Anaerobic digestate impacts on soil microbiology and Nitrogen retention

This field lab is looking at the impacts of digestate on soil microbiology and Nitrogen retention by cover crops

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Field Lab Timeline

    8/11/2019 11:00:00 PM
  • Field lab proposal submitted

    Field lab proposal submitted
  • 8/26/2019 11:00:00 PM
  • Cover crop drilling

    Cover crop drilling
  • 9/1/2019 11:00:00 PM
  • Funding approved

    Funding approved
  • 9/11/2019 11:00:00 PM
  • Group meeting

    Group meeting
  • 9/17/2019 11:00:00 PM
  • Soil sampling 1

    Soil sampling 1
  • 10/15/2019 11:00:00 PM
  • Soil sampling 2

    Soil sampling 2
  • 11/20/2019 12:00:00 AM
  • VESS and worm count

    VESS and worm count
  • 12/10/2019 12:00:00 AM
  • Group meeting

    Group meeting
  • 2/12/2020 12:00:00 AM
  • Soil sampling 3

    Soil sampling 3
  • 3/18/2020 12:00:00 AM
  • VESS and worm count

    VESS and worm count
  • 5/27/2020 11:00:00 PM
  • Results meeting and Yr 2 planning

    Results meeting and Yr 2 planning
  • 6/23/2020 11:00:00 PM
  • Soil sampling 4

    Soil sampling 4
  • 9/29/2020 11:00:00 PM
  • End of year report

    End of year report
For further information hover over the above milestone marks
  • Discussion

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  • Achievements

    May 2021

    Final report published

    Three years of data has been gathered to ascertain the benefits of cover cropping combined with ADD autumn applications.

    The use of cover crops to reduce erosion, improve soil structure, improve soil organic matter and an active soil microbial community, added to mitigation of potential diffuse pollution from the ADD application has proved beneficial in this field lab.

    While the evidence for nitrogen retention was not quite as pronounced as was documented in the first field lab, it is clear that use of cover crops in combination with ADD made a positive difference in retention of Nitrogen within upper soil horizons when applied in the autumn.

    Participating farmers have good pilot data to indicate that this approach is safe and potentially advantageous in terms of utilisation of ADD, whilst also seeking to build soil structure and function. This indicates that sustainable use of ADD in combination with cover crops is a viable management strategy and was effective in highly contrasting soil types.

    The lack of any precipitation meant that the handkerchief method to test soil fungal activity unviable, as no degradation had occurred in dry soil.

    To read the full report, please go to the Documents section at the top right corner of this page (sign-in required, registration is free).

    Milestone: End of year report

    March 2020

    Third soil samples to be taken

    Handkerchiefs have been sent out to the farmers. These will be buried in the ground for three to four weeks, dug up and photographed to show the microbial breakdown of the handkerchief. Alongside this, the farmers will do a third round of VESS, worm counts and assessments of Green Area Index (GAI).

    Cover crop will be terminated soon and three of the farmers are looking to sow maize in late April/early May.

    The cover crop is looking a lot better after having some initial struggles with establishment

    Milestone: Soil sampling 3

    February 2020

    Initial microbial analysis results

    Farmers have taken a closer look on the impact of digestate on soil microbiology, using a ‘dirty’ approach with the handkerchief methods, and using sensors to get a better idea of the microbial communities populating the soil.

    The sensors essentially provide a platform onto which micro-organisms can concentrate . The sensors are then taken back to the lab for DNA sequencing and comparison against reference databases enables the identification of those species.

    The first round of testing showed the dominance of certain fungal species, without considerable differences between treatments. It appears that low soil temperatures at the time of sampling may have affected which species are captured. The group is planning on a second round of testing at the end of March, when the soil will be warmer. Digestate from each of the trial sites is also being tested.

    Milestone: Soil sampling 2

    December 2019

    December group meeting

    The group met in December and their discussions are summarised below.

    - Cover Crop Establishment:
    Poor weather conditions at critical period results in poor establishment. Despite a good initial establishment, the cover crop has suffered from the dry weather at the time of drilling and the subsequent rain in the Autumn. Hot spots of single species (seed mix is made up of black oat, vetch, radish, buckwheat) are now being observed in certain areas at each site, with some species establishing much better than others. This poor establishment is reflected in the low Green Area Index measures and will likely impact nitrogen levels through the soil profile.

