Long-term organic matter effect and the response to compressive stress
Long-term field trials provide an ideal means to assess effects of cultivation practises (e.g., fertilisation, tillage, crop rotation etc.) on soil physical properties and soil fertility. To build upon the knowledge of the role of organic carbon (OC) and other soil properties on soil response to compressive stress, undisturbed soil cores were collected from a long-term fertilisation experiment in Bad Lauchstädt in Germany, including combinations of animal manure and mineral fertilisers. The cores were drained to -100 hPa matric potential and exposed to uniaxial confined compression (200kPa). Investigated indicators for compression resistance included compression index, precompression stress, and resistance and resilience indices based on measured soil physical properties (bulk density, air-filled porosity, air permeability, and void ratio). Soil resilience was assessed following exposure of compacted cores to freeze-thaw (FT) and wet-dry (WD) cycles. The OC content increased with increased fertilisation and resulted in decreased initial bulk density, higher air-filled and total porosities, and increased organisation of the pore space. Soil resistance decreased with increasing OC content but the correlation was not significant. However, initial bulk density (ρbi) and initial gravimetric water content (wi) were significantly positively correlated to the indices of soil compression resistance, with the effect of ρbi being significantly stronger. Significant recovery of airfilled porosity and air permeability was observed following exposure to FT and WD cycles, with the latter cycle showing higher recovery levels. The OC and ρbi significantly influenced the magnitude of recovery following FT cycles, with ρbi showing contrasting trends on void ratio after both WD and FT cycles. It was concluded that the main drivers influencing soil response to compressive stress are ρbi and wi. No direct influence of OC was observed, rather the indirect effect of OC was seen through lower ρbi and greater wi associated with higher OC levels. Further studies are required to differentiate the relative effects of OC, ρbi and wi for variably-textured soils.