5 years ago

Die Wirksamkeit von Boden

Die Wirksamkeit von Boden

Performance of farmland

Performance of farmland terraces in soil fertility maintenance erosion induces selective removal of soil particles on steeper slopes living behind coarser materials (sand, gravel, stones), while the transported material is deposited as the slope steepness decreases. Medium-sized soil particles (very fine sand and silt) settled ahead, while finer particles (mainly clay) and solutes (soluble salts) settled on the flatter slope, as the runoff water stayed for a relatively longer period or flowed at a considerably slower speed or the accumulated water infiltrated down (Gebremichael et al. 2005). The soil texture pattern across the slope of the terrain indicates that terracing did not prevent an accumulation gradient; nevertheless it considerably reduced watererosion-induced texture gradients. 5.3.3 Soil fertility variation at different positions within a terrace Among physico-chemical properties analyzed, only bulk density and pH [KCl] showed statistically significant differences between the three terrace positions. The soil properties that showed non-significant differences include OC, TN, exchangeable bases, CEC, pH [H2O] and texture (Table 5.4). This trend is different from reported by other studies, where the studies reported statistically significant nutrient gradient across a terrace (Gebermichael et al. 2005; Vancampenhout et al. 2006; Nyssen et al. 2007). At the beginning of terrace construction the basins excavated in front of the terrace risers (in the upslope direction) not only reduce the energy of incoming runoff but also serve as a storage tank for sediment-loaded runoff. Consequently, the water detained in the basin and at the low-terrace position would have adequate time to unload the sediments and infiltrate into the soil (Gebermichael et al. 2005; Vancampenhout et al. 2006; Nyssen et al. 2007). Thus, a soil accumulation gradient develops until the slope differences between the two edges of the terrace are minimized (Gebermichael et al. 2005). These processes cause higher sediment accumulation in front of terraces at the beginning of terrace construction, which results in nutrient gradient within a terrace. However, the accumulation decreased with terrace development. In the study site, it was observed that farmers keep increasing terrace heights until formation of bench terraces. This leads to a considerable slope difference reduction across a terrace. The slope gradients measured across the terraces in the study site were mostly less than 2%. With the slope gradient reduction, the incoming runoff has been distributed uniformly, thus 76

Performance of farmland terraces in soil fertility maintenance the runoff have adequate time to deposit the sediments uniformly throughout the terrace (Herweg and Ludi 1999; Gebermichael et al. 2005; Cao et al. 2007). Table 5.4 Average (n = 16) topsoil (20 cm depth) properties across terraces Position pH pH EC TN OC av. P CEC (H2O) (KCl) (ds/m) (%) (%) (ppm) (cmol(+)/kg) A 6.6 4.96 dc 0.09 0.18 1.56 13.5 43.7 B 6.7 5.02 d 0.07 0.18 1.48 11.1 43.0 C 6.8 5.09 0.07 0.19 1.69 11.8 44.4 F -value 2.267 ns 2.90 * 2.15 ns 0.221 ns 0.974 ns 0.214 ns 0.899 ns Position Exchangeable bases (cmol(+)/kg) Db (g/cm 3 Na ) Particles size distribution (%) + K + Ca 2+ Mg 2+ Sand Silt Clay A 0.24 0.49 23.9 5.15 1.2 ad 21 38 41 B 0.35 0.51 23.8 5.07 1.6 a 24 37 39 C 0.32 0.48 23.8 5.56 1.2 24 36 40 F -value 1.71 ns 0.09 ns 0.014 ns 1.04 ns 14.78 *** 0.703 ns 0.926 ns 0.313 ns Note: A = low-terrace, B = mid-terrace, C = up-terrace positions; for other abbreviations and significance levels see Table 5.2 But this does not mean that soil depths in terraces are the same. In fact, they decrease in upslope direction. A soil depth gradient develops during the early stage of sediment accumulation. As the terraces develop to bench terraces, the topsoil receives a proportionally similar sediment load and has a uniform topsoil nutrient status. As a result, non-significant topsoil fertility differences between the three positions of the terraces were observed. Thus, it could be concluded that the topsoil fertility gradient within a terrace decreases during this process. However, it also important to note that bench terrace formation does not prevent volumetric soil and nutrient differences within a terrace. The topsoil fertility uniformity occurs as elevation differences within a terrace reduce to a minimum level. Soil profile depth variations definitely result in overall soil nutrient and moisture reserve gradients. In contrast, bulk density (Db) significantly differed (P < 0.001) at the three terrace positions. The average soil Db of the farmland terraces was 1.35±SD gm/cm 3 . The highest value (1.6 gm/cm 3 ) was measured at the mid-terrace position while the two edges of the terrace had nearly equal Db. The average Db measured at the low- and upterrace positions were 1.21 gm/cm 3 and 1.24 gm/cm 3 , respectively. The �Db between the mid- and the two edges of the terraces was nearly 0.4 gm/cm 3 . The mid-terrace 77

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