BioCycle March 2009, Vol. 50, No. 3, p. 44
Sally Brown

I ended Part I of this column in the February issue drawing distinctions between pyrolysis and anaerobic digestion, setting the stage for this month’s comparison of the end products from each process – char versus digestate, or more generally, char versus compost (which can be produced from digestate). As it turns out, Part II is really a discussion about soils, and what each by-product brings to the table in terms of soil benefits.
Both pyrolysis and anaerobic digestion produce a high carbon residual that can be used as a soil amendment. So in both cases, if you add the char from pyrolysis or the digestate (straight or composted) to soils you are adding carbon to the soil, which is always a good thing. Carbon in soils has a wide range of functions and adding carbon has beneficial effects on almost every soil property you can name – chemical, physical and biological. These cover things like soil tilth, nutrient availability, water holding capacity, cation exchange capacity, bulk density, aggregation and the list goes on. In some cases, these properties are all connected, like the hipbone being connected to the knee bone. In other cases, they can function independently.
But all carbon is not created equal. The same way that all potatoes are not created equal. Comparing a naked baked potato to one mashed with heavy cream and lots of butter and salt is a way to explain differences in potatoes. Maybe a good way to explain differences in soil is to go through some major soil properties that carbon can influence, and explain how char might work in comparison to compost.
NUTRIENTS FIRST
Nutrients are a fine place to start. Both char and compost are likely to provide a fraction of necessary plant nutrient requirements with one major exception. All of the nitrogen in the feedstocks that undergo pyrolysis is lost; it turns into nitrogen gas when the temperature gets high. For anaerobic digestion, nitrogen is conserved and is generally added as organic N to soils. If you compost digestate before land application, a fraction of the nitrogen can be lost when it gets transformed from organic nitrogen into ammonia. However, a very large portion of the nitrogen stays put as organic N. In fact, compost is often used as the only nitrogen source for a crop. When that organic carbon with the nitrogen seasoning is added to the soil, soil microbes begin to feast, and as they eat this organic matter, they release nitrogen that is taken up by plants, which then return the N back to the soil as organic nitrogen and the cycle begins again. This same type of cycle goes on with all of the other nutrients in compost-amended soils.
Compost also holds onto nutrients in the soil because of its high surface area and internal charge, like a sponge with magnets. With these magnets (the technical term for these is cation exchange capacity), the carbon in the compost can hold nutrients and prevent them from leaching out of the soil. On a recent soil sampling field trip in California, we saw that compost increased plant available phosphorus, zinc, copper, iron and sometimes manganese and magnesium in soils. So with compost or digestate application, you get nutrients via the slow release breakdown of organics, increased microbial activity to recycle those nutrients, and magnets to hold onto those nutrients that are just waiting around.
Char also adds these other nutrients, but holds them via this cation exchange capacity rather than as part of the organic compounds. In other words, you just get the magnets and none of the cycling stuff. It’s like getting your nutrients from vitamins instead of a feast. Char is not in the least bit appetizing to soil microorganisms. The carbon in the char is very difficult to digest. The nutrients that don’t volatilize from the feedstock are added to the soil with the char, most likely as oxides. As these oxides dissolve, the magnets in the char can hold onto the nutrients and make them plant available. Char does nothing for nutrient cycling via microbial decomposition. So for char, nutrients are all the vitamins, with no calories and no flavor.
SOIL WATER HOLDING CAPACITY
Next up in the comparison is soil water holding capacity. The amount of water that falls on top of the soil – and actually goes into the soil and stays there – determines how drought resistant a soil is, how efficiently it can supply water to plants and how much water is needed for plants to prosper. This is a function of many factors. For example, how much water infiltrates into the soil depends on the bulk density of the soil. Soils are made mostly of different types of ground up/weathered rocks (clay, silt and sand particles) and pore space. If the soil only consisted of the rocks and no pore space, each cubic centimeter of soil would weigh 2.6 grams. Because of the pore space, a much more common weight is 1.5 g. Now, depending on how the clay, silt and sand particles are held together, that bulk density can get down below 1 g per cm3. The less dense the soil, the more room there is for water and the faster the water can flow into the soil.
The glue that holds the particles together and helps reduce bulk density is organic matter. As bacteria eat the organic matter, they produce waste and the waste carbon works like a glue to cement particles of soil together. This is what gives you good tilth in soil. This same type of action is one of the things that makes earthworm castes so desirable. As the carbon in compost is very tasty, compost addition to soils supplies plenty of that glue action. It also doesn’t hurt that the weight of organic matter is pretty low, usually about .25 g per cm3. Better yet, the organic carbon itself can hold an order of magnitude more than its weight in water, and then some. Just like a sponge.
Now you probably have guessed this, but I am going to tell you anyway. Compost gives you the glue and the water holding capacity. Char just gives you the water holding capacity, not the structure and tilth. Water holding capacity is good, don’t get me wrong. It just helps if the water can get into the soil first so that the soil can store water instead of just letting it run off of the surface. In other words, the glue (aka organic matter) holds the particles together, making the soil porous so that the water can flow in.
AND THEN THERE’S CARBON STORAGE
You may be catching on to a pattern here: The compost serves up all the benefits, whereas the char supplies a minor helping. Well how about carbon storage in soils? Char is often touted as a wonderful means to sequester carbon in soils. Soil carbon storage is great, a fast and largely underappreciated way to store carbon. And when you put char in the soil it stays there a very long time. Why? Well who would want it, that is the main reason. Because it is unavailable to microbes and other soil biota, char sits there.
As I’ve made abundantly clear, compost is tasty and is eaten up. Of course, compost isn’t nearly as tasty as fresh plant residues or even digestate because the carbon in the compost has already gone through several digestive cycles, making it more recalcitrant than fresh organic matter. But it is a whole lot better to eat than char. So the carbon in compost gets eaten. A portion of what gets eaten volatilizes as CO2; another portion gets returned to soil as more recalcitrant organic matter, and humic and fulvic acids. But here’s the deal: While that piece of compost may get eaten and some of it may volatilize, that meal makes the soil a happier and better functioning environment. So plants grow better, and deposit even more carbon, and more of that gets eaten and some of that stays behind. And the cycle continues, building soil tilth, recycling nutrients and increasing soil carbon reserves.
Those biochar-enriched Terra preta soils in Brazil that I mentioned in Part I are vastly improved over their neighbors. But their neighbors are in very bad shape. It may be that the improved ability of the Terra preta soils to hold onto nutrients and water was enough to get that improved growth and cycling going. Our soils, while depleted, are nowhere near as weathered and nutrient poor as the Brazilian soils. So while our soils are almost definitely going to respond to compost addition, there is a good chance that char application to soils will get them as excited as a naked baked potato.
Sally Brown – Research Associate Professor at the University of Washington in Seattle – is a member of BioCycle’s Editorial Board, and authors this regular column on the connections of composting, organics recycling and renewable energy to climate change. E-mail Dr. Brown at slb@u.washington.edu. Andrew Trlica, who is working with Sally Brown on his Masters of Science, provided much of the basics on pyrolysis for Part I.