Biotic and Abiotic Controls on Soil Nutrient Diffusion Fluxes Based on Microdialysis Measurerments

Abstract

Diffusion is acknowledged as the principal mechanism for the soil solution transport of limiting nutrients in terrestrial ecosystems. This process is influenced by the interplay among the chemical, biological, and physical properties of soil, where alterations in these properties can variably impact other factors, thereby influencing diffusive fluxes. This study, based on theoretical analysis and the review of existing literature, explores how soil biological properties such as microbial activity and soil enzyme activity, as well as abiotic soil properties like soil pH, soil texture, soil cation and anion exchange capacity, and moisture, influence the diffusion and availability of nutrients in soils. We first formalize the drivers of diffusive solute fluxes into three contributors according to Fick’s first law of diffusion (the diffusion coefficient controlled by soil physicochemistry, the path length by pore size distribution and soil water content and the concentration gradient related to source-sink relationships) and then discuss and study the effects of soil biological and abiotic properties on these three principal drivers and on nutrient diffusion. Microbial activity plays a crucial intermediary role in the diffusion of nutrients in soils, significantly influencing their availability and distribution. Soil microorganisms, by decomposing soil organic matter, alter the form and availability of soil nutrients, thereby impacting the concentration gradient for nutrient diffusion. Additionally, the competitive relationship between plants and soil microorganisms affects the forms and quantities of available nutrients. Abiotic soil factors also significantly influence the migration and diffusion of nutrients. Soil chemical properties, such as soil pH and surface charge which vary among different forms of nitrogen, including inorganic forms such as nitrate and ammonium as well as organic forms such as amino acids. These forms exhibit considerable differences in net charge, hydrophobicity, and molecular weight, affecting their interaction with the soil matrix. Through processes like ion exchange, adsorption, and hydrophobic interactions, these interactions consequently alter their individual diffusion coefficients based on soil properties. Additionally, the physical structure of soil, such as the porosity, pore size distribution and aggregate structure, determines the mobility of water and nutrients within the soil through affecting the diffusive path length, together with soil water content, thereby affecting nutrient diffusion. In conclusion, this study underscores the importance of understanding and evaluating the interplay between soil biotic and abiotic properties when conducting nutrient diffusion research using soil microdialysis techniques. This comprehensive analytical approach is crucial for enhancing the effectiveness of soil nutrient management, as well as for its long-term implications on agricultural production and environmental conservation. The conceptual/analytical approach will be applied to a wide range of soils differing in texture, mineralogy, pH, chemistry, management and microbial activity, and diffusive fluxes of multiple elements and solute forms determined simultaneously at similar soil moisture and temperature, and then be linked - using machine learning approaches - to those key soil properties potentially controlling nutrient diffusive fluxes to develop a generalized model of controls of soil diffusive fluxes of elements.