5 years ago

Die Wirksamkeit von Boden

Die Wirksamkeit von Boden

Performance of farmland

Performance of farmland terraces in soil fertility maintenance positions had not only higher Db but smaller standard deviation. This indicates that most terraces had higher Db at the mid-terrace positions (Figure 5.8). Figure 5.8 Soil bulk density variability across terraces The higher standard deviation between soil bulk densities of the two edges of the terraces indicates existence of wide variation among terraces. On the other hand, Db at the mid-terrace position had a smaller standard deviation as compared to the Db at the two edges of the terraces, which indicates that terracing had a similar compaction effect at the mid-terrace position (Figure 5.8). Bulk density differences (�Db � 0.4 gm/cm 3 ) between the midpoint and the two edges of the terraces likely to influence soil physical properties, particularly soil-water movement, aeration and root proliferation (Kaufmann et al. 2010). According to Kaufmann et al. (2010), critical Db above which plant root development is affected range from 1.4 gm/cm 3 in clay soils to 1.8 gm/cm 3 in soils coarser than loamy sand. Clay and OC content differences (�clay � 2% and �OC � 0.21%) between mid-terrace positions and the two edges of the terraces were very small. Moreover, the Db also correlated insignificantly with both clay and OC. Very low content and the non-significant correlation of soil OC with Db within a terrace could indicate that it is unlikely to has a relevant influence on Db (Catherine and Ouimet 2007; Keller and Håkansson 2010). Statistically significantly higher Db at the mid- 78

Performance of farmland terraces in soil fertility maintenance terrace position could be due to differences in soil aggregate packing. On average sand, silt and clay contents at the mid-terrace positions were 24%, 37% and 39%, respectively. Keller and Håkansson (2010) reported that the mechanical behavior of soil changes at about 29.3% clay content. A nearly proportional soil particle distribution could enhance soil packing and thereby result in a higher Db. With the development of bench terraces, sediments are deposited uniformly across the terrace, thus the midterrace receives average sized-particles. The proportional soil aggregates received at the mid-terrace positions could enhance packing of soil voids and thereby increase Db. Hence, higher soil Db at the mid-terrace positions could be attributed to the soil aggregates received. On the other hand, the proportional sediment deposition at the midterrace positions indicates the balance between erosion and deposition within a terrace. Soil pH [KCl] also showed statistically significant differences (P = 0.07) between the three terrace positions. Values tended to increase in the upslope direction where the lowest pH was measured just at the low-terrace positions and the highest at up-terrace positions. The mean soil pH [H2O] ranged from 6.6 low-terrace to 6.8 at the up-terrace positions. However, the pH differences (�pH [KCl] � 0.13 and �pH [H2O] � 0.2) low-terrace and up-terrace positions were too small to cause differences in soil nutrition status and heavy metals toxicity (Dong et al. 1995; Xu et al. 2006). 5.3.4 Soil fertility variation with age of farmland terraces The results of the present study reveal nearly stable soil nutrient levels over two decades of farmland terracing at the MSCRS. There was a slight decline in some nutrients while others showed an increasing trend. Soil properties showed a decreasing trend were OC, exchangeable bases and bulk density. On the other hand, an improvement was observed in soil pH and TN. Accordingly, on average, soil acidity decreased (pH increased 6.4 to 6.7) and TN increased 0.16 to 0.19%. However, other soil properties such as OC, exchangeable bases and av. P showed a decreasing trend: OC declined from 1.98% to 1.58% while av. P dropped by 5.16 ppm (5.1 g/ton). Likewise, exchangeable K + , Ca 2+ and Mg 2+ decreased by 0.15 cmol (+)/kg, 10.35 cmol (+)/kg and 6.01 cmol (+)/kg, respectively. Average bulk density increased from 1.02 g/cm 3 to 1.35 g/cm 3 . Generally, the soil nutrient status showed a slightly declining trend with time (Table 5.5). The soil bulk density differences with age of terraces and within a terrace showed clear 79

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