2011 | Charlotte Robertson

Charlotte has been awarded a Gold CREST award for her project:

“Watercress – The Nitrogen Junkie?” 

Supervising teacher: Dr. Heather Meikle

Her Executive Summary states:

The aim of this project was to determine the effectiveness of watercress (Nasturtium officinale) to reduce nitrogen pollution in waterways by quantifying the mass balance and determining how much nitrogen the plants can take up from hydroponic solution over a 10-week period.

Many New Zealand waterways are polluted by farm run off and human activities. This pattern of water quality exists throughout the world to such an extent that water quality is now a global issue.  World leaders and scientists ask whether there will be enough clean water to support future human populations.

A review of the literature showed that nitrogen and phosphorus are strong contributors to nutrient pollution in waterways, particularly those near sites of human activities such as agriculture and commercial vegetable production, as well as from urban sewerage treatment plants.  The Parliamentary Commissioner for the Environment stated that “…once nitrogen is leached to the environment there is no effective way to remove it – it is simply too late, and the consequences must be dealt with”.

Watercress is an edible aquatic herb and belongs to the Brassicaceae (cabbage) family.
Watercress grows rapidly and is regarded as a ‘luxury’ feeder because it can absorb more nutrients than it needs to survive.  As watercress can take up nitrogen at levels which exceed its growth requirements, it could be an ideal plant to help remove pollutants such as nitrogen from waterways.  Watercress is an edible herb, therefore it may also be possible to harvest the watercress leaves for human consumption, thus removing the associated nutrients from the waterway.

Watercress was grown hydroponically in a glasshouse using a controlled nutrient supply of known composition.  This setup was designed to determine nitrate uptake in an environment whereby nutrient levels and environment are controlled in a manner not possible in the natural world.  Environmental factors which could interfere with nitrate uptake, for example nutrient fluctuations, wind variability, possible frost conditions, rain were controlled.

A four-week pilot trial was carried initially to assess the feasibility of this approach and to ‘fine-tune’ the experimental design and methodology.  Subsequently, plant growth (dry mass) and nitrate uptake were followed over a 10 week period during late summer/early
autumn during the main trial. The total plant nitrogen levels at 10 weeks reached an estimated 2 g of nitrogen per plant.

The results showed that nitrate uptake paralleled the increase in plant dry mass.  The nitrate uptake of 4 – 7 % plant mass, by weight, is very high but within the range of other published data.  The variation in percent nitrogen uptake was attributed to the stage of plant growth.  These trials showed that nitrogen percentage uptake in plants was greatest in young, very actively growing plants (7 % of plant dry mass), and was lower in more fibrous, flowering, mature plants (~4 % of plant dry mass).

These results support the hypothesis that watercress could potentially be used effectively to remove nitrogen (and potentially other nutrients such as phosphates), from waterways contaminated by excess nutrient pollution.

Future work to further investigate use of watercress to remove pollutants should
include:

• Determining the fate of ‘locked-in’ nitrogen if watercress dies down at the end of season, rather than being harvested.
• Designing a trial to assess effectively, the effect of controlled watercress harvesting on nitrate uptake during the months of growth (mid-November to mid-February).  That is, does harvesting enhance or reduce nitrate uptake?
• Calculating the volumetric uptake of nitrogen, relative to the river flow (flux) to assess capture potential and to design systems to increase capture efficiency.