Deep Drainage Lowers Methane and Nitrous Oxide Emissions from Rice Fields in a Semi-Arid Environment in Rwanda

Few studies have explored greenhouse gas (GHG) emissions from arable land in sub-Saharan Africa (SSA), and particularly from rice paddy fields, which can be a major source of methane (CH4) and nitrous oxide (N2O) emissions. This study examined the effect of drainage on CH4 and N2O emissions from rice fields in Rwanda under shallow drainage to 0.6 m, with the drain weir open four times per week, and deep drainage to 1.2 m with the weir open four times or two times per week. CH4 and N2O fluxes from the soil surface were measured on nine occasions during rice flowering and ripening, using a closed chamber method. Measured fluxes made only a minor contribution to total GHG emissions from rice fields. However, drainage depth had significant effects on CH4 emissions, with shallow drainage treatment giving significantly higher emissions (~0.8 kg ha−1 or ~26 kg CO2-equivalents ha−1) than deep drainage (0.0 kg) over the 44-day measurement period. No treatment effect was observed for N2O fluxes, which ranged from low uptake to low release, and were generally not significantly different from zero, probably due to low nitrogen (N) availability in soil resulting from low N fertilization rate (in the region). Overall, the results suggest that deep drainage can mitigate CH4 emissions compared with traditional shallow drainage, while not simultaneously increasing N2O emissions

Effects of forest restoration tree species on soil properties in Southern Rwanda

Abstract Effects of forest restoration tree species on soil properties in Southern Rwanda 1,2 Peter Rwibasira*, 2Francois Xavier Naramabuye, 2Donat Nsabimana, and 1Monique Carnol 1University of Liège, Belgium – Plant and Microbial Ecology; 2University of Rwanda, Rwanda. *Corresponding Author: [email protected] Understanding the effects of tree species on soil properties is primordial for the development of forest restoration policies regarding the choice of species that will meet both environmental and local livelihood needs. We investigated the effects of exotic and native tree species, planted in 27 plots (31-82 years old), on selected soil properties (pH, SOM, water-extractable C and N) and soil exchangeable base cations (EBC) at two soil layers (0-5 cm and 5-10 cm) in the arboretum of Ruhande, Southern Rwanda. Given that trees were planted on one site with similar land-use history, climatic conditions, parent material, and soil type, we expect current differences in soil characteristics to reflect the influence of the tree species. The effects of tree species were most apparent in the upper thin layer (0-5 cm) and the values of parameters were higher in this layer compared to the lower soil layer. The influence of tree species at 5-10 cm depth, was less marked and the values were lower for all measured parameters except for Al3+ and Fe2+ concentration. Eucalyptus species stands had the lowest soil pH (3.7) and mixed native species had the highest soil pH (5.8) as well as the highest exchangeable base cations (EBC) at both 0-5 cm depth (63±1.4 cmol (+)/kg) and 5-10 cm depth (9.3±0.2 cmol (+)/kg). Water extractable C and N fractions were higher at the upper soil layer compared to the lower layer. At 0-5cm, hot and cold water-extractable C and N fractions were positively correlated to soil organic matter (SOM) and negatively correlated to pH, but no significant correlation was observed at 5-10 cm. Our results showed that: (i) the two soil layers (0-5 cm and 5-10 cm depths) which were visibly undistinguishable under most of the species significantly differed in soil properties. This highlights the importance of the upper thin layer (0-5 cm depth) in tropical soils, which are generally poor and rely heavily on internal nutrient cycling through rapid decomposition of above-and belowground litter from vegetation (ii) Eucalyptus had negative consequences on soil pH and EBC, whereas soil under mixed natives had higher pH and EBC (iii) The sensitivity of water-extractable C and N fractions to detect tree species effects and their correlation with SOM indicates that these fractions could be used as substitutes to SOM analysis. Keywords: Eucalyptus; organic matter; water-extractable C and N; exotic tree species; soil nutrients; soil quality; Ruhande ArboretumARES - Rwanda (Result1

Application of remote sensing data to improve the water and soil resource management of Rwanda

The Rwandan agriculture strongly relies in the dry seasons on the water stored in artificial reservoirs of various sizes for irrigation purposes. Furthermore, the success of irrigation depends on a wide range of soil properties which directly affect the moisture regime of the growing medium. By integrating remote sensing and auxiliary data the objectives of our study are to monitor the water level fluctuation in the reservoirs, estimate the volume of water available for irrigation and to combine this information with soil property maps to support the decision making for sustainable irrigation water management in a study area in Southern Rwanda. For water level and volume estimation a series of Sentinel-1 (product type: GRD, acquisition mode: IW, polarizations HH and VH) data were obtained covering the study area and spanning over a period of two years. To map the extent of water bodies the Radar-Based Water Body Mapping module of the Water Observation and Information System (WOIS) was used. High-resolution optical data (Sentinel-2) were used for validation in cloud-free periods. To estimate the volume changes in the reservoirs, we combined the information derived from the water body mapping procedure and digital elevation models. For sustainable irrigation water management, digital soil property maps were developed by the application of wide range of environmental covariates related to soil forming factors. To develop covariates which represent the land use a time series analysis of the 2 years of Sentinel-1 data was performed. As auxiliary soil data, the ISRIC-WISE harmonized soil profile database was used. The developed digital soil mapping approach is integrated into a new WOIS workflow

