Soil and water conservation management on hill slopes in southwest Ethiopia. II. Modeling effects of soil bunds on surface runoff and maize yield using AquaCrop.

On the cultivated slopes of the highlands of southwest Ethiopia, soil degradation due to water erosion is a challenge for crop production. To limit surface runoff and soil erosion, soil bunds often in combination with trenches, constructed along contour lines, are common. In addition to the interception of surface runoff, soil bunds may affect crop yield. Here, we evaluate effect of soil bunds on surface runoff and maize yield, using FAO's AquaCrop model, calibrated based on field experiments in the Bokole-Karetha watershed, in SW Ethiopia. Experiments were conducted in 2018 and 2019 on three neighboring fields, each comprising plots in triplicate without and with soil bunds. Experimental data from 2018 to 2019, which were average and above average with respect to rainfall, indicate that water availability was sufficient or even in excess for maize production. Soil bunds significantly (p < 0.05) reduced surface runoff, but maize yield did not differ significantly. In plots without soil bunds, the AquaCrop model described surface runoff satisfactorily after slight adjustment of the curve number (related to infiltration capacity) in one of the three fields. Maize yields were reproduced adequately after calibrating soil fertility and adjusting water productivity. After calibration and validation, the AquaCrop model was used to hindcast surface runoff and grain yield from 1999 to 2017, given available climatic data for the region. Hindcasts show that maize yield in the Bokole-Karetha watershed, with its relatively high rainfall, is not significantly affected by rainfall in two of the three fields. In the third field maize yield decreases slightly, but significantly (p < 0.05) with rainfall. In the short run, yield differences between plots with and without soil bunds are not significant. However, eventually high surface runoff from plots without soil bunds are expected to result in unsustainable crop production, due to significant erosion and degradation of the often nutrient-poor soils. Implementation of soil and water management techniques, combined with fertilization, are important to prevent soil degradation and nutrient stress on sloping land.

Biochar dispersion in a tropical soil and its effects on native soil organic carbon.

Although biochar application to soils has been found to increase soil quality and crop yield, the biochar dispersion extent and its impacts on native soil organic carbon (SOC) has received relatively little attention. Here, the vertical and lateral migration of fine, intermediate and coarse-sized biochar (<0.5, 0.5-1 and 1-5 mm, respectively), applied at low and high doses (1.5-2 and 3-4% w/w, respectively), was tracked using stable isotope methods, along with its impact on native SOC stocks. Biochar was homogeneously mixed into the surface layer (0-7 cm depth) of a loamy sandy Acrisol in Zambia. After 4.5 y, 38-75% of the biochar carbon (BC) was lost from the applied layer and 4-25% was detected in lower soil layers (7-30 cm). Estimating BC mineralization to be no more than 8%, 25-60% was likely transported laterally out of the experimental plots. This conclusion was supported by observations of BC in the control plot and in soils up to 2 m outside of the experimental plots. These processes were likely progressive as recovery of BC in similar plots 1 year after application was greater in both surface and lower soil layers than after 4.5 y. Fine and intermediate-sized BC displayed the greatest downward migration (25.3 and 17.9%, respectively), particularly when applied at lower doses, suggesting its movement through soil inter-particle spaces. At higher dosages, fine and intermediate-sized particles may have clogged pore, so coarse biochar displayed the greatest downward migration when biochar was applied at higher doses. In the BC treatment plot soil profiles, native SOC stocks were reduced by 2.8 to 24.5% (18.4% on average), i.e. positive priming. However, some evidence suggested that the soils may switch to negative priming over time. The dispersion of biochar in soil should be considered when evaluating biochar's agronomic benefits and environmental effects.

Life cycle assessment to evaluate the environmental impact of biochar implementation in conservation agriculture in Zambia.

Biochar amendment to soil is a potential technology for carbon storage and climate change mitigation. It may, in addition, be a valuable soil fertility enhancer for agricultural purposes in sandy and/or weathered soils. A life cycle assessment including ecological, health and resource impacts has been conducted for field sites in Zambia to evaluate the overall impacts of biochar for agricultural use. The life cycle impacts from conservation farming using cultivation growth basins and precision fertilization with and without biochar addition were in the present study compared to conventional agricultural methods. Three different biochar production methods were evaluated: traditional earth-mound kilns, improved retort kilns, and micro top-lit updraft (TLUD) gasifier stoves. The results confirm that the use of biochar in conservation farming is beneficial for climate change mitigation purposes. However, when including health impacts from particle emissions originating from biochar production, conservation farming plus biochar from earth-mound kilns generally results in a larger negative effect over the whole life cycle than conservation farming without biochar addition. The use of cleaner technologies such as retort kilns or TLUDs can overcome this problem, mainly because fewer particles and less volatile organic compounds, methane and carbon monoxide are emitted. These results emphasize the need for a holistic view on biochar use in agricultural systems. Of special importance is the biochar production technique which has to be evaluated from both environmental/climate, health and social perspectives.

