Effects of a deep-rooted crop and soil amended with charcoal on spatial and temporal runoff patterns in a degrading tropical highland watershed

Abstract

Abstract. Placement and hence performance of many soil and water conservation structures in tropical highlands has proven to be challenging due to uncertainty of the actual location of runoff-generating areas in the landscape. This is the case especially in the (sub-)humid areas of the Ethiopian highlands, resulting in limited success of such conservation measures. To improve understanding of the effect of land use on spatial and temporal runoff patterns in the Ethiopian highlands, we monitored runoff volumes from 24 runoff plots constructed in the 113 ha Anjeni watershed, where historical data of rainfall and stream discharge were available. In addition, we assessed the effectiveness of charcoal amendment of the soil and crop rooting depth in reducing runoff, and we compared the effect of lupine (a deep-rooted crop) to that of barley. We also measured daily rainfall, surface runoff, and root zone moisture contents during the monsoon seasons of 2012 and 2013 (with all plots being tilled in 2012, but only barley plots tilled in 2013). In addition, we analyzed longterm surface runoff from four plots, and outlet discharge data from the research site (1989)(1990)(1991)(1992)(1993) were analyzed and compared with our observations. Results showed that the degrees of soil degradation and soil disturbance (tillage) were significant factors affecting plot-scale runoff responses. As expected, runoff was greater from more degraded soils. Overall, under the commonly applied lupine cropping practice, runoff was higher than under the commonly applied barley cropping practice. In particular, considerable difference was observed during smaller rainfall events (approximately < 20 mm) in 2013, when lupine plots (non-tilled) had greater runoff than barley plots (tilled). Charcoal tended to decrease runoff, but results were not significant.

Nitrous Oxide and Methane Fluxes from Smallholder Farms: A Scoping Study in the Anjeni Watershed

“…While soils in the Anjeni watershed are generally degraded with acidic pH, low organic matter content, high clay content, and shallow depth, as described by Bayabil et al [28][29][30], there was no clear and consistent trend in N 2 O-N emission rates between transects ( Figure 5) regardless of apparent differences in soil degradation status based on differences in soil depth, organic matter content, and bulk density between the three transects, as shown in Table 1. Since nitrous oxide emissions are regulated by nitrification and de-nitrification rates, which in turn are controlled by soil nitrogen status [11,20,33], our findings demonstrate, in accordance with [11], that elevated fluxes under charcoal and lupine plots (both with a slowly releasing nitrogen source) are directly related to the degraded status of the soil with low organic matter contents.…”

“…Each replicate consisted of three treatments (barley grown on non-amended soils and charcoal amended soils, and a leguminous [28]). Dashed lines in (c) are elevation contours.…”

“…While soils in the Anjeni watershed are generally degraded with acidic pH, low organic matter content, high clay content, and shallow depth, as described by Bayabil et al [28][29][30], there was no clear and consistent trend in N2O-N emission rates between transects ( Figure 5) regardless of Average nitrous oxide fluxes along the slope catena of three transects showed that N 2 O-N emission rates from plots at the downslope position tended to be slightly greater compared to upslope and mid-slope slope observations ( Figure 5). Compared with reports by other studies outside Ethiopia [7,12,13], nitrous oxide fluxes observed in the humid climate in the Anjeni watershed were relatively greater.…”

“…Charcoal was prepared from Eucalyptus camaladulensis and manually crushed to obtain relatively uniform particle size (2 mm diameter) and then manually incorporated on the top 20 cm of the soil (more details can be found in Bayabil et al [28,30]). …”

“…However, Figure 1. Map of the Anjeni watershed in the Northern Ethiopian Highlands (a), with the location of downslope transects and runoff plots indicated in (b) and (c) (not to scale) (source: [28]). Dashed lines in (c) are elevation contours.…”