WHAT’S CONTRIBUTING TO THE CARBON DEPLETION OF OUR SOILS?

The adoption of no-till farming has been great for many reasons, but why are carbon levels of soils still depleting despite the return of much carbon back to the soil via stubble?  This week I examine why high rates of synthetic nitrogen fertiliser is contributing to soil carbon depletion, as well as what to consider in transitioning from high N agriculture to a lower input system.

The liquid carbon pathway, which is described by Christine Jones and I have reported on in ‘There’s Carbon, then there’s Carbon”, is the way that plants take carbon from the atmosphere and convert it via photosynthesis to carbon rich plant sugars.  These sugars which provide two functions for the plant:

1. providing immediate fuel to the plants
2. are exuded from the plant roots to feed soil microbiology.  In turn, these soil microbes make nutrients (including nitrogen), available to the plant.  One particularly important outcome of this process is that these root exudates are the substances in which carbon enters the soil in a stable humate form.

When we bypass nature and supply nitrogen in an inorganic form (synthetic fertilisers), there is no requirement for the plant to supply exudates to the soil microbes that in turn supply nitrogen to the plant.  This is because the plant already has access to luxury levels of nitrogen.  This means we have interrupted the process by which plants deliver stable carbon to the soil, and soils are subsequently being depleted of their carbon stores.¹
This becomes a vicious cycle, as the lower carbon stores mean the CEC of the soil is lowered and less nutrients (including nitrogen) are able to be stored in the soil.  Even more nitrogen then needs to be added to compensate for less being held in the soil – and so the cycle continues.

Trial sites from the University of Illinois in the U.S. that have been running for 100 years support this explanation.  Some plots have been fertilised with manure, limestone and rock phosphate for this complete 100 year length of time, while other plots were changed to synthetic nitrogen fertiliser in the 50’s and 60’s and fertilised this way since.  Scientists from the University have analysed soil cores from the plots to look at long term effects of fertiliser application on soil carbon.  The results showed that over the whole trial period, carbon levels had declined in the synthetic fertiliser plots, with losses averaging 12.1T/ha (4.9T/ac) of carbon, and declines were greater in the higher rate plots and this is despite the inputs into the system of anywhere from 222 T/ha (90T/ac) to 306t/ha (124t/ac) of residue carbon as stubble  – depending on crop rotations.²

So, how can we arrest this problem – to reinstate the natural nitrogen cycle and cease depleting our soil carbon levels?

The ways that we can reinstate these things are through the means that I have spoken of before.  The things being practiced and advocated by Colin Seis, the U.S. cover croppers and renowned scientist of soil carbon, Christine Jones.  These are:

1. Ensure living ground cover all year round – with cover cropping or pasture cropping
2. Reduce inputs of high analysis fertilisers – as these interrupt the natural process of the plant signalling soil microbiology that certain nutrients are required.
3. Maximise biodiversity – a variety of plants supports a great variety of microorganisms, which in turn supply a greater variety of nutrients to the plants.
4. Add animals to the system – this adds another layer of biodiveristy and can help to maximise the liquid carbon pathway.³

Be aware however, that by practicing the above things, we are not going to achieve a completely healthy, microbe rich functioning soil in just one crop season or year.  So, we need to transition to it in a way that maximises our crop yields and profitability.

Given that regenerative farm practices focus on building organic matter and soil microbiology, we need to anticipate and be aware of what effect this can have on our nutrient availability and productivity – especially whilst in the transition phase.   Let’s continue to look at carbon and nitrogen – the ratio of the two is something to be aware of – and let’s consider this with relation to nitrogen tie up.

Most farmers are pretty aware of the fact that nitrogen tie up can occur under certain situations, but how many of us really understand what is going on with nutrient tie up and what does it mean for some of our regenerative farm practices?

What is nitrogen tie up?

The microbes in our soils have a carbon to nitrogen ratio of around 8:1.  In order to maintain this ratio in their bodies they must source both of these nutrients from their surrounding environment, which can come from organic matter in the soil.  Some of both the carbon and nitrogen is consumed and remains in their bodies, while some carbon is also used as an energy source after which it is released as carbon dioxide through the process of respiration.  To maintain this ratio of 8:1 and also have enough extra carbon for energy consumption – the microbiology need to consume a diet with a C:N ratio of 24:1.  At the consumption of food stuffs containing this ratio, 16 of the 24 carbon are used for energy and 8 are for maintenance (the 8 of which ends up in their bodies).  So 24:1 is the perfect ratio on which the soil microbiology can feed.

