RETHINKING PERMANENT SOIL COVER – JOEL WILLIAMS Permanent soil cover! One of the three pillars of conservation agriculture and rightly so. There are numerous benefits to soil health and crop production that stem from this foundational soil protective principle. However, in recent years there has been an expanding body of research into soil ecology and soil organic matter [SOM] formation and this emerging evidence warrants and important rethinking and clarification regarding the use of residue cover vs living roots.
If you spend some time reading online into conservation agriculture, you might notice some subtle inconsistencies in the terminologies used when discussing the permanent soil cover principle. Often the principle is framed as ‘keep the soil covered with residue or living plants’ and sometimes it will be listed as ‘keep the soil covered with residue and living plants’. You might think I’m splitting hairs regarding the use of ‘or’ vs ‘and’, but the emerging evidence is highlighting how very important living roots and their associated root exudates are for soil function and SOM formation. Living roots should not be an option, they are compulsory. It’s not to detract from stubble conservation or to suggest the practice should be abandoned; perhaps the suggestion is simply that stubbles are only half the job. This aboveground strategy must be equally matched with an intentional belowground strategy. Strive to implement both practices, not one or the other. Let’s explore some of the nuance why...
#RootsNotShoots Rightly so, we have traditionally placed significant focus on stubble retention and maintaining shoot litter on the surface of the soil for a host of important reasons – protecting the soil from erosion, conserving soil moisture, providing habitat for soil dwellers and to build SOM. However, there are numerous studies that highlight that although surface shoots do help to build SOM [mainly indirectly], there is a much more efficient pathway, but we must shift our attention to belowground residues – roots not shoots1–6. One particular study reviewed a selection of other studies that have explored this relationship between roots vs shoots. Table 1 summarises their findings and highlights what percentage of above- or below- ground carbon [C] was captured into SOM. Overall, they suggest that root inputs are approximately five times more likely than shoot inputs to become
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integrated into SOM6. There are a few nuanced factors involved
about root exudates? How much of a contribution do root exudates make
Table 1: Proportion of aboveground and belowground biomass contributing to SOM formation in agricultural field studies performed in situ using primarily isotopic approaches6
but overall, there is no magic secret as to why roots have a disproportionate influence rather than shoots – the primary driver is simply the fact that roots reside in the soil and that’s where the bulk of the living organisms are also found. So the spatial accessibility and point of entry of roots and exudates5 to the soil biology means they are more effectively processed into microbial biomass as compared to surface shoot-C which is far more prone to being oxidised off into the atmosphere as CO2. Additionally, the constant drip feed of root exudates stimulates more steady assimilation and lower microbial respiration as compared to larger but infrequent C additions which can induce greater respiration losses5. Down in the soil, root litter C is also far more likely to be entangled and embedded within aggregates where it is physically protected from oxidation and occluded from microbial degradation7.
What about Root Exudates?
So if root litter plays a more important role than shoot litter, the next logical question would lead us to – what
toward building SOM? It appears that root exudates may have historically been rather overlooked in many studies exploring SOM dynamics – and there are two key reasons for this. Firstly, the sampling of root exudates ‘in situ’ is incredibly difficult hence making them extremely hard to study8; and secondly, previous thinking was that root exudates were unlikely to ever be stabilised into SOM as they were too labile [structurally simple] and not recalcitrant [structurally complex] enough – the thought being that small, simple substrates would rapidly oxidise off as CO2, while complex substrates would slowly decay and hence remain in the soil as SOM. However, this paradigm that only complex forms of carbon are more important for soil carbon sequestration has been displaced by a growing body of evidence that recalcitrance is not solely the most important factor for carbon stabilisation in soils7,9–11. These recalcitrant carbon compounds [such as plant structural residues] have a lower carbon use efficiency [CUE] than root exudates12–14. In other words, what www.directdriller.co.uk 11