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# Die Wirksamkeit von Boden

Die Wirksamkeit von Boden

## Performance of farmland

Performance of farmland terraces in maintaining crop productivity production. Similarly, other studies indicated that 10% to 15% of land lost by terracing depending on slope (Herweg and Ludi 1999; Vancampenhout et al. 2006). Nevertheless, this does not affect the plots yield comparison. 6.2.3 Statistical data analysis Crop yield is a function of bio-physical factors and management such as soil, climate, inputs, crop rotation, etc. The physical factors are the terracing and the farmers´ practices, while the natural factors are governed by prevailing conditions. Crop selection, rotation and other management practices are based on farmers´ decisions and experience. Thus, the crop yield performance evaluation is an observational study. As a consequence, various known and unknown confounders such as variations in sample sizes across season and year due to farmers´ decisions and other naturally occurring as well as management-based variations are involved. This calls for use of a model that considers these variations and disturbances. Therefore, the yield data were statistically analyzed mainly by a mixed linear model using the statistical software package SAS (Version 9.2). Multiple pair-wise testing was conducted using the Tukey-Kramer adjustment. The mixed model used yield (grain and biomass) as dependent variables, slope of the terrain and terrace position as fixed factors of interest, and age as a covariate. Since the plots are inherently heterogeneous, they were considered as subject effect. Therefore, the interpretation in this analysis is based on the Tukey-Kramer adjustment that used adjusted P values. A mixed model is defined as: y � X� � Z� � � (6.1) where y , � , � and � are vectors; y = observation with E( y ) = X � ; � = fixed effect; and� = random effect X and Z are fixed and random effect model matrices, respectively, in relation to the observation y to � and � . Note that a grand mean � is mostly incorporated through an extra column within 1 in X and a parameter � in � The above mixed-effect model assumes that: � � and� are mutually uncorrelated, normally distributed variables with zero expectation 88

Performance of farmland terraces in maintaining crop productivity � but covariance of � and � could have a general (unstructured) form Therefore, the explicit mixed liner model for our data is: � y1 � � 1 � � � � ... � �... � � � ... �... � � � � � � � yn � � 1 slope ... ... slope 1 n position ... ... position 1 n age � � μ 1 � � ... � � ß � � ... � ß � � age � � n � � ß slope position age 89 � � z1 � �� 1 � � � � � � � � ... � � ... � � � � � � ploteffect � ... � ... � � � � � � � � � � � � � zn � �� n � (6.2) where y i = yield at plot i (i = 1, ..., n) with a specific terrain slope, terrace position and age � = ( � slope, ß position, ß age) fixed effects � = random plot effect. 6.3 Results and discussion 6.3.1 Impact of farmland terracing on crop yield across terrain Grain and biomass yield data were collected and analysed separately. But as the results follow similar trend, the presentation of the grain and biomass yield analysis results and the discussion are combined. The analysis revealed that most crops showed insignificant yield differences across the terrain. Among the seven crop types analyzed, only the grain yields of barley (P = 0.01), field pea and emmer wheat (P = 0.05) showed significant differences across the terrain. Similarly, barley and field pea biomass showed higher significant (P = 0.05) differences, and emmer wheat biomass showed only marginal (P = 0.1) differences (Table 6.1). Both grain and biomass yield of most crops except wheat tends to decrease in the upslope direction. Although the yield tends to decrease in the upslope direction, the pair-wise comparison shows a nonlinear gradient across the slope of the terrain. For example, unlike other crops, a very low wheat yield (biomass and grain) was observed on gently sloping (3-5%) terrain; yields were over 50% lower than the average values. Other crops showed higher yields on gently sloping (3-5%) and sloping (5-8%) terrain. Also, the highest maize, emmer wheat and horse bean yield was measured on gentle slopes (3- 5%), while the highest barley, wheat, teff and field pea yields were measured on sloping (5-8%) terrain. However, the yield differences between gently sloping and sloping terrain were very small except in the case of wheat. The comparison of yields on strongly sloping (8-15%) and moderately steep slopes (15-30%) showed an irregular

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