Changes in mechanical and resilience characteristics of degraded arable land under long-term grassland management

Abstract

The sustained intensification of agricultural production to meet increasing food, feed and fibre demands has aggravated soil deformation, thereby accelerating soil degradation. The conversion of some of these degraded arable lands to permanent grassland has been recommended to recover the soil functions. However, there is still a considerable gap in understanding the timeline for the effective recovery of degraded land in terms of its stability (resistance and resilience to disturbance). Moreover, the dynamics of the recovery process in ameliorative grasslands are still not fully understood. In this study, the physical, hydraulic, and mechanical properties including the coefficient of compressibility (C_n) and precompression stress were investigated in degraded arable land at three different depths (0–5, 10–15 and 20–25 cm) after 1-, 2-, 8-, 13-, 19-, and 25-years ameliorative grassland conversion. To fully understand and finalise the dynamics of the recovery process as a function of time since the amelioratory conversion, we combined the analysed data from 2 different sets of measurements (loading conditions) on samples predrained to − 60 hPa matric potential. The loading conditions were (a). static - confined compression with normal stresses applied for 4 h in steps of 1, 20, 50, 100, 200, and 400 kPa without stress relaxation on each sample, and (b). dynamic - cyclic loading at 50 kPa with 30 seconds of loading and unloading (relaxation). We included data concerning porewater pressure dynamics under the cyclic loading condition to document possible changes in elasticity. Our results showed that settlement during loading and the elastic rebound during unloading were related to the sward age and the sampled depth. Before the cyclic loading experiment, higher values of effective stress were recorded in the older swards, but the values changed after loading in response to the change in the porewater pressure. The effective stress values were less negative during loading than when unloading. At soil depth of 0–5 cm in the 25 years old sward, the rebound rate (values) and the coefficient of compressibility were higher due to changes in soil properties, particularly the soil bulk density, while at the 10–15 and 20–25 cm depths, the mean values were much closer. When the rebound rate was considered, the highest mean value occurred at 13 years after conversion. In addition, significantly higher values of pre-compression stress were observed in the 8-year-old sward under static loading, which decreased by 19 years. Higher values of pre-compression stress were mostly recorded at the lower depths under static loading. Finally, the results showed that a period between 8 and 13 years is needed to document the starting of strength regain and the recovery of the physical properties and functions, after conversion to grassland. This recovery was observed even up to deeper depths of 20–25 cm for precompression stress and for the soil compressibility/rebound in the top 5 cm