ارزیابی آستانه‌های فرسایش پذیری خاک متاثر از نوسانات سطح ایستابی آب‌های زیرزمینی

Introduction
Land subsidence caused by excessive extraction of groundwater resources is a worldwide problem in many arid or semiarid countries such as Iran, which depend on groundwater resources. One of the most important consequences of groundwater table drawdown is profound changes in the soil physical properties due to soil compaction associated with land subsidence that has critical role in the acceleration of desertification and intensification of water erosion because of lack of water infiltration into soil. As over time, the over-exploitation of groundwater resources may lead to a declining water table and the associated enhanced loss of water within soil layers. This in turn reduces soil porosity volume and causes soil inelastic compaction in the aquifer system. These conditions promote land subsidence and result in horizontal land deformation and the associated soil compaction. Land subsidence is due to compaction of clay beds within the aquifer systems. When groundwater level is considerably high, the gravel and sands are buoyant. As water table reduces resulting from over extraction, the rate of coarse fractions buoyancy is decreased and therefore additional weight from the gravel and sand stimulates descending pressure on clay beds that are between the sand and gravel strata from which water has been extracted. When the water held in the clays can no longer withstand the pressure from the increased weight of the gravel and sands above, the clays are compressed and water is squeezed from them. These clays will never absorb again the water that has been expelled from them. The aim of this study was to examine how fluctuation in groundwater level drawdown and subsequently land horizontal deformation associated with land subsidence can impact on soil erodibility level via changes in soil physical and biochemical attributes.
Materials and methods
The study sites were located in Neyshabur County, Khorasan-Razavi province, northeastern Iran. Statistic of the piezometric wells taken from Regional Water Organization of Khorasan-Razavi province demonstrate that the selected sites exhibit statistically significant differences (P < 0.05) in the rates of groundwater level drawdown. At each the site 8 earth fissures were selected. Soil samples were collected from 20 replicate 1 × 1‐m quadrats. Soil physical properties are recognizes as indices of soil compaction i.e., bulk density, total soil porosity, macro pores volume, micro pores volume, soil moisture, and infiltration capacity also the associated some biochemical attributes were measured.
Results and discussion
The statistical results of the measured indicators of soil compaction illustrated the appearance of different degrees of soil erodibility along fluctuation of the groundwater level drawdown. It is noteworthy that different soil compaction levels associated with land subsidence were considered as an index for assessing erodibility level. The bulk density significantly (P < 0.05) increased in the site A that had the greatest value of the drawdown compared with the site B with decreased values of the drawdown. Macro porosity volume significantly decreased in the site A that had the greatest value of the drawdown compared with the site B. Microporosity volume and soil moisture significantly increased in the site A with the light rates of the drawdown in comparison with site B. Also, infiltration capacity significantly (P < 0.05) increased in the sites that had light drawdown of groundwater level compared with the deep drawdown. These findings illustrated the impacts of land subsidence related to fluctuations of groundwater level drawdown in the occurrence of different rates of soil erodibility as changes in the level of soil compaction. Although, there is no dedicated study regarding impacts of fluctuations in groundwater level drawdown on the soil compaction degrees (as an index of soil erodibility level), other studied in relation to soil compaction resulting from agricultural field traffic can confirm these findings. Our findings is in agreement with the mentioned studies that soil compaction can strongly affects the level of total porosity and volume of macro and micro porosity within soil profile. Further, the statistical results of the biochemical indicators from soils belonging to earth fissures in the sites with different rates of the drawdown explain that how changes in soil compaction degree due to variation in the rate of groundwater level drawdown can affect soil productivity indicators. Microbial biomass carbon and microbial biomass nitrogen showed the lowest values in the deep groundwater level (site A), demonstrating the critical impacts of sever soil compaction resulting from deep of groundwater level on reducing microbial activities and microbial nitrogen immobilization. There is a significant relationship between the continuous drawdown of groundwater level with soil erodibility indicators related to soil compaction and biochemical attributes. This means that total porosity and macro porosity volume decreased as groundwater level has decreased more and more.
Conclusions
Fluctuation in groundwater level over time can critically affect soil erodibility level via increasing soil compaction. At the site A that had more severe drawdown of groundwater level in comparison with the site B with the decreased rates of the drawdown during different years, it was seen that total porosity, macro porosity volume, and infiltration capacity significantly decreased, causing reduction in microbial activity level as significant decreasing MBC and MBN. The findings explain the critical role of land subsidence related to groundwater level drawdown and the associated fluctuation in increasing erodibility level of dryland soils that are vulnerable to environmental harshness.