Plant and microorganism biodiversity increases the availability of phosphorus in the soil

A Chinese study reveals the mechanisms that influence phosphorus mobilization capacity. Fertilization reduces the presence of absorbable phosphorus, while forest regeneration increases it

by Matteo Cavallito

In forest systems, phosphorus mobilization is promoted by the interaction between plants and microorganisms, which gives rise to a “trophic cascade” that increases the availability and therefore the absorption of the element. This complex process is currently illustrated by a study from the Institute of Subtropical Agriculture of the Chinese Academy of Sciences, which, on this occasion, has clarified for the first time some of the mechanisms that allow organisms to adapt to phosphorus limitation in subtropical ecosystems.

The study, published in the Journal of Advanced Research, demonstrates in particular how the effectiveness of mobilization is influenced by factors such as climate and land use, but also by lithology, i.e., the type of rock present—carbonate (karst) or clastic (non-karst).

Only a small portion of phosphorus is assimilable

Phosphorus is an essential nutrient for the productivity of terrestrial ecosystems. However, it is mainly present in the soil in immobilized forms, for example as part of minerals or complex organic matter, which cannot be absorbed by plants. On the other hand, the element is available for absorption if it undergoes mobilization, i.e., the process that releases it in the form of soluble phosphates, which can then be absorbed by roots.

There are many organisms responsible for mobilization. These include bacteria and fungi, as well as nematodes that fuel processes such as enzyme production or the decomposition of organic matter.

Understanding how they work and what factors influence their activity is therefore essential. This is especially true in tropical and subtropical areas where soil phosphorus availability is extremely low, but also in areas subject to intensive fertilization. This practice, the study notes, does not solve the problem, since most of the phosphorus applied in this way ends up being “precipitated by minerals (e.g., calcium, iron, and aluminum), leached, or eroded,” contributing to the depletion of mineral resources and the loss of nutrients.

Forest regeneration frees up available fraction

To understand whether and how the biodiversity of organisms and the complexity of soil food webs can promote the transformation of less available forms of phosphorus into more assimilable fractions, the authors examined various soils in subtropical southwestern China characterized by carbonate and clastic rocks.

They discovered, in particular, that prolonged fertilization increases the accumulation of moderately labile or stable forms of phosphorus in all cases, while weakening the biological capacity for mobilization.

In other words, ultimately, it reduces the availability of the element to plants. On the other hand, after conversion from agricultural land to forest, it “significantly increased soil labile P fractions by 43.8% in karst regions, but decreased soil moderately labile and stable P fractions by 62.6–79.1% and 34.8–36.6%, respectively, in both karst and non-karst regions.”

Biodiversity is crucial

Ultimately, therefore, forest restoration seems to trigger a process of transforming immobilized phosphorus reserves into more absorbable forms, despite the general scarcity of this element observed in mature forests. The study argues that the explanation lies in the greater multitrophic biodiversity found in forest and karst soils compared to non-karst soils. “Forest restoration in the karst regions enhanced the cascade interactions among phosphate-mobilizing bacteria, mycorrhizal plants, and nematodes,” the authors explain in a statement.

This synergy, in particular, “enhanced biological P mobilization and uptake, which reduced P precipitation by calcium/magnesium and consequently alleviated P limitation.”

The study, they conclude, “highlights the vulnerability of karst ecosystems, where human disturbances like tillage and deforestation readily cause species loss and disrupt critical multitrophic interdependencies.” Finally, the findings “underscore the critical roles of reducing mineral P inputs and enhancing legacy P mobilization through biological pathways in promoting agricultural sustainability and supporting the recovery of degraded ecosystems under global change.”