April 21, 2011 | General

Climate Change Connections: Carbon Cycling 101

BioCycle April 2011, Vol. 52, No. 4, p. 52
Sally Brown

Time for another lesson. Last month was all about the basics of N2O formation. This month will be your carbon cycling 101 class. I have recognized the need for this type of lesson for awhile now, but what really brought it home is my recent interactions with Seachar, a local group promoting use of biochar to improve soils and sequester carbon.
Let me remind you that, in my humble opinion, char is not the answer to this or much else (see “Pyrolysis For Char” and “The Char And Compost Face-Off,” February and March 2009). But when Seachar called and asked me to tour some research plots they had established at a local community college, I was happy to have a look. As I walked through the plots, I really wanted to be able to pick out the compost alone treatments. I couldn’t. If anything, the plots that had gotten both char and compost looked better than the others. That was last summer.
This winter, Jim Ippolito, a scientist from USDA’s Agricultural Research Service who has been analyzing the soil and plant samples from these plots, came to the University of Washington to talk about results. He included data from his own biochar plots at the USDA station in Idaho. There was a nice crowd including some students as well as many people from Seachar who really wanted to hear about how special char was. Jim’s talk concluded that from a scientific perspective, nothing significant happened on the Seattle plots with regard to the use of biochar. He added that on his plots in Idaho, the main result was that the char reduced plant yield in comparison to compost and fertilizer, which yielded about the same. This was not easy for representatives from Seachar to hear.
What really struck me in this whole interaction was not the results about the char – those were no surprise – but how dedicated the people were and how much they really wanted to store carbon in soils. And the way that they knew to do this was through biochar addition. There is a real perception that the only way to store carbon in the soil is to add it in a form that will be inert and inactive and will last through eternity. Char may be that type of carbon, but that isn’t the type of carbon that is good for soil health. It isn’t the optimal way to store carbon in soils.

So this column is about how carbon works in soils. The key to understanding how carbon works is to understand how it cycles. There is a fixed amount of carbon that goes between different pools. Every year, massive amounts of carbon cycles between the terrestrial sphere (soils, plants and living organisms) and the atmosphere. Soils alone are the third largest carbon pool (about 5%) behind oceans (81%) and geologic (including fossil fuels) reserves (11%). In contrast, the atmosphere contains a measly 2 percent of the total carbon. The 5 percent in soils amounts to 2,500 million tons of carbon. And as plants fix CO2 from the atmosphere and deposit a portion of this in soils, and as soil organisms use that carbon for energy and release it back to the atmosphere as CO2, about 76 million tons of it cycles back and forth between the terrestrial pool (soils and plants) and the atmospheric pool.
Now the 2 percent that the atmosphere contains (760 million tons) is about 266 million more than it used to contain before the industrial age. And while each molecule of CO2 only hangs around in the atmosphere for a limited amount of time (estimates range from 7 to 400 years), it is interesting to note where this CO2 has come from. Total emissions from fossil fuels from 1850-2000 were 275 billion tons. Emissions from soils and plants from the time we started messing around with them to 2000 total about 450 billion tons. In other words, more carbon has been released from the terrestrial sphere over the course of human influence than from burning fossil fuels. A range of factors is responsible for this loss including deforestation, urbanization and large-scale agriculture with conventional tillage equipment.
The net effect is we’ve caused climate change by taking carbon that is supposed to be stored in one of the pools down here and pumped it into the atmospheric pool up there. This loss down on earth has left a hole. A hole that we can fill relatively cheaply and easily.

Active carbon in soil systems doesn’t hang around for thousands of years. In fact, average residence times for organic matter in soils is more like 20 to 30 years. This is how it works. Plants fix carbon from the atmosphere. This is organic matter. A portion gets deposited in the soil from plant roots and plant parts dying. A good portion of that gets eaten. The stuff on top of the soil and below the soil gets eaten by animals that live on top of the soil. Think bears and humans, berries and potatoes. The stuff under the soil also gets eaten by a range of soil-dwelling animals including nematodes and fungi. All eating releases CO2 and produces waste materials that are also cycled back into the soils. Think of the age-old question about the bear and its waste materials. We all know the answer to that one.
This means that the carbon cycling process is constantly fixing more CO2 (plant growth) into organic matter and that organic matter and the nutrients it contains are always being transformed, cycled and returned. It is a dynamic system. The more organic matter you have in the system, and here I mean active organic matter and not charcoal, the more robust and vigorous your system is. High organic matter in soils improves a wide range of soil properties including water holding capacity, water infiltration rates, soil aggregation and nutrient status. This healthy soil contains higher numbers of microorganisms that outcompete disease-causing organisms.
All of this cycling and living makes the plants growing in the soil grow larger. In turn, these plants deposit more organic matter that further increases the eating and cycling and makes the plants grow even bigger until some type of equilibrium is reached. (By the way, the official term for bigger plants is increased net primary productivity).
The vast majority of our soils is not at equilibrium, i.e., where soils store as much carbon as they used to before we started tilling them and taking organic matter away from them. Remember those billions of tons of carbon lost from the terrestrial system? By adding fresh organic matter to those systems, we can speed up the cycle and increase primary productivity that will increase organic matter deposition and lead to soil carbon accumulation. And even if that added carbon only lasts for 25 years, you have increased the rate of deposition so that until equilibrium is reached, the balance will be shifted with more of the carbon from the bigger plants staying in the soil and less going to the atmosphere. The plants will keep adding more and more carbon to replace that short-lived stuff with more short-lived stuff. As some gets eaten more gets added. This is also good because increased net primary productivity may mean that we will have enough to eat.
What this all means is that not only is it okay to add active carbon to a soil system, adding active carbon improves soil health which leads to more productivity. Keeping soils healthy and productive will not only get them to store more carbon, it will allow soils to contribute to the wide range of ecosystem services (things like nutrient cycling, water storage and filtration, supporting biodiversity) that will allow our planet to keep functioning.
The people that I have met through Seachar have great energy and wonderful intentions. I would love to see this devotion directed to areas that promise high productivity, even in the short term.

Sally Brown – Research Associate Professor at the University of Washington in Seattle – authors this regular column. E-mail Dr. Brown at slb@u. washington. edu.

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