Nitrate nitrogen and phosphate concentrations in drainflow: An example of clay soil

Nutrients dissolved in water and not taken by plants leach into deeper soil layers or flow out to surface water through pipe drainage systems, causing ground or surface water contamination. Thus, drainflow from agricultural areas has significant influence on surface water eutrophication. The objectives of this study were to evaluate nitrate nitrogen and phosphate concentrations and load changes in drainflow using as an example clay soil analyzed in period spanning the years 2010 and 2013. Field research was conducted at an experimental site in Lidzbark Warmiński, in the Province of Warmia and Mazury (województwo warmińsko-mazurskie) in Poland. Mollic Gleysols developed from loam and clay dominate in this area. The experimental field has a tile drainage system with 21 m drain spacing and average 0.9 m drain depth. Winter wheat (Triticum L.) and oilseed rape (Brassica napus) were cultivated in 2009–2012 and in 2012–2013, respectively. Chemical analysis of water samples was performed with a Hach Lange DR 3900 spectrophotometer. Annual rainfall ranged from 555 mm in 2013 to 814 mm in 2012. Average nitrate nitrogen daily loads ranged from 0.07 to 0.58 kg ha-1, while the total annual nitrate load varied from 7.5 to 34.6 kg ha-1. Daily loads of phosphate were about ten times lower than daily loads of nitrate and the total annual phosphate load ranged from 0.1 to 2.0 kg ha-1. Neither nitrate nor phosphate concentrations are strongly depended on drainflow, but the nitrate nitrogen concentration indicates some relationship with the season. A substantial increase in the nitrate nitrogen concentration appears at snow melting (March) and continues until the end of May, peaking in the third decade of April, when the cultivated crops begin the vegetative growth. The phosphate concentration did not undergo significant changes during the investigated period.