Biotic and Abiotic Controls on Soil Nutrient Diffusion Flux Based on Microdialysis

Diffusion is a key mechanism controlling nutrient transport and availability in terrestrial ecosystems. This process is controlled by interactions among soil chemical, biological, and physical properties, through which biotic and abiotic factors jointly influence the diffusion of organic nutrients (e.g., amino acids and organic carbon and phosphorus compounds) as well as inorganic ions (anions and cations). Thus, identifying the key soil factors controlling the diffusion flux of different nutrient forms is important. However, most previous studies have examined the diffusion fluxes of individual nutrients and fewer studies have considered multiple organic nutrient forms together, but only in a few soils at maximum.

In this study, 63 soils covering different geologies and land management, with a wide texture and pH range, were collected across Austria. After transport to the laboratory, intact soil cores were brought to field capacity and after 48 hours subjected to microdialysis measurements of solute diffusion rates over 60 hours. Dialysate samples collected during the experiment were analyzed for amino acids (20 proteinogenic amino acids) as well as inorganic anions (Cl^-, NO₃^-, SO₄^2-, PO₄^3-) and inorganic cations (NH₄⁺, K⁺, Mg²⁺, Ca²⁺). In parallel, a duplicate set of soil cores were analyzed for soil physicochemical and biological properties, including soil pH, texture, pore size distribution, water content, soil organic carbon, total nitrogen, and total phosphorus, exchangeable cations, microbial biomass and respiration, and soil enzyme activities.

Across the 63 soils, soil pH values ranged from 3.9 to 8.2, and soil textures from sandy to clayey. Solute flux determination has been finished and final soil physicochemical properties are currently under analysis. We expect that (i) differences in soil texture lead to changes in pore structure and pore size distribution, as well as the continuity of water films and diffusion path lengths under field capacity, shorter path lengths at finer soil textures promoting nutrient diffusion in soil. (ii) Negative charges on clay minerals and soil organic matter increase cation adsorption and exchange, reduce cationic solute concentrations in the soil solution, and thereby slow down effective cation diffusion. (iii) Soil pH affects the surface charge of soil organic and mineral particles, which in turn affects the adsorption and release of inorganic ions and amino acids, thereby influencing their mobility and availability. 

To test these predictions, we will conduct an integrated data analysis of solute properties (e.g., mass, pKa, charge, hydrophobicity, solubility, %C, %N, %O, other structural properties) and soil properties (see above). We will test diffusive fluxes of all solutes quantified across the 63 soils with principal component analysis. In parallel, we will use LASSO (Least Absolute Shrinkage and Selection Operator), multiple linear regression, and structural equation models to understand direct and indirect controls on solute and nutrient fluxes by diffusion. By integrating the diffusion behavior of multiple organic and inorganic nutrients across a wide range of soils differing in soil physical, chemical and biological properties, this study will greatly improve our understanding of nutrient transport in heterogeneous soils, identify key drivers and thereby help clarify the processes influencing soil nutrient availability.