The Conference featured eleven papers, covering topics such as the measurement of soil carbon and nitrogen usage efficiency, the practical role of regenerative agriculture, the agronomic performance of P recycled fertilisers, the role of boron, a review of biostimulants, long tern trial results on the impact of pH management, and options to reduce nitrous oxide emissions from crop production.

The presentations were augmented by a varied display of posters, while the Conference also hosted the final of the 2021 Brian Chambers International Award for Early Career Researchers in Crop Nutrition.

Paper abstracts of the presentations made at the Conference can be read below.

THURSDAY

Recent advancements in Nitrogen Use Efficiency research – a review of what is known; and what is not

Hein ten Berge / Renske Hijbeek, Wageningen University and Research, the Netherlands

This paper reviews recent research at Wageningen University and Research relating to nitrogen use efficiency (NUE). This covers the development of models to predict  agronomic NUE, based on large datasets from different countries; interactions between fertiliser-nitrogen and organic inputs and/or soil organic matter; long term N recovery from synthetic fertilisers; nitrogen cycling in dairy and arable systems; the effects of organic and mineral inputs on soil biota and the N fertiliser values of upcoming organic inputs from waste streams.

This paper will collate and illustrate recent findings in these areas, and will end by discussing the key remaining unknowns, in the opinion of the authors..

Options to reduce nitrous oxide emissions from crop production – a review

Nicholas Cowan, Centre for Ecology and Hydrology, UK

Co-authors: Peter Levy, Julia Drewer, Juliette Maire, Edward Carnell, Ute Skiba.

This paper will review the current options that exist to reduce N₂O emissions on farms, highlighting their advantages and disadvantages in the context of maintaining crop yields and productivity. The methods that current net-zero reports propose should be used to meet emissions targets will be discussed and evaluated, to identify if these efforts can realistically reduce N₂O emissions and at what cost. These topics will include precision farming, microbial/urease inhibitors, pH, fertiliser types, pollution swapping and the potential for future technologies.

Impact of pH management on productivity, soil biology and soil health – findings from a long term trial

Christine Watson¹

Co-authors: Fraser, F.¹, Walker, R.L.¹, Topp, K.², Stockdale, E.A.³ and Bhogal, A.⁴

¹ SRUC, Craibstone Estate, Aberdeen AB21 9YA

² SRUC, West Mains Road, Edinburgh, EH9 3JG

³ NIAB, 93 Lawrence Weaver Road, Cambridge, CB3 0LE

⁴ ADAS, Gleadthorpe, Mansfield, Notts. NG20 9PD

Managing soil pH is fundamental to the maintenance of crop production. Soil acidification is a major cause of soil degradation across the world. The chemical principles of soil acidification are well understood but changes in atmospheric deposition and climate are constant challenges. The impact of lime on soil respiration and CO2 emissions is also a cause for concern in terms of climate change. It is well known that pH strongly affects both carbon and nitrogen cycling, however, there is significantly less understanding of the effect of pH on soil biology in relation to soil function, particularly the relationships between soil biology, soil health and crop production.  

The pH plots at Craibstone have provided land-users and biological/environmental scientists, both in Scotland and worldwide, with a uniquely powerful resource to understand this pivotal role of soil pH in regulating agricultural productivity and environmental pollution. The long-term pH trial in Woodlands Field at SRUC Craibstone near Aberdeen was established in 1961 and investigates the impact of different pH levels (ranging from 4.5 to 7.5 in 0.5 increments) on soil properties and crop performance of an 8 course rotation comprising: 3 year grass/clover ley, winter wheat, potatoes, spring barley, swede and spring oats (undersown with grass/clover). Each crop in the rotation is present every year enabling a comparison of the response of all crop types within the same season. The influence of soil pH on crop yield is available over a 60 year period. More recent studies have explored the influence of soil pH on crop and grass quality.  As part of the AHDB Soil Biology and Soil Health partnership we have unique information about the impact of pH on the soil microbiome.

This paper will explore the impacts of soil pH on productivity, soil biology and soil health in the Woodlands Field experiment in the light of information from other long-term pH studies across Europe.

The benefits of increasing soil carbon

Andy Neal, Rothamsted Research, UK

One potential unintended consequence of the current debate around whether the 4 per Mille initiative is that it acts as a distraction from the development of a practicable approach to involving soils in society’s response to the climate crises, and the central role that organic carbon plays in soil.  Soil plays an important role in the carbon cycle and is one of the most complicated reservoirs of carbon. More carbon is stored in the world’s soils than is currently present in the atmosphere. However, the issue is broader that just how much carbon can be stored in soil, the attendant co-benefits that carbon brings to soil influence the water cycle and nutrient use efficiency.  Much of this is known and accepted, but the fundamental processes supporting soil function are understood less well. This paper describes the diverse benefits that soil organic matter (SOM) provides for soils and agricultural production.