    - Results coming in from microbial analysis:
    Hankies provide a good indicator for microbial activity. One method used was burying handkerchiefs into the soil and using degradation as an indicator of microbial activity. Hankies were buried in the top layer of soil, where microbial activity occurs, for approximately 4 weeks from the end of October. Visual assessment showed that more degradation was evident in the cover crop treatment with a high dose of digestate.

    The second method made use of sensors, and these were placed in the ground for two weeks approximately, before being sent back for analysis. We will be hearing from the results in January.

    - First set of results from nutrient analysis:
    Inconclusive so far. Results were available from one of the sites. Differences were expected for the treatments including a cover crop however this was not observed and was likely due to the poor cover crop establishment. Nevertheless, there was no evidence of large amounts of nitrogen leaching into the deeper soil layers.

    Milestone: Group meeting

    November 2019

    Vess and worm count done

    VESS (Visual Assessment of Soil Structure) and worm counts have been undertaken. A VESS score gives an indication of root penetration, aeration, and water availability to plants.

    Milestone: VESS and worm count

    October 2019

    Microbial Analysis underway

    Over the duration of the trials farmers will be using two different methods to assess the impact of digestate on microbial populations.

    1. Handkerchiefs have been buried in the ground and will be taken out in the spring. The extent of degradation will provide a good indication of microbial activity. Update: Pictures of this can be found in the 'Field Lab Documents' section at the top of this page.

    2. Sensors have been deployed and these will identify which fungal species are present in the soil, as well as provide some relative quantities on population numbers for each species identified.

    Milestone: Soil sampling 1

    October 2019

    Soil health training session

    The participating farmers attended a successful soil health training session on Friday 11th October kindly hosted by Nick Sheppard at Upton Suffolk Farms. The meeting provided the opportunity to learn about the different assessments they will be undertaking during the delivery of the trial which includes:

    - VESS - this stands for VESS stands for Visualisation Evaluation of Soil Structure, a useful indicator of top soil structure. In the trial, we would expect the cover crop treatment to have a better soil structure than the control for example, due to the deep rooting properties of the cover crop mixture

    - Worm count - worms are a good indicator of soil health and doing a count will to quantify population numbers.

    - GAI - Green Area Index is the ratio of green leaf area to ground area. We would expect the high digestate dose system to have a higher GAI, since more Nitrogen is available to the crop for development.

    Led by Lydia Smith, head of Innovation Farm at NIAB, the farmers spent some time practising their new skills. The participating farmers reported that they found the day really useful and confirmed that they now have a better idea of what to look for when assessing soil structure.

    Milestone: Group meeting

    September 2019

    Cover crop establishment

    On three of the participating farms the cover crop has been established. It was drilled late August/early September. All farmers have reported that it has grown fairly well in all treatments with some reports of nettles getting established too. Green Area Index will be assessed later on, once the cover crop has developed more.

    Milestone: Cover crop drilling

    August 2019

    Field lab proposal

    This group are conventional and organic farmers and industry leads who either own or work with digestate plants in East Anglia. Many of them have been part of a previous field lab, "Increasing nutrient efficiency from anaerobic digestate" that ran from 2017-2019.

    The group have discussed the outcomes of the 2017-2019 field lab and have identified the following challenges that they would like to address:

    • Have a better understanding of the effect of digestate (ADD) on soil microbiology (pathogens and beneficials)
    • Stabilising N between time of application and when it is needed by the cash crop
    • Further define the influence of cover crops in improving the utilisation of ADD and reducing potential diffuse pollution and nutrient losses.
    • Modes of application: liquid (injection) vs solid (top)
    • Understanding the economics behind the practices relating to ADD management is crucial – agronomic benefits vs economic impact of storage / movement and how this affects nitrogen retention in the ADD. Identifying the cost-benefit ratio for the field lab trial will be important.
    • Identifying the simple, easily measureable, cost effective metrics to demonstrate impact of treatments.

    In brief, the group will address these questions by:
    • Continuing to use cover crops to improve N fixation and reduce volatilisation.
    • Monitoring changes in soil nutritional levels (N in particular, but also P, K and minor nutrients) to assess N stabilisation
    • Monitoring soil microbiology and soil health (e.g. worm count)
    • Establishing trial protocols in conjunction with the research partner for consistent and accurate data collection

    Their aim is to have solid data to rely on when assessing whether the treatments are worth investigating further or using in long term ADD management on their farms.

    A proposal has been submitted by the group and they have applied for a research fund grant to cover costs such as soil microbiology screening, soil nutrient analysis and cover crop seed.

    Milestone: Field lab proposal submitted

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