Short-term effect of the soil amendments activated carbon, biochar, and ferric oxyhydroxide on bacteria and invertebrates.

The aim of the present study was to evaluate the secondary ecotoxicological effects of soil amendment materials that can be added to contaminated soils in order to sequester harmful pollutants. To this end, a nonpolluted agricultural soil was amended with 0.5, 2, and 5% of the following four amendments: powder activated carbon (PAC), granular activated carbon, corn stover biochar, and ferric oxyhydroxide powder, which have previously been proven to sequester pollutants in soil. The resulting immediate effects (i.e., without aging the mixtures before carrying out tests) on the springtail Folsomia candida, the earthworm species Aporectodea caliginosa and Eisenia fetida, the marine bacteria Vibrio fischeri, a suite of ten prokaryotic species, and a eukaryote (the yeast species Pichia anomalia) were investigated. Reproduction of F. candida was significantly increased compared to the unamended soil when 2% biochar was added to it. None of the treatments caused a negative effect on reproduction. All amendments had a deleterious effect on the growth of A. caliginosa when compared to the unamended soil, except the 0.5% amendment of biochar. In avoidance tests, E. fetida preferred biochar compared to all other amendments including the unamended soil. All amendments reduced the inhibition of luminescence to V. fischeri, i.e., were beneficial for the bacteria, with PAC showing the greatest improvement. The effects of the amendments on the suite of prokaryotic species and the eukaryote were variable, but overall the 2% biochar dose provided the most frequent positive effect on growth. It is concluded that the four soil amendments had variable but never strongly deleterious effects on the bacteria and invertebrates studied here during the respective recommended experimental test periods.

Soil and water conservation management on hill slopes in Southwest Ethiopia. I. Effects of soil bunds on surface runoff, erosion and loss of nutrients.

On the steep hill slopes of southwest Ethiopia, soil erosion may cause significant declines in soil organic carbon (SOC) and nutrients, negatively affecting cropland productivity. Soil bunds are advised as an effective means to reduce surface runoff and soil erosion. However, the effects on SOC and nutrients are rarely quantified. The objective of this study was to assess the quantitative effect of soil bunds on surface runoff as well as soil and nutrients losses from cropland in the region. Data was collected from experimental fields on three farms (fields 1, 2 and 3) in the Omo-Gibe River basin in southwest Ethiopia. On each farm, effects of soil bunds on runoff and erosion were investigated and compared with adjacent plots without soil bunds in the 2018 and 2019 growing seasons. Soil bunds effectively reduced surface runoff (by 80-92%). Without soil bunds, soil losses in the growing season were 5-22 t ha-1 in 2018 and 15-43 t ha-1 in 2019, on average removing 1.3-4 mm soil per year. Soil bunds decreased soil losses by about 96%. Observed soil losses from fields without soil bunds were well described by the Universal Soil Loss Equation (USLE; R2 = 0.92; p < 0.01). Of the total soil loss, 47-69% was removed in suspended form. Suspended material had significantly larger (p < 0.05) SOC, and plant available potassium (K) and phosphorus (P) concentrations than coarser, rapidly settling sediment and bulk soil. In 2019, up to 733 kg SOC ha-1, 77 kg total nitrogen ha-1 and 21 kg K ha-1 were lost per season from plots without soil bunds. For SOC this amounts to 6% of its stocks in the topsoil. Soil bunds are important controls on surface runoff, strongly limiting losses of SOC and nutrients in sloping croplands of southwest Ethiopia.

Carbon stocks of above- and belowground tree biomass in Kibate Forest around Wonchi Crater Lake, Central Highland of Ethiopia.

Forests play an important role in the global carbon (C) balance, but their biomass has decreased globally mainly because of deforestation and a reduction in forest cover. However, little is known about the C stock of tree biomass related to environmental factors in the remnant forest patches. Thus, the present study aimed at assessing the status of C stocks of tree biomass using an allometric equation in Kibate Forest (Ethiopia). Sixty-six plots (30×30 m) were laid out at 100 m interval distance along the altitudinal gradients in five transects. The results revealed that the highest C stocks (67.4%) per species were contributed by Juniperus procera, Ilex mitis var. mitis, Nuxia congesta, and Olea europaea subsp. cuspidata. The mean total tree biomass was 91.9 ± 10.01 Mg ha-1. The mean total C stock was 45.9 ± 5.17 Mg ha-1, out of which 38.3 ± 4.31 and 7.7 ± 0.91 Mg ha-1 were stored in above- and belowground C pools, respectively. Anthropogenic factors were negatively associated with the C-stock distribution in the study area. Thus, the status of the C stock of tree biomass related to anthropogenic factors indicates that sustainable forest management practice is needed in the study area to conserve biodiversity and mitigate climate change.

