Unraveling the Edaphic Factors Driving Organic Material Decay: Insights from Long-Term Manure Application Studies

Organic material (OM) decomposition is crucial to soil fertility. We evaluated the hypothesis that long-term manure application induces changes in soil properties which affect the decomposition of OM in the first three months. We buried standardized plant litter with different C/N ratios, i.e., green tea (high-quality OM) and rooibos tea (low-quality OM), in five long-term organic fertilization experiments across different soil types in Europe. Intact 100 cm³ soil cores and bulk soil around the buried OM were analyzed for soil properties, including the physicochemical environment (nutrient contents, pore structure, etc) and microbiological properties (biomass of arbuscular mycorrhizal fungi, fungi, actinobacteria, Gram-positive and Gram-negative bacteria, and fluorescein diacetate [FDA] enzyme activity). Despite the difference in microbial growth and activity and soil pore structure between treatments and crops, the effect of manure on OM decomposition was inconsistent across the fields and varied with soil texture and standing crop species. Decomposition of high-quality OM was reduced by 5–7% in two sandy fields with manure treatment and that of low-quality OM was reduced by 22% in one silty manured field, while in the other fields, the decomposition was not affected by manure. The decomposition of both OM types was higher in the maize field than in the barley and grass fields in one sandy site. Soil texture and electrical conductivity were negatively linked to the mass loss of both OM types. For the high-quality OM, its decomposition was also negatively linked to soil organic carbon and nutrient content, but positively linked to FDA enzyme activity. In contrast, the decomposition of low-quality OM was positively impacted by the bacterial biomass and soil total porosity. In conclusion, the effect of long-term manure application on OM decomposition depends on the soil texture and the standing crop species, and the edaphic drivers for OM decomposition vary with OM quality.