1. Introduction
Litter from trees, crops, and other plants is typically mixed and decomposed together in the soil [
1]. However, the quality and quantity of residues produced by each plant species differ and, in turn, affect the rate of decomposition and nutrient release [
2]. The recent literature suggests that decomposition processes are influenced by a combination of physical, chemical, and biological factors, especially when leaf litter from various species is mixed [
3]. This interspecific mixing alters the chemical content and modifies the physical characteristics of the litter surface, thereby affecting decomposition dynamics [
4]. Studies highlight that such alterations can significantly influence the abundance and activity of decomposers. Litter mixtures from different species create unique microenvironments that can either facilitate or hinder microbial colonization and activity, depending on the specific combinations of chemical compounds present [
5].
The rate of litter decomposition and nutrient release are affected by the species variety in the litter mixture. Agroforestry species exhibit diverse litter qualities, which are distinguished by their specific carbon-to-nitrogen ratios, lignin concentrations, and other chemical features. These differences impact the activity of microorganisms and the speed at which decomposition occurs, ultimately affecting the way nutrients are processed in the soil [
6].
In an agroecosystem, the decomposition and nutrient release patterns from litter can exhibit non-additive effects. This means the actual outcome in mixed litter differs from the predicted outcome based on individual species in monoculture. Non-additive effects can be synergistic, where the observed effect is greater than expected, or antagonistic, where the observed effect is less than expected [
7,
8]. Nevertheless, the non-additive effects in litter mixture decomposition are not yet fully understood [
9,
10]. The nutrient transfer hypothesis often explains non-additive mass loss in litter mixtures and suggests that decomposers favor high-nitrogen (N) litters, releasing N that can then be transferred to low-N litter. This process accelerates the decomposition of more resistant litter, leading to non-additive mass loss by increasing the overall decomposition rate [
11]. According to [
12], the soil fauna can accelerate carbon (C) and nitrogen (N) release by improving litter quality across different elevations, thereby enhancing the decomposition process. On the other hand, a study conducted in
Eucalyptus and
Acacia plantations found that changes in microbial community structure and diversity did not affect decomposition rates, suggesting that nutrient transfer between litter types might not always lead to non-additive effects in decomposition [
13].
In Rwanda, a recent study showed that the plantation of exotic species, particularly
Eucalyptus species, negatively affects the soil while native trees improve soil properties and microbial processes [
14]. The study revealed that there was increased N mineralization under
Eucalyptus maidenii, despite reports on the detrimental effects of
Eucalyptus species on the growth and activity of soil microorganisms, due to their soil-acidifying effects and secretion of allelopathic compounds [
14].
According to [
15], in Rwanda,
Eucalyptus species dominate forest plantations, accounting for 89%. Pines make up 6.5%, mixed exotic forests constitute 3.1%, and plantations of native species represent only 1.4%. From these concepts, we hypothesized that studying these species and considering their decomposition dynamics from a mixed-litter perspective could provide more insight into how litter species mixtures can promote nutrient release in an agroforestry ecosystem during the decomposition process.
Despite the evident benefits of litter mixtures for nutrient cycling, this area has received limited research attention in eastern Africa and in Rwanda in particular. There is a lack of comprehensive studies examining the decomposition dynamics of mixed litter from different agroforestry species and this study aims to address this gap by determining the effects of mixed-leaf litter of selected tree species on decomposition and nutrient release patterns in Rwanda.
In our study, we explored the potential of enhancing decomposition rates of diverse quality leaf litter, including a few native and exotic species that are already adopted in agroforestry landscapes, to improve organic resource management. By focusing on mixed-species litter decomposition, we aim to provide insights for better agroforestry practices and address the existing research gap. Most decomposition studies on mixed litter reported mass losses and nitrogen dynamics [
16]. Far less attention has been given to the release of other nutrients during decomposition processes and no studies, so far, reported on litter decomposition dynamics based on mixtures of agroforestry tree species in Rwanda. This study compared litter decomposition rates of native tree species (
Markhamia lutea,
Croton megalocarpus), exotic tree species (
Eucalyptus globulus,
Grevillea robusta), and exotic N
2-fixing tree species (
Alnus acuminata,
Calliandra calothyrsus). We tested the following hypotheses:
A larger trait divergence of the litter quality of the species in a mixture results in a faster mass loss of the mixture than expected based on the single species;
Mixed leaf litter results in a more balanced and sustained nutrient release compared to single-species leaf biomass, due to complementary decomposition rates and nutrient profiles.
5. Conclusions
This study demonstrates the synergistic effects of mixing leaf litter on decomposition, highlighting how mixtures of litter types with contrasting chemical properties enhance decay rates. Specifically, mixtures of A. acuminata and M. lutea showed the highest mean mass loss at 65.77% ± 3.35, suggesting strong complementary interactions between these species. Similarly, the combination of A. acuminata, M. lutea, and E. globulus achieved a 60.78% ± 2.29 mass loss, further supporting the role of diverse species combinations in accelerating decomposition.
On the other hand, the mixture of C. calothyrsus and C. megalocarpus resulted in a 53.68% ± 4.02 mass loss, whereas the three-species mixture of C. calothyrsus, C. megalocarpus, and G. robusta resulted in 59.54% ± 3.99, indicating that species with varying substrate qualities can significantly enhance mass loss through complementary interactions. The results align with previous studies, which demonstrated that litters with better initial substrate quality, such as C. calothyrsus and M. lutea, can positively affect litters with poorer substrate quality, like G. robusta and E. globulus. These combinations significantly influenced nutrient release, especially in Musanze, where mixtures of the native species M. lutea with A. acuminata and E. globulus showed lower lignin release compared to E. globulus alone, which had higher lignin release. This is likely due to complementary interactions that promote greater microbial diversity and activity.
In agroforestry systems, understanding these non-additive litter effects is crucial for selecting optimal plant mixtures. Mixtures of native species, such as M. lutea and A. acuminata, not only improve nutrient cycling but also contribute to long-term soil fertility. The ability to maximize nutrient release through carefully selected species combinations can support restoration and rehabilitation efforts in degraded areas, contributing to sustainable nutrient cycling and enhanced soil development.
Future research should include long-term monitoring of microbial biomass and biota abundance to provide deeper insights into the mechanisms driving nutrient cycling in mixed-species litter. This understanding will be key to optimizing agroforestry practices, improving carbon sequestration, and reducing greenhouse gas emissions.
