Direct Driller Magazine Issue 12

Page 11

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


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 11

Articles inside

What to read? article cover image

What to read?

pages 97-100
New Analyis to Unlock Soil Health article cover image

New Analyis to Unlock Soil Health

pages 94-96
Putting Covers in Context article cover image

Putting Covers in Context

pages 91-93
Effective Soil Sampling Program article cover image

Effective Soil Sampling Program

pages 88-90
Pasture for Life article cover image

Pasture for Life

pages 84-87
Farmer Focus: Steve Lear article cover image

Farmer Focus: Steve Lear

pages 76-77
Putting a Price on Regenerative Agriculture article cover image

Putting a Price on Regenerative Agriculture

pages 73-75
What's in a Name article cover image

What's in a Name

pages 70-72
Farmer Focus: Adam Driver article cover image

Farmer Focus: Adam Driver

pages 68-69
AHDB - Sharing the Strategic Farm Results article cover image

AHDB - Sharing the Strategic Farm Results

pages 58-61
Online Soil Health Guide article cover image

Online Soil Health Guide

pages 64-67
Bring back the Cow article cover image

Bring back the Cow

pages 62-63
Farmer Focus - Neil White article cover image

Farmer Focus - Neil White

pages 56-57
It's in our Nature article cover image

It's in our Nature

pages 54-55
Biology Up, Costs Down article cover image

Biology Up, Costs Down

pages 48-49
The Future of Soilid Urea article cover image

The Future of Soilid Urea

pages 46-47
Farmer Focus: Clive Bailye article cover image

Farmer Focus: Clive Bailye

pages 44-45
Helping your Buying article cover image

Helping your Buying

pages 41-43
Farmer Focus: Edwin Talyor article cover image

Farmer Focus: Edwin Talyor

pages 35-36
Wet on Top, Dry Underneath article cover image

Wet on Top, Dry Underneath

pages 37-40
Farmer Focus: Tom Sewell article cover image

Farmer Focus: Tom Sewell

pages 20-23
Featured Farmer: George Young article cover image

Featured Farmer: George Young

pages 6-10
Climate Smart Farming article cover image

Climate Smart Farming

pages 24-33
Effect of Farm Management on Topsoil Organic Carbon article cover image

Effect of Farm Management on Topsoil Organic Carbon

pages 18-19
Climate Change Effects on Soil Biota article cover image

Climate Change Effects on Soil Biota

page 34
Rethinking Permanent Soil Cover article cover image

Rethinking Permanent Soil Cover

pages 11-13
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