Reducing nitrate leaching with cover crops

While there is evidence that cover crops reduce nitrate leaching, it remains unclear when this nitrogen is released across an arable rotation. Our trials at Strategic Cereal Farm East are looking at the role cover crops play in reducing nitrate leaching losses.

How do cover crops effect nitrate leaching?

  • Start: August 2019
  • End: Ongoing

Objective

To determine the role of cover crops in reducing nitrate leaching losses across a rotation. 

Essex & Suffolk Water analysed water collected from land drains in 2017/2018 and showed that cover crops improve water quality. There is evidence that cover crops reduce nitrate leaching, but their benefits across the rotation are still not clear. The establishment of cover crops and the next cash crop is not always straight forward and this impacts nitrogen uptake. There are questions about when this nitrogen is released across the rotation.

Testing impact of cover crops on nitrate – trial summary

This split field trial uses two fields:

  • Appletree
    • 06-hectare field
    • Cultivated using a plough
    • Sandy loam soil
  • Blacksmiths
    • 32-hectare field
    • Crop established using a one-pass system
    • Sandy loam soil

Strategic Cereal East host, Brian Barker, drilled a cover crop mix on 24-28 August 2019 at 40 kg/ha:

  • Rye (32%)
  • Buckwheat (40%)
  • Phacelia (8%)
  • Oil Radish (8%)
  • Sunflowers (12%)

The cover crop was destroyed on 13 March 2020. Spring barley was undersown with herbage grass on 28 March 2020 and harvested on 10 August 2021.

Cover crop results from harvest 2020 on nitrate levels

A well-established cover crop is effective at improving water quality. Nitrate concentrations in drainage water were below 50 mg/l.

Reductions were found in spring crop yields after cover crops because of slug damage and crop establishment in wet conditions.

Strategic Cereal Farm East Harvest 2020 Report

Watch: Cover crop results 2020 from Brian Barker and Kate Smith (ADAS)

Assessing nitrate levels in Harvest 2021

Autumn soil nitrogen supply

The soil nitrogen supply was assessed in November 2020. The results show that the above-ground biomass had captured between 15 and 45 kg N/ha. The soil mineral nitrogen ranged between 33 to 76 kg N/ha.  

Soil nitrogen supply is the soil mineral nitrogen (SMN) from 0-90 cm and above-ground biomass nitrogen content.

Our Nutrient Management Guide explains how to determine the nutrient requirements of cereals, oilseeds, peas and beans, sugar beet and biomass crops.

The results from the cover crop one-pass system have likely increased the risk of N leaching to water:

  • Autumn herbage grass biomass was 0.44 t/ha compared to 1.1 to 1.7 t/ha in the other treatments
  • The above-ground biomass N content was lower and SMN was higher
  • Higher nitrate (NO3) concentrations in drainage water. The high winter rainfall (276 mm Oct-Mar) is also likely to have exacerbated the risk of SMN leaching into the water. More work is needed to understand these results.

Spring soil nitrogen supply

April 2021 assessments showed that the crop had captured between 53 and 62 kg N/ha. The soil mineral nitrogen ranged from 30 to 56 kg N/ha.

The average crop biomass was 1.5 t/ha in all treatments apart from the cover crop one-pass system which resulted in 1.3 t/ha.

The cover crop treatments resulted in increased SMN by around 20 kg/ha. This resulted in total nitrogen content of 113 kg N/ha compared to 86 kg N/ha where there hadn’t been any cover crops.

How has cover crops impacted nitrate levels so far in the trials?

Spring 2021

We are monitoring soil nitrate flow at 30 and 60 cm using solid-state nitrate sensors. Researchers from John Innes Centre are testing the probes as part of a three-year Innovate UK project

Soil assessments include penetrometer, soil moisture, bulk density, VESS, and VSA and earthworms.

Before harvest

Crop nitrogen biomass

The field team will collect and weigh leaf and stem samples. They will dry the samples, send a sub-sample for total N content analysis and calculate dry matter yield and N uptake.

Yield

At harvest, the farm will collect combine yield data for NIAB to analyse.

Cost of production

The Farm Economics team will calculate the economic cost of production of the winter wheat in each trial field. Using Farmbench, they will produce costs per hectare and per tonne. The calculations will use:

  • Seed, fertiliser, and crop protection
  • Farm labour, machinery, and equipment
  • The regional average for property, energy, and administration

Previous nitrate level results from Strategic Farm East

In August 2018, Brian established four ground cover treatments in two fields. The fields that were used were Big Lawn and Hills. Brian drilled a cover crop mix of oil radish and rye in a split field trial design on 25 August 2018 and destroyed using glyphosate on 22 February 2019. Linseed was drilled on 12 April 2019 and winter wheat was drilled on 23 October 2019.

The results showed that a well-established cover crop is effective at improving water quality by bringing nitrate concentrations in drainage water below 50 mg/l.

Useful resources

Strategic Cereal Farm East Harvest 2019 Results

Opportunities for cover crops in conventional arable rotations

Our Strategic Farms are an opportunity to see how to use our research on a commercial farm. Find out more about our Strategic Cereal Farm East Programme

Image of staff member Teresa Meadows

Teresa Meadows

Knowledge Exchange Manager - Cereals and Oilseeds
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