Dr Ranvir Singh, senior lecturer in environmental hydrology and soil science at Massey University, is overseeing a series of student research projects focused on understanding nutrient flow pathways and their potential attenuation on their journey from farms to rivers and lakes. This collaborative project between Massey University Fertiliser and Lime Research Centre and Horizons Regional Council started five years ago with Singh at the helm.
Singh said he wanted to get Massey and Horizons working together to build up a student research programme to advance their knowledge on issues in the space of water quality.
Current student projects are based around better understanding of nitrogen flow pathways and its potential attenuation or reduction in the subsurface environment, ie below the farm root zone.
“One of the major water quality issues relates to the elevated levels of nitrate-nitrogen in the river. This may lead to excessive periphyton growth, which then becomes an issue for the river ecology, often depleting the dissolved oxygen and aquatic life.”
Previous research on nitrogen leaching, put into models like Overseer, estimates how much nitrogen or nutrients are likely being leached from the root zone of the farm, but doesn’t consider what happens to the nitrate-nitrogen once it’s leached from the root zone and makes its way to the river, Singh said.
Nitrate is a reactive compound so is able to react and change form on its flow pathway from the farm to the river. In theory the process of denitrification is well understood. Nitrate-nitrogen can change form through to nitrous oxide, and to dinitrogen, which mainly happens in low-oxygen conditions where there is a carbon source and denitrifying bacteria. However, there is limited knowledge and understanding about how nitrate-nitrogen is transformed and transported in the sub-surface environment.
“Previous studies have found when Overseer estimates are compared with loads in the river, up to 50% of leached nitrogen is not accounted for. There is a significant difference between the two numbers, so where is it going? Is it being accumulated or reduced in the system and if it’s being reduced (attenuated), is this reduction uniform or variable across the catchments and sub-catchments. These are the questions we are looking into.”
The Manawatu and Tararua regions were chosen as the case study catchments for the projects because the area has higher nitrogen concentrations in surface water, with a large share of nitrogen load estimated to come from agricultural areas.
River flow and water quality data collected by Horizons and estimates made by Overseer were used to calculate a ‘Nitrogen Attenuation factor’. This shows how much nitrogen disappears between the root zone and the river load. The factor sits between 0, no reduction and 1, 100% reduction. Calculations from 13 sub-catchments showed the factor was variable with figures between 0.29 and 0.75, showing some regions have more natural nitrogen reduction potential than others.
To build on this, groundwater surveys were done in 2014. PhD Student Aldrin Rivas took the lead, sampling 57 wells in the area. Water quality was measured to classify them into one of two categories: oxidised conditions, where nitrate shouldn’t reduce because of the high amount of dissolved oxygen in the water and reducing conditions, where there are low levels of dissolved oxygen in the water, increasing the likelihood of nitrogen attenuation.
“The results show the number of wells that were oxidised and non-oxidised aligned quite nicely with the attenuation factor figures. Reducing wells matched with high attenuation factor areas.”
Economically there is value in better quantification and mapping of nitrogen attenuation in the subsurface environment. With rough estimates of about 3000 tonnes of nitrogen being reduced somewhere in the system in the Tararua region, and considering the approximate cost of nitrogen reduction through farm management practices is about $15/kg, about $45 million worth of nitrogen is being reduced by natural processes.
“It’s being reduced somewhere in the system and we don’t understand it, so we can’t use it smartly. So economically it makes sense to figure out what’s happening and the areas we need to be focusing on and where we shouldn’t because it’s already being reduced in the system.”
Profiles from the soil surface to the groundwater are being measured to see what happens to the nitrate-nitrogen as it works its way through the system. Field sites have been set up with suction cups and shallow groundwater piezometers at varying depths from soil surface to groundwater. Aldrin collects samples every month and analyses the groundwater chemistry and measures the dissolved oxygen and nitrate-nitrogen levels.
“Some of the sites show high dissolved oxygen and high nitrate levels, so it’s again proving that where oxidised water is present, it’s likely nitrate is being leached and transported into the groundwater and going into the river. But in other areas where there is low dissolved oxygen, there is also low nitrate in groundwater.”
“Regardless of land-use on the top, there could be different reducing conditions in the subsurface environment which kind of puts a control on the flow of nitrate-nitrogen leached from the farm to the river. It’s not only the land use; it’s the transport and transformation process that’s important also.”
“In some cases it’s possible that the leaching is not a problem due to the reducing conditions under the ground, so the nitrogen being leached isn’t necessarily making its way to groundwater and then on to the surface water.”
Assessing the capacity of land to reduce nitrogen means at some point it will be possible to match land use with land suitability, to increase agricultural production while reducing environmental impacts in catchments.
“Some areas where there is naturally high nitrogen reduction would not benefit environmentally or economically from huge changes in farm practices for nitrogen mitigation. In others areas with low natural reduction and good connectivity to lakes and rivers there would be benefit.
“It becomes a handy tool to see where we should be spending the limited resources we have in a way that’s going to make the most impact and improve the productivity and reduce the environmental footprint.”
Singh said the research was on-going with much more to be discovered and he hoped it would be taken on by other councils.
“I would like to see more regional councils getting on board and working together the way that we are with Horizons so that we can build more case studies as the idea it to keep building the work slowly and get more understanding about it – what controls the natural attenuation, is it the soil type, geology or something else? Once we get enough knowledge built up we will be able to assess and map out these areas and be able to target out mitigations.”
Key findings so far
Nitrogen loads measured in the river are significantly smaller than estimates by Overseer.
• The difference in those figures is likely because of the nitrogen reduction process that happens in the subsurface.
• The capacity of land for denitrification is variable.
• Has opened the door to many management questions and has widened the scope for further research.
Collaboration with Horizons Regional Council
Dr Jon Roygard, freshwater and science manager at Horizons Regional Council, says the project is making great steps towards filling the gaps in knowledge around the process of nitrogen transport from farms to waterways.
Horizons has a role in increasing its understanding around catchments to inform policy development and this type of understanding is increasingly required through legislation like the national policy statement for freshwater management, Roygard says.
“We need good information. The better the information we have the better we can target things correctly and really understand the linkage between farms and rivers and the other inputs that all come together to influence water quality outcomes. We are also looking for opportunities to increase production and profitability on farms and the types of information we are getting from projects like this allows us to explore those avenues.
“Bringing in industry partners makes sense for us to ensure what we are doing is practicable and sensible. A great thing about this is the connectivity – the governance team at the council have the opportunity to view the research and in early March students will be in front of council to discuss it. So the decision makers are in front of the research, it’s linking the research to the practice side of it, ensuring we are adapting and creating policies that are going to work.”