Soil organic matter binds mineral particles and colloids together and is the fundamental causative agent generating structural complexity. Plant and animal residues are processed by microbes before joining the SOM pool, a continuum of progressively more extensively oxidized compounds. Much of this SOM is associated with 30–100 μm diameter pores. As a result, the effect of microbial processes—metabolism, extracellular degradation of compounds, polymer secretion and cell lysis—on soil structure is particularly evident at the scales less than 80 μm responsible for regulating convective and diffusive flow rates, as well as the balance of air and water at any given matric potential. These hierarchical processes exhibit characteristic properties of self-organizing and emergent systems.

We observe soil to be a self-organizing adaptive system, modulated by texture.  In high organic input systems such as pastures and manure-amended arable soils, total porosity and connected porosity are increased providing a greater capacity to store water and soluble nutrients.  This improves soil system resilience during periods of low rainfall or nutritional inputs. In addition, the extensive and highly connected pore network selects for assimilatory, and against dissimilatory, processes by permitting greater flux of O₂ through the system: it thus improves the efficiency of metabolic processes and C_org conversion into biomass while reducing potential losses of nutrients arising from leaching or emission of methane and nitrous oxide to the atmosphere—both potent greenhouse gasses.  Soils which experience low inputs of organic carbon develop less extensive and less connected pore networks through which O₂ and nutrients flow at reduced rates.  Consequently, anaerobic respiration becomes more prevalent and the efficiency of metabolic processes is reduced.  Dissimilatory processes such as denitrification and methanogenesis become more influential, increasing the loss of nitrogenous plant nutrients from the soil as nitrous oxide.

Microbial metabolism of organic carbon acts as the determining factor in adaptive soil systems since it creates the simple molecules which act to glue mineral particles together to create structure.  This indicates that continual inputs and flux, rather than soil organic carbon content, acts as the critical factor in soil systems.

How to measure, report and verify soil carbon change

Pete Smith, University of Aberdeen, UK

One proposed option for removal of carbon dioxide from the atmosphere is by increasing the amount of carbon retained in the soil organic matter, an option known as soil organic carbon sequestration. Given that soils already contain a lot of carbon, and changes in soil organic carbon are slow, it is difficult to measure increases in soil carbon against the large background soil carbon stock. Because of this difficulty in measuring changes in soil organic carbon, a key barrier to implementing programmes to increase soil organic carbon is the need for credible and reliable measurement/monitoring, reporting and verification platforms.

I present methods for measuring soil organic carbon change directly in soils, examine novel developments for quantifying soil organic carbon change, and describe how surveys, long-term experiments and chronosequences (sites of different ages with changes at various stages of carbon gain) can be used for testing models and as benchmark sites in global frameworks to estimate soil organic carbon change. I present some of the measurement/monitoring, reporting and verification platforms for soil organic carbon change already in use and describe a new vision for a global framework for measurement/monitoring, reporting and verification platform of soil organic carbon change. The proposed platform builds on existing repeat soil surveys, long-term experiments, remote sensing, modelling and novel measurement methods and could be applied at national, regional or global scales.

How much carbon can realistically be sequestered in arable soils and can we measure it?

David Powlson, Rothamsted Research, UK

Sequestration of carbon (C) into organic matter in soils is seen as a means of mitigating climate change by removing carbon dioxide from the atmosphere. Initially the focus was on removal of land from agriculture and conversion to grassland or forests under schemes such as set-aside in the EU or the Conservation Reserve Program in the USA. But currently there is more consideration of sequestration of C in soils that continue in agriculture, sometimes linked with practices such as so-called “regenerative agriculture”. Schemes for paying farmers for increased soil organic C (SOC) are being developed as part of carbon offset schemes. There is considerable debate and conflicting claims about the extent to which SOC can be increased in arable soils; evidence on this will be briefly reviewed with respect to soils in arable cropping.

If such an approach is to be pursued, it will be necessary to establish reliable methods to measure SOC increases. Direct measurement is difficult because increases are relatively small and occur against a large background of existing SOC: it might typically be 5 years, and often longer, before a change in management leads to a change that can be reliably detected. Infra-red reflectance analyses should make measurement of SOC quicker and easier, but not necessarily giving more accurate detection of changes. There is evidence that various fractions within the total SOC change more rapidly in response to altered management and can give “early warning” before total changes become measurable. Microbial biomass C, typically comprising 2-5% of total SOC, is well established in this regard. Other possibilities include fractions obtained by physical separation, fractions easily oxidised chemically, and a wide range of biological measurements. Biological approaches range from detailed DNA-based methods to detect changes in specific microbial groups to coarse-scale methods such as the burst of CO₂ evolution after rewetting dry soil. All such “early warning” methods may be successful in detecting trends (e.g. whether or not a soil is increasing in SOC) and this is helpful. But they will probably not provide direct evidence of the absolute change in total C that has occurred. Linking such methods with models of C turnover, and the use of benchmark sites where detailed measurements are made under controlled conditions, may offer the best practical approach.