Long-Term Effects of Forest Plantation Species on Chemical Soil Properties in Southern Rwanda

peer reviewedUnderstanding the long-term effects of tree species on soil properties is crucial for the development of forest restoration policies in relation to the choice of species that meet both environmental and local livelihood needs. This study was performed in the Arboretum of Ruhande, Southern Rwanda, where monocultures of 148 deciduous and 56 conifer species have been established in 0.25 ha replicated plots from 1933 onwards. We investigated the effects of six exotic and two native tree species planted in monoculture plots and native species mixed within one self-regenerated plot on soil properties in two layers (0–5 cm and 5–10 cm depth). We measured general soil properties (pH, SOM, exchangeable base cations) and water-soluble C and N as a proxy for soil functioning. Changes in soil properties were observed in the upper soil layer for all tree species. Planting Eucalyptus species caused soil acidification, whereas soil exchangeable cations and pH were higher under native species (Entandrophragma excelsum and Polyschias fulva) and mixed native species. The effects of tree species were more pronounced for hot water-extractable C and N than for other soil properties. Their analyses could be used for detecting changes in soil functioning linked to vegetation types

Supplementing grass-based cattle feeds with legume leaves and its effects on manure quality and value as a soil improver for an Anthropic Ferralsol in Rwanda

Combined use of lime, animal manure and inorganic fertilisers is effective in replenishing the fertility of degraded acid soils. However, many smallholder farmers lack access to sufficient amounts of these inputs to improve the fertility and reduce the aluminium toxicity of Ferralsols. Organic manures are available but often have low nutrient content, which limits their ability to supply nutrients to soils. In a two-factor field experiment over four seasons on an Anthropic Ferralsol in Southern Province, Rwanda, we assessed (i) the effect of cattle manure on soil properties at a reduced rate affordable to smallholder farmers compared with that of NPK fertiliser applied, with and without lime also at a reduced rate, and (ii) the effect of supplementing grass in a basal cattle diet with legume leaves on manure quality and its effect on soil properties. Manure from cattle fed only the grassChloris gayana(grass-only manure) and from cattle fedC. gayanasupplemented withAcacia angustissimaleaves (grass+legume manure) was applied at 5 t dry matter ha(-1)(25% of the recommended rate) at the beginning of each growing season. NPK was applied as split doses supplying a total rate of 70 kg N ha(-1). Lime was applied annually at a rate of 2.0 t CaO ha(-1), which was 25% of the rate required to neutralise total acidity at the site. All amendments were applied only to the soil surrounding the maize plants (planting stations), which is estimated at 25% of the plot area. Maize stover was left on plots after harvest and planting stations were retained over all growing seasons. All treatments altered soil properties at the planting stations. Lime generally increased pH but there was no significant difference between lime plus manure treatments and non-limed manure treatments. Soil organic carbon concentration and cation exchange capacity were higher in manure and NPK treatments than in non-fertilised treatments. The manure treatment increased soil water-holding capacity compared with the NPK and non-fertilised treatments. There was no significant difference in total N, Ca2+, Mg(2+)and K(+)between the NPK and manure treatments. Micro-dosing animal manure can thus replace mineral fertiliser plus lime for soil fertility replenishment in smallholder farming. Grass+legume manure contained higher concentrations of total N, Ca, Mg, K and Na than grass-only manure, but its effect on soil properties did not differ significantly from that of grass-only manure

Effect of grass-diet and grass-legume-diet manure applied to planting holes on smallholder maize production in Rwanda

Animal manure provides plant nutrients and also affects soil nutrient availability, pH buffering and soil physical properties through its contribution to soil organic matter pools. However, the quality and quantity of manure are often low on smallholder farms in sub-Saharan Africa and the initial effect of manuring on crop yield may be small or even negative. In a two-factorial experiment over four seasons in southern Rwanda, the fertiliser value to a maize crop of manures produced by cattle fed a basal diet of only Chloris gayana grass or a mixed C. gayanaAcacia angustissima diet was compared with that of NPK 17-17-17 and no fertiliser. The potential liming effect of the manures was also evaluated through inclusion (or not) of travertine as the second factor. All amendments were applied only to maize planting holes. The crop failed in season 1 due to drought, but manure application (5 t ha-1) approximately doubled maize yield compared with the unfertilised control during seasons 2-4, while NPK (70 kg N ha-1) increased yield by 3- to 4-fold, with corresponding improvements in crop performance indicators. The mixed diet increased manure quality and maize yield compared with the grass diet in season 4. Liming showed a consistent tendency to improve crop performance indicators and yield, but significant differences were only identified in some cases, possibly because the pH increase was small. The results suggest that in regions where manure availability is limiting, application of reduced rates only to planting holes may be an efficient technology. Enhanced animal feed can result in higher quality manure, and ultimately increase crop yield, if nutrient losses during manure handling and storage can be limited

