ways to reduce agricultural greenhouse gases

[Robert Northup]

Minimize Greenhouse Gas Emissions Associated with Agriculture

Agricultural operations emit large amounts of carbon dioxide and nitrous oxide, either directly or indirectly. Agricultural nitrogen comes at a high cost of carbon dioxide and nitrous oxide emissions during fertilizer production, and nitrous oxide emitted as a by product from excess nitrogen in the field, or in the runoff to surface water. High yield crop breeds require chemical fertilizer because they do not produce very much root mass. Input of new organic carbon through root turnover is not enough to keep pace with decomposition of preexisting soil carbon, causing net loss of soil organic matter and a net release of carbon dioxide. Fertilizer uptake by high yield breeds is inefficient, averaging less than one third of the applied nitrogen getting into the crop, and the other two thirds getting into the environment. Excess agricultural nitrogen is a major source of water pollution and nitrous oxide emissions, and the loss of soil organic matter is an important source of carbon dioxide emissions. By accepting lower yields, we could go back to using crop breeds that produce a large enough root mass to derive adequate nutrition from unfertilized soil, and very efficiently take up any applied fertilizer. The reduction in yield would be partly compensated by the reduction in greenhouse gas emissions, reduced water pollution, and increased soil organic matter content, with its associated fertility benefits and measurable carbon offset value. There are such large areas of land under cultivation that managing them in a way to increase soil organic matter by even a small fraction would sequester enormous amounts of carbon dioxide. To minimize adverse impacts of agricultural nitrogen, variable rate technology can be used to ensure that larger amounts are given only to the limited areas that need it, rather than applying uniformly over the whole field, in order to get maximum yield. Agroecosystems need to be selected to be compatible with unique conditions of different environments, including steep, high rainfall regions. Compared to the mechanized tillage, chemical-intensive, monocrop plantations that have largely replaced them, indigenous agroforestry practices that maintain perennial ground cover are superior in their capacity to protect water quality, minimize soil erosion and nutrient loss, sustain productivity, and sequester carbon.

Manage Drained, Cultivated Wetlands to Minimize Greenhouse Gas Emissions

Wetlands sequester atmospheric carbon dioxide through photosynthesis, and the organic matter they produce is accumulated and stored because the waterlogged, low-oxygen conditions prevent much of the dead plant material from decomposing. Over centuries, enormous deposits of organic matter can accumulate. When wetlands are drained for cultivation, oxygen becomes available to decompose the stored organic matter, and large amounts of carbon dioxide are released. It is now believed that 10% of annual global carbon dioxide emissions, and significant nitrous oxide emissions, are released from drained wetlands. To reduce this important contribution to global warming and to mitigate subsidence of land surface elevation in the face of rising sea levels, drained wetlands need to be managed in a manner that minimizes the loss of carbon. This can be achieved in some cases by restoring them to the wetland condition. Restored wetlands can be managed for flood control, wildlife habitat, fisheries, and to act as a carbon “sink” to sequester carbon dioxide. Unfortunately, the carbon dioxide that a restored wetland can sequester in a year is one or two orders of magnitude less than the amount of carbon dioxide that an equal area of drained, cultivated wetland can emit. Furthermore, drained wetlands include the world’s most productive cropland. Mitigation of global warming can be achieved by greatly reducing greenhouse gas emissions from cultivated wetlands, without taking them out of production. For example, a layer of mineral-rich dredged sediment can be used to raise the land surface elevation and “cap” the organic-carbon-rich peat soil, to prevent wind erosion, impede aeration, oxidation and decomposition of organic matter, and minimize greenhouse gas emission from drained, cultivated wetlands.

Plant Tannin-rich Woody Perennials to Maximize Carbon Sequestration

Long-term sequestration of carbon dioxide can be accomplished by ensuring that organic carbon produced by photosynthesis does not decompose (or combust) to release carbon dioxide back to the atmosphere. Whereas wetlands sequester carbon due to waterlogged, low-oxygen conditions, other ecosystems sequester carbon by producing organic matter that is highly resistant to decomposition. The convergent evolution of tannin-rich woody perennial plant communities has occurred on highly leached and infertile soils throughout the world. In extreme cases, such as fern thickets in rain forests, plant litter piles up to a depth of a meter or more, despite warm, wet, well-drained conditions to favor rapid decomposition. Due to its exceptionally high tannin content, the litter is unpalatable to detritivores, and difficult for microbial decomposers to degrade. The raw humus litter layer that accumulates can contain significantly more (sequestered) carbon than the live biomass, and can be vital for storing and retaining nutrient capital, protecting the soil against erosion, and absorbing water to maximize infiltration and minimize runoff. Nitrogen cycling in tannin-rich ecosystems is regulated in such a manner that losses are minimized and nutrient availability is synchronized with uptake capacity. Most nitrogen in tannin-rich leaf litter is immobilized as protein-tannin complexes, does not easily release ammonium, and is rarely oxidized to nitrate. Whereas ammonium and nitrate can easily transform to forms that leave the ecosystem, including loss as nitrous oxide emissions, preservation of nitrogen in protein-tannin complexes minimizes losses.
As we shift away from using crops that are dependent on being supplied with chemical fertilizers, nutrient cycling dynamics of these natural ecosystems can serve as models for selection of appropriate agroecosystems. Particularly in high rainfall areas, where tannin-rich litter provides benefits for protecting soil and water quality, we may want to select agroecosystems, forest and rangeland management practices that mimic the carbon sequestration dynamics of tannin-rich woody perennial plant communities. As regulators of organic matter decomposition, tannins can be utilized to mitigate global warming.

