Compared impact of compost and digestate on priming effect and hydrophobicity of soils depending on textural composition

Abstract. Anaerobically fermented digestates as well as aerobically composted organic substances (OS) are used as valuable organic fertilizers in agriculture. Besides their benefits for plant nutrition and carbon sequestration potential, these amendments are also suspected to interfere negatively with the soil matrix. To compare the relevance of digestates and compost for priming effects and water repellency of soils, a moderate (40 m³ ha⁻¹) and a threefold (120 m³ ha⁻¹) amount of digestate derived from mechanically pre-treated silage from 80 % maize and 20 % sugar beet or 10 t and 30 t of compost, respectively, was mixed with homogenized samples of a loamy Cambic Luvisol (Ut3) and a sandy Podzol (Ss) in a laboratory experiment. The basal respiration rate (BAS) and the repellency index (RI) of moist (pre-dried to −60 hPa) soil-digestate-mixtures (SDM) or soil-compost-mixtures (SCM) were analyzed to determine the effect of digestate and compost on microbial activity and hydrophobicity of soils. Additionally, the content of organic carbon (C_org) was investigated using air-dried and finely milled mixtures. The Ss showed quantitative reduction of C_org in the SDM and SCM and an increased BAS, which could be explained by a beginning priming effect through microbial stimulation. As a result of enhanced OS protection in the Ut3, constant amounts of C_org and a subsequent declined BAS could be detected. The wettability was reduced in both soils; directly in the Ut3 by the supply of amphiphilic components and indirectly in the Ss by increased incorporation of microbial exsudates and mucilages. The supply of higher contents of available organic compounds with digestate and higher amounts of hydrophobic humic acids applied with the compost could be assumed to be the controlling factors decisive for the impact of this amendment on soil wettability. But also the inherent textural composition of the soil controlled the microbial activity and subsequent decomposition and release processes at high degree, since the Ut3 exhibited higher incorporation of OS in finer pores and contributed to the protection against microbial decay.