David writes: Does Synthetic Nitrogen Fertilizer Destroy Soil Carbon? – THE SURVIVAL GARDENER
“According to a recent article at Grist”
While I can’t tell if David is being sarcastic about the recent part of the Grist article since it’s from 2010, I totally agree with what he says and quotes.
Surface application of synthetic nitrogen alone has been shown to deplete the surface layer of organic carbon and increase the amount of dissolved organic carbon. This then leaches into the lower layer and initially increases C mineralization there. As a result there is then less carbon in the surface layer for microbes to use and to bind nitrogen from air and so more fertilizer is then needed, leading to even more leaching over time and the creation of the more-on farmer.
Now there are studies showing that if synthetic nitrogen is added in moderation with organic matter that it can actually increase carbon mineralization, I just can’t find where I filed them. 🙂 What really matters is the form of organic matter added.
Recent studies[2,3] indicate a C:N of 100:1 or less is good for carbon mineralization depending on the clay in your soil, with between 11:1 and 50:1 being near ideal for priming in lab conditions. Split the difference and you get 30:1 which is often recommended for compost piles for microbes to break them down, not a coincidence!
Responses of priming of organic matter (OM) decomposition to OM C:N ratios (horizontal axis) and labile C:N ratios (vertical axis).
This contour figure was made based on all priming results of four OM forms from , using C:N ratios in OMs as x-axis, C:N ratios in the labile inputs as y-axis, and all priming data as z (color) axis. Priming effects vary strongly among substrates along the white dashed line, where labile carbon inputs are high and nitrogen is low. Priming effects do not vary strongly among substrates along the black dashed line where labile carbon is low and nitrogen is high. The dashed pink line indicates the substrate C:N threshold ratio where priming changes from negative to positive.
Ramial(small branch) chipped wood tends to be at the upper end of the region between 30:1 and 170:1 and can be a good choice long-term. However getting the C:N down should build soil faster. Chopping and dropping is one approach. Growing and harvesting cover crops before grazing and trampling them in with animals like Gabe Brown does would be another.
There are also mechanical versions like those used in biodynamics and regenerative practice. Hand operated versions of the latter are also possible.
Growing lawn grass and leaving any mower clippings can even build soil carbon. However it may take 30 years to raise Total SOC levels by 1% doing it this way…
However in cases where you’re adding heartwood chips like many Back to Eden peeps, then nitrogen addition may help, I believe this is why many like Mr. Back to Eden himself Paul Gautschi has found that high nitrogen chicken manure helps.
Another example would be after forest fire or biochar creation where you have a lot of carbon but are nitrogen, phosphorus and sulfur limited as these tend to boil off at fire temperatures.
In the comments on David’s post Bob also mentions the N impact on ectomycorrhizal fungi.
I found a carbon-13 labelled study that seems to suggest that by adding nitrate in the form of calcium nitrate, that trees significantly reduce below-ground C allocation, probably because the trees are getting their nitrates from the fertilizer and don’t need the microbes to fix the nitrates for them. As a knock on effect the fungi also reduce their C allocation to soil biota by 60%, likely because fungi need continuous carbon input to grow and fruit and can only afford to give up so much. This all suggests that nitrate addition short-circuits the carbon-nitrogen cycle when it doesn’t increase below-ground plant C allocation. Those papers Bob quotes suggests the effect is not limited to nitrate but also ammonium further up the nitrogen cycle.
I’ve also read that the symbiosis between legumes and their rhizobia breaks down with use of nitrogen fertilizer. And that there’s a consistent change in soil microbial communities with additions of N and P.
If N doesn’t do it, I’m wondering what will actually increase below-ground C allocation to build soils fast… any suggestions? [Other than legumes or perennials with long thin roots]
Off to Google Scholar I go…
 Carbon mineralization in response to nitrogen and litter addition in surface and subsoils in an agroecosystem http://www.tandfonline.com/doi/abs/10.1080/03650340.2016.1145792
 Is the fate of glucose-derived carbon more strongly driven by nutrient availability, soil texture, or microbial biomass size? http://www.sciencedirect.com/science/article/pii/S0038071716302103
 Carbon and nitrogen additions induce distinct priming effects along an organic-matter decay continuum, Figure 5 doi: 10.1038/srep19865 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4726261/figure/f5/
 Quantification of effects of season and nitrogen supply on tree below-ground carbon transfer to ectomycorrhizal fungi and other soil organisms in a boreal pine forest – Högberg – 2010 – New Phytologist – Wiley Online Library http://onlinelibrary.wiley.com/doi/10.1111/j.1469-8137.2010.03274.x/full
 Long-term nitrogen fertilizer use disrupts plant-microbe mutualisms http://www.rdmag.com/news/2015/02/long-term-nitrogen-fertilizer-use-disrupts-plant-microbe-mutualisms
 Consistent responses of soil microbial communities to elevated N & P nutrient inputs in grasslands across the globe PNAS | Mobile http://m.pnas.org/content/112/35/10967.abstract
 Turfgrass Selection and Grass Clippings Management Influence Soil Carbon and Nitrogen Dynamics https://dl.sciencesocieties.org/publications/aj/abstracts/0/0/agronj2016.05.0307