[screener]

Hi Robert, welcome to the ag forums. Very interesting stuff. Compatability of the farm to its products and vice versa. as opposed to compatability with the market demand that farmers see. I'm for it.

Here's a place where people can see more thoughts along similar lines.

Environmentally Sustainable Food and Agriculture

We've been growing clovers and alfalfas for their nitrogen fixation capabilities and fescues for their soil fibre production. An old practise that is often overlooked in the sequestration discussion.

How persistant is tannin, this is where the tanning of leather products comes from isn't it? It works really well for preservation but might it be too long lasting for potential agricultural soils?

I read somewhere that 97 % of carbon in a rainforest is in the growing things there. Are you being overly optimistic about our ability to influence that?

[Robert Northup]

Just have a moment for a short answer. In the future, there is much I could share about soil management. Meanwwhile, part of why deforestation is so devastating regarding greenhouse gas emissions is the enormous carbon pool in dead biomass, virtually always exceeding that of the live biomass at least several fold. It is subject to decomposition and combustion with the trees cleared, relatively rapidly exhausted, and slow to replace.

I don't know where a 97% figure for rainforest carbon in live biomass came from. An example I like to use to display the decomposition regulation power of tannin is the case of equatorial rainforests on acid white sands. These strongly acidic soils are virtually devoid of nutrients, at least in the mineral soil which is essentially clean quartz sand, without even any roots in it. A meter or two or three of raw humus is piled on top of the mineral soil, and all the root growth and nutrient cycling takes place above the mineral soil surface in the organic medium that the forest itself produces. Despite hot, wet, well aerated conditions, the organic matter decomposes very slowly, and the dead biomass contains the vast majority of the carbon in the whole system. In disturbed rainforest sites, fern thickets can establish that create conditions comparable to the acid white sand, rapidly accumulating carbon.

[screener]

I've tried to find the old reference for the carbon percentages in the rainforest but it's lost. What I find is more agreeable to what you stated. It's possible I uh mis-remember.

[Robert Northup]

As I think about it, I realize it is a pretty common misconception that live biomass and a very rapidly decomposed and relatively small pool of newly dead biomass contain virtually all the carbon and other nutrients in rainforests. If I take the time, there are plenty of recent references regarding this issue I can provide, but the following is a "classic" review (1986) of literature about carbon and nitrogen storage in a broad range of different kinds of forests. It is authored by Kristina Vogt, of the Yale school of Forestry, a highly respected expert on carbon dynamics with whom I have co-authored in the past. My obsession at the time I typed up the reference for my annotated bibliography was nitrogen cycling, so my exerpts are focused on that, but the same review article gave values for carbon storage, and it's pretty clear that there is a whole lot of carbon stored in dead biomass, above and below ground.


Vogt, K.A., C.C. Grier, and D.J. Vogt. 1986. Production, turnover, and nutrient dynamics of above- and belowground detritus of world forests. Advances in Ecological Research 15:303?377

* greater N inputs to most forest soils (especially evergreens) from root turnover than from litterfall, review of many data sets
"..climatic factors appear to have no correlation with organic matter accumulation across a broad range of forest types. The larger data pool of this article demonstrated the high variability in litter accumulation that occurs under similar conditions, explaining why a high correlation should not be expected between just climatic factors and forest floor accumulation. The range of forest floor mass estimates was greater for evergreen forests (e.g. 3,200-54,000 kg/ha for tropical broad-leaved evergreen, 4,520-47,500 kg/ha for warm temperate broad-leaved evergreen, and 4,279-50,780 kg/ha for warm temperate needle-leaved evergreen) than deciduous forests (e.g. 2,075-16,480 kg/ha for tropical broad-leaved deciduous, 1,340-13,120 for subtropical broad-leaved deciduous, and 1,500-27,000 for warm temperate broad-leaved deciduous). The high variability in forest floor accumulation within similar climatic zones may be due to several factors: (1) incomplete quantification of total litter input by the exclusion of the belowground (root turnover) component, and (2) different decay rates of litter due to a (a) variation in litter quality, for instance, increasing content of lignin and/or other secondary metabolites... Generally, tropical evergreen forests are restricted to sites of low nutrient availability. Trees characterized by evergreen behavior tend to dominate nutrient-poor sites...(refs refs) suggested that slow decomposition rates in tropical evergreen forests were related to litter substrate quality. ...needle-leaved evergreen forests tend to be restricted to a narrower latitudinal range and on nutrient-poor sites. Forest floor mass accumulation could not be accurately predicted from climatic factors or latitude, but was related to tree behaviro (evergreen or deciduous). Evergreen forests accumulated higher forest floor masses than deciduous in similar climatic zones. When deep organic matter accumulations did occur in lower latitude forests, Low P..appeared to limit organic matter decomposition, and higher belowground litter inputs required a greater total amount of litter to be decomposed. ..higher total mean root masses occurred in tropical broad-leaved evergreen forests. ...deciduous forests averaged 5000 kg per ha less total root mass than evergreen forests. Root turnover contributed 29-255 kg/ha/year of N to the forest soil across all forest climatic zones. Except for cold temperate broad-leaved deciduous forests, 18-58% more N was added to the soil through root turnover than by litterfall. If the mean residence time of organic matter in the forest floor was estimated with and without root inputs, roots decreased estimates of organic matter mean residence time from 19 to 77% in cold temperate forests."

