Earthworm-enhanced phosphorus availability in soil

Background

The decline in reserves of rock phosphate, the source of P fertiliser, is one of the most serious threats to global food security. Phosphorus is a key nutrient for plants, but very difficult to access by plant roots as P-fertiliser quickly precipitates, adsorbs to soil mineral particles, or is incorporated in organic matter. This results in typically only 10-20% of fertiliser P taken up by plants. In the past, large amounts of P fertiliser have therefore been applied to many soils, resulting in a large pool of relatively inaccessible P. This pool can in theory supply plant growth for decennia. It has been shown that earthworms can, through burrowing and defecating, temporarily and locally make this P available, leading to increased plant growth. However, the mechanisms are still unclear.

Description

In this research project, I explore how earthworms can contribute to a more sustainable form of P use by sustaining optimal P acquisition for plants, thereby reducing the need for external P fertiliser inputs through increasing the level of plant-available P in soil. To do this, understanding of the various mechanisms involved in the earthworm-induced increase of plant-available P is needed, both from chemical and ecological perspectives and considering different spatial scales. Therefore, a variety of experimental approaches ranging from greenhouse pot experiments to a field experiment and from chemical analysis of earthworm casts to surface complexation modelling is used.

Results

This showed a large variation in the ability of earthworm species to alter the extent to which P is present in plant-available forms. Multiple pathways contribute to earthworm-enhanced P-availability and all controlling mechanisms are directly or indirectly related to the capacity of an earthworm to mineralise the organic material it ingests. This research project revealed a so far unconsidered mechanism: the decrease of the reactive surface area (RSA) of metal-(hydr)oxides in casts by particle growth, resulting in a decrease of the surface area that is available for P adsorption. This reduction of the RSA was only observed for Fe-(hydr)oxide-dominated soils, whereas it was absent or minor in Al-(hydr)oxide-dominated soils. This demonstrated that soil mineralogy influences earthworm-enhanced P-availability and suggests that earthworms have the largest potential to improve the sustainability of P use in Fe-(hydr)oxide-dominated soils.

The field experiment considered the effect of earthworm diversity on grass biomass production and P uptake. Thereby I identified two earthworm species as keystone species to increase grass P uptake and biomass production. Furthermore, this study showed that the effect of earthworms on grass P uptake was not only present under controlled greenhouse conditions but can also be observed at the larger scale of more realistic field conditions. Therefore earthworm-enhanced P-availability has potential to contribute to a more sustainable P nutrition for agricultural grasslands in (the future of) the Netherlands. However, the challenges associated with agricultural P use are likely too large to be solved by a single approach. A combination of several approaches is needed by agriculture to provide an adequate P nutrition to grass on low agronomic P-status soils. This project shows that stimulating earthworm populations to increase the utilisation of P in soil could be one of those approaches.