Long-Term Soil Ecosystem Studies

David Powlson, Rothamsted Research, UK

• I started my career in soil science working at Rothamsted Research, UK. This is the site of several long-running experimental field sites, the oldest being the Broadbalk Wheat experiment started in 1843. It was originally designed to compare crops yields achievable from (the then newly developed) chemical fertilizers with those from animal manure. In its first few decades the experiment laid the foundations for our current understanding of plant nutrition.
• My first research was not directly concerned with long-term sites but rather with the development of a method to measure the quantity of organic carbon held in the cells of living organisms in soil (the soil microbial biomass). However some of the soils I used came from the long-term sites so I have always appreciated the immense values of these sites as resources for other research.
• One of the great strengths of long-term sites is that they are a source of material, and background knowledge, that is of value in research projects quite different from those for which they were originally designed – and often for work that could never have been envisaged by the initiators of the experiments. Thus, if I wish to conduct process level research or test hypotheses using soils that differ in only factor only I can easily obtain them. For example, I can sample soils with high or low organic matter content, high or low content of specific nutrients or different pH but having the same texture and mineral content. I do not have to spend years getting soils into the required contrasting states - what a benefit!
• Although types of work, and must, be done using soils from non-experimental farms or forests, the advantage of long-term experimental sites is that they normally have their history well recorded. So research requiring a contrast in properties can be conducted with confidence that there are a minimal number of confounding factors.
• Climate change caused by enhanced emissions of carbon dioxide and other greenhouse gases is probably the biggest challenge facing humanity and the planet on which we depend for our survival. The amount of carbon held in the organic matter in the world’s soils is very large – about twice the amount held in the carbon dioxide in the atmosphere. Thus changes in land use that cause a change in soil organic matter content, if reproduced over large areas, can influence the global carbon cycle – either slowing climate change if additional carbon is locked up (sequestered) or making it worse if soil organic carbon declines and is released as carbon dioxide. However changes in soil carbon content occur slowly, over periods of years or decades (depending on climate and other factoras) so generally cannot be reliably measured over short periods. Long-term experiments (or networks of space-for-time sites or long-term monitoring efforts) are the only ways of obtaining information on soil carbon changes and thus assessing the likely climate change impacts of different changes in land use and management practices. I have been involved in efforts to use results from long-term experiments for this purpose. Initially we simply extrapolated results from contrasting treatments in several long-term experiments in Europe to estimate changes in soil carbon stocks for the land area of Europe for a range of possible land-use scenarios. A more satisfactory way of assessing future trends is to use models of soil carbon dynamics. But before doing this it is essential to check that the models correctly simulate changes in soil carbon content for the environments concerned. This can only be done by comparing model simulations with changes actually measured in the past – long-term experiments are the ideal source of such data. I was involved in a major international effort to assemble information from over 100 long-term experiments worldwide and use data from some of them to evaluate several models. The initiative was called SOMNET – Soil Organic Matter Network. I think this was a significant service to soil science, and to humanity, through the increased confidence with which we can now predict the impact of land use changes on soil carbon stocks in the context of climate change. And it could not have been achieved if earlier generations of scientists had not had the foresight to initiate long-term experiments – and others that followed had not had the vision and persistence to continue them.
• Producing sufficient food for the world’s population (probably about 9 billion later this century) is another major challenge. Long-term experiments are one important source of information on the crop yields that are achievable with different levels of input and the impacts on soil and the wider environment of a range of management practices.