FRIDAY

Regenerative Agriculture – separating practicalities from polemic

Alastair Leake, The Loddington Estate

Farming systems employ different techniques to produce food. Historically rotations were driven by the need to incorporate sufficient fertility building phases alternated between exploitative phases. The development of the Haber-Bosch process removed the need for rotations to be limited by biologically fixed nitrogen, although organic farming systems continue this approach. In recent years focus has shifted from a gross output approach where maximum yield or profitability were the principle drivers to more agro-ecological approaches where biodiversity, water quality, gaseous emissions, soil heath, animal welfare, carbon storage and the nutrient density of food are considered to be important.

Reconciling these sometimes conflicting factors with other societal demands such as afforestation for climate change mitigation, rewilding for biodiversity or re-wetting land for flood prevention presents a major challenge to modern farming and land management. This presentation will explore these challenges and trade-offs from a practical perspective.    

The role of Regenerative Agriculture in food production systems

Ken Giller¹

Co-authors: Renske Hijbeek¹, Jens A. Andersson¹ and James Sumberg²

¹ Plant Production Systems Wageningen University PO Box 430 6700AK Wageningen, The Netherlands

² Institute of Development Studies (IDS), University of Sussex, Brighton, UK

Agriculture is in crisis. Soil health is collapsing. Biodiversity faces the 6th mass extinction. Crop yields are plateauing. Against this crisis narrative swells a clarion call for Regenerative Agriculture. But what is Regenerative Agriculture, and why is it gaining such prominence? Which problems does it solve, and how? This paper addresses these questions from an agronomic perspective. The term Regenerative Agriculture has actually been around for some time, but there has been a resurgence of interest over the past five years, and particularly since 2016. It is supported from what are often considered opposite poles of the debate on agriculture and food. Regenerative Agriculture has been promoted strongly by civil society and NGOs as well as by many of the major multi-national food companies. Many practices promoted as regenerative, including crop residue retention, cover cropping, and reduced tillage are central to the canon of ‘good agricultural practices’, while others are contested and at best niche (e.g. permaculture, holistic grazing). Worryingly, these practices are generally promoted with little regard to context. Practices most often encouraged (such as no tillage, no pesticides or no external nutrient inputs) are unlikely to lead to the benefits claimed in all places.

The paper argues that the resurgence of interest in Regenerative Agriculture represents a re-framing under the same banner of what have been considered to be two contrasting approaches to agricultural futures, namely agroecology and sustainable intensification. This is more likely to confuse than to clarify the public debate. More importantly, it draws attention away from more fundamental challenges. We conclude by providing guidance for research agronomists who want to engage with Regenerative Agriculture.

Predicting the agronomic performance of P recycling fertilisers – challenges and potential solutions

Sylvia Kratz, Institute for Crop and Soil Science, Julius Kühn Institute, Germany

Co-authors – Paul Keßeler, Kira Jabs, Rüdiger Anlauf, Helge Schultz, Elke Wilharm, Thade Potthoff, Bernd Steingrobe and Elke Bloem.

Mineral rock phosphate has become increasingly scarce and is seen as a critical raw material by the European Union. In line with the EU Circular Economy Action Plan and related political initiatives, the production of P recycling fertilisers from renewable secondary phosphate sources is therefore gaining in importance. In order to assess their agronomic performance, reliable test methods are needed. This paper reviews factors affecting the agronomic performance of P recycling fertilisers and presents tools to predict it. Standard chemical extraction methods and possible future alternatives as well as greenhouse vegetation trials are discussed. Ongoing research aiming to work towards a standardisation of greenhouse vegetation trials for fertiliser testing is presented, using struvite-based fertilisers originating from wastewater treatment as a model case.

