SOS Forests

Biochar and Forests

We have posted three new studies on biochar at the W.I.S.E. Colloquium: Forest and Fire Sciences. Biochar is charcoal worked into soil as an amendment for increased fertility and productivity.

Biochar has some interesting implications for forests, and the three new studies express those.

The first is Soil respiration curves as soil fertility indicators in perennial central Amazonian plantations treated with charcoal, and mineral or organic fertilisers by Christoph Steiner, Murilo Rodrigues de Arruda, Wenceslau G. Teixeira, and Wolfgang Zech [here]. The lead author, Dr. Christoph Steiner, Ph.D., is Research Associate at the Department of Biological and Agricultural Engineering, University of Georgia, and a co-editor of Amazonian Dark Earths: Wim Sombroek’s Vision [here, here].

Amazon dark earths, or terra preta, are ancient human-developed soils found in pre-Columbian settlement sites throughout the Amazonia. The deep, rich terra preta soils are in stark contrast to the nutrient-poor, red clay latisols that represent the unmodified soil condition. As noted in many research reports, including Amazonian Dark Earths: Wim Sombroek’s Vision, terra preta contains abundant charcoal (biochar) as well as compost and pottery shards. Scientists hypothesize anthropogenic terra preta supported intensive agriculture for hundreds and perhaps thousands of years, whereas the common latisols can’t support intensive agriculture at all.

To test that hypothesis, Steiner et al. planted banana and guarana starts in holes filled with charcoal, chicken manure, super phosphate, and lime, and top dressed the plants with potassium, zinc sulphate, ground charcoal, bone meal, and chicken manure. The idea was to create terra preta from scratch. They then tested the soil for microbial activity, specifically substrate (glucose) induced respiration. If microbes are present, they eat the glucose and emit the metabolic byproduct, CO2 (much like you and I do).

Different mixtures of charcoal, organic, inorganic fertilizers were tested. Both plantations received charcoal, the guarana plantation got organic (chicken manure and bone meal) fertilizers, and the banana plantation got inorganic (mineral) fertilizers.

The presence of charcoal increased microbial respiration (due to increased microbial biomass) in the banana plantation (inorganic fertilizers), but not so much in the guarana plantation. The organic fertilizers (chicken manure and bone meal) had more effect than charcoal in the guarana plantation.

Charcoal is not a plant nutrient, but it can bind to nutrients and prevent them from leaching out of soils. The propensity of charcoal to bind with metallic oxides (cations) is the reason it is used in many filtration systems.

The soil testing was done about a year after planting. Terra preta is thought to have been built up over many human generations. The upshot is that a one-time, short-term charcoal incorporation treatment is not sufficient, in and of itself, to create terra preta.

The second study posted at the W.I.S.E. Colloquium: Forest and Fire Sciences is Nitrogen retention and plant uptake on a highly weathered central Amazonian Ferralsol amended with compost and charcoal by Christoph Steiner, Bruno Glaser, Wenceslau Geraldes Teixeira, Johannes Lehmann, Winfried E.H. Blum, and Wolfgang Zech [here].

In that study Dr. Steiner and his collaborators prepared 15 different soil treatments using combinations of mineral fertilizer, organic fertilizers (compost) and charcoal. The charcoal was derived from secondary forest wood to mimic what might be produced by slash-and-burn agriculture. Two harvests of rice and sorghum were tested to see how the plants took up nitrogen.

The most N uptake occurred on plots with a combination of compost and mineral fertilizers. The charcoal did not improve plant growth. The soils with more charcoal retained more N, however. It could be that the electrostatic (binding to cations) properties of soil carbon reduced leaching. It is also true that a large C:N ratio can limit plant growth by reducing N uptake by plant roots, too.

The authors conclude that in some soils, such as the nutrient-poor Amazon latisols, additional carbon can enhance soil fertility. It is clear, however, that charcoal (biochar) is not the only ingredient that makes terra preta so productive. Organic wastes applied over decades and centuries made all the difference. Slash-and-burn alone does not lead to the vastly improved terra preta soils.

