Discussion
Spraying oil-mulch significantly increases vegetation cover and litter percents while it simultaneously decreases the percent of bare soil (Table 3 and Fig. 2). These positive effects of oil-mulch treatment have probably resulted from three mechanisms include 1) stabilizing effects on sand dunes (Wuddivira et al., 2013), 2) decreasing water evaporation and preserving available soil moisture (Kowsar et al., 1969; Mulumba and Lal, 2008; Liu et al., 2020) and 3) increasing the rate of seedling survival (Akbarian and Mosavi, 2006) and germination (Farahpour et al., 2005; Jafarian, 2006). Arid and semi-arid regions are known as water-limited areas (Shojaei et al., 2020), and thereby, soil moisture is an important factor that controls the seedling germination, survival, and growth (Zhenghu et al., 2004). It has been reported that oil-mulch treatment could provide more benefits in encouraging seed germination than other tested sand stabilizers (Farahpour et al., 2005), mainly due to the presence of more soil moisture and temperature. In addition, the positive effects associated with the use of oil-mulch were reported in terms of soil organic matter, soil water holding capacity, and the amount and activity of soil organisms (Pouyafar and Asgari Moghadam, 2006; Hashemimanesh and Matinfar, 2012).
Although it is not expected that after short-term oil-mulch treatment, an increase occurrs in amount of plant litter, our finding may even indicate negative effects (i.e., drying effects on the plants) of oil-mulch treatment on the initial vegetation. On the other hand, however, the increase in litter accumulation may increase along time after oil-mulch treatment, resulting in an increase in soil fertility, infiltration, and stability as well as a decline in soil erosion (Shojaei et al., 2020). The positive effects of plant litter in increasing soil water holding capacity, water retention, and moisture loss reduction from evaporation were also reported (El-Kader et al., 2010; Simmons et al., 2010; Rodrigo‐Comino et al., 2020). Similarly, a positive linear effect of mulch application rate on soil organic matter concentration was reported by Duiker and Lal (1999). In addition, an increase in litter accumulation may increase the hydraulic conductivity of soil stability (Moazzemi et al., 2013) and improve the specific mass and soil porosity (Shojaei et al., 2020), increasing the strength of the soil surface and reduction of the soil erosion (Singh and Agrawal, 2008; Mulumba and Lal, 2008).
Our results indicate that the short-term application of oil-mulch significantly decreases bare soil percent (Table 3 and Fig. 2). In line with this result, Hashemimanesh and Matinfar (2012) reported that the petroleum mulch project in southwestern Iran decreased sand dune areas from 66.4% in 1991 to 56% in 2002. Similarly, Azoogh et al. (2018) results indicated that petroleum mulching-biological fixation practices promoted the restoration of vegetation cover in the sand dunes. Therefore, this result is likely explained by the positive effects of oil-mulch treatment on vegetation cover and litter.
Also, results indicate that species diversity and evenness are negatively, and species richness is neutrally related to oil-mulch treatment. Generally, in a short-term oil-mulch application, negative (Karami khaniki, 2009; Adeli, 2012; Gholami Tabasi et al., 2015) and neutral (Jafari et al., 2017) effects on vegetation diversity are reported. In addition, Gholami Tabasi et al. (2014) found that applying the plantation and oil-mulching negatively and neutrally affects vegetation attributes (i.e., cover and density) of Astragalus squarrosus and Convolvolus hamadae , respectively. In contrast, Tabasi (2014) indicated that oil-mulch did not negatively affect plants; however, it may increase vigor and biomass in the plant’s stands. Therefore, it seems that these negative effects of oil-mulch treatment are likely related to the drying effects of oil-mulch on the plants, especially in the early period after the application of oil-mulches. While, by the passage of time since mulching, plant diversity could increase in the mulch-treated area even more than the control area. For example, Khalilimoghadam and Bodaghabadi (2020) reported that ’Shannon’s diversity index increased over the time period of 30–40 years compared to the initial years of sand dunes stabilization (< 5 years).
The current study was designed to fill the gap in our knowledge about oil-mulching effects on RCS. The results showed that RCS increased one year after the oil-mulch treatment, i.e., it changes from very poor class (RCS = 12) to poor class (RCS = 22) in control and oil-mulch treated. In this study, RCS was determined based on four parameters, including BS: bare soil, LF: litter frequency, WPC: weighted palatability classes, BP: biomass production (Table 1). As shown previously, results demonstrate that short term oil-mulching increase vegetation cover and litter and decrease bare soil percent. In addition, a strong positive association between vegetation cover and biomass production was reported (Flombaum and Sala, 2007; Pordel et al., 2018; Louhaichi et al., 2018). Therefore, it can be said that oil-mulch treatment leads to better RCS by affecting vegetation attributes (i.e., cover, biomass). On the other hand, however, among all RCS parameters, only WPC was not significantly different between control and mulch treated sites. This difference mainly related to non- or low-palatable plants that reintroduced and planted with mulching such asCitrullus colocynthis L., Prosopis juliflora (Sw.) DC.,Eucalyptus camaldulensis and Ziziphus spina-christi (L.) Desf. Considering that native species had a greater chance to survive, establish and grow in such arid condition (Jafari et al., 2018; Bazgir et al., 2020), the management practices after oil-mulch treatment (i.e., selection of suitable plants) had a vital role in achieving successful results.
To compare the vegetation cover pre and post oil-mulch treatment (one year before (2017) and one year after (2019), we used the remotely sensed vegetation index extracted based on Landsat 8 OLI imagery (i.e., MSAVI). Results demonstrated that vegetation cover greatly increased in 2019 compared to those found in 2017, indicating positive effects of oil-mulching and confirmed our previous results. This increase in vegetation cover compared to the control area mainly resulted from two reasons, including 1) positive effects of oil-mulch treatment and 2) increased amount of precipitation in 2019 compared to 2017 (210.7 and 638.8 mm, respectively). As previously noted, soil moisture and instability are the main limitations in an arid area, and any factor like oil-mulch treatment which increases the soil moisture will lead to an increase in vegetation cover (Akbarian and Mosavi, 2006; Mulumba and Lal, 2008; Wuddivira et al., 2013; Shojaei et al., 2020). This conclusion could be confirmed by the two-way ANOVA results that revealed that vegetation cover was greater for 2019 than 2017 and in the oil-mulch treated site in 2019 compared to the correspondence control site (Fig. 5). In addition, no significant difference was observed between controls and mulch sites in 2017, highlighting the importance of oil-mulch treatment. Therefore, the greater vegetation cover in 2019 synergically responded to the joint effects of oil-mulching and an increase in precipitation.