In the Province of Flevoland, the Netherlands, land
subsidence poses a problem to agriculture and water management. The peat
layers in the soil are susceptible to compression and oxidation causing
further subsidence. Applying subirrigation through the tile drain system to
maintain saturation of the peat may be a measure to slow down subsidence. A
study was therefore carried out at two sites, Nagele and Zeewolde, to assess
the impact of subirrigation in the peat on the seasonal variation in soil
moisture content, and corresponding redox conditions. Bacterial community
analysis was carried out to verify the hydrochemical observations.
Subirrigation proved to be an efficient measure to maintain a high water
level in the peat soil as long as the permeability in the upper part of the
peat was sufficient to allow transmission of water into the inter-drain area
and when the peat layer extended enough below the minimum regional water
level to prevent drainage to the sand layer underneath. The peat showed dual
porosity and water levels could well be maintained by subirrigation at the
Nagele site. At the Zeewolde site, the variability in the thin peat layer
allowed drainage to occur in the sand layer, preventing subirrigation to
maintain high water levels. However, at both sites the peat layer remained
close to saturation throughout the summer, which may be caused by the
fine-grained mineral layer isolating the peat from water extraction via
evapotranspiration. Nitrate concentrations of up to 100 mg L
The Flevoland polders were reclaimed in the mid-20th century from the former Zuiderzee, which was closed off from the North Sea to form the fresh water IJsselmeer lake. The land surface is currently about 3–4 m below sea level, and subsidence continues (Fokker et al., 2015) posing a significant threat to agricultural practices (Vogelenzang et al., 2019), complicating water management and threatening water quality by increased upward seepage of saline water. Main causes of subsidence were the loss of pore pressure following reclamation and subsequent gradual lowering of phreatic water levels by the Water Board to create and maintain favourable conditions for agriculture. This resulted in compression and compaction of the clayey top soil layer and the peat layer underneath. An important factor for continued subsidence is the oxidation and permanent shrinkage of shallow humic clay and peat layers (Fokker et al., 2015). Peat oxidation can account to up to 68 % of loss in volume (Leifeld et al., 2011) and is fastest at high oxygen availability and high soil temperatures. Peat oxidation conditions are therefore most favourable during summer when phreatic levels decrease in response to evapotranspiration and near-surface peat layers partially dry out. Land use in Flevoland is mainly agricultural. About 71 % of agricultural land is used for intensive arable farming of potatos, wheat, sugar beet and onions, which requires phreatic levels of at least 0.8 m below the surface. Peat meadows for production of grass or corn fodder cover about 21 % (Vogelenzang et al., 2019). For arable farming, tile drain systems and regulation of the regional water level by the Water Boards keep groundwater levels low, even during high rainfall events. Inter-drain distances are typically between 12 and 30 m. In peat meadows, drainage is accomplished by a system of shallow ditches dug at typical inter-distances of more than 30 m (Couwenberg, 2018).
In summer, the lowering of the regional water level by the Water Board and high crop evapotranspiration cause a decline in phreatic water level, thereby allowing oxygen to enter the soil pores causing potential for enhanced decomposition and shrinkage of peat. A possible method to reduce the risk of subsidence in agricultural areas with shallow peat layers is to adapt the tile drain system for subirrigation, such as to allow active infiltration into the peat and maintain a higher water level in the soil. This requires transmission of water from the drains into the inter-drain space. The degree of success of subirrigation therefore depends on if the infiltration capacity is high enough to balance the extraction of soil moisture in the inter-drain space for evapotranspiration. Peat often exhibits dual porosity, with higher permeability in interconnected macropores and low permeability in the matrix (Rezanezhad et al., 2016). Permeability generally decreases with depth and low permeabilities require small inter-drain distances to maintain moist conditions in the area between the drains. Within the “Spaarwater Flevoland” project the effects of active tile drain infiltration on phreatic water levels and soil moisture content between drains and on peat oxidation processes were investigated at two agricultural plots in the dry summers of 2018 and 2019. The objectives were to assess the rate of infiltration through the tile drain system to maintain a uniform phreatic level, and if subirrigation could prevent the soil water from becoming oxic. The latter was studied using hydrochemical and microbiological analyses.
