Strategy to minimise nitrogen load to finish a zero discharge cultivation

Abstract

The Dutch authorities established a plan for the reduction of water emissions from the greenhouse sector to become zero by 2027, to enforce the European Union Water Framework Directive. At the end of the cultivation period, a considerable amount of nitrate and phosphate is still present in solution in substrate slabs and in the irrigation system. The left over nutrient solution is generally discharged into the sewage system or surface water. The end of cultivation strategy was developed to use the plant uptake to reduce this nitrate and phosphate in the last 5 weeks of the cultivation period. This reduction should be realised without affecting the production or quality of the last fruits to be harvested. The strategy consisted of a gradual decrease over the last five weeks of the cultivation of nitrate, phosphate and water quantity supplied to the plant. Chloride was used to replace nitrate in the supply water. The anion-shift allowed maintaining a sufficient quantity of cations in the root environment to meet the plant's needs and to avoid production loss. A progressive shift from nitrate-nitrogen to ammonium-nitrogen was realised. The nitrogen-shift acidified the substrate slabs to release precipitated phosphate. A double irrigation cycle at the start of the day was implemented during the strategy to anticipate nutrient accumulation in the root environment. In 2016, the strategy was applied to a sweet-pepper cultivation. The water volume in the substrate slabs was reduced from 6 to 3.5 L m-2. Nitrate quantity was reduced from 109 to 55 mmol m-2. Phosphate quantity was reduced from 0.5 to 0.2 mmol m-2. In 2017, the strategy was applied to a cucumber cultivation. The average water volume was reduced from 8 to 5.3 L m-2. Average nitrate quantity was reduced from 113 to 45 mmol m-2 and average phosphate quantity from 4.5 to 1.65 mmol m-2,.