Determination of soil dispersion caused by anaerobic digestates: interferences of pH and soil charge with regard to soil texture and water content

Purpose As a by-product of biogas production, liquid digestate is used as an organic fertilizer on agricultural fields. The digestates that contain, amongst others, monovalent cations which transferred to the soil matrix can have negative impacts since they increase the risk of weakening an intact soil structure, resulting in a lasting damage of soil functions. The aim of the study was therefore to investigate the effect of anaerobic digestates (AD) on the soil stability derived from particle-particle interactions. Materials and methods The content of readily dispersible clay (RDC) is a parameter to quantify the vulnerability of a soil structure to disrupt and may be used to assess the potential risk of crusting and erosion by impact of raindrops and mechanical forces. Therefore, a study was conducted to assess the influence of 30 m 3 ha −1 of ADs on clay dispersion of structured samples of a loamy Cambic Luvisol and a sandy Podzol from parental material. After the application of digestates on the surface of four samples (1841 cm 3 ) and an infiltration time of 10 days, the samples were subdivided into five depths to determine the pH, cation exchange capacity (CEC), and the amount of RDC as a function of depth. Results and discussion The results showed both dispersing and aggregating effects on both soils after digestate application in dependence of pH and CEC. Especially, digestates containing high amounts of sodium contributed to the dispersion of soil particles. Nevertheless, the soil texture appeared to be the most sensitive factor determining the propensity to disperse, since the clay-rich Cambic Luvisol showed a 30-fold higher amount of RDC compared to the sandy Podzol. Both soils showed a lower content of RDC after drying compared to moist soil. The rising amount of RDC as a result of digestate amendment may be due to the supply of monovalent salts and an increase of pH. Both mechanisms influenced the clay particle charge according to the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. With a more negative matric potential, the menisci between soil particles strengthened the linkages between soil particles and stabilized the soil. Conclusions The soil with a higher amount of clay showed a higher vulnerability toward dispersibility which was reflected by the higher amount of RDC compared to the sandy soil. Thus, clay-dominated soils exhibit a higher risk to silt up, especially in a moist state. The consequence is a reduced infiltration and surface r