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5 years ago

Die Wirksamkeit von Boden

Die Wirksamkeit von Boden

Performance of farmland

Performance of farmland terraces in soil fertility maintenance Even if runoff is slow at the down-slope positions, the solutes need adequate time to precipitate. Thus, exchangeable base suspensions may not have time to precipitate into the soils of terraces located at the upper-slope positions, and the exchangeable bases could therefore be washed down the slope. Erosion, leaching and accumulation of soluble salts are determined by topographic conditions and surface-water and groundwater flow directions (Chen et al. 1997; Olarieta et al. 2008). At the lower slope positions, water has a relatively longer residence time and as a result, soluble materials precipitate down (Olarieta et al. 2008). Consequently, relatively higher contents of soil exchangeable bases were observed in terraces located on 3-8% slope (Chen et al. 1997). It can thus be deduced that terracing could be unlikely to enhance in-situ conservation of soil exchangeable bases. Rather, terracing may play an important role in enhanced deposition of eroded or leached bases within a watershed, particularly in the soils of terraces located on flat land or gentle slopes. Exchangeable bases lost via erosion and leaching from upper slope terraces may partly be deposited in the watershed at down slope. Terraces in the lower slope areas have gentler slopes and wider spacing, and as a result the incoming runoff could remain for longer period and partly deposit suspended and dissolved materials. If the terraces were not there, sediment-loaded runoff would continue to flow down the slope until it encountered low-lying land or a stream course. Generally, statistically significant differences in soil pH and exchangeable bases on the terraces across the terrain could be largely attributed to erosion and/or leaching of basic cations from the steep slope to flat and/or gentle slope positions. However, the small differences between terraces located on upper and lower slope positions could be due to the inherent soluble salt level of the soils. The CEC of soil is the result of the interactive effect of different soil attributes such as exchangeable bases, clay and organic colloids. Thus, it was not clear which property influenced the CEC differences across the terrain. Soil organic matter (OM) determines soil quality, physical properties, crop nutrition and the link between these (Bergmann 1992; Loveland and Webb 2003). The soil physical properties affected by soil OM include aggregate stability, infiltration, water-holding capacity, soil workability, bulk density, aeration and water movement (Bergmann 1992; Loveland and Webb 2003). The analysis revealed a low OC (mean 72

Performance of farmland terraces in soil fertility maintenance OC = 1.58 %) content and slight increase with slope of the terrain. Loveland and Webb (2003) reported that a 2% soil OC is a critical level for crop production and soil aggregate stability. The OC content increased significantly (P = 0.008) with increase in slope. However, TN and plant-available P did not show significant changes with slope. Although soil OC content increased with slope, the increase was not uniform but skewed (Table 5.2). A comparison between terraces for different slope categories revealed statistically significant OC differences between terraces on moderately steep slopes (15-30%) and terraces in the other slope categories (0.016 � P � 0.034) with mean differences of 0.50% to 0.56%. Nevertheless, soil OC content differences (0.50% � mean �OC � 0.56%) between terraces were too small to cause fertility gradients. The differences could be related to input differences. The farmlands on the moderately steep slopes are near to exclosures and/or grasslands. These terraces could gain additional OM due to natural erosion. Terraces could significantly reduce further transportation of the added litter from non-arable land situated above. The difference in OC content could possibly be related to input differences and effect of terracing. Terracing reduced soil erosion and improved deposition and trapped material such as plant litter. Hence, the plant litter transported from non-arable land covered by trees or grasses could be stopped by the terraces at the upper slope from further translocation. The overall low OC level could be due to oxidation as a consequence of continued tillage and residue removal (Bergmann 1992; Loveland and Webb 2003). Unless soil nutrient export is compensated for through use of fertilizer, residues�and manure, OC degradation will continue (Gabrielle et al. 2005). The statistical analysis also revealed that TN and available P showed nonsignificant differences across slope. Nevertheless, values followed the same trend as OC across slope of the terrain (Table 5.2). The Pearson correlation revealed positive, significant (P = 0.01) and strong (r2 = 0.9) correlation of OC and TN (Table 5.3). The correlation between OC and TN indicate their interdependency. The topsoil on average had 1.9 kg t -1 (0.19%) TN, a little higher than the absolute minimum (Alexander 1991; Bergmann 1992; Sys et al. 1993). Like OC, higher TN (mean 0.21%) contents were measured in the soils of the moderately steep slopes (15-30%) than those on less than 15% slopes (mean TN of 0.18%). The difference in soil TN between terraces of moderately steep slopes and the other two middle slope categories was too small 73

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