Sorption and desorption of organic matter in soils as affected by phosphate

Marie Spohn,Kateřina Diáková,Felipe Aburto,Sebastian Doetterl,Jakub Borovec

doi:10.1016/j.geoderma.2021.115377

Abstract

The contribution of different adsorption processes to soil organic matter (SOM) stabilization and the consequences of the intensification of land use on the adsorption of SOM are not yet fully understood. Therefore, this study aimed to explore the adsorption of dissolved organic carbon (DOC) in soils as well as desorption of organic carbon (OC) caused by phosphate addition. We conducted desorption and sorption experiments with DOC, phosphate (Na2HPO4), and chloride (KCl) in topsoil and subsoil samples from a Ferralsol, an Andosol, and a Podzol. Furthermore, we quantified the size of DOC by size-exclusion chromatography.We found that phosphate addition caused a strong increase in the DOC concentration in all soils. The DOC concentration was elevated by up to a factor of 4.5 compared to a control (water only) within five minutes after phosphate addition and kept increasing further with time, particularly in the Ferralsol. After 10 days, the DOC concentrations in the P addition treatment were between 5.5 and 18.0 times higher than in the control treatment. In contrast, chloride addition did not lead to increased DOC concentrations compared to the control (water only). Phosphate addition led mostly to desorption of organic matter of medium molecular size (10–100 and 100–1000 kDa) in the Ferralsol and the Andosol and of large molecular size (>1000 kDa) in the Podzol. In contrast, potassium chloride addition shifted the size distribution of DOC in the soil solution towards small compounds (<10 kDa), likely because KCl addition affected the aggregation of DOC compounds, in contrast to Na2HPO4 addition. Furthermore, phosphate addition also decreased subsequent sorption of DOC in the Ferralsol and the Andosol by a factor of up to 2.9 and 2.1, respectively.Our results have far-reaching implications because they show that phosphate addition can lead to desorption, and thus to destabilization of SOM, particularly in Ferralsols, and can also prevent further sorption of organic matter in soils. Our study provides a mechanistic explanation of why phosphate addition can decrease soil organic carbon sequestration and stabilization. This mechanism should be considered in the analysis of land-use and tillage effects on SOM sequestration in the future. Furthermore, our study indicates that also medium and large molecular size organic matter is stabilized through adsorption in soils.

Highlights

Organic matter is stabilized in soils against microbial decomposition through several processes (Sollins et al, 1996; Torn et al, 1997; von Lützow et al, 2006)
This study shows that phosphate addition causes desorption of sor bed soil organic matter and decreases further adsorption of organic matter
We found that the amount of dissolved organic carbon (DOC) that desorbed by phosphate addition was higher in a Ferralsol than in an Andosol, which is due to the higher content of hematite in the Ferralsol

Summary

Introduction

Organic matter is stabilized in soils against microbial decomposition through several processes (Sollins et al, 1996; Torn et al, 1997; von Lützow et al, 2006). The relative contributions of different pro cesses to organic matter stabilization are not fully understood. In addi tion, the consequences of land-use intensification for organic matter stabilization, and for the adsorption of organic matter in soils with different mineral composition have not been fully explored yet. The number of sorption sites in a given soil determines to a considerable extent the amount of soil organic matter (SOM) that the soil can store (Mayer, 1994; Sollins et al, 1996; von Lützow et al, 2006; Jackson et al, 2017). Recent synthesis studies claim that adsorption stabilizes mostly small organic substances with a molecular size < 600 kDa against decomposition in soils (Sokol et al, 2019; Basile-Doelsch et al, 2020), but empirical evidence for this is slender

Objectives

Methods

Results

Discussion

Conclusion