How a small area of ground can support multiple plant species with similar resource requirements is a question that has challenged ecologists for many decades.  The Long Term Experiments at Rothamsted Research are a unique resource for studying this kind of question because the plant communities on the plots have had over 170 years to adapt to the different fertiliser treatments and are now relatively stable.

The Broadbalk winter wheat experiment begun in 1843 and is one of Rothamsted Research’s Long Term Experiments, which collectively are a National Capability funded by BBSRC. The Broadbalk experiment tests the effects of various combinations of inorganic fertiliser (supplying the elements N, P, K, Na and Mg) and farmyard manure on the yield of wheat: a control strip has received no fertiliser or manure since it started.

Originally the weeds were controlled by hand weeding but later by periodically bare-fallowing and cultivating different parts of the field in different years. Since early 1960’s herbicides have been used but they are withheld from one part of the field. This section of plots has never received herbicides and now has weed communities adapted to different levels of soil fertility.

Weed surveys were conducted in wheat crops on Broadbalk for 50 years from 1930, representing one of the longest continuous surveys of arable weeds worldwide.  Since 1991, Rothamsted Research scientists have conducted more thorough surveys and now have over 20 years of data, which allow analyses of weed dynamics.  These analyses have shown clearly that, even in the absence of herbicides, the frequency of different species varies greatly from year to year. 

Previous research has established the filtering effect of fertiliser on weeds communities between plots and shown how increasing fertiliser use is partly responsible for the decline of a number of UK plant species that have become nationally rare, including corn buttercup and shepherd’s needle.

Dr Stephen Moss, senior researcher at Rothamsted said: “What is striking is that on the plots receiving high amounts of N fertilizer, similar to most commercial wheat crops in the UK, only a small number of weed species exist – primarily black-grass, scentless mayweed and chickweed and these are typical weeds of commercial wheat crops in the UK too”. 

“Significantly, on Broadbalk, this limitation in number of weed species occurs on areas that have never been treated with herbicides, highlighting the fact that it is not necessarily herbicides that have eliminated many of the older, less competitive weed species, but that the use of high amounts of N fertilisers has been an even more potent factor” Stephen added.

Dr Jonathan Storkey, lead Rothamsted Research scientist of the recently published work said: “ The new research on the Broadbalk weed communities has helped to answer the question of how a single plot of land can support up to 20 different weed species. ”

“We have found evidence for the idea that co-existence is supported by variation in the weather, especially whether we have a warm or cool spring. Some species, for example scentless mayweed, grow relatively more quickly in warm springs so gain a competitive advantage over species that are found on the same plots and have similar resource requirements such as poppies”, Jonathan added.

This interaction between weather and the competitive balance between weed species explains the cycling of the populations of different species and the change in the character of the experiment between years.

This work is a great example demonstrating the unique value of the Long Term Experiments at Rothamsted.  They are an irreplaceable resource for understanding the interactions of land use with the environment and will be especially important in the future for predicting future impacts of climate change.

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