The effect of raised groundwater tables on load-bearing capacity of peat grassland on a dairy farm

Dutch agricultural peatlands are typically drained to promote production and trafficability, however, this leads to peat oxidation, CO2 emissions and soil subsidence. Rewetting peatlands by raising the groundwater table (WT) to 0.20 m reduces these environmental impacts but may compromise load-bearing capacity (LBC). An LBC of at least 0.7 MPa is required to prevent sward damage during field access, thus inadequate LBC affects optimal farm management. However, mechanisms determining LBC under a raised WT remain poorly understood. This study examined the relationships between an elevated WT (to 0.20 m, using an active water infiltration system), soil moisture content (SMC), season, grassland use (grazing or mowing), and LBC measured as penetration resistance in a four-year on-farm experiment. Biweekly measurements in spring and autumn were conducted on sixteen
experimental plots. A regression model was developed, and structural equation modelling was applied to understand relationships among variables. Results showed that raising the WT decreased LBC by 1 MPa m 1, but this was strongly mediated by SMC and highly dependent on season. Moreover, LBC was higher on pasture fields
than silage fields. This study highlights how the complex interplay between SMC, WT, grassland use and season jointly affect the LBC of peat soils at one experimental location. Moreover, measuring impact depth was compared to penetration resistance, showing greater sensitivity under wet conditions and to variations in sward characteristics. The findings can provide input for further studies on field management decisions and compensation schemes to support a wide range of farmers to successfully farm under raised WT conditions.