and ranged from 2.3 and 5.9 in different treatments. These results indicated subcritical soil water repellency in soil. Soil water repellency index increased 38 and 67 percent in treatments with 2 and 5 g residues 100g-1 soil compared to control treatment (no residue) (Figure 3). In addition, soil hydrophobicity was 10% higher in the pomegranate peels treatments than in grape leaves treatments (Figure 4). Soil organic carbon and soil hot water and diluted acid extractable carbohydrates concentration increased by the plant residues addition. The soil organic components have a hydrophobic and a hydrophilic parts and the orientation of hydrophobic parts on the soil particle surface can make a repellent soil surface (6).
Soil calcium (Ca) concentration increased by salinity. This divalent cation in the soil solution could act as a bridge between the soil particles and functional groups of dissolved organic matters. This bridge could facilitate covering of soil particles by hydrophobic compounds and make a more stable soil structure by flocculating soil particle at high salinity levels (7). The hydrophobic coatings on the soil surfaces increased the solid–liquid interfacial free energy (γsl) and decreased the solid surface free energy (γsg) as indirect effects of salinity on repellency. In addition, water entering into the soil immediately dissolved the soluble salts which had precipitated in the initially dry soil. There is some evidence showing that surface tension of water (γlg) increases with salts. Decrease of γsg and the increase of γsl and γlg might cause the repellency increment (43).
Calcium bridge between soil particles could improve the soil structure with salinity increment. Increasing the SE by salinity in this study confirms this hypothesis. The soil SW depends on pore geometry and hydrophobic coating on soil particles, but the soil SE only depends on pore geometry. Thus, increasing the SE might be an indicator for better pores connection and stable structure (15).
Conclusion Many grape leaves and pomegranate peels are produced in Iran every year. These plant residues are potentially a good source for increasing the soil organic carbon. Our results showed that incorporation of these plant residues in to the soil could increase the soil organic carbon and carbohydrate concentration and improve the soil aggregates stability. However incorporation of residues into the soil increased the soil water repellency. In addition salinity increment induced soil hydrophobicity. More detailed studies are needed to understand the positive or negative effects of this subcritical hydrophobicity development in saline soils.