Assessing the assimilative capacity of catchments for nitrate

Lincoln Agritech, a subsidiary of New Zealand’s Lincoln University, has conducted research into the impact of denitrification - the conversion of nitrate into gaseous forms of nitrogen - on catchment management.

Currently, New Zealand resource planners are limited to making spatial intensity decisions based only on estimates of the amount of nitrate that leaches out of the soil zone. This means the focus of resource planning has been largely on the soil and climate in a catchment, ignoring the processes up to 1 m below the soil zone.

But Lincoln Agritech’s research shows that not all nitrate leached from the soil zone ultimately pollutes groundwater systems, as processes below the soil zone can actually help groundwater to safely assimilate nitrate without lasting environmental damage.

“While soil zone denitrification can result in significant nitrous oxide emissions, much less of this greenhouse gas is produced in the groundwater zone and even less is emitted into the atmosphere,” said principal scientist Dr Roland Stenger.

“It is anticipated that properly understanding this process and its implications will result in better resource management by enabling spatial land use intensity patterns that can better protect environmental, economic, social and cultural values, while more efficiently using resources.”

Assimilative capacity is determined by two major biogeochemical and hydrological factors. The first is the existence of conditions that allow for denitrification: oxygen-depleted conditions, availability of suitable electron donors and the existence of microbes with the required metabolic capacity. The second is the possibility for dilution with clean groundwater from other sources, eg, water originating in the Southern Alps diluting polluted recharge from the Canterbury Plains.

Dr Stenger said that reliable information on the presence of these factors would enable better use of land that does have high assimilative capacity but is currently not able to be used because of high root-zone leaching estimates.

“Significant variations in assimilative capacity do exist, but due to lacking procedures for cost-effective assessments they are not yet reflected in regulatory decisions, which are currently based on nitrate leaching estimates alone,” he said.

“What our research aims to do is to develop cost-effective procedures for assessing the assimilative capacity of a catchment for nitrate. Having that information easily available will facilitate far better decision-making, so that we can improve economic outcome while maintaining environmental quality.”

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