In this post, Peter Manning discusses a paper he recently handled by Stephen Wood and colleagues “Agricultural intensification and the functional capacity of soil microbes on smallholder African farms

Around 900 million of the world’s poorest people are smallholder farmers in the tropics, and their lives are a tough reality of uncertain crop yields and worries about sustainability. Soils are often mismanaged and become rapidly degraded, which in turn leads to land abandonment, further habitat conversion, malnutrition and even starvation. It’s therefore important that we find effective strategies to maintain the soil health and fertility of such farms.

One problem with developing such strategies is that healthy and sustainable soils are a tough thing to define: soil processes are multifaceted and so simple and useful measures of soil health and function have proven elusive for soil scientists and ecologists alike. In their recent article ‘Agricultural intensification and the functional capacity of soil microbes on smallholder African farms’ Wood et al. (2015) advance upon this problem by bringing a range of new laboratory and statistical tools to the table that, to my knowledge at least, have not yet been used in this type of work.

Photo credit: Stephen Wood
Photo credit: Stephen Wood

Their study looked at how soil microbial community composition and the ability of the soil to degrade a wide range of organic substrates varies according to different types and rates of fertiliser management, both in experimental plots and in real farms. They then apply a measure of ecosystem ‘multifunctionality’ on this data to produce measures of the soil performance that they term functional capacity.

This functional capacity measure counts the number of compounds that degraded at a rate beyond a defined threshold value and by doing so summarises a wealth of information about the soil into a single value that allows easy comparison between management practices (see Byrnes et al. 2014, for details of this technique). What they find is that the functional capacity of the soil is greatest when fertiliser is combined with the planting of legumes, a strategy that, encouragingly, typically corresponds with increased crop yields.

What I personally find particularly interesting about this work is that until now multifunctionality measures have been used almost exclusively to relate biodiversity to ecosystem functioning, and typically in experimental settings. This study, along with several other recent papers (e.g. Bradford et al. 2014 and Constán-Nava et al. 2014), helps show that their application has the potential to go much further than this. Because multifunctionality measures capture the ability of an ecosystem to deliver a range of useful functions in a single measure they could help to inform a diverse range of environmental management issues, e.g. by providing a simple metric for decision makers in cases where multiple benefits are required.

Photo credit: Stephen Wood
Photo credit: Stephen Wood

The approach is not without its problems though. As the lead authors point out elsewhere (Bradford et al. 2014) the aggregation of function measures can obscure our understanding of their component parts. Trade-offs between functions are also common and so we need to be extremely careful to ensure that such metrics are meaningful and informative. For example, in this case the soil functional capacity measure reflects processes that an African farmer would probably encourage, such as rapid nutrient turnover. This could mean that farmers could reduce the use of expensive inorganic fertilisers if functional capacity is encouraged. Simultaneously, the measure also contains some functions that the farmer, and wider society, might not be so keen on, such as carbon turnover, which could deplete the soil organic matter stocks that bind the soil together and hold water, whilst also releasing greenhouse gases. While I agree with the conclusions of Wood et al. that this functional capacity measure reflects a generally healthy and active soil, it’s not currently clear if maximising this property could also bring disservices.

Future measures of ecosystem multifunctionality need to carefully consider their component parts, what drives these processes, how they relate to one another and also how the individual functions that they comprise should be weighted and measured. Producing these reliable measures of ecosystem multifunctionality is not a trivial challenge but a worthwhile one given their great potential for future use in ecosystem management.

Literature cited:

Bradford, M.A., Wood, S.A., Bardgett, R.D., Black, H.I.J., Bonkowski, M., Eggers, T., Grayston, S.J., Kandeler, E., Manning, P., Setälä, H. & Jones, T.H. (2014a) Discontinuity in the response of ecosystem processes and multifunctionality to altered soil community composition. Proceedings of the National Academy of Sciences of the United States of America, 111, 14478-14483.

Byrnes, J.E., Gamfeldt, L., Isbell, F., Lefcheck, J.S., Griffin, J.N., Hector, A., Cardinale, B.J., Hooper, D.U., Dee, L.E. & Emmett Duffy, J. (2014) Investigating the relationship between biodiversity and ecosystem multifunctionality: Challenges and solutions. Methods in Ecology and Evolution, 5, 111-124.

Constán-Nava S., Soliveres, S, Torices, R., Serra, L & Bonet (2015) Biological Invasions. Direct and indirect effects of invasion by the alien tree Ailanthus altissima on riparian plant communities and ecosystem multifunctionality. Biological Invasions DOI 10.1007/s10530-014-0780-4

Wood, S.A., Bradford, M.A., Gilbert, J.A., McGuire, K.L., Palm, C.A., Tully., K.L., Zhou, J., & Naeem, S. (2015) Agricultural intensification and the functional capacity of soil microbes on smallholder African farms. Journal of Applied Ecology. DOI: 10.1111/1365-2664.12416