The Identification of the Reactive Pore Space in Soils

Introduction

Ensuring the quality of our soils is essential for a sustainable world. Testing the soil quality traditionally occurs by soil sampling, sieving, and extraction, thereby disturbing the soil’s hierarchical pore size structure.

Issues with Traditional Methods

Soil sieving and extraction disrupt the macro-aggregates and overestimate the accessible reactive surfaces of soil. Plant roots are exposed to only a fraction of these soil reactive surfaces; hence, traditional soil tests for bioavailability underscore the physical non-equilibrium of nutrients and contaminants in soil.

Project Objectives

EXPOSOIL aims to identify the reactive pore space to which roots are exposed in undisturbed soil. The research objectives are to:

1. Quantify the effects of soil structure and mobile colloids on bioavailability of nutrients and contaminants.
2. Develop methods to understand and diagnose these effects.

Hypotheses

We speculate that these effects create local heterogeneities that are most important in soils with stronger aggregate structure, for contaminants or nutrients that are relatively immobile and less aged in soil, and for elements strongly associated with mobile colloids.

Experimental Studies

Experimental studies will be set up to test these hypotheses in soils with surface amended trace metal contaminants, fertilizers, or lime, using isotopes to trace local provenances and using novel visualization tools.

Development of New Methods

Novel reactive membranes acting as zero sinks for solutes and colloids will be developed to mimic plant roots and to make 2D images of the locally available elements. This development is of high risk but high gain because no other method has yet assayed diffusive fluxes of solutes, let alone of colloids, in unsaturated and undisturbed soils.

Expected Outcomes

The new method will disclose the enigmatic roles of soil physical factors and colloids on bioavailability. This knowledge will advance the practical use of soil chemistry in environmental applications, e.g., to improve existing soil testing assays and to facilitate the development of novel, more efficient fertilizers.