Allometric equations, wood density and partitioning of aboveground biomass in the arboretum of Ruhande, Rwanda

Open Access Journal; Published online: 10 Nov 2020There is growing interest in plantation forests throughout Africa because of their role in environment, economy and people's livelihoods. However, the contribution of planted forests to climate mitigation is poorly understood, partly due to lack of allometric equations for biomass estimation. This study aimed to determine wood density and biomass fractions in aboveground components, and to develop biomass estimation equations for multispecies plantation forests in the arboretum of Ruhande in Rwanda. Allometric equations were developed by regressing diameter at breast height (DBH) alone or in combination with height or wood density or age of trees against the biomass of 45 trees harvested from a 200-ha site. Biomass estimates obtained from destructively sampled trees were up-scaled to estimate the amount of carbon stocked in the arboretum of Ruhande, assuming a stem density of 250 stems per ha. Wood density varied among the species but not tree size. The greatest fraction of aboveground biomass was allocated to stems (71–77%) compared to branches (19–27%) and leaves (1–8%) and varied by species. Equations developed fit the data well with DBH explaining over 90% of the observed variation in aboveground and stem biomass. Including height or wood density as supporting parameters reduced the relative error for aboveground biomass by 6.4 and 8.0% and improved model fit by 2.1 and 2.9%, respectively. Akaike information criterion (AIC) showed that wood density (AIC=63.6) and height (AIC=48.2) were the most suitable parameters to support DBH as a proxy for aboveground and stem biomass, respectively. Allometric equations developed in this study are useful tool for estimating carbon stocks of plantation forests in Rwanda and can enhance the accuracy of biomass predictions where site-specific equations rather than generalized models are recommended. Further studies focusing on development of allometric equations on belowground biomass in such systems are recommended

Disentangling the positive and negative effects of trees on maize performance in smallholdings of Northern Rwanda

In the sub-humid parts of East Africa, high population density and pressure on land have led farmers to integrate multipurpose trees on farm. Although mixing trees and crops generates numerous benefits (e.g., fuelwood, timber), it often reduces crop yields. Whereas the effects of mature trees on crops are well studied in semi-arid parklands, there are only few studies for the sub-humid environment. The effects of mature Alnus acuminata (Kunth) and Markhamia lutea (Seem.) on crops were studied on-farm for four seasons in the sub-humid environment of northern Rwanda. Five sampling points for A. acuminata and M. lutea were: (i) 1 m from tree trunk without maize, (ii) 3 m from tree trunk without maize, (iii) 1 m from tree trunk with maize, (iv) 3 m from tree trunk with maize and (v) sole maize away from any tree. Nutrient availability and uptake, soil water, air temperature, solar radiation, crop growth and yields were measured. The APSIM-maize module was used to assess the sensitivity of maize yields to changes in these variables. Nutrients availability was higher under A. acuminata compared with M. lutea, because of higher litter fall but maize nutrient uptake increased only under A. acuminata 3 m from tree trunk during a wetter season. None of tree species affected water availability for maize in the topsoil. Photosynthetically active radiation (PAR), total solar radiation and day air temperature were reduced by both tree species. Maize crop at 1 m and 3 m from the tree trunk was shorter in height but had the same number and size of leaves when compared to sole maize plots. Crop yield was generally reduced more at 1 m than at 3 m from the tree trunk. A positive interaction between A. acuminata and maize was only apparent at 3 m from the tree in one of the four seasons following higher litter fall, suggesting that the negative effect of shade was offset by extra N input during that season. In a modelled scenario under low N fertilization, larger N input from trees could compensate for yield loss caused by reduction in radiation and temperature in about 60% of the seasons. Our findings suggest that adequate pruning and high leaf litter recycling can reduce the negative effect of shade in low intensity farming systems

