Aspects of feeding crops webinar

This webinar will be comprised of two presentations: Soil microbial dead zones induced by N fertilisation application by Tina Roose, University of Southampton, UK, and Boron in the soil and plant: a review by Philip White, James Hutton Institute, UK.

Abstracts

Soil microbial dead zones induced by N fertilisation application

S. Ruiz¹, D. McKay Fletcher¹, A. Boghi^1,5, K. Williams¹, S. Duncan¹, C. Scotson¹, C. Petroselli¹, T. Dias¹, D.R. Chadwick^2,3, D.L. Jones^2,4, T. Roose¹

¹Bioengineering Sciences Research Group, Department of Mechanical Engineering, School of Engineering, Faculty of Engineering and Physical Sciences, University of Southampton, Southampton SO17 1BJ, UK
²School of Natural Sciences, Bangor University, Bangor LL57 2UW, UK
³Interdisciplinary Research Centre for Agriculture Green Development in Yangtze River Basin, Southwest University, Chongqing, China.
⁴SoilsWest, UWA School of Agriculture and Environment, The University of Western Australia, Perth, WA 6009, Australia
⁵Computational Science Ltd, 30a Bedford Place, Southampton SO15 2DG, UK

Nitrogen (N) released into soils from fertiliser pellets is prone to loss via volatilization, leaching, denitrification, and surface run-off. These loss pathways result in low N use efficiency (NUE), environmental degradation, and negative effects on human health. While multiple field and farm-scale models describe N flows and losses in the plant-soil system, these fail to adequately capture spatial heterogeneity in N reactions surrounding fertiliser pellets and impacts on soil function.

Our aim was therefore to develop a mathematical model to describe N transport and reactions in soil at the pore-scale. Using X-ray Computed Tomography (XRCT) scans, we reconstructed a microscale description of a dry soil-pore geometry as a computational mesh. Solving two-phase water/air models produced pore-scale water distributions at 15, 30 and 70% water-filled pore volume. The model considers ammonium (NH₄⁺), nitrate (NO₃^–) and dissolved organic N (DON), and includes N immobilisation, ammonification and nitrification processes, as well as diffusion in soil-solution. We simulated the dissolution of a fertiliser pellet and a pore scale N cycle at the three different water saturation conditions. To aid interpretation of the model results, microbial activity at a range of N concentrations was measured experimentally.

The model showed that the diffusion and concentration of N in water films is critically dependent upon soil moisture and N species. We predicted that the maximum NH₄⁺ and NO₃^– concentrations in soil solution around the pellet under dry conditions are in the order of 110³ and 110⁴ mol m^-3 respectively (1-10 M), and under wet conditions 210² and 110³ mol m^-3, respectively (0.1-1 M). Supporting experimental evidence suggests that these concentrations would be sufficient to reduce microbial activity in the zone immediately around the fertiliser pellet (ranging from 0.9 to 3.8 mm depending on soil moisture status), causing a major loss of soil biological functioning.

This model demonstrates the importance of transport processes in regulating N movement in soil with special capability to predict the effects of fertilisers on soil processes and the root microbiome. We will also discuss and expand what this study means for the field scale fertiliser application and resulting soil microbiome dynamics.Nitrogen (N) released into soils from fertiliser pellets is prone to loss via volatilisation, leaching, denitrification, and surface run-off. These loss pathways result in low N use efficiency (NUE), environmental degradation, and negative effects on human health.

While multiple field and farm-scale models describe N flows and losses in the plant-soil system, these fail to adequately capture spatial heterogeneity in N reactions surrounding fertiliser pellets and impacts on soil function. Our aim was therefore to develop a mathematical model to describe N transport and reactions in soil at the pore-scale. Using X-ray Computed Tomography (XRCT) scans, we reconstructed a microscale description of a dry soil-pore geometry as a computational mesh. Solving two-phase water/air models produced pore-scale water distributions at 15, 30 and 70% water-filled pore volume. The model considers ammonium (NH₄⁺), nitrate (NO₃^–) and dissolved organic N (DON), and includes N immobilisation, ammonification and nitrification processes, as well as diffusion in soil-solution. We simulated the dissolution of a fertiliser pellet and a pore scale N cycle at the three different water saturation conditions. To aid interpretation of the model results, microbial activity at a range of N concentrations was measured experimentally. The model showed that the diffusion and concentration of N in water films is critically dependent upon soil moisture and N species. We predicted that the maximum NH₄⁺ and NO₃^– concentrations in soil solution around the pellet under dry conditions are in the order of 110³ and 110⁴ mol m^-3 respectively (1-10 M), and under wet conditions 210² and 110³ mol m^-3, respectively (0.1-1 M). Supporting experimental evidence suggests that these concentrations would be sufficient to reduce microbial activity in the zone immediately around the fertiliser pellet (ranging from 0.9 to 3.8 mm depending on soil moisture status), causing a major loss of soil biological functioning. This model demonstrates the importance of transport processes in regulating N movement in soil with special capability to predict the effects of fertilisers on soil processes and the root microbiome. We will also discuss and expand what this study means for the field scale fertiliser application and resulting soil microbiome dynamics.

Boron in the soil and plant: a review
Philip White, James Hutton Institute, UK

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. 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.

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.

There will be a written paper accompanying this webinar.