5 years ago

Die Wirksamkeit von Boden

Die Wirksamkeit von Boden

Performance of farmland

Performance of farmland terraces in maintaining crop productivity yield pattern that varied with crop type. Barley, wheat, maize and field pea yields on moderately steep slopes were higher than on the strongly sloping terrain. Although mean wheat yield on 3-5% slope terrain position was considerably smaller than those on other positions (5-8%, 8-15% and 15-30%), differences were statistically nonsignificant, i.e. non-significant F value. This is due to small sample size (onlay 6 samples at the 3-5% slope terrain position) and the large standard error (Figure 3). Table 6.1 Crop yield (t ha -1 ) on terraces across terrain Biomass yield (t ha Crop type -1 ) by slope (%) 3-5% 5-8% 8-15% 15-30% F -Value Barley 4.6 dbd 4.7 bd 2.9 d 4.3 3.62 ** Wheat 1.3 4.3 3.0 4.0 2.01 ns Maize 6.5 5.0 4.2 5.3 1.06 ns Teff 2.8 4.1 3.5 2.7 1.45 ns Emmer wheat 5.0 cdc 2.6 dd 3.4 d 2.9 2.54 * Field pea 2.9 ddd 3.9 bd 1.6 d 2.4 0.09 ** Horse bean 3.7 2.5 2.9 2.8 0.42 ns Grain yield (t ha Crop type -1 ) by slope (%) 3-5% 5-8% 8-15% 15-30% F -Value Barley 2.3 dbd 2.2 bd 1.3 d 1.9 4.40 *** Wheat 0.3 1.6 1.2 1.5 1.63 ns Maize 2.0 1.8 1.6 1.8 0.15 ns Teff 1.2 1.8 1.4 1.2 1.66 ns Emmer wheat 2.4 ddb 1.4 dd 1.5 d 1.2 2.89 ** Field pea 0.7 ddd 1.9 bd 0.6 d 1.0 3.85 ** Horse bean 2.2 1.2 1.6 1.5 0.78 ns Note: The letters a, b, c and d indicate that biomass and grain yield of a given crop type was different (a) at P = 0.01, (b) at P = 0.05, (c) at P = 0.1 and (d) non-significant as compared with the subsequent slope categories at P < 0.1 (Tukey-Kramer). F -value is *** significant at P = 0.01, ** significant at P = 0.05, * significant at P = 0.1, and ns nonsignificant. The non-significant yield differences across the terrain may indicate that terracing reduced the impact of soil erosion on soil fertility and thereby on crop productivity. The rate and degree of soil erosion largely depends on the slope of the land, which could result in crop productivity gradients (Gebremichael et al. 2005; Nearing et al. 2005; Vancampenhout et al. 2006; Nyssen et al. 2007; Olarieta et al. 2008). However, the current analysis shows that the gradient was not significant across the terrain, which suggests a positive impact of terracing. 90

Performance of farmland terraces in maintaining crop productivity Even though terracing contributes to reducing soil erosion, it also has negative impacts on crops sensitive to certain effects, such as water-logging. This negative impact of terracing has been observed with respect to wheat. The lowest wheat yield on gently sloping terrain could partly be attributed to the negative effects of terracing, in this case water-logging. Herweg and Ludi (1999) reported that level structures (e.g., level bund, level terraces and level Fanya Juu) could result in water-logging problems at lower terrain positions. Other studies also show that water-logging critically limits wheat yield (Olgun et al. 2008; Ghobadi and Ghobadi 2010). The slight yield gradient across the terrain, i.e., yield decrease towards the upper slope position, may be a result of the erosion before terracing. Although terracing reduces soil erosion and nutrient loss by changing slope angle, length and shape, the effect of erosion before the terracing remains for a long time after the terracing (Nearing et al. 2005; Vancampenhout et al. 2006; Nyssen et al. 2007; Olarieta et al. 2008). The yield decrease towards the upslope position of the terrain is likely related to soil depth differences resulting from soil erosion and deposition processes before the intervention (see Chapter 5). Soil depth influences water and nutrient storage. Studies indicate that on cultivated land, erosion induces soil depth gradients across the slope of the land, which in turn result in soil fertility and crop productivity variation (Nearing et al. 2005; Vancampenhout et al. 2006; Nyssen et al. 2007). Soil washed from upper slopes is partly deposited down-slope, as the runoff energy is reduced, which in turn results in soil depth and nutrient storage gradients at micro-watershed level (Chen et al. 1997; Nearing et al. 2005). Thus, the terraces on the lower slope positions were deeper than those on the upper slopes, which in turn influenced overall plant-available nutrients (Calvi�o and Sadras 1999; Tesfaye and Walker 2004). Moreover, lands situated on lower slope positions were less affected by erosion. As a result, this land had deeper soil even before terracing (Gebremichael et al. 2005; Nyssen et al. 2007). This also applies to the soils in this study (Chapter 5). 6.3.2 Impact of farmland terracing on crop yield within a terrace The analysis showed significantly higher yield (grain and biomass) differences within a terrace even after the Tukey-Kramer adjustment. Crops on low-terrace (A) positions had better stands and higher density than those on the mid- (B) and up-terrace (C) positions. 91

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