Muck & Slurry


Scientists finally unearth why soil carbon is so valuable


A


radical new way of think- ing has solved the mys- tery of why adding ma- nure to soil improves crop yields – and offers other benefi ts too. Scientists at Rothamsted Re-


search found that relatively low nitrogen levels in healthy soils limit the ability of microbes to uti- lise carbon compounds, excrete them as glue-like polymers to create a porous, interconnected structure which allows water, air, and nutrients to circulate. Writing in the journal Sci-


entifi c Reports, the research- ers reveal that the Victorian-era switch from manure to ammonia and phosphorous based fertilis- er caused microbes to metabolise more carbon, excrete less poly- mers and fundamentally alter the properties of farmland soils.


Lost carbon As carbon is lost from soil, the scientists found that the pores within it become smaller and less connected. They found that low carbon, poorly connected soils, are much less effi cient at supporting growth and recycling nutrients – limiting yields. Lead researcher Professor Andrew Neal said: “This results in fundamental changes in the fl ow of water, nutrients and oxy- gen through soil and forces sev- eral signifi cant changes to micro- bial behaviour and metabolism.” A lack of oxygen in soil re- sults in microbes having to turn to nitrogen and sulphur com- pounds for their energy – inef- fi cient processes, says Professor Neal. Among other issues, this re- sults in increased emissions of the greenhouse gas nitrous oxide The closed soil structure also


means microbes need to expend more energy on degrading less easily accessible organic matter for nutrients. Conversely, in car- bon-rich soil there is an exten- sive network of pores which al- low for greater circulation of air, nutrients and retention of water.


Nutritious manure


“Manure is high in carbon and ni- trogen, whereas ammonia-based


22 MIDLAND FARMER • DECEMBER 2020


Manure delivers more benefi ts than ammonia-based fertilisers, say researchers


• Carbon encourages healthy soil structure • Artifi cial fertilisers deplete carbon stocks • Adding manure boosts benefi cial microbes


“


Carbon dynamics and soil water were poorly understood


fertilisers are devoid of carbon. Decades of such inputs – and soil processes typically act over dec- ades – have changed the way soil microbes get their energy and nu- trients, and how they respire.” While soil carbon was already known to drive climate and water cycles the world over, Prof Neal said it took a chance discussion


between experts working at very different scales to discover the reason why. The idea to look at this link


between the living and non-liv- ing components of soil came about through a discussion be- tween Prof Andrew Neal and Prof John Crawford – now at Glasgow University – who studies the way complex systems behave. Prof Neal says: “Carbon dy- namics and the link to soil wa- ter were poorly understood. So- ciety struggles with the concept of what soil is and how it can be


Researchers develop ‘Theory of Soil’


These latest fi ndings comple- ment previous work showing how microbial action infl uences soil structure – helping Rotham- sted scientists develop an over- arching Theory of Soil. In healthy soils, relatively low nitrogen levels limit the ability of microbes to metabo- lise carbon compounds. These are instead excreted by the or- ganism in polymers (called ex-


tracellular polymeric substanc- es, or EPS) that can act as a kind of glue.


EPS stabilises the aggrega- tion of soil particles in the en- vironment adjacent to the mi- crobe. This makes the physical structure of the environment around these microbes more sta- ble to disruption, such as occurs when the soil wets up and dries, or when plant roots pass nearby.


managed effectively because it is such a complex combination of biological, chemical and phys- ical processes.”


Dynamic interaction The two scientists took inspira- tion from a theory proposed by Richard Dawkins in the 1980s that many structures we encoun- ter are in fact products of organ- isms’ genes. This helped them understand soil as a product of microbial genes, incorporating organic materials. “We have shown for the fi rst


time a dynamic interaction be- tween soil structure and micro- bial activity – fuelled by carbon – which regulates water storage and gaseous fl ow rates in soil with real consequences for how microbes respire.” The group, which also in- volved scientists from Notting- ham University, is the fi rst to se- riously study the details of this intimate two-way relationship between the microscopic life in soil and its structure at scales relevant to microbial processes. The results also have implica-


tions for farmers, where the ad- dition of nitrogen and phospho- rous fertilisers – and not carbon – may be leading to a degrada- tion of natural soil fertility that could be detrimental to long term productivity.


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