Subsidence of low moor peat soils in the western Netherlands

Abstract

Three experimental fields were laid out to study the effects of deeper drainage on the processes of subsidence. It was found that 6 years after ditchwater levels had been lowered, surfaces had subsided 6–10 cm. Of this subsidence, 65% could be ascribed to shrinkage and oxidation of organic matter in the layer above the groundwater level and 35% to compression of the layer below groundwater level. Compression accounted for 1–4 cm of subsidence after 6 years. Because the deep peat is affected only very slightly by deeper drainage, it may be assumed that compression of it caused little or no subsidence over the past 1000 years in areas with high groundwater levels. By comparing the bulk density of organic matter in the layers above and below groundwater level, it was found that about 0.30 m, or approximately 15% of the total subsidence of 2 m in the past 1000 years, could be ascribed to shrinkage of the upper layer. This leaves 85% to be ascribed to the oxidation of organic matter. If it is assumed that higher contents in mineral elements in the upper layer result from oxidation of organic matter, the rate of oxidation can be calculated by comparing the bulk density of mineral elements in the layers above and below groundwater level. In this way oxidation of 1.75 m was estimated which would explain surface subsidence of about 1.75 mm per year. This theory is supported by the fact that organic soils supply more nitrogen to the crop than mineral soils. At deeper drainage of organic soils this additional N-supply increases considerably. The additional N-supply is calculated from the N-content of the dry matter yield of grass of the experimental fields. The loss of organic matter can be calculated from the additional N-supply, taking the N-supply of mineral soils as a reference level. According to this calculation, the annual losses of organic matter per hectare are 4 tons in shallowly drained and 12 tons in more deeply drained (0.5 m more) peat soils. With a bulk density of organic matter of the toplayer of 0.2 g cm −3, the rate of loss of organic matter is 2 mm per year at shallow drainage and it increases to 6 mm per year after 0.5 m deeper drainage. These rates of 2 and 6 mm per year are in good agreement with the soil losses above groundwater level measured by means of disks placed at various depths in the profile, some years after drawdown of the ditchwater levels. In this manner a plausible explanation can be given for the surface subsidence of the low moor peat soils in the western Netherlands.