Controlled Drainage As Measure to Reduce Nitrate Leaching in Winter Wheat Production System in Denmark.

Subsurface drainage of soil to avoid water logging is a prerequisite for crop cultivation for a large proportion of the agricultural land. The tile-drain systems are often the main transport pathway for dissolved nitrate from the field to the surface waters. To reduce the risk of nitrate losses to the aquatic environment effective nitrogen mitigation measures are needed. Raising the upper ground water level by managing the drain depth (referred to as Controlled Drainage (CD)) during periods with low crop growth and high leaching rates has been found to be effective measures to reduce losses of nutrients through the drain systems. CD was tested in a three-year period (2012-15) in winter wheat on a loamy soil. Impacts of CD on crop growth, yields, groundwater levels, drainage of phosphorous and nitrate, nitrous oxide emissions from soil, when applying regulation levels of 50 and 70 cm above drain pipes were obtained using a before-after control-impact (BACI) study design. Four separate drain systems were used as separate blocks. Two were treated with CD and two were control blocks.

The field trials showed that the drain regulation level had to be 70 cm above drain pipes to significantly elevate groundwater levels and reduce drain outflow and nitrate loss, which were reduced by 38-52 % (97-127 mm) and 36-48 % (6-9 kg nitrate-N ha^-1), respectively. CD did not have an effect on yield and did not cause pollution swapping e.g. increased phosphate losses or higher nitrous oxide emissions. Stable isotope analysis of δ¹⁵N and δ¹⁸O in nitrate from drain water supported by measurements of nitrate, sulphate and ammonium concentrations in drain water revealed that denitrification was not enhanced markedly at the impacted blocks. The results showed an effect of CD on nitrate drain outflow without yield losses in the winter wheat production system .