Soil microbial properties and soil nutrients content under exotic and native tree species in Southern Rwanda

Erosion has been ranked as the primary cause of soil degradation in Rwanda, a land that has naturally inherited very acidic soils with low contents of organic matter. With its hilly landscape and heavy rainfall, Rwanda loses annually millions of tons of its fertile soil from unprotected slope hills. Forests plantation seems to be an efficient option, not only for land protection and restoration, but also as a timber and household energy source. Tree species may influence soil quality and soil microbial activity via litter decomposition and root exudation. Although most of these introduced fast growing exotic tree species are now scattered all over the country, their effects on soil microbial processes is unknown. A 200 ha arboretum of Ruhande with various exotic and native tree species was used in this study to assess the impacts of tree species on soil properties. The aims of this study were to investigate (i) the effects of tree species on soil physico-chemical properties, (ii) compare microbial processes under different tree species. Tree species were selected based on their importance, adaptability throughout the country and relevance to daily use. Soil was sampled in 3 plot replicates per species (grouped into exotic (Exot), agroforestry (Agro), native (Nat) and mixed native species (MNS)) and in each sample 2 horizons (organic and mineral) were taken separately. Samples were analysed for physico-chemical properties (pH, moisture, organic matter content, exchangeable cations) and microbial properties (net N mineralization, potential bacterial and archaeal nitrification, respiration potential, microbial biomass C and N, metabolic quotient). We observed a higher nutrient content in the thin organic horizon: Ca2+= 5215.3 vs 2396.8 (MNS), 3242.9 vs 507.5 (Exot) mg kg-1 in organic versus mineral respectively. Mean values of pH were 5.1 vs 4.2 (Agro), 5.3 vs 4.7 (MNS), 4.4 vs 3.8 (Exot), and 5.4 vs 4.3 (Nat) in organic versus mineral respectively. Mean values for soil microbial biomass carbon were 1065.2 vs 326.4 (Agro), 1733.4 vs 490.6 (MNS), 1638.7 vs 271.5 (Exot), and 1463.3 vs 267.4 (Nat) µg C g-1 of soil in organic versus mineral horizon respectively. Preliminary analyses indicate higher soil microbial activities and alleviation of soil acidity under native tree species compared to the exotic species

Biomass production and nutrient content of three agroforestry tree species growing on an acid Anthropic Ferralsol under recurrent harvesting at different cutting heights

Agroforestry systems may alleviate challenges relating to soil degradation and low livestock production for smallholder farmers. Species-adjusted management regimes will determine how agroforestry fits in farming systems. Long-term productivity of biomass in agroforestry systems managed for fodder production requires tree species that coppice after repeated cutting. This study evaluated the effect of different cutting heights (0.3 and 1.0 m) and repeated harvests (1-5) on biomass production and chemical composition of the leguminous trees Acacia angustissima, Leucaena pallida and Mimosa scabrella in a field study on an Anthropic Ferralsol in Southern Rwanda. Shoot biomass production was highest at 0.3 m cutting height for A. angustissima and L. pallida, but M. scabrella could not survive that cutting height. Shoot biomass was highest for A. angustissima and lowest for M. scabrella, which did not adapt to repeated harvests. Leaf:stem ratio was not affected by cutting height. Cutting height did not affect crude protein (CP), but neutral detergent fibre (NDF), acid detergent fibre (ADF) and total polyphenol (TP) concentrations were higher at 1.0 m cutting height than at 0.3 m. Crude protein was highest in A. angustissima and lowest in M. scabrella, while NDF and ADF were highest in M. scabrella. Although all species provided high feed quality in terms of high CP content at both cutting heights, low cutting height (0.3 m) is recommended for A. angustissima and L. pallida for higher overall quality and biomass production

Deep Drainage Lowers Methane and Nitrous Oxide Emissions from Rice Fields in a Semi-Arid Environment in Rwanda

Few studies have explored greenhouse gas (GHG) emissions from arable land in sub-Saharan Africa (SSA), and particularly from rice paddy fields, which can be a major source of methane (CH4) and nitrous oxide (N2O) emissions. This study examined the effect of drainage on CH4 and N2O emissions from rice fields in Rwanda under shallow drainage to 0.6 m, with the drain weir open four times per week, and deep drainage to 1.2 m with the weir open four times or two times per week. CH4 and N2O fluxes from the soil surface were measured on nine occasions during rice flowering and ripening, using a closed chamber method. Measured fluxes made only a minor contribution to total GHG emissions from rice fields. However, drainage depth had significant effects on CH4 emissions, with shallow drainage treatment giving significantly higher emissions (~0.8 kg ha−1 or ~26 kg CO2-equivalents ha−1) than deep drainage (0.0 kg) over the 44-day measurement period. No treatment effect was observed for N2O fluxes, which ranged from low uptake to low release, and were generally not significantly different from zero, probably due to low nitrogen (N) availability in soil resulting from low N fertilization rate (in the region). Overall, the results suggest that deep drainage can mitigate CH4 emissions compared with traditional shallow drainage, while not simultaneously increasing N2O emissions