Determining and managing maize yield gaps in Rwanda

Smallholder maize growers are experiencing significant yield gaps due to sub-optimal agricultural practices. Adequate agricultural inputs, particularly nutrient amendments and best management practices, are essential to reverse this trend. There is a need to understand the cause of variations in maize yield, provide reliable early estimates of yields, and make necessary recommendations for fertilizer applications. Maize yield prediction and estimates of yield gaps using objective and spatial analytical tools could provide accurate and objective information that underpin decision support. A study was conducted in Rwanda at Nyakiliba sector and Gashora sector located in Birunga and Central Bugesera agro-ecological zones, with the objectives of (1) determining factors influencing maize yield, (2) predicting maize yield (using the Normalized Difference Vegetation Index (NDVI) approach), and (3) assessing the maize yield gaps and the impact on food security. Maize grain yield was significantly higher at Nyakiliba (1.74 t ha−1) than at Gashora (0.6 t ha−1). NDVI values correlated positively with maize grain yield at both sites (R2 = 0.50 to 0.65) and soil fertility indicators (R2 = 0.55 to 0.70). Maize yield was highest at 40 kg P ha−1 and response to N fertilizer was adequately simulated at Nyakiliba (R2 = 0.85, maximum yield 3.3 t ha−1). Yield gap was 4.6 t ha−1 in Nyakiliba and 5.1 t ha−1 in Gashora. Soil variables were more important determinants of social class than family size. Knowledge that low nutrient inputs are a major cause of yield gaps in Rwanda should prioritize increasing the rate of fertilizer use in these agricultural systems

Applicability of conservation agriculture for climate change adaptation in Rwanda’s situation

Improving food security and environmental conservation should be the main targets of innovative farming systems. Conservation agriculture (CA), based on minimum tillage, crop residue retention and crop rotations has been proposed against poor agricultural productivity and soil degradation. This paper discusses the applicability and potential benefits of CA in Rwanda under the unfolding climate change scenario. The potential and benefits from CA may vary with rainfall regime. In high rainfall areas (For example North and West of Rwanda), the soils are susceptible to soil erosion and face fertility decline while in low rainfall areas (For example East of Rwanda) crops fail due to sub-optimal water use efficiency. Furthermore, low organic carbon content lower fertilisers response and government targets of increasing production through Crop Intensification Program, is limited. It has been shown that CA can: Reduce soil loss from 35.5 to 14.5 t/ha/year, have 50-70% greater infiltration and increase 42% of organic carbon. Long term analysis using Agricultural Production System Simulator showed that CA can increase yield from 3.6 to 4.4t/ha in areas having >770 mm. Based on the evidence from regional research, CA has a good potential for climate change adaptation in both high and low rainfall areas of Rwanda. However, decreased yield observed in high rainfall areas, increased labour requirements when herbicides are not used and lack of mulch due to priority given to feeding of livestock constrained CA adoption. We conclude that there is a need for critical assessment under which ecological and socio economic conditions CA is suited for smallholder farming in Rwanda

Lowland rice yield and profit response to fertilizer application in Rwanda

Open Access Article; Published online: Dec 10 2019Rice (Oryza sativa ) production in Rwanda increased by 70% while yield ha−1 decreased during the past decade. Yield has biotic and abiotic constraints including inadequate nutrient supply. Yield response functions for N, P, and K were determined in eight marshlands grouped into four clusters. Additional treatment allowed for the diagnosis of response to Mg–S–Zn–B (MgSZnB). Rice grain yield with no fertilizer applied was 2.27 Mg ha−1. Mean yield increases were 2.35, 1.53, and 1.71 Mg ha−1 with N, P, and K application, respectively. The mean economically optimal rates (EOR) were 58 to >150, 11–30, and 21–35 kg ha−1 for N, P, and K, respectively, depending on cluster and the cost of fertilizer. Yield responses to nutrient rates were similar across marshland clusters, and a single response function for each of P and K can serve all four clusters, while the response to N differed for Cluster B compared with A, BC, and C. Net returns to applied P and K were greater than for N, but the application of N is likely needed for such responses to P and K. The MgSZnB resulted in a mean grain yield increase of 1.72 Mg ha−1 with increases in all marshlands, but the information was not sufficient to determine which nutrients of MgSZnB were deficient or their optimal application rates. Fertilizer use can be very profitable for rice production in Rwanda. Profit can be enhanced with the application at less than EOR when fertilizer use is financially constrained

