Soil Carbon on-farm: missed opportunity

This post is in response to NZ government’s missed opportunity to mitigate rising atmospheric carbon. Despite access to the world’s science, and despite the opportunity represented by 105,000 km² of farmland, government persists in denying farmers agency in helping solve the problem. Soil carbon is ignored in all govt. statements, there’s a perception in govt that “soil is probably an even riskier way of storing carbon in a warmer world than trees” NZ Parliamentary Commissioner for the Environment’s initial response to my query re farm soil carbon sequestration in 2019.

Email sent today to James Shaw
In 2019 I corresponded with Simon Upton PCE about deep-soil carbon sequestration on farm. I wrote as there appears to be a perception that soils are unsuitable for long-term soil carbon storage. As far as I’m aware no-one else communicated with PCE on this topic as they said they had not heard of the approaches I discussed. This email has first my email to PCE, then Simon Upton’s response, and finally my reply with comments and links to peer-reviewed documents. I have copies of all the documents if they are wanted.

I am growing very concerned that the current focus on trees and trees alone as a carbon sink is flawed, and an existential threat to our future, if only where nations with better carbon responses can use our flawed approach as a reason to not trade .

Email sent Tue 12/10/2019 to Simon Upton PCE
“ Dear Simon
While shallow well-drained soils are a poor sink, anaerobic and cool, deep soils >1m depth hold [significant] recalcitrant carbon fractions and their cooler, stable temperatures help retain carbon to millennia timescales.

Conversely ‘developing’ anaerobic soils ¹, or thoughtless tree planting would perturb these deep soils resulting in the loss of their total C storage ².

The other side of this is political/social engagement; yes results are needed but who carries them out? Removing farmers’ agency and some degree of control from farmers disincentivises them – for a farmer the idea that they can help recover C where they are is a big part of the mentality of the modern farmer.

Carbon as an issue needs to become almost a religion, deeply embedded in culture, life and business; intergenerational family farms could be the forefront of this shift.

1 – Anaerobic microsites have an unaccounted role in soil carbon stabilization, Nature Communications volume 8, Article number: 1771 201

2 – Root penetration in deep soil layers stimulates mineralization of millennia-old organic carbon. Shahzad etal., Soil Biology and Biochemistry 124 (2018) 150–160 Shahzad, T., Imtiaz Rashid, M., Maire, V., Barot, S., Perveen, N., Alvarez, G., Mougin, C. and Fontaine, S.

The PCE’s core response was:

“Many thanks for your message. … The short answer is no, I haven’t devoted much attention to on the basis that soil is probably an even riskier way of storing carbon in a warmer world than trees.

But I am more than interested in what you may have to share if the angle is: how could large scale land-use change through afforestation destabilise soil carbon and thereby add to our problems? Any articles you care to share would be much appreciated.

I suppose my starting point is that soil conservation and tree planting are both good things (done in the right way in the right place). It’s just that we should do them in any case – not as a way of trying to prolong the emission of fossil fuels.

I fully agree with the desirability of incentivizing farmers to take a joined up approach to managing land with a sustainable, inter-generational outcome in mind. That is what my proposal for recycling a biogenic GHG levy to the catchments and sub-catchments that pay them was all about (see chapter 6 of farms, Forests & Fossil Fuels).

Do let me know if there is further literature you think I should read.”

I responded to Simon (Tue 12/10/2019) with study links, comments and context on many of the studies.

“Hi Simon,
I do have some extra recommended reading. I have winnowed these back and forth to tease out the (hopefully) most helpful ones and added a short note where relevant.

Should the PCE need an out-of-the-box reader and finder of lesser-known research please contact me as my current path of changing the world one landscape at a time is not fast enough.

Long-term soil sequestration of recalcitrant carbon forms:
Bartosz et. al, 2017, “that plant-mycorrhiza interactions increase recalcitrant pool of organic N in SOM due to root-derived tannins” : How mycorrhizal plant-soil interactions affect formation and degradation of soil organic matter in boreal forest. 19th EGU General Assembly, EGU2017, proceedings from the conference held 23-28 April, 2017 in Vienna, Austria., p.12053. Have 1 page FT. Bartosz’s research is often on ‘pines’, forestry, mycorhiza and carbon.

Once formed recalcitrant carbon may not gasify:
Katharine L. Stuble., Shuang Ma., Junyi Liang., Yiqi Luo., Aimee T. Classen and Lara Souza. 2019. Long-term impacts of warming drive decomposition and accelerate the turnover of labile, not recalcitrant, carbon. Ecosphere, 10, 5, 1-11.

Old carbon and soil carbon destabilization:
Shahzad et al. 2018 seem to be the only researchers looking at the issue of old carbon; with ancient deep carbon most researchers seem to form a loose association. In the short time since their publication several researchers have taken the ideas further and these are cited below.

The Case for Digging Deeper: Soil Organic Carbon Storage, Dynamics, and Controls in Our Changing World. Gross and Harrison. Soil Syst. 2019, 3, 28
Readable and thoughtful:
“…we highlight the present need to sample (often ignored) deeper soil layers. Contrary to long-held biases, deep SOC—which contains most of the global amount and is often hundreds to thousands of years old—is susceptible to decomposition on decadal timescales when the environmental conditions under which it accumulated change.”

What do we know about soil carbon destabilization? Bailey Priese Lajtha 2019, 14, 8, Environmental Research Letters Journal.
Deeply thought-provoking, many C-destabilization pathways explored:
“… we lack a strong, generalizable understanding of the mechanisms through which soil organic carbon is destabilized in soils.”

These papers below are the most helpful citations in Shahzad et al’s paper:

The turnover of organic carbon in subsoils. Part 1. Natural and bomb radiocarbon in soil profiles from the Rothamsted long‐term field experiments.
Jenkinson, Poulton and Bryant, Euro. J of Soil Science, 2008, 59, 2, 391-399. paywalled but “The mean radiocarbon ages of all the pre‐bomb samples from cultivated land were 1210 years (0–23 cm), 2040 years (23–46 cm), 3610 years (46–69 cm) and 5520 years (69–92 cm).” Site was the Rothamstead Long-term study. Shahzad et al 2018 do cite other similar old-carbon studies.

Stability of organic carbon in deep soil layers controlled by fresh carbon supply.
Fontaine, S., Barot, S., Barré, P., Bdioui, N., Mary, B., Rumpel, C., 2007. Nature 450, 277–280 [Fontaine a co-author of Shahzad et al 2018]. “we show that the supply of fresh plant-derived carbon to the subsoil (0.6–0.8 m depth) stimulated the microbial mineralization of 2,567 ± 226-year-old carbon

Rumpel, C., Kögel-Knabner, I., 2011. Deep soil organic matter-a key but poorly understood component of terrestrial C cycle. Plant and Soil 338, 143–158. “With few exceptions, the carbon-to-nitrogen (C/N) ratio is decreasing with soil depth” – a number of papers refer to biological activity at depth limited by low N, but also to a range of mechanisms that lead to deep, ancient C being protected from root and biological activity. I have Full Text.

Keiluweit et al., 2018. Anaerobic microsites have an unaccounted role in soil carbon stabilization. Nature Communications,8: 1771. Explains how many soils contain multiple microscopic zones of recalcitrant carbon (protecting anaerobic soils from development and exposure resulting in recalcitrant C forms).
I believe these ‘anaerobic microsites’ include some of the hidden carbon fractions commonly missed in studies.”

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