On Top, or On the Side

Pyrolysis is often looked at as the perfect solution for biosolids -- a true form of green incineration.  Pyrolysis is combustion without the flame.  By combining heat and pressure in a low oxygen environment, organic materials transform into three products -- gas, liquid and solid.  The gas and liquid can be transformed into fuels.  The solid can be used as a soil amendment or to grill steak; it is a version of charcoal. 

This charcoal like substance is called biochar. It is credited with bringing the tropical soils in the Brazilian jungle back to life.  The char is inert in soil, and, so, one of the benefits of adding char to soil is that it doesn’t go away.  In other words, put enough char in soil and right away you have dramatically increased soil carbon storage and that isn’t going to change with time.  No need to worry about carbon cycling. With char added to soil, that carbon is in lock-down.   

Devotees of char claim that adding it to soils will not only store carbon, but deliver other benefits.  It will be a source of eternal youth for soil, much in the same way those black soils in Brazil have stayed rich and fertile through the centuries.  Those first reports of the terra preta or black earth soils in Brazil first surfaced a few years back, and char mania is still going strong. But now there are much more data to see if the Brazilian miracle works in North America.

Not exactly North America, but the first article in the library is based on research in Australia.  Although English is the spoken tongue, the highly weathered soils down under have more in common with Brazil than they do with N. Dakota.  However, the paper is likely relevant for all soils.  The authors studied char amended soils over a 10 year period to see if they could figure out the mechanisms associated with carbon accumulation in char amended soil, a carbon accumulation above and beyond the char just sitting there.  They found that char can work as an excellent construction substrate.  The char helps the soils to form microaggregates, meaning very small clusters of soil particles.  The char works like scaffolding for these microaggregates.  The carbon that is contained within the microaggregates is less accessible to soil microbes and so less of the char is used as a fuel source and mineralized.  So this research confirms that biochar increases soil sequestration and provides information on the mechanism (besides just sitting there) that is responsible for this.

Now, as an aside (not in the library), other research has shown that when carbon in soils forms association with clay particles that are rich in iron or aluminum, the carbon is also protected from microbial attack by the same mechanism associated with biochar (Kleber et al., 2007).  And that would explain why biosolids carbon can also be long-lived in soils.