The third study posted is Effect of low-temperature pyrolysis conditions on biochar for agricultural use by J. W. Gaskin, C. Steiner, K. Harris, K. C. Das, and B. Bibens.

In that study the researchers prepared charcoal (biochar) from poultry litter (phew!), pelletized peanut hulls, and raw pine chips. They used a batch pyrolysis unit (a kiln?) at different temperatures and with different carrier gases (nitrogen and steam).

Gaskin et al. found that, in general, poultry litter biochar had the most plant nutrients, including nitrogen, calcium, phosphorus, and potassium. They also found that the hotter they cooked it, the less yield of biochar remained. At temperatures above 500°C (932°F), much of the N volatilized and cation exchange capacity decreased.

Gaskin et al. also cited other studies of forest fires which found that only 3 percent of the (unburned biomass) N remained in the ash. Forest fires, particularly severe fires in dense fuels, can reach 1200°C (2200°F).

In a study posted at the W.I.S.E. Colloquium: Forest and Fire Sciences last October, Intense forest wildfire sharply reduces mineral soil C and N: the first direct evidence by Bernard T. Bormann, Peter S. Homann, Robyn L. Darbyshire, and Brett A. Morrissette [here, here], the researchers reported that in places the Biscuit Fire (2002) consumed ~60 percent of the carbon and nitrogen from mineral soil horizons and stripped the topsoil away in the plume vortexes.

Fire-related soil changes — including losses of soil organic and inorganic matter — were so large that they became complicated to measure. The 51 Mg ha–1 of loose rocks on the soil surface after fire suggests erosion of 127 Mg ha–1 of fine mineral soil, some of which likely left in the fire plume.

I draw following conclusions from all that good science:

1. Building good soil requires more than biochar

I have large gardens and build the soil with compost, aged steer manure, leaf litter, and wood ash. I occasionally apply chemical fertilizer (N-P-K) to certain plants (roses, mainly). The compost is made from garden and kitchen wastes, leaf litter, and chicken litter (phew!). I top-dress and also till in soil amendments.

That’s the time-honored way to build soil, and by time honored I mean going back deep into the Holocene, following the traditional methods used to create terra preta.

2. Wildfire debilitates soils

Intense fire can significantly degrade soils. Fire can volatilize N and C and suck the ash and soil into fire plumes and transport them somewhere else. The soil can be glazed by the intense heat, which subsequently impedes percolation of rain and snow melt. Post-fire erosion can also strip the remaining soil off the bedrock.

It is specious nonsense to say that wildfire “recycles” nutrients. More often than not, forest fires lead to significant nutrient depletion.

Wildfires can also cause dramatic upward shifts in the C:N ratio. Fires in old-growth forests not only volatilize as much as 100 tons per acre of carbon, they leave as much as five time that much dead carbon on the ground. For decades afterward all the available N is bound up in dead wood decay, unless subsequent fires rip through the debris. Repeat fires are a common occurrence.

Even the mild slash-and-burn fires of modern swidden agriculture in Amazonia do not create terra preta.

3. Biochar is not the solution to global warming

The globe is not warming, CO2 is not a significant driver of global temperatures, and biochar is not made from fossil fuels. Biochar is a part of the natural, organic, carbon cycle. There will never be enough man-made biochar produced to make a detectable difference in atmospheric CO2.

Warmer is better, anyway.

4. Terra preta has other charms

The most significant finding from terra preta research is the reconstruction of human history. There is no such thing as wilderness. “Wilderness” is a modern conceit without foundation in the real world.

Terra preta opens a door to a new and exciting exploration of our own past. The where, when, why, and how of human history is so much more than a list of kings and potentates. Real people, just like you and me, have lived here for millennia. Their story needs to be told.

The study of historical human influences on landscapes is shattering old myths and engendering a new paradigm in ecology. The repercussions and implications for modern stewardship are profound.

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The new studies posted at W.I.S.E. are sterling; they represent the cutting edge in soil science today. We are grateful to the researchers for their capable and exacting research, because we can learn many useful lessons from it and them.