Divergence in physical, chemical, and biological soil properties caused by different long-term bare fallow management and natural succession

• Systemic evaluation of chemical, biological and hydraulic soil properties. • Missing organic input, more than tillage, drives soil degradation. • Tillage had greater impact on microbiome composition than herbicides. • Tillage had greater impact on fungi than on bacteria. • Soil structure of no-till, herbicide fallow similar to that of natural succession. The absence of plants has profound effects on many ecosystem functions of soil. Long-term bare fallow trials are valuable tools for studying the dynamics in soil carbon decline and associated soil degradation. However, it is challenging to disentangle the contribution of missing organic inputs from the frequent physical disturbance caused by soil tillage or herbicide application to keep the soil free from vegetation. In this study, we evaluate a unique long-term experiment (36 years) in which a bare fallow was established using different methods: i) mechanically through soil tillage, ii) chemically with herbicides, and iii) a combination of both methods. The aim was to separately assess the effects of tillage and herbicide application on various soil properties. Additionally, the bare fallow treatments were compared with natural succession to evaluate the effect of missing organic inputs. We monitored the annual dynamics of carbon and nitrogen contents in the topsoil (0–30 cm) and subsoil (30–60 cm). In addition, we analyzed the shallow topsoil (5–10 cm) comprehensively by integrating physical properties (microstructure and hydraulic properties), chemical properties and biological properties. All bare fallows were declining in carbon contents at very similar rates while physical disturbance by conventional tillage did not accelerate this effect. In both soil depths of all bare fallows a fast decline in C content during the first ten years was followed by a more gradual or no decline. A large share of the long-term stable carbon was contributed by pyrogenic carbon and to a lesser degree by microbially processed carbon. In the natural succession, the annual increase in soil organic carbon contents was more pronounced in the topsoil than in the subsoil and had not reached a plateau after 36 years. Irrespective of the bare fallow treatment, the absence of plants ceased the nutrient uplift by roots and the supply of carbon, which drastically reduced all indicators of biological activity like basal respiration, mesofauna abundance, and feeding activity. Soil tillage had a greater impact on the diversity of soil organisms than the application of herbicides. This was due to the disturbance of soil structure, the resulting changes in physical soil properties and the structure of habitats. In addition, tillage effects were stronger for fungal than bacterial communities, as fungal hyphae might be more susceptible to physical disturbance. The bioporosity and especially the amount of empty root channels was elevated in the herbicide fallow to values in the same range as the natural succession, despite the sparse vegetation cover. The fragmentation of the soil matrix by plowing drastically decreased the unsaturated hydraulic conductivity without affecting water retention. In summary, the frequent physical disturbance by soil tillage had a surprisingly small effect on the quantity and quality of organic carbon, as the decline and shifts in molecular composition were dominated by the absence of organic inputs. In turn, strong effects of physical disturbance were observed for soil properties that depend on pore structure and its persistence in time, like hydraulic conductivity and microbial communities.