عنوان مقاله [English]
Overgrazing affects the functionality of rangeland ecosystems. One method for monitoring an ecosystem is landscape function analysis (LFA). In this method, using 11 soil surface indices, three functional properties including soil stability (soil ability to resist erosion and reform after disturbance), infiltration (soil capacity to retain rainfall for plant-available water) and nutrient cycling (how efficiently organic matter is cycled back into the soil) are assessed. In this order, the current study aimed at assessing soil functional properties under various grazing intensities in Sahand mountains rangelands in East Azerbaijan Province, Iran. Data analysis was done using SPSS version 20. The result showed that soil stability, infiltration and nutrient cycling were significantly different in different grazing intensities (p < 0.01). An increase in grazing intensity led to the reduction of functional properties. In rangelands with hight, moderate and light grazing intensity, the average soil stability index was 36.66%, 48.26% and 52.85%, respectively, the average infiltration index was 31.11%, 34.9% and 37.62%, respectively. The average nutrient cycling index was 20.85%, 27.85% and 30.68%. In general, based on the results of the present study, with increasing the intensity of livestock grazing, soil functional properties decrease, so controlling the balance of livestock and rangeland and reducing the intensity of livestock grazing in the study area is recommended.
Rangelands are natural ecosystems and cover 84 million ha of the Iran territory which is equal to 52 percent of its total area (Rostami et al., 2014). Grazing intensity is one of the factors which affects rangeland ecosystems. Intense grazing in rangelands particularly in arid and semiarid lands is considered among the most important degrading factors (Mesdaqi, 1996). Rangeland degradation leads to the ecosystem instability and threats rangeland sustainable exploitation for the near future. One of the agents that make rangeland condition volatile is overgrazing which cause a decline in the functionally of ecosystems (Bagheri et al., 2010). The degradation of vegetation patches owing to overgrazing fascilitates desertification (Tongway and Ludwig, 2002). Hence, assessing soil surface characteristics and indices has an important role in evaluation of rangeland functionality (Ludwig et al., 1997). It is possible to use simple and visible indices of soil surface such as soil stability, infiltration and nutrient cycling to evaluate the ecosystem condition (Tongway and Hindley, 2004). With this respect, landscape function analysis (LFA) is one of the simplest methods to determine ecological function of plant vegetation and soil condition after being exposed to various disturbances. Therefore, this study aimed at assessing the effects of livestock grazing on soil function in Sahand mountains rangelands in Maragheh County in East Azerbaijan Province, Iran.
The current study was conducted in Sahand mountains rangelands located at 30 km distance from Maragheh county in N 37° 27′ and E 46° 33′. The climate of the region is semiarid with an average altitude of 1900 m a.s.l. Based on the statistical data from the nearest meteorological station, the average annual precipitation is 322.4 mm with cold and snowy winters. The maximum and minimum monthly precipitation are 64.6 and 1.6 mm in April and August. The maximum and minimum monthly temperature are 19 and 5.4 °C with an average annual temperature of 12.5 °C (Iran Meteorological Organization, 2019).
Three sites with three different grazing intensities were selected. Selection of these sites was done based on the stocking rate per area unit and the length of the grazing periods. All the sites were located on the southern slope of Sahand Mountains with an average slope between 10-25 percent and an altitude of 1900-1950 m a.s.l. According to the interviews conducted with local sheep herders and rangeland experts, the three sites had been traditionally grazed for more than three decades by pastoralists’ flocks which were constituted mostly from sheep and goat. The three grazing intensities were defined as follows:
Lightly grazed: the pastureland was almost completely covered with plant species (vegetation cover over 80 percent) and the stocking rate was between 2-2.5 heads of livestock/ha.
Moderately grazed: this pastureland was relatively far from the settlements and pens, however, the grazing was done relatively regularly. The stocking rate was between 3-5 heads of livestock/ha. The land was covered with 60-75 percent vegetation cover.
Highly grazed: the area included lands adjacent to the settlements and pens under constant livestock grazing and trampling. The vegetation cover was less than 35 percent and the stocking rate was between 4.5-5 heads of livestock/ha.
Five 50 meter long trasects were installed in each site with 100 meter distance from each other. The length of vegetation patches and inter-patches were measured. Then, the 11 indices of LFA method were measured in each transect. The indices included: (1) perennial vegetation canopy and rock cover; (2) perennial vegetation basal (grass) or canopy (trees and shrubs) area; (3) litter cover, depth, origin and decomposition; (4) cryptogams (biological crust cover); (5) crust brokenness; (6) erosion type and severity; (7) deposited materials; (8) soil surface roughness (microtopography); (9) surface resistance to disturbance; (10) slake test and (11) soil texture. Data analysis was done using SPSS version 20.
Evaluation of soil and vegetation factors may shed lights on potentials of rangelands to subsequent determination of rangelands conditions. Vegetation are affected directly by soil factors. Overgrazing affects the functionality of rangeland ecosystems. The result showed a significant difference between the three sites with different grazing intensity regarding soil stability, infiltration and nutrient cycling. It was found that the higher grazing pressure leads to a decline in all soil function indices. The comparison between all sites showed the average infiltration index of 31.11 for the lighly grazed site, 34.9 for the moderately grazed site and 37.62 for highly grazed site. It was also found that the average nutrient cycling for lighly, moderately and highly grazed sites were respectively 20.85, 27.85 and 30.68. The soil stability index showed an average of 36.66, 48.26 and 52.85 for lightly, moderately and highly grazed sites. The highest figure for the three indices was observed in the lighly grazed site. Although there was a significant difference between all three sites, the decline in the indices for the highly grazed site was very high. Hence, continuing the current traditional grazing systems are recommended with light and moderate grazing intensity.