Effects on biota

Biochar Effects on the Abundance, Activity, and Diversity of the Soil Biota

The soil is not just a chemical matrix. It is alive—with bacteria, fungi, protozoa, nematodes, and a host of invertebrates interacting through food webs, symbioses, and decomposition processes. These organisms influence nutrient cycling, disease suppression, carbon stabilization, and soil structure. Adding biochar alters their habitat. That change, in turn, influences what lives, multiplies, and functions in the soil.

Biochar modifies the physical and chemical properties of the soil environment—pH, water-holding capacity, nutrient retention, and available carbon. These shifts can affect the abundance, diversity, and activity of soil biota, often in complex and sometimes unexpected ways. Understanding these interactions is key to predicting when and how biochar will deliver agronomic or ecological benefits.

Biochar’s physical structure plays a foundational role. The porous surface of biochar provides shelter for microorganisms, protecting them from predation and environmental stress. Microbial colonization is influenced by pore size, surface area, and water retention capacity. For instance, bamboo biochar exhibits a wide range of pore diameters—some small enough to host bacteria, others large enough to support fungal hyphae or nematodes. Biochars with higher porosity, especially those with leachable ash content, tend to offer more habitable microenvironments over time

Water retention is also important. Biochar can increase soil water-holding capacity, especially in lighter-textured soils. Water availability supports microbial growth and metabolism. In laboratory tests, some biochars retained more moisture than even activated carbon or pumice, improving microbial access to the liquid phase of the soil. This increased water retention indirectly enhances microbial habitability and function.

But biochar is not a food source in the conventional sense. Its recalcitrant carbon content is resistant to microbial degradation. Once residual bio-oils or labile compounds are consumed, biochar contributes little metabolizable carbon. As a result, microbial communities respond more to the habitat conditions biochar creates than to its nutritional value. That said, biochar may facilitate carbon cycling by adsorbing dissolved organic compounds and concentrating them near colonizing microbes.

In terms of microbial abundance, field and mesocosm studies show that biochar amendments often lead to increases in microbial biomass. In Amazonian Dark Earths (ADE), for example, microbial biomass is consistently higher than in adjacent unamended soils. The increased abundance does not always coincide with increased respiration. In fact, biochar-amended soils frequently show lower CO₂ evolution per unit microbial biomass—indicating higher metabolic efficiency. This pattern suggests that biochar can stabilize microbial activity, reduce respiration losses, and contribute to long-term soil carbon retention

Biochar also appears to influence microbial activity. Experiments measuring soil respiration—a proxy for biological activity—have shown mixed results. In some cases, basal respiration remains unchanged or declines following biochar addition. However, when labile substrates such as glucose are introduced, soils with biochar often show enhanced substrate-induced respiration. This suggests that while biochar-amended soils may support lower baseline metabolic rates, they retain the capacity for rapid microbial response when energy sources become available

Microbial diversity and community composition are harder to generalize. Studies show that biochar can shift the balance between bacterial and fungal populations, though the direction and magnitude of the effect depend on the type of biochar, soil conditions, and plant presence. In some systems, biochar promotes colonization by arbuscular mycorrhizal fungi (AMF), which improve nutrient uptake, especially phosphorus. The physical refuge offered by biochar pores may be particularly beneficial to AMF spores and hyphae, enhancing their establishment and persistence in the rhizosphere

The chemical properties of biochar introduce another layer of complexity. Biochar surfaces can adsorb a wide range of substances—dissolved organic carbon, inorganic nutrients, gases, and even microbial enzymes. These adsorption processes can enhance or inhibit microbial activity, depending on whether they increase substrate proximity or restrict access to active sites. For instance, strong adsorption of toxic compounds or allelopathic substances may relieve microbial inhibition, whereas excessive sorption of nutrients could limit availability

Not all effects are beneficial. In some cases, biochar may reduce respiratory activity or shift microbial ratios in undesirable ways. It may also adsorb and immobilize microbial signaling compounds or extracellular enzymes, complicating trophic interactions and microbial communication. Additionally, the sorptive capacity of biochar introduces methodological challenges. Many standard assays for microbial biomass or activity rely on measuring extracted compounds—such as CO₂, DOC, or nitrogen forms—that may be sequestered by biochar during testing. This can lead to underestimation of microbial activity in biochar-amended soils unless properly accounted for

Beyond microbes, soil fauna may also be affected. Earthworms, for instance, can ingest biochar particles, with unknown effects on their gut microbiota. Shifts in microbial populations can ripple through food webs, altering the distribution of protozoa, nematodes, and larger invertebrates. Fungal-to-bacterial ratio changes can influence which energy channels dominate the soil food web—affecting not only nutrient cycling, but also disease suppression and carbon storage dynamics.

The influence of biochar on soil biota is therefore multifaceted. It alters habitat structure, moisture regimes, chemical sorption dynamics, and biotic interactions. These changes can enhance or suppress biological processes, depending on the match between biochar properties and soil context. For practical application, this means that biochar should not be treated as a universal microbial enhancer, but as a soil conditioner whose biotic effects depend on feedstock, pyrolysis conditions, soil type, and co-applied amendments.

To maintain or improve long-term soil fertility, biochar applications should aim to support the functions that soil biota perform—nutrient cycling, organic matter turnover, pathogen suppression, and aggregation. Selecting the right type of biochar and integrating it properly into the system is essential. Further research is needed to understand species-specific responses, community-level shifts, and the role of biochar in structuring soil food webs under field conditions.

The biological legacy of biochar is not a single function or interaction, but a system-wide restructuring of the soil environment. Its value lies not just in what it adds to the soil, but in how it changes what the soil—and the life within it—can do.