Wellbeing of Future Generations Indicator-13: Soil Carbon

ERAMMP, and its predecessor programme GMEP, provide data to contribute to several of the Welsh Wellbeing of Future Generation indicators (WFG). Links to some of our contributions include: 

ERAMMP Report-23: Options for a Synthetic ‘Well-being of Future Generations’ Indicator-44 (Biodiversity) 
ERAMMP Report-78: Interim Report on the Development of Indicator-44 (Status of Biological Diversity in Wales) 
ERAMMP Report-85: Development of Indicator-44: Status of Biological Diversity in Wales Final Report 
 

We also provide the data for Indicator-13 as described on Welsh Government web pages here and here and summarised below.

WFG Indicator-13 is the measured quantity of soil carbon and organic matter content of topsoil (0-15cm) measured in grams of carbon per kilogram (gC per Kg). It is measured from soil samples using the loss on ignition methodology to determine the soil carbon concentration.

Samples were taken from across all of Wales' 26 land classes, for the Countryside survey element of the ERAMMP National Field Survey. This is conducted in 300 1km sample squares and is intended to cover all of Wales. The survey does however exclude densely developed city areas and therefore it should not be considered as an inventory for brownfield sites. This survey is a direct resurvey of the nationally representative sites within the Glastir Monitoring and Evaluation Programme (GMEP) national field survey (2013-16) using identical methods.

What does the data say?

According to the latest information from the ERAMMP, the concentration of carbon and organic matter in soil is generally stable at 80.4 gC per kg in 2021-23. This is very similar to the concentrations found in the 2013-16 GMEP at 81.8 gC per Kg. Reported values are lower than the concentrations found in the 1998 and 2007 Countryside surveys (109 gC per Kg in both) due to an improvement in field survey coverage.

Carbon concentration status and change was assessed using repeat survey data from the GMEP and ERAMMP field survey programs. This differed to previous reporting, where change was assessed by statistically linking carbon concentration in GMEP to earlier monitoring from the Countryside Survey, which used a compatible sampling approach but contained fewer monitoring sites. The current method, made possible by ERAMMP, is considered to give a more robust and representative value for Wales.

Interpretation of change 

In brief, an increase in soil carbon concentration indicates an improvement in soil health. No change indicates stability. A decrease suggests a decline in soil health. 

Method and Process 

The current assessment of Indicator-13 is based on data from the ERAMMP National Field Survey (NFS) conducted between 2021-23 funded by Welsh Government (ERAMMP Report-105: Wales National Trends and Glastir Evaluation; in prep). The NFS includes a subset of 150 1 km squares which form a nationally representative sample which provide the data which inform the WFG Soil Carbon Indicator-13. The ERAMMP NFS survey revisits sites initially sampled during the GMEP NFS (2013-16), ensuring consistency and comparability over time. Soil samples are collected from up to five sites within each square, co-located with botanical surveys and habitat recordings. 

Topsoil (0-15cm) carbon concentration is measured in grams of carbon per kilogram of dry soil (gC per kg). Organic matter content is determined via loss-on-ignition and converted to carbon concentration using a known ratio of 55% carbon to organic matter for these soils. 

Data Analysis 

To analyse the data, a linear mixed effect model is employed. This model accounts for the nested structure of soil data within the 1 km squares and the identity of resampled sites. It provides estimates of mean carbon concentration for each survey cycle and assesses changes over time. Statistical tests ensure that any observed changes are robust and significant. 

Change in methodology and baseline national estimate 

The revised methodology for calculating Indicator-13, based on the ERAMMP NFS, offers a more robust and representative estimate of soil carbon concentration in Wales. The changes in sampling design, increased sample size, and improved statistical analysis have led to a significant adjustment in the reported mean values, providing a clear understanding of soil carbon dynamics over time.  

The previous approach which provided the original national figures and trends exploited a long running monitoring programme called Countryside Survey delivered by UKCEH. Data from this programme have now been removed to focus on the data from NFS. This has resulted in a change in the national baseline estimate for soil C. No change in the direction of the long term trends has resulted from this change.  

Key Points 

• Stability of Topsoil Carbon: The concentration of carbon in topsoil has remained stable, with current estimates showing 80.4 gC per kg soil for 2021-23, similar to the 81.8 gC per kg reported for 2013-16. 
  
  Table: Topsoil carbon concentration (gC per kg of soil) 
  

• Improved Robustness: The current methodology offers a more robust and representative estimate of Indicator-13 for Wales, thanks to enhanced field survey coverage and increased sample size. 

• National Representation: The NFS design ensures a nationally representative sample, excluding densely developed urban areas, thus providing a comprehensive overview of soil carbon status across various land classes in Wales. 

• Consistency in Sampling: The same sites are revisited in each survey cycle, allowing for accurate tracking of changes over time. 

• Enhanced Monitoring: The Welsh Government's investment in expanding the number of monitored squares has improved the ability to detect changes in soil carbon concentration. 

Indicator-13 is a vital tool for understanding soil health in Wales. The methodological improvements and robust data analysis ensure that the current estimates provide a reliable picture of soil carbon status, aiding in effective environmental management and policy decisions. 

Further information  

More information on the role of soil carbon for climate change mitigation can be found in the ERAMMP short note “The opportunities and limitations of carbon capture in soils and peatlands”.

For more information about the Well-being of Future Generations Wales Act see https://www.gov.wales/wellbeing-wales. 

For the summary and information about the GMEP survey see https://gmep.wales/summary-gmep-results.