Effect of conservation farming and biochar addition on soil organic carbon quality, nitrogen mineralization, and crop productivity in a light textured Acrisol in the sub-humid tropics.

Conservation farming (CF), involving basin tillage, residue retention and crop rotation, combined with biochar may help to mitigate negative impacts of conventional agriculture. In this study, the effects of CF on the amount and quality of soil organic matter (SOM) and potential nitrogen (N) mineralization were investigated in a maize-soya-maize rotation in an Acrisol in Zambia. A large field was run under CF for 7 years and in the subsequent three growing seasons (2015-2018), four management practices were introduced to study effects on soil characteristics and crop yield. We tested i) a continuation of regular CF (CF-NORM) ii) CF without residue retention (CF-NO-RES); iii) Conventional (CONV), with full tillage and removal of residues; and iv) CF with 4 ton ha-1 pigeon pea biochar inside basins and residue retention (CF-BC). The experiment involved the addition of fertilizer only to maize, while soya received none. Soya yield was significantly higher in CF systems than in CONV. Maize yields were not affected by the different management practices probably due to the ample fertilizer addition. CF-NORM had a higher stock of soil organic carbon (SOC), higher N mineralization rates, more hot-water extractable carbon (HWEC; labile SOC) and particulate organic matter (POM) inside basins compared to the surrounding soil (outside basins). Our results suggest that the input of roots inside basins are more effective increasing SOM and N mineralization, than the crop residues that are placed outside basins. CONV reduced both quality and quantity of SOM and N mineralization as compared to CF inside basins. CF-BC increased the amount of SOC as compared with CF-NORM, whereas N mineralization rate and HWEC remained unaffected. The results suggest benefits on yield of CF and none of biochar; larger impact of root biomass on the build-up of SOM than crop residues; and high stability of biochar in soil.

Significant build-up of soil organic carbon under climate-smart conservation farming in Sub-Saharan Acrisols.

Conservation farming (CF) involving minimum tillage, mulching and crop rotation may offer climate change adaptation and mitigation benefits. However, reported effects of CF, as applied by smallholders, on storage of soil organic carbon (SOC) and soil fertility in Sub-Saharan Africa differ considerably between studies. This is partly due to differences in management practice, soil type and adoption level between individual farmers. Where CF involves planting basins, year-to-year changes in position of basins make SOC stock estimates more uncertain. Here we assess the difference in SOC build-up and soil quality between inside planting basins (receiving inputs of lime and fertilizer; basins opened each year) and outside planting basins (no soil disturbance or inputs other than residues) under hand-hoe tilled CF in an Acrisol at Mkushi, Zambia. Seven years of strict CF husbandry significantly improved soil quality inside planting basins as compared with outside basins. Significant effects were found for SOC concentration (0.74 ±â€¯0.06% vs. 0.57 ±â€¯0.08%), SOC stock (20.1 ±â€¯2.0 vs. 16.4 ±â€¯2.6 t ha-1, 0-20 cm), soil pH (6.3 ±â€¯0.2 vs. 4.95±â€¯0.4) and cation exchange capacity (3.8 ±â€¯0.7 vs. 1.6 ±â€¯0.4 cmolc kg-1). As planting basins only occupy 9.3% of the field, the absolute rate of increase in SOC, compared with outside basins, was 0.05 t C ha-1 yr-1. This corresponds to an overall relative increase of 2.95‰ SOC yr-1 in the upper 20 cm of the soil. Also, hot water extractable carbon (HWEC), a proxy for labile organic matter, and potential nitrification rates were consistently greater inside than outside basins. The significant increase in quantity and quality of SOC may be due to increased inputs of roots, due to favorable conditions for plant growth through input of fertilizer and lime, along with increased rainwater infiltration in the basins.

Woody species composition and diversity of riparian vegetation along the Walga River, Southwestern Ethiopia.