Soils in Flevoland show considerable heterogeneity due to temporal and spatial variations in the depositional environments in the Holocene. Sand and peat deposits occurred in tidal marshes, which were overlain by fine-grained deposits of varying thickness. The soil in Flevoland is characterised by a sequence of a fine-grained mineral top layer on a peat layer formed on sand. Two pilot sites were selected with different soil layering and thickness of the peat layers. The locations of the Zeewolde and Nagele pilots are presented in Fig. 1.
Locations of the Zeewolde and Nagele sites in the Flevoland Province, east of Amsterdam, in the Netherlands. © MapTiler and © OpenStreetMap contributors 2020. Distributed under a Creative Commons BY-SA License.
The soil at the Nagele site (
The soil at the Zeewolde site (
For subirrigation, the tile drains were connected to a collector drain and
buffer tank, in which provisions were made to allow fixing water levels at
0.1 m level increments, independent of the ditch water level. A solar pump
was used to supply water to the buffer tank from the ditch to maintain a set
water level. Percolating rain water was removed by overflow in the drain
well to the ditch. This system allows subirrigation in the experimental plot
to be better regulated and at higher water levels than possible in the
regular ditch approach. Flows into and out of the drains were continuously
monitored with flow meters (Octave; Arad, Israel). Phreatic water level
measurements were made in transects perpendicular to tile drain directions.
Piezometers were installed next to the drains and at 0.5–1.0 m intervals in
the inter-drain space for continuous phreatic water level measurements
(CTD-10; Decagon, USA). Soil moisture content profiles were measured (GS3;
Decagon, USA) down to a depth of 0.75 m. Soil moisture and groundwater
samples were extracted at two-week intervals using rhizons (Rhizosphere
Research Products, the Netherlands) installed at 0.25, 0.50. and 0.75 m
depths. Deeper groundwater was sampled at 1.5 and 3.5 m depth (sand layer)
from piezometers, which were duly flushed before sampling. Samples were
filtered (0.2
Bulk soil samples (three locations at Nagele) for identification of bacterial communities in view of their potential to reduce nitrate and sulphate were taken at depths of 0.7 and 2.2 m in the peat layer. Sampling was carried out at the end of the winter season (6 March 2019) in prepared sample bottles and were analysed using next generation sequencing (ORVIdecode) at Orvion (the Netherlands). This procedure allows characterization of DNA and identification of bacteria populations.
Ground water level measurements at Nagele showed discharge towards the drain
from the inter-drain space in the winter period, with higher water levels in
the piezometer between the drains than that at the drain (Fig. 2). In the
summer of 2018, the pump was not capable to supply enough water to the
drains and the water level between the drains decreased following the deeper
regional trend. In the summer of 2019, the farmer decided to increase the
water level to the bottom of the clay layer at about 0.5 m below the surface
and infiltration occurred such that the peat remained saturated despite the
decrease in the regional ground water level. The situation in summer 2019
showed that the peat layer was permeable enough at drain depth (Ks
Groundwater levels in the experimental field at Nagele, where the drain well was used to maintain fixed water levels in summer. Drain is on the drain, centre is between the drains and deep is the groundwater level in the deep sand layer (regional). Note that the piezometers were dry at water levels below 1.1 m.
In Zeewolde, where the peat layer was less deep and the drains were locally in the base sand layer, it was not possible to supply enough water to maintain a high level in the peat layer because of leakage to the regional groundwater system through the sand layer. The water levels in the peat layer therefore followed the regional trend (Fig. 3).
The decline in phreatic level during the dry period did not have a large
effect on
Groundwater levels in the experimental field at Zeewolde, where the drain well was used to maintain fixed water levels in summer. Drain is on the drain, centre is between the drains and deep is the groundwater level in the deep sand layer (regional). Note that the piezometers were dry at water levels below 1.1 m.
In Zeewolde, the experimental site showed a fluctuating moisture content,
peat being saturated in wet periods (
Variation in soil moisture content (
Contribution of bacterial communities to nitrogen transformation processes in the peat layer at depths of 0.7 m (sub-oxic) and 2.2 m (anoxic) in Nagele, March 2019.