[screener]

Thanks Robert,
I may have heard a figure for % of biomass in living matter in relation to decomposing biomass, as opposed to carbon.

If I understand your reference then, roots were some of the material that rotted fastest across the spectrum of forest types?

Can you reference similar information for various ag crops?

[Robert Northup]

It may take a little time to pull up the references, but there has been a LOT of work studying the decomposition of various kinds of crop residues, roots or shoots. My favorite papers among these, of course, are the ones that cite my work, so I am somewhat biased. Among them, are studies combining tannin-rich organic matter with low-tannin organic matter to optimize decomposition rates and nitrogen release. It's not some new theoretical concept, it has been well investigated, and it has also been practiced for thousands of years already. Peasant agricultural science has much to teach us, as indigenous agroforestry systems have been able to sustain productivity on poor soils, whereas our more modern agriculture depletes, sometimes rapidly, the capacity of otherwise fertile soils to sustain productivity.

[UrbanFarmboy]

While I am all for sustainable management of agricultureal resources, one must realize that all geographies are not created equal. In areas of limited rain fall, all available water is needed for agricultrual production. I am from west texas where average rainfall is less than 20 inches. While we realize decent yields, we need every bit of that water to produce a crop. We substitute green manuer with real manuer because we simply cant afford to lose soil moisture to less valuable crops. We also utilize no till conservation tillage in conjunction with biotechnology to increase soil carbon and reduce input costs associated with heavy tillage.

Biotechnology can also drastcally reduce methan emissions from rice paddies by creating varities that more efficiently use available nitrogen.

I think that if people are really concerned about the future and are truely interested in the best methods for reducing our impact on the earth, that every approach needs to be looked at and studied adn given a fair seat at the table. I think the integration of organic practices with modern biotechnology could have an amazing impact on greenhouse emissions.

[screener]

one of the big strains on farmers and then their lands is a kind of self generating circular stress caused by low returns for the food produced. As Urban Farmboy points out, it is difficult for farmers to justify putting land into soil regenerating crops because in the short term ie this growing season there is no money coming in from the growth.

Farmers then substitute, usually commercial fertilizers, in an attempt to get another crop off depleting fields. The cost of inputs goes up as more farmers use more fertilizers and farmers are less able to chance taking a year to replenish some of their fields through raising rotational crops that improve the soil.

I had a look at the EU farm programs concerned with environmental degradation and thought that some of the ideas were way ahead of anything in Canada. Payments to farmers de-coupled from production to encourage farmers to take care of their soils and waterways. Does anyone have any experience with dealing with those programs?

[UrbanFarmboy]

Originally Posted by screener

one of the big strains on farmers and then their lands is a kind of self generating circular stress caused by low returns for the food produced. As Urban Farmboy points out, it is difficult for farmers to justify putting land into soil regenerating crops because in the short term ie this growing season there is no money coming in from the growth.

Farmers then substitute, usually commercial fertilizers, in an attempt to get another crop off depleting fields. The cost of inputs goes up as more farmers use more fertilizers and farmers are less able to chance taking a year to replenish some of their fields through raising rotational crops that improve the soil.

I had a look at the EU farm programs concerned with environmental degradation and thought that some of the ideas were way ahead of anything in Canada. Payments to farmers de-coupled from production to encourage farmers to take care of their soils and waterways. Does anyone have any experience with dealing with those programs?

The only thing really lacking in our soil is N, P, and K. Soil carbon is high, but corn and wheat stover, don't do much for nitrogen generation. Substituting manure or even synthetic fertilizer, which in my area is fairly rare considering the proximity of dairies and feed lots, is the only thing we need to produce a very healthy crop. There is really no problem with synthetic fertilizer or manure application as long as other soil agents are present.