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The Long Term Sludge Experiment (LTSE) at Hartwood

Photograph of the field site at Hartwood
Studying the long term impact of wastewater sludge on soil fertility and grassland productivity.

PDF file: The LTSE leafletSoil supports a range of ecosystem services that underpin the sustainability of our production systems and environmental security. The recycling of organic materials by applying them to land as soil conditioners and sources of nutrients has long been practised but there are potential risks if inappropriate rates of application are used and/or the material also introduces excess nutrients and potentially toxic chemicals. Although the application of organic materials is regulated there is concern about what are safe limits of application in relation to toxins and their long term effect on ecosystem services such as C sequestration and biodiversity. Long term (chronic) effects to soil may take decades for their full effect to be seen so we need to be sure that current regulations protect soils from harmful levels of toxins and their long term effects.

The Long Term Sludge Experiment (LTSE) at Hartwood (a part of the UK Sewage Sludge Network) was set up to investigate the effects of adding (treated) waste water sludge to agricultural land. In this work, waste water sludges with high levels of individual heavy metals were added to soils at different rates to establish dose response curves. The plots were originally established in 1994 and the sludge application to most plots was completed by 1997.

What we have found so far

By 2005 there was a significant reduction in the numbers of N-fixing Rhizobia, chosen as an indicator of microbial activity, in most treatments but in particular in those plots to which Zn-containing sludges had been added. By 2011 there were no N-fixing Rhizobia remaining in most treatments, including some at concentrations below that allowed under the current legislation.

Continuing the experiment

The plots at Hartwood are being maintained over the long term as part of the Underpinning Capacity funding from the RESAS strategic research programme. This is to allow the soil/organic matter/metal interactions to reach an equilibrium and provide evidence of the dose-response relationships between heavy metals and selected biological indicators which are measured every two years. In addition, advanced analytical techniques such as metagenomics are being used to understand better the interactions between metals, organic matter and the microbial populations.

A rich resourceImage of the National Soils Archive logo

The long term nature and uniqueness of the experiment make it a valuable resource with several international research groups using it for their studies on microbial diversity. The National Soil Archive contains both soil and DNA samples taken over the lifetime of the project and which are available for further study and which are available for further study. Please contact Pat Cooper for further information.

More information on Hartwood and the experiment is below.

We would like to thank the Scottish Government for the support of this work.


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The James Hutton Research Institute is the result of the merger in April 2011 of MLURI and SCRI. This merger formed a new powerhouse for research into food, land use, and climate change.