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5 years ago

Die Wirksamkeit von Boden

Die Wirksamkeit von Boden

Performance of exclosure

Performance of exclosure in restoring soil fertility SWC structures constructed during/before exclosure implementation have played vital role to intercept of runoff and enhanced infiltration. The SWC structures enhance in-situ soil conservation and minimize material redistribution through erosion from the upper towards the lower landscape positions. Thus, structures retain existing or newly dropped litter in-situ than transported down-slope. It could be possible to conclude that the structures played complementary conservation effect to that of exclosure restoration. Therefore, the non-significant soil properties differences across landscape position could be attributed to the complementary effect of the physical SWC structures. Converse to other research, this study indicates that soil nutrients are fairly uniformly distributed across the landscape positions of the exclosures. The lower landscape positions are mostly in close vicinity to settlements and cultivated and grazing lands. Consequently, these parts of the exclosures are prone to occasional human and livestock disturbances. The interferences remove organic matter through cutting and livestock grazing, which in turn negatively influenced soil fertility restoration. As a result, most soil nutrients except exchangeable bases were low at lower landscape positions (Table 7.7). These soils on average had 4.5 g/kg less OC, 0.6 g/kg less TN and 11% less available P than soils in the middle landscape positions. Similarly, soils in the lower landscape positions on average had 2 g/kg less OC, 0.4 g/kg less TN, 150% less available P and 0.13 g/kg less exchangeable K + than the soils in the upper landscape positions. 7.4 Summary and conclusions Among the different SWC interventions, exclosure is a widely implemented activity for rehabilitating degraded lands. Despite the massive SWC interventions, their impact has not been sufficiently studied. Thus, this study focused on analyzing the impact of exclosure to restore soil fertility and evaluated the restoration variation across age, agroecological zone and landscape position of the exclosures. The study was conducted in Gubalafto district of the North Wello zone in Amhara National Regional State of Ethiopia. Soil data were collected from exclosures (10- and 27-year-old) and control (open) sites in the mild and cool agro-ecological zones. The exclosures and the controls sites were further subdivided in upper, middle and lower landscape positions. 118

Performance of exclosure in restoring soil fertility Composite soil samples to 15 cm depth were collected from sampling plots (20 m 2 each) for each age, agro-ecological zone and landscape position in three replicates. The samples were analyzed for texture, pH, OC, av. P, TN, exchangeable bases and CEC using standard laboratory methods. The soil laboratory analysis results were statistically analyzed by analysis of variance (ANOVA) using the general linear model (univariate), in SPSS 17. Among the different soil properties analyzed and statistically tested, only OC and TN contents showed statistically significant differences with exclosure age, but the differences were nonlinear. Soils of the exclosures had significantly higher OC and TN contents than the open sites but no significant difference was observed between the 10and 27-year-old exclosures. Accordingly, soils of the 10-year-old exclosures had statistically significantly higher OC and TN contents than those of the open sites. Similarly, soils of the 27-year-old exclosures had significantly higher OC and TN contents than those of the open sites. The differences between soil OC and TN content of the control sites and 10-year-old exclosures were 41% (6.7 g/kg) and 44% (0.7 g/kg), respectively, while differences in OC and TN between soils of 10- and 27-year-old exclosures were only 9% (2.1 g/kg) and 22% (0.5 g/kg), respectively. This implies that soil OC and TN restoration rate of the exclosures increased at a decreasing rate after some time and reached a steady state with time. Retardation of the soil OC and TN restoration rate with exclosure age could be attributed to change in biomass quantity and quality, which in turn determine OM input and mineralization. Soils of the 27-year-old exclosures had a lower C/N ratio than soils of the 10-year-old exclosures and the control sites OC and TN were highly interdependent as manifested through their positive, strong (r 2 = 0.92) and significant (P = 0.01) correlation. Comparison of soil properties across agro-ecological zones of the exclosures revealed statistically significant variation. The soils of exclosures in the mild zone had statistically significantly higher pH, EC, OC, TN and available P, silt and clay contents than those in the cool zone. Although exchangeable bases (Ca 2+ , Mg 2+ and K + ) contents showed non-significant differences between the two agro-ecological zones, higher mean values were observed in the mild zone. The higher soil nutrient contents of exclosures in the mild zone can be attributed to the effect of agro-ecological zone/climate on biomass 119

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