The recent legal requirement for developments to achieve 10% biodiversity net gain (BNG) post-development has created a demand for purchasable biodiversity units (BUs) from elsewhere, where this target cannot be achieved on site. To provide these, designated offsite biodiversity areas are being created, known as ‘habitat banks’, from which a limited number of units may be bought, corresponding to specific areas of a particular habitat type. To ensure these habitat banks are maintained for the statutory 30-year management and monitoring period, the land is entered into a legal contract known as a ‘Conservation Covenant’, binding the land to a set of agreements detailed in a Habitat Management and Monitoring Plan (HMMP). This process is overseen by a ‘Responsible Body’ and RSK Wilding/RSK Biocensus became one of the first Responsible Bodies in 2024.
When land is chosen for a habitat bank, habitat creation is informed by local ecological and geological data including soil type and conditions, as each distinctive habitat type has specific requirements and is often highly sensitive to soil acidity and nutrient availability. In light of this, an interesting question has recently been raised regarding the effect of intensive livestock farming near habitats banks, as aerial ammonia deposition from these units can cause an increase in nutrient-nitrogen and changes in soil acidity, altering soil chemistry and potentially impacting the likelihood of a target habitat reaching required condition.
Eighty-one percent of global emissions are a result of agriculture, with cattle the largest single source, followed by poultry and pigs (1). Ammonia emissions from poultry and pigs, despite being lower in total than cattle, are concentrated around housing units, creating hotspots reaching 60 µg/m³ – an increase of 118% from normal atmospheric levels of around 0.5 µg/m³ (2). These areas of elevated atmospheric ammonia can stretch up to 2km from livestock units (3).
The critical level of atmospheric ammonia for plants, above which direct adverse effects may occur, is generally 3 µg/m³, while for more sensitive habitats such as those containing lichens and bryophytes it is 1 µg/m³ (4). This sensitivity to atmospheric ammonia is due to its direct correlation with plant-available nitrogen in soils. An estimated 80 kg/ha of ammonia-associated nitrogen has been recorded deposited annually 30m from a poultry unit and 14 kg/ha 650m downwind of the unit (3). Although less locally concentrated, a similar effect would be expected from intensive cattle farms, with studies showing nitrogen deposition from cattle-associated atmospheric ammonia increasing background levels by over 100%, with levels 50% elevated at a 2km distance (10).
Also to be taken into consideration is the fact that large parts of the UK are already exceeding the critical level of ammonia for sensitive habitats due to intensive agricultural practices, with most of the south falling between 1 and 3 µg/m³, and hot spots exceeding this. Similarly, nitrogen critical loads, which fall between 5kg/hectare(ha)/year and 20kg/ha/year (depending on the nitrogen sensitivity of the habitat) (5) are exceeded throughout the UK with an average of 7.4 kg/ha/year in 2021, and the majority of south of the UK exceeding 14kg/ha/year.
So, with ammonia levels already nationally critically high, additional ammonia from livestock units is potentially an extra strain, but how exactly are botanical assemblages in habitat banks affected? The level of disruption is likely to be dependent on the habitat type and species assemblage, but the expected effect of increased nutrient availability is a change to these botanical communities, including significant reduction in species richness (6).
Distinctive habitat types are characterized by a group of ‘indicator species’, often highly sensitive to the specific conditions of that habitat, on which habitat classification is based, the loss of which results in less desirable habitat types. As an example, 50% of Other neutral grassland (ONG) indicator species are adapted to low to moderate soil nutrient levels (7). Raising nitrogen levels has been shown to decrease the abundance or even eradicate some of this grassland’s species (8). This could result in the downgrading of ONG to a less species rich grassland type such as Modified grassland.
When created, habitats (and corresponding BUs) are also assigned a condition from ‘good’, ‘moderate’ or ‘poor’, which must also be maintained for the 30-year duration of the offsite habitat bank. To achieve moderate or good condition, ONG requires species indicative of optimal conditions to be in greater abundance than those indicative of suboptimal conditions. A third of the species likely to remain when nutrient levels are increased are found to be categorised as indicative of suboptimal conditions. Excess soil nutrient levels could, therefore, also make it difficult to reach overall habitat conditions above poor.
ONG is known to be one of the least sensitive habitat types, with a nitrogen critical load of 20kg/ha/year, while raised and blanket bogs have a critical load of 5kg/ha/year (9). It is likely that even more extreme adverse effects would therefore be seen in more sensitive habitats exposed to the high ammonia associated with intensive livestock farming.
Intensive livestock farming near habitat banks may, therefore, pose a challenge to the habitat creation and/or enhancement works and the close proximity of these should be given sufficient consideration by both habitat bank owners and the Responsible Body as part of its ecological due diligence process.
References
1 – Misselbrook et al (2023) Inventory of Ammonia Emissions from UK Agriculture 2021. Defra contract SCF107. Inventory Sumissions Report. Rothamsted Research
2 – Pitcairn et al. 2002. Defining the spatial impacts of poultry farm ammonia emissions on species composition of adjacent woodland ground flora using Ellenberg Nitrogen Index, nitrous oxide and nitric oxide emissions and foliar nitrogen as marker variables.
3 – Pitcairn, C.E.R., Leith, I.D., Sheppard, L.J., Sutton, M.A., Fowler, D., Munro, R.C., Tang, S. and Wilson, D., 1998. The relationship between nitrogen deposition, species composition and foliar nitrogen concentrations in woodland flora in the vicinity of livestock farms. Environmental pollution, 102(1), pp.41-48.
4 – Rowe, E.C., Hina, N.S., Carnell, E., Vieno, M., Levy, P., Raine, B., Sawicka, K., Tomlinson, S., Martin-Hernandez, C. and Jones, L., 2022. Trends Report 2022: Trends in critical load and critical level exceedances in the UK. Report to DEFRA under contract AQ0849, UKCEH project, 7617
5 – Hall, J., Curtis, C., Dore, T. and Smith, R., 2015. Methods for the calculation of critical loads and their exceedances in the UK.
6 – (Pitcairn et al. 2002. Defining the spatial impacts of poultry farm ammonia emissions on species composition of adjacent woodland ground flora using Ellenberg Nitrogen Index, nitrous oxide and nitric oxide emissions and foliar nitrogen as marker variables.)
7 – (Hill, M.O., Mountford, J.O., Roy, D.B. and Bunce, R.G.H., 1999. Ellenbergs’ indicator values for British plants. ECOFACT. Institute of Terrestrial Ecology, Huntingdon, UK (ISSN.)
8 – (Stevens, C.J., Payne, R.J., Kimberley, A. and Smart, S.M., 2016. How will the semi-natural vegetation of the UK have changed by 2030 given likely changes in nitrogen deposition?. Environmental pollution, 208, pp.879-889.)
9 – (Hall, J., Curtis, C., Dore, T. and Smith, R., 2015. Methods for the calculation of critical loads and their exceedances in the UK).
10 – Fahey, T.J., Williams, C.J., Rooney‐Varga, J.N., Cleveland, C.C., Postek, K.M., Smith, S.D. and Bouldin, D.R., 1999. Nitrogen deposition in and around an intensive agricultural district in central New York (Vol. 28, No. 5, pp. 1585-1600). American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America.