Boron in the soil and plant: a review

Philip White^1,2

Co-authors – Guangda Ding^2,3, Lei Shi^2,3 and Fangsen Xu^2,3

 ¹The James Hutton Institute, Invergowrie, Dundee DD2 5DA, UK

²National Key Laboratory of Crop Genetic Improvement, Huazhong Agricultural University, Wuhan 430070, China

³Microelement Research Centre, Key Laboratory of Arable Land Conservation (Middle and Lower Reaches of Yangtze River), Ministry of Agriculture and Rural Affairs, Huazhong Agricultural University, Wuhan 430070, China

Boron (B) is a plant micronutrient. It is essential for plant growth and reproduction. Commelinid monocots, such as cereals and grasses, require tissue B concentrations greater than about 10 mg kg^-1 dry weight (DW), whereas non-grasslike monocots and eudicots require tissue B concentrations greater than 20-30 mg kg^-1 DW. In general, the tissue B requirements of plant species parallel the amounts of B in their cell walls. The main reason for this is the unique role of borate esters in cross-linking rhamnogalacturonan II monomers in the pectin fraction of plant cell walls. Rhamnogalacturonan II is a dominant component of the cell walls of eudicots and non-Commelinid monocots, but not of Commelinid monocots. Frequent symptoms of B deficiency in plants include impaired root and shoot elongation, chlorosis of developing leaves, necrosis of meristems and aberrant reproductive growth, reflecting the physiological role of B in cell walls.

Soils differ greatly in their phytoavailable B concentrations. The main factors affecting B availability to plants include the B content of the soil’s parent material, soil texture, soil organic matter content, clay mineralogy and pH. Boron deficiency is often considered one of the most common micronutrient deficiencies in crops and generally occurs on deep sandy soils in regions of high rainfall, through which B readily leaches. Boron deficiency in crops can be addressed through the application of soil or foliar B fertilisers. Boron toxicity to plants occurs in arid regions where the B content of the soil’s parent material is high, such as the Middle East, Southern Australia, parts of California, Chile and the Eastern Mediterranean. It has been addressed both by improved soil management and by the development of crop genotypes with greater abilities to prevent B uptake or tolerate high tissue B concentrations.

Boron can cross biological membranes either (1) by diffusion of uncharged boric acid through the lipid membrane, (2) by facilitated diffusion of uncharged boric acid through protein channels of the Nodulin26-like Intrinsic Protein (NIP) subfamily, which are also known as aquaporins, or (3) by the transport of borate through borate exporter (BOR) proteins driven by the electrochemical gradient. In the soil solution, most B is present as boric acid, B(OH)₃. In the cytoplasm of plants cells some B (about 2%) is present as borate, B(OH)₄^–. Plant root cells can control B uptake by regulating the entry of boric acid and the efflux of borate through the expression of genes encoding the B transport proteins in their plasma membranes. Boron is relatively immobile in the phloem of most plant species, with the exception of plants that transport sugar alcohols (polyols) in the phloem. Thus, a constant B supply is required to prevent the occurrence of B deficiency symptoms, which will occur first in developing tissues. There is considerable variation among species, and among genotypes of a species, in the ability to control B uptake, the efficiency by which B is utilised physiologically, and the ability to tolerate large tissue B concentrations. This variation has been used to develop crops for soils with low or high B phytoavailability.

This talk will describe the occurrence of B in soils and factors affecting B availability to plants, the molecular mechanisms by which plants control B uptake and its distribution between organs, and the agronomic and genetic strategies that are being used to mitigate B deficiencies and B toxicities in agriculture.

Claims and mechanisms of plant biostimulation

Patrick du Jardin, Gembloux Agro-Bio Tech, University of Liège, Belgium

Plant biostimulants have been accepted as a new regulatory category of fertilising products in different parts of the world, but their properties and use still need to be clarified. The main reason for this is that their definition is based on claims of agricultural effects – which are increased nutrients use efficiency, improved tolerance to abiotic stress, enhanced product qualities and increased availability of soil nutrients, according to the new regulation (EU) 2019/1009 – but how these claims should be substantiated in practice is still unclear, generating skepticism among growers. The EU approach consists of defining harmonized standards, relating to agreed definitions, quality criteria and protocols for generating data on product efficacy. Compliance of the data package provided by the companies with the adopted standards will be regarded as sufficient to validate product claims, as indicated on the labels of CE-marked products.

Notwithstanding the regulatory developments, achieving confidence in the products will depend on the scientific knowledge of the mechanisms of biostimulation. Although many scientific articles on biostimulants have been published in peer-reviewed journals over the last decade, several bottlenecks can be identified in the understanding of biostimulants’ action. For example, many articles use single substances (which can be multicomponent), whilst most marketed products combine several biostimulant substances, sometimes added to micro-nutrients. This makes it difficult to assign the observed effects to individual constituents or to specific interactions between constituents. Secondly, biostimulants aim to interact with the life processes of plants, which respond to many environmental factors and also depend on the genetic makeup of the variety. Thirdly, a plant is now regarded as a ‘superorganism’, associated with microbes, and many biostimulants will interact with the plant-associated microbiota. Other difficulties stem from the blurred borders between biostimulation and biocontrol, and also between nutrients and biostimulants, with some nutrients acting on physiological processes in ways that are similar to those of biostimulants.

A roadmap to tackle some of the above issues will be discussed.

We are grateful for the support being provided to the Conference by these organisations.

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