effect of grazing intensity on some soil chemical characteristics in gardaneh zanburi rangeland of arsanjan

introduction:changes caused by grazing on range ecosystem are generally assessed based on the soil conditions and vegetation. livestock as one of the major elements in range land ecosystems has different effects on different parts of this ecosystem. one of these impacts is excessive livestock grazing capacity which can have different effects on soils and plants in various intensities. materials and methods:gardaneh zanboorirangelandis located in arsanjan in fars province. this isanareaof mountains, hillsandplains with the maximum height of 2280 meters and minimum height of 1640 meters above sea level. related areas were separated under three different management methods of enclosure, moderate grazing and heavy grazing. these three areas are considered as symbolic areas of grazing intensity including the reference area where no grazing intensity was observed, the key area where medium to heavy grazing was applied and critical area where heavy grazing was used. these areas were similar in all characteristics such as topography, soil type and rainfall and differed only in their grazing intensity factor. then, soil samples were collected. random systematic soil sampling was conducted at two horizons of 0 -15 and 15 -30 cm. therefore, five profiles in each area (enclosure, moderate grazing and heavy grazing), a total of 15 soil profiles, were excavated and two samples were taken in each profile (one sample from each horizon). finally, the thirty soil samples were transported to the laboratory. samples were dried in the air laboratory and passed a two millimeter sieve after smashing. factors such as n, p, k, om, ec and ph were measured in each sample in the laboratory, the percentage of p was determined by the olsen method while the percentage of k was determined using the flamephotometry method. moreover, n was measured using the kjeldhal method. c was measured by the walkley and black method. the percentage of om was found by carbon multiplying percentage at 1.72 numbers. ph was determined with measuring the ph of saturated soil by ph meter machine. measurement of soil ec was performed by the electrical conductivity meter. data analysis was conducted by spss software. comparing of mean values for each factor and between areas with different grazing intensity was done by the tukey test. results and discussion: two-way analysis of variance and tukey test showed no significant differences in term of n, om and ph between critical and reference areas. but the amount of n and om in the key area is lower than that of the reference and critical areas. while value of ph is higher than the other regions. also values of p and k decreased within creasing grazing intensity but the ec factor increased. conclusion: since vegetation removal and its exclusion from the ecosystem followed by considerable effect on the cycle of nutrient elements and their absorbability, it seems that in the studied ranges, the p and k elements decrease through the use and leaving of vegetation in the area. the results of n and om showed that moderate grazing causes further decomposition of plant residues and organic nitrogen mineralization but there was no difference between the two treatments of heavy grazing and enclosure areas. in heavy grazing intensity, the amount of om and n increases by several mechanisms. first, with soil bulk density and increased soil compaction, the oxygen supply and degradation rate decreases. in the second mechanism, intensive grazing changes the vegetation composition and root to shoot ratio. in the third mechanism, animal urine and feces can speed up the nitrogen cycle in grassland ecosystems. it seems that the simultaneous effect of the above factors studied in the related range causes no significant difference between heavy grazing and enclosure areas in the percentage of total nitrogen. ec is the lowest in the enclosure area. this is due to the absence of livestock and therefore no stepping on the soil and also more vegetation. the ph level of enclosure area is less than that of the moderate grazing area. this may be caused by more presence of organic matter in the soil of enclosure. when organic matter decomposes, organic acid and mineral acids are produced. permanent production of acids in the soil in places where the root density is high causes dissolution of limestone and the soil is washed and so it reduces the ph.