I have always known that lucerne hay is a really great choice for mulching a vege patch (for many reasons), but wasn’t aware that it has the virtual ideal C:N ratio for soil microbiology – at 25:1.

More C than N

Something like wheat stubble however, has a much higher C:N ratio, at 80:1.  Given that the microbiology consume these two nutrients in the 24:1 ratio, it means that for every nitrogen consumed from the wheat stubble, there will be just 24 carbon consumed.  Leaving a spare 56 carbon.  The microbiology will therefore need to seek nitrogen from elsewhere in the soil in order to be able to feed on this remaining carbon.  In being consumed, this nitrogen then becomes contained in the bodies of these microbes and is immobilised or unavailable to growing plants.  If enough of the nitrogen is immobilised, it can create a deficiency for the plants that are growing in that soil.

The nitrogen in the microbiology will eventually become available again as it is released through the natural death and decomposition of the microbes via mineralisation.

 Less C than N

Conversely, if we were to supply an organic matter to the soil that had a low C:N ratio, like a vetch cover crop which has a C:N ratio of 11:1 (much less than the ideal 24:1), then the carbon will be consumed by the microbes, leaving excess nitrogen available in the soil and for subsequent crops.  Christine Jones says that this excess nitrogen can have the same effect as applying high level inorganic fertilisers, in that it can result in the decline of soil carbon through the process talked about above.⁴  The scientists from the University of Illinois study (mentioned earlier), say that excess nitrogen may actually stimulate soil microbes that then consume 24 carbon for every excess nitrogen – thus depleting soil carbon.²

In field implications of C:N ratio?

Consideration of the C:N ratio may be relevant in crop rotations.  “A low C:N ratio cover crop containing legumes (pea, lentil, cowpea, soybean, sunn hemp, or clovers)  and/or brassicas (turnip, radish, canola, rape, or mustard) can follow a high C:N ratio crop such as corn or wheat, to help those residues decompose, allowing nutrients to become available to the next crop. Similarly, a high C:N ratio cover crop that might include corn, sorghum, sunflower, or millet can provide soil cover after a low residue, low C:N ratio crop such as pea or soybean, yet decompose during the next growing season to make nutrients available to the following crop.”³

The C:N ratio might also be considered if planning for a cover crop.  What is the ratio of the cover crop being grown and what will be the impacts of this on the subsequent cash crop.  You may recall that Gabe Fuller suggesting in a previous blog, that after a first time cover crop you may choose to put slightly MORE fertiliser on to compensate for mineral tie up.  Alternatively, choose a cover crop with the most appropriate ratio.

Once the amount of overall soil microbiology and organic matter builds, such issues will not concern us, but it is important to be aware of them whilst transitioning to a new type of system.

A little bit of technical stuff this week, but if we can understand such things, we can then understand the implications of our farming practices and choices.  I have called this blog ‘The Conscious Farmer’ because we need to be ‘conscious’ of our decisions and the knock on effects that these have.  I have mentioned before that maybe some people avoid regenerative farming practices because it does take an increased level of understanding and maybe a little more mind work.  I say, give yourselves a pat on the back for stepping up and seeking to understand!

In writing the blog, my husband Derek and I always have a discussion about the relevance of my writings to in paddock situations, and it can often end with lots of questions and discussion.  I hope that my blogs stimulate similar questions and discussion around your kitchen table.

Whatever new things you may be introduced to through this blog, I encourage everyone to at least take a 20 ha block and try something different – and this needs to happen over several years.

As farmers, we don’t have a long history of testing carbon in our soils, but what have you notice about the changes in your soils over time?

Have your nitrogen application rates changed over time?

References

1&4. Jones,C.  2014.  Nitrogen: the double-edged sword.  28^th Aug 2014 http://www.amazingcarbon.com/PDF/JONES%20%27Nitrogen%27%20(21July14).pdf

2. Study reveals that Nitrogen Fertilisers Deplete Soil Organic Carbon.  2007. http://news.aces.illinois.edu/news/study-reveals-nitrogen-fertilizers-deplete-soil-organic-carbon.  30th Aug, 2014. 

3. Carbon to Nitrogen Ratios in Cropping Systems.  2011. file:///C:/Users/User/Downloads/C_N_ratios_cropping_systems.pdf  (28^th Aug. 2014)  USDA.