Assessing soil and land health across two landscapes in eastern Rwanda to inform restoration activities

Land degradation negatively impacts water, food, and nutrition security and is leading to increased competition for resources. While landscape restoration has the potential to restore ecosystem function, understanding the drivers of degradation is critical for prioritizing and tracking interventions. We sampled 300–1000 m2 plots using the Land Degradation Surveillance Framework across Nyagatare and Kayonza districts in Rwanda to assess key soil and land health indicators, including soil organic carbon (SOC), erosion prevalence, vegetation structure and infiltration capacity, and their interactions. SOC content decreased with increasing sand content across both sites and sampling depths and was lowest in croplands and grasslands compared to shrublands and woodlands. Stable carbon isotope values (δ13C) ranged from −15.35 ‰ to −21.34 ‰, indicating a wide range of historic and current plant communities with both C3 and C4 photosynthetic pathways. Field-saturated hydraulic conductivity (Kfs) was modeled, with a median of 76 mm h−1 in Kayonza and 62 mm h−1 in Nyagatare, respectively. Topsoil OC had a positive effect on Kfs, whereas pH, sand, and erosion had negative effects. Soil erosion was highest in plots classified as woodland and shrubland. Maps of soil erosion and SOC at 30 m resolution were produced with high accuracy and showed strong variability across the study landscapes. These data demonstrate the importance of assessing multiple biophysical properties in order to assess land degradation, including the spatial patterns of soil and land health indicators across the landscape. By understanding the dynamics of land degradation and interactions between biophysical indicators, we can better prioritize interventions that result in multiple benefits as well as assess the impacts of restoration options

Recommended modification of porridge and mixture to improve nutrient intake in the rural area of Northern Rwanda

In Rwanda, nutritional problems are increasingly drawing attention, and the National Nutrition Policy focuses on the solutions to reduce the prevalence of malnutrition and to improve household food intake. Since rural Rwandans typically have meals at home and household food intake is known to be affected by their socioeconomic status (SES), care should be taken to ensure that homemade meals are healthy. This study aimed to assess the current nutrient content of porridge and mixture so as to recommend modifications to be made to improve nutrient intake within rural households in Rwanda. A crosssectional study was conducted; anthropometric measurements and one-day weighed food records (WFRs) were collected from 30 participants in four households with different SES in the Musanze district in the Northern Province of Rwanda. The first objective of this study was to compare nutritional status and food intake among households with different SES. The study results indicated that SES did not solely explain the nutritional status of the household members, and co-existence of over-nutrition and undernutrition was observed within the better-off household. Although meal frequency per day and the number of dishes and ingredients were positively related to household SES, rural Rwandans consumed monotonous diets characterized by porridge for breakfast and mixture (a dish boiled some foods together) for lunch and dinner as a whole. These two familiar dishes, porridge and mixture, greatly affected their energy and nutrient intakes. The second objective was to compare energy and nutrient contents in the same dishes with different ingredients and cooking methods. Porridges were made by dissolving mixed flour (maize and sorghum flours) in hot water. The porridge did not contain vitamin A. The energy, protein, and iron contents of the porridge were affected by flour concentration. Thick porridge whose flour concentration is 13% is recommended. Beans and potatoes were popular ingredients of mixture. Beans were major sources of energy, protein, and iron intakes. Contrary to general assumption, roots and tubers were also the major sources of protein and iron intakes among the participants. To cook mixture that is well boiled and contains beans and potatoes is a feasible way to increase energy and nutrient intakes regardless of household SES. In order to provide good vitamin A intake, addition of yellow plantain, palm oil, and/or tomato is recommended. This study presents locally and economically feasible recommendations to make popular dishes more nutritious for rural Rwandans