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  • Rootzone Reality Measuring N Leaching From Crops

Rootzone reality - measuring N leaching from crops

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FluxmeterA research programme measuring nutrient losses from cropping farms is up and running, with a network of special measuring devices being installed underground in paddocks up and down New Zealand.

The Rootzone Reality project is funded by the Ministry for Primary Industries’ Sustainable Farming Fund and led by the Foundation for Arable Research (FAR). It aims to scientifically prove what is happening under our cropping systems and ensure accurate reporting of nutrient losses from them.

Project Manager, Diana Mathers from FAR, says the project grew out of a need to ground truth nutrient loss models being developed by regional authorities looking for ways to improve freshwater quality.

“Many councils are looking at the development of Farm Environment Plans, complete with an Overseer® nutrient budget and a commitment to address nutrient losses through a change in management practice. Cropping farmers are often suspicious of models and question whether the results truly represent losses from their farms. This is fair enough, because in reality, there has been little measurement of nitrogen losses from the root zone of cropping rotations and we are short of data to calibrate the cropping components of the models.

“That is where this SFF project comes in, funding the installation of a permanent network of fluxmeters under cropping rotations in Canterbury, Manawatu, Hawkes Bay, Waikato and Pukekohe. Fluxmeters are a simple but clever design for the collection of drainage water. They are essentially a drainage pipe with a funnel, and have a wick which draws water into the collection pipe during drainage events. Drainage volume can be measured and samples of drainage water tested for nitrate concentration.”

The project will collect data from a diverse range of crops and cropping rotations across sites and seasons. These will include arable and vegetable rotations, with crops including grains and seeds, onions, maize, potatoes, beetroot and process and green vegetables. The impact of stock grazing will also be measured, along with drainage and nutrient loss data, weather, soil moisture, crop management details, crop biomass accumulation throughout the season, and crop yields.

At each site, one paddock is selected and 12 fluxmeters are installed in groups of four. Variability between individual fluxmeters is reduced by targeting areas within the paddock with similar soils and soil profiles. The top of the fluxmeters sit 1 metre below the soil surface, well below the cultivation management zone. Once they are in and the soil has re-settled, they have no influence on the crop above them. Tubes for collecting the drainage samples come from the fluxmeter to the soil surface, and samples are collected throughout the season after drainage events.

Diana says installing the fluxmeters is hard work.

“Because we want the data to represent the cropping system, the fluxmeters must be installed with minimal disturbance of the soil in the rootzone. A hole is augered out, and the soil horizons are preserved so they can be repacked, in order, after the fluxmeter is dropped in.

There are some limitations to consider in site selection. Paddocks need to be flat with no artificial drainage, the rotation must be representative of the main arable and vegetable rotations and the host farmer must be willing to share information about the cropping system. Paddocks with shallow top soils and stony secondary layers are unsuitable because of the difficulty in repacking the stones and soil above the fluxmeter in a way that guarantees the integrity of the drainage data. Soils with high water tables are also problematic because of the risk of the fluxmeter being flooded every time the water table rises.

During the life of the project we will be running Overseer budgets for the crop system that the fluxmeters are under. This will give us two sets of numbers, the actual N loss from the rotation and the modelled N loss from Overseer.

It is important to remember that Overseer uses long term average weather data to model nutrient losses over a 12 month period while the actual leaching losses relate to real-time rainfall. When comparing the two sets of data, we would expect the real data values to be either higher or lower than the Overseer values, depending on whether it has been a wet or a dry season. This data pattern will enable us to see whether the Overseer model is doing a good job. If we find that the real data is always either higher or lower than the Overseer values, we will suspect that model is either over-estimating or under-estimating the nitrogen losses for the rotation. Whatever the outcome, the data collected in this project will be a valuable asset for the cropping sector providing insights into the relationships between soils, crops and management practices.

Rootzone Reality is funded by the Ministry for Primary Industries’ Sustainable Farming Fund and led by the Foundation for Arable Research in partnership with Plant & Food Research, HortNZ and five Regional Councils (Environment Canterbury, Hawkes Bay Regional Council, Horizons Regional Council, Waikato Regional Council and Auckland Regional Council).

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