Impacts of Shade Trees on the Adjacent Cacao Rhizosphere in a Young Diversified Agroforestry System

Alana Firl,Hamran Hamran,Nur Insana Imaniar,Taylor M Crow,Samantha J Forbes,Jennifer E Schmidt

doi:10.3390/agronomy12010195

Abstract

Cacao agroforestry systems offer the potential to diversify farmer income sources, enhance biodiversity, sequester carbon, and deliver other important ecosystem services. To date, however, studies have emphasized field- and system-scale outcomes of shade tree integration, and potential impacts on the rhizosphere of adjacent cacao trees have not been fully characterized. Interactions at the root–soil interface are closely linked to plant health and productivity, making it important to understand how diverse shade tree species may affect soil fertility and microbial communities in the cacao rhizosphere. We assessed the impacts of neighboring shade tree presence and identity on cacao yields and physical, chemical, and biological components of the cacao rhizosphere in a recently established diversified agroforestry system in South Sulawesi, Indonesia. Stepwise regression revealed surprising and strong impacts of microbial diversity and community composition on cacao yields and pod infection rates. The presence of neighboring shade trees increased nitrogen, phosphorus, and pH in the rhizosphere of nearby cacao trees without yield losses. Over a longer time horizon, these increases in rhizosphere soil fertility will likely increase cacao productivity and shape microbial communities, as regression models showed nitrogen and phosphorus in particular to be important predictors of cacao yields and microbiome diversity and composition. However, neither presence nor identity of shade trees directly affected microbial diversity, community composition, or field-scale distance-decay relationships at this early stage of establishment. These results highlight locally specific benefits of shade trees in this agroecological context and emphasize the rhizosphere as a key link in indirect impacts of shade trees on cacao health and productivity in diversified systems.

Highlights

Cacao (Theobroma cacao, L.) agroforestry systems (CAFS), in which cacao grows under one or more tree species, are a biodiverse alternative to monoculture production
Positive impacts of shade trees on soil fertility are frequently cited as a benefit of CAFS, as leaf litter and decomposing fine roots generate an important influx of nutrients into the soil
PH was affected by shade tree species (p < 0.01), and Tukey’s HSD tests revealed that pH was higher in the rhizosphere of cacao trees with neighboring red teak, sengon, or banana trees than that in cacao with neighboring cacao

Summary

Introduction

Cacao (Theobroma cacao, L.) agroforestry systems (CAFS), in which cacao grows under one or more tree species, are a biodiverse alternative to monoculture production. Integration of shade trees provides ecosystem services such as carbon sequestration, preservation of biodiversity, and pest management, in some cases these may come at the expense of short-term productivity [1,2,3]. Positive impacts of shade trees on soil fertility are frequently cited as a benefit of CAFS, as leaf litter and decomposing fine roots generate an important influx of nutrients into the soil. Inputs of C from leaf litter, fine roots, and other decomposing mass can be stored in soil aggregates and contribute to C sequestration [5]. Decomposition, nutrient mineralization rates, and impacts on soil fertility and functions can vary by shade

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Conclusion