Nutrient leaching

Dr Ross Monaghan from AgResearch at Invermay in Otago describes the problems caused by nutrients leaching into water systems. He also explains why winter is a high-risk period for nitrogen leaching.

Transcript

DR ROSS MONAGHAN

Nutrient leaching is an environmental issue for two reasons. The first is too much nitrate getting into our groundwater aquifers that are used for drinking water supplies. High concentrations of nitrate in drinking water that’s used for making infant formula poses a risk to young infants because they can’t process that nitrate that we as adults can. So we want to make sure that the drinking water we provide has a concentration that the World Health Organisation recommends is less than about 10 milligrams of N per litre.

The second aspect that is coming to attention more frequently in New Zealand and abroad is the problem of eutrophication. Eutrophication is a term that describes the nuisance growth of weed and algae in surface waters, in streams and estuaries. That process is really driven by two nutrients – nitrogen and phosphorous. So nitrate that leaves this farm ends up in a lake or an estuary. If it arrives with phosphorous, together, that’s quite a fertile combination that will promote that nuisance weed and algal growth. And the growth can have lots of undesirable effects in terms of how we might be able to use that water for recreational purposes, and habitat value, choking out all the other wildlife that can occur.

Winter is a high-risk period for nitrogen leaching because of at least two reasons. The main one is that it’s the time of year when we have a lot of surplus rainfall arriving and thus a lot of drainage and transport of any nitrogen that’s sitting in the soil, potentially available for leaching, and secondly, it’s a time of year when plant uptake is quite low, so particularly if you’ve got a relatively bare paddock, for example, a forage crop paddock that’s been recently grazed, there’s a lot of nitrogen that’s being returned to that paddock in the form of urine but very little plant uptake until temperatures warm up in the following spring. So those two factors cause winter to be a high-risk leaching loss period of the year.

Acknowledgements:
Dr Ross Monaghan, AgResearch, Invermay
Juliet Milne, Otago Regional Council
Tamara Douglas