Electrical conductivity of soil moisture in the peat at 0.75 cm depth was
relatively high at
Salinity and redox status influence the microbial biomass and metabolic
activity related to the mineralization of carbon in peat, with pH being an
important factor controlling the microbial diversity (DeAngelis et al.,
2010; Preston et al., 2012; Tecon and Or, 2017). In the upper part of the
peat oxygen seems to be absent, but nitrate percolating down from the
unsaturated top layer can provide the oxygen used for decomposition by
nitrate reducing bacteria. Lower in the profile, nitrate has been consumed
and the bacterial population could change to include species that specialise
in the reduction of, for instance, sulphate. At Nagele, the analysis
indicated that bacteria were present in much larger numbers in the shallow
suboxic peat sample than in the deeper anoxic sample. In both samples
proteobacteria were dominant accounting for 70 % of the population DNA,
with alpha- and betaproteobacteria accounting for about 50 %.
Terrabacteria groups (mainly actinobacteria) accounted for another 10 %.
Differences were observed in the presence of
The fractions of bacteria specialised in denitrification (mainly
The study shows that several conditions need to be met for keeping a peat layer saturated through subirrigation by infiltration of water through the tile drain system. The first condition is that the inter-drain distance is proportional to the permeability of the peat. The present study showed that the 6 m inter-drain distance was sufficiently low to cause a fast response of the phreatic level at the centre of the drains. Soil moisture measurements suggested a dual porosity system that would allow relatively fast transfer of water through the peat. The second condition is that leakage towards the lower regional phreatic level should be limited, which requires a low permeability layer to extend below the minimum water level. This condition was not met at Zeewolde, allowing the supplied water to infiltrate in the sand layer below the peat and causing the water level to drop. In spite of this, the moisture content remained high in the peat layer at both sites, even though 2018 was an exceptionally dry year. The fine-grained mineral layer capping the peat seems to have provided sufficient moisture to the crop for evaporation, while largely preventing the extraction of moisture from the peat underneath. The thin layer of sand on top of the peat layer at Zeewolde may have prevented upward flow through capillary rise, whereas drainage to the sand layer may have been minimized by the low permeability of the peat/clay layer at the base of the peat. Cracks formed in the top soil but were not deep enough to reach the peat layer. Furthermore, the farmer aims to keep cracks at minimum through surface irrigation to avoid tilt damage, in particular to onion crop.
Both the biogeochemistry and the bacterial community analyses indicated that oxygen levels in the peat remained low even in the dry summer season (sub-oxic – anoxic conditions), with very low nitrate concentrations being observed in the peat, and a decrease of sulphate concentrations observed with depth. The bacterial analysis supported the hydrochemical observations confirming the absence of nitrification microbes and indicating that decomposition of peat in the absence of oxygen is through nitrate and sulphate reducing bacteria. The lower DNA count at depth, relative to that in the shallow sub-oxic region, suggests a decrease in the rate of peat mineralisation with depth.
Measured groundwaterlevels are publicly accessable at
FH and AR are responsible for the hydrological data analyses, AR is the projectleader. BLG and MW are responsible for the geochemical data analyses. MW and FH did the text writing and editing. SV performs the lab analysis of waterquality. PM performs the field measurements and installation of equipment. JV provided assistance for data analyses.
The authors declare that they have no conflict of interest.
This article is part of the special issue “TISOLS: the Tenth International Symposium On Land Subsidence – living with subsidence”. It is a result of the Tenth International Symposium on Land Subsidence, Delft, the Netherlands, 17–21 May 2021.
We are grateful to Aleida de Vos from Orvion for the contribution towards the interpretation of the microbial diversity in the soil. Stijn Groeneweg and Saline Verkerk are thanked for their assistance with analyses of water samples.
This research has been supported by the Province of Flevoland (decision no. 1957339, file no. 1947426), LTO Noord Fondsen (project no. 16.29) and Zuiderzeeland Reigonal Water Authority (case nos. 479411 and 519214)