Agroforestry Systems in Tropical Cash Tree Plantations: A Study on Soil Fungal Communities and Carbon and Nitrogen Pools

Agroforestry is a land use system that integrates trees and crops on the same land, aiming to enhance productivity, biodiversity, and ecosystem services.

It has been proposed as a sustainable alternative to monoculture cash tree plantations, such as rubber, which are often associated with deforestation, soil degradation, and loss of biodiversity in tropical regions.

However, how agroforestry affects soil carbon and nitrogen dynamics, which are key processes for soil health and climate regulation, remains poorly understood.

A new study by researchers from the Chinese Academy of Sciences has shed some light on this question by comparing different land use types in Xishuangbanna, southwest China.

How the researchers assessed soil carbon and nitrogen dynamics in different land use types

(Photo : Arthur Edwards - Pool/Getty Images)

The study, published in Science of The Total Environment, involved collecting soil samples from three soil aggregates (macro-, meso- and micro aggregates) in a natural forest, 12- and 24-year-old rubber monocultures, and corresponding agroforestry systems.

Soil aggregates are clumps of soil particles that vary in size and structure, and influence soil physical, chemical, and biological properties.

The researchers measured carbon and nitrogen fractions, fungal communities, enzymatic activities, and chemical properties within each soil aggregate.

They also used a technique called uranium-thorium dating to determine the age of the soil carbon, which they then analyzed to know how these variables differed among land use types and soil aggregate sizes.

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What the researchers found and why it matters

The researchers found that land use change had significant effects on soil carbon and nitrogen dynamics, mediated by fungal functional guilds (groups of fungi that perform similar functions) across soil aggregate sizes.

They reported the following main findings:

•  The conversion of natural forest to rubber monoculture reduced the carbon and nitrogen pools in all soil aggregates, especially in the micro aggregates. The carbon and nitrogen pools were partially restored in the agroforestry systems, but the microbial biomass was still lower than in the natural forest.
•  The carbon- and nitrogen-degrading enzyme activities were higher in the agroforestry systems than in the rubber monocultures, indicating enhanced nutrient cycling. The enzyme activities increased as the soil aggregate size decreased, reflecting higher microbial activity in smaller aggregates.
•  The fungal diversity and composition were influenced by both land use type and soil aggregate size. The natural forest had the highest fungal diversity and richness, followed by the agroforestry systems and the rubber monocultures. The micro aggregates had the highest fungal diversity within each land use type. The pathogenic fungi increased while the saprotrophic (decomposing) and symbiotic (mutualistic) fungi decreased with forest conversion.
•  The carbon and nitrogen pools within soil aggregates were affected by different fungal functional guilds, mainly driven by soil pH. The saprotrophic fungi had a positive effect on carbon pools, while the symbiotic fungi had a negative effect on nitrogen pools. The pathogenic fungi had no significant effect on either carbon or nitrogen pools.

The researchers concluded that their study provides an important insight into the early establishment of agroforestry systems in tropical cash tree plantations, as they offer greater benefits in nutrient cycling and pathogen control than rubber monocultures.

They also suggested that more research is needed to understand how other factors, such as climate change, management practices, and plant diversity, may influence soil carbon and nitrogen dynamics in agroforestry systems.

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