Explicit simulation of the impact of subsoil compaction on root growth dynamics

Agriculturally induced subsoil compaction degrades soil structure, negatively affecting spatiotemporal soil moisture dynamics and root growth. Natural and mechanical recovery of the soil structure in sandy soils is limited, making the degraded layers very persistent and hard to alleviate. Subsoil compaction compresses the soil resulting in a decrease in pore sizes and pore connectivity while increasing the penetration resistance. In the compacted layer and the soil layer above, subsoil compaction intertwines the effects on important soil physical and biological processes related to the water retention, water and air transport, and soil strength. These processes influence root development and thereby reduce crop yields by increased aeration-, drought-, and mechanical stresses exerted on roots. This complexity makes it urgent to further understand fundamental processes regarding subsoil compaction in order to accurately simulate both soil moisture dynamics and root growth. Earlier research showed that root growth is affected by subsurface compaction but this effect is often missing in agro-hydrological models.

This study aims to bridge this gap by implementing the effect of soil strength and soil moisture regime on root penetration in the SWAP (Soil Water Atmosphere Plant) model to simulate the influence of soil compaction on the root growth. Therefore, the existing SWAP concept of soil moisture dependent root growth will be extended with a module for mechanical strength dependent root growth based on a theoretical equation of the influence of bulk density, soil moisture and soil texture on root growth (Jones et al., 1991). In a first step the influence of the extended module on soil moisture regime and root growth will be validated against root development data from greenhouse soil column experiments where silage maize was grown under different subsoil compaction scenarios. For the soil physical parameterization of SWAP, the soil physical characteristics were measured for these specific soil columns.

After validation, the new module will be used to simulate the effect of subsoil compaction in an agricultural field. Field samples from a Dutch sandy soil are collected and artificially compacted with a load of 300 kPa in the laboratory, representing loadings exerted on the subsoil because of agricultural machinery. For these samples the water retention and hydraulic conductivity curve, and the bulk density will be determined. These results are used to parameterise a soil profile with and without compaction, in order to simulate the influence of compaction on soil moisture dynamics and root growth for silage maize under natural hydrological conditions in the field.

This study aims to improve the overall understanding and simulation of changes in soil moisture dynamics and root penetration due to compacted sublayers. The addition of a soil strength module in SWAP-WOFOST will enable soil hydrological- and agronomical simulations, potentially improving assessment of agricultural practices and their resilience to different hydrological scenarios.

 

References

Allan Jones, C., Bland, W. L., Ritchie, J. T., & Williams, J. R. (1991). Simulation of Root Growth. Modelling Plant and Soil Systems, 31, 91-123.