The primary objective of this study was to examine the status of woody species composition and diversity along the Walga River of Wonchi, Southwestern Ethiopia. Fifty quadrats of 10 m x 50 m were laid at 500 m interval through systematic sampling method along the river line. Vegetation height (≥2.5 m) and DBH (≥2.5 cm) of only tree species were measured and altitude, ecological disturbances such as, grazing intensity and human impacts were included as main environmental variables at each of the sampled plots. The data was analyzed using different R statistical packages. Ninety-nine woody vascular plant species belonged to 81 genera and 45 familieswere recorded in Walga riparian vegetation. Only 10% of specieswere endemic to the Flora area. Asteraceae and Fabaceae had the highest number of species. Majority of the species (52.5%) were shrubs. Four major plant community types were identified: Euclea divinorum-Maytenus arbutifolia (1), Pterolobium stellatum- Calpurnia aurea (2), Brucea antidysenterica-Prunus africana (3), Erica arborea-Hagenia abyssinica (4). Species richness, true diversity and importance values were highestin community type 2(the lowest altitude ranges between 1976-2212 m a.s.l.) while evenness was highestin community type 3(mid altitude ranges between 2359-2676 m a.s.l.). Both community typeswere comprised of 56% of all recorded species and all endemic taxa except two. The highest percentage of species in lower frequency classes indicates a higher degree of floristic heterogeneity. There was a strong negative correlation (r = -0.65, p<0.001) between species richness and altitude with 42% of the variation in species richness per plot being explained by altitude. Our findings suggest that human disturbances and excessive livestock grazing are the main threats in community types1 and 2. We conclude that identifying major plant community types and underlying environmental conditions may help to manage and conserve forest resources in the area.

Fading positive effect of biochar on crop yield and soil acidity during five growth seasons in an Indonesian Ultisol.

Low fertility limits crop production on acidic soils dominating much of the humid tropics. Biochar may be used as a soil enhancer, but little consensus exists on its effect on crop yield. Here we use a controlled, replicated and long-term field study in Sumatra, Indonesia, to investigate the longevity and mechanism of the effects of two contrasting biochars (produced from rice husk and cacao shell, and applied at dosages of 5 and 15tha-1) on maize production in a highly acidic Ultisol (pHKCl3.6). Compared to rice husk biochar, cacao shell biochar exhibited a higher pH (9.8 vs. 8.4), CEC (197 vs. 20cmolckg-1) and acid neutralizing capacity (217 vs. 45cmolckg-1) and thus had a greater liming potential. Crop yield effects of cacao shell biochar (15tha-1) were also much stronger than those of rice husk biochar, and could be related to more favorable Ca/Al ratios in response to cacao shell biochar (1.0 to 1.5) compared to rice husk biochar (0.3 to 0.6) and nonamended plots (0.15 to 0.6). The maize yield obtained with the cacao shell biochar peaked in season 2, continued to have a good effect in seasons 3-4, and faded in season 5. The yield effect of the rice husk biochar was less pronounced and already faded from season 2 onwards. Crop yields were correlated with the pH-related parameters Ca/Al ratio, base saturation and exchangeable K. The positive effects of cocoa shell biochar on crop yield in this Ultisol were at least in part related to alleviation of soil acidity. The fading effectiveness after multiple growth seasons, possibly due to leaching of the biochar-associated alkalinity, indicates that 15tha-1 of cocoa shell biochar needs to be applied approximately every third season in order to maintain positive effects on yield.

Biochar improves maize growth by alleviation of nutrient stress in a moderately acidic low-input Nepalese soil.

We studied the role of biochar in improving soil fertility for maize production. The effects of biochar on the alleviation of three potential physical-chemical soil limitations for maize growth were investigated, i.e. water stress, nutrient stress and acid stress. Experiments involved soils with two dosages of biochar (0.5% and 2% w:w), as well as ones without biochar, in combination with four different dosages of NPK fertilizer, water and lime. Biochar was produced from the invasive shrubby weed Eupatorium adenophorum using flame curtain kilns. This is the first study to alleviate one by one the water stress, nutrient stress and acid stress in order to investigate the mechanisms of biochar effects on soil fertility. Biochar addition increased soil moisture, potassium (K) and plant available phosphorous (P-AL), which all showed significant positive relationship (p<0.001) with above ground biomass of maize. However, biochar was much more effective at abundant soil watering (+311% biomass) than at water-starved conditions (+67% biomass), indicating that biochar did increase soil moisture, but that this was not the main reason for the positive biomass growth effects. Biochar addition did have a stronger effect under nutrient-stressed conditions (+363%) than under abundant nutrient application (+132%). Biochar amendment increased soil pH, but liming and pH had no effect on maize dry biomass, so acidity stress alleviation was not the mechanism of biochar effects on soil fertility. In conclusion, the alleviation of nutrient stress was the probably the main factor contributing to the increased maize biomass production upon biochar addition to this moderately acidic Inceptisol.