By Vestine Mukamparirwa
Assessing the use of biomass from Agroforestry tree species as organic fertilizer
The majority of smallholder farmers in Eastern Africa lack the financial resources to purchase sufficient fertilizers to replace soil nutrients exported with harvested crop products. Organic resources are proposed as alternatives to commercial mineral fertilizers but they are usually not available in sufficient quantities to reverse soil fertility decline[1]. In Rwanda, little is known on the utilization of such biomass as potential organic input from available agroforestry tree species, especially when using biomass mixture including agroforestry native trees.
Key messages
- Agroforestry tree biomass transfer is useful for soil enrichment with nutrients and can serve as an alternative source of organic inputs. Biomass characterization of such species is important but there is an urgent need for more and better data, as well as quantification of biomass traits, particularly those with power to improve tree-ecosystem processes.
- Species-specific decomposability focusing on chemical traits of the leaf; is an expression of the quality of litter as a substrate for microorganisms as well as nutrients release patterns.
- Measurements show that potential interactions in mixed leaf litter affects decomposition through species interactions. Studies on effects of adding mixtures of organic materials of contrasting quality are still few in the region and not well understood in Rwanda.
Why is it important to characterize such Agroforestry species; considered as alternative source of organic inputs?
When trying to understand how environmental variables shape vegetation characteristics, or how vegetation characteristics affect local flows of matter (e.g. mineral nutrient cycling), the main criterion for species selection should be local abundance. Selected species should collectively make up for ~80% of cumulative relative abundance. The first key message of the current brief informs about the selection of agroforestry tree species, e.g. Alnus acuminata; Markhamia lutea ; Eucalyptus globulus subsp. maidenii at Musanze and Calliandra calothyrsus; Croton megalocarpus ; Grevillea robusta at Kayonza research sites in Northern and Eastern parts of Rwanda.
The need for organic inputs that can improve soil nutrients requires a good understanding of the potential of different available agroforestry tree biomass in Rwanda.
Little has been studied to prioritize indigenous and exotic tree species, commonly planted tree species in the cropland agroforests of Rwanda. Quantifying their biomass and nutrient contents is essential.
A species-specific decomposition and nutrients release.
Decomposition and nutrient release rates are determined by the resource quality of the organic material, the environment, and the decomposer organisms present.
The organic constituents and nutrient contents define the resource quality of a given material. Under the current study, technique of Litterbag exposure in the field as well as in greenhouse was used to determine the rate of decomposition and changes in the chemical characteristics in the leaf litter of selected tree species.
Litter quality is the most important determinant of decomposition rates at a regional scale[2]. This study will provide data on the specific decay rate of N, P, C, and lignin and will assess how the litter quality influences decomposition in this particular ecosystem. Moreover, it provides important inputs, from a region where little is known about litter decomposition especially considering litter mixture in an agroforestry system.
The analysis of litter quality prior to decomposition and the chemical composition of decomposed litter over time is done in the Rwanda Agriculture Research Institute laboratory.
OM, C, N, P, lignin, and Polyphenol concentrations of the litter are parameters considered to determine the litter quality.
Visit from my supervisors at Rwanda Agriculture Research Institution/ Soil and Plant analytical laboratory
Data on this research are still being collected.
It is expected through this research, sources of organic inputs that enhance other major pathways of improving soil nutrients including some native tree species. Green manure as untapped potential will be valued; hence improved soil structure due to tree biomass application could reduce the demand for inorganic fertilizers and thereby reduce greenhouse gas emissions. The research will have a positive policy implication by informing ongoing tree-based restoration efforts in Rwanda and across the sub-region at large.
[1] Jama et al., 2000. Tithonia diversifolia as a green manure for soil fertility improvement in western Kenya: a review. Agroforestry systems, 49 (2), 201-221
[2] Guendehou, G. S. et al., 2014. Decomposition and changes in chemical composition of leaf litter of five dominant tree species in a West African tropical forest. Trop Ecol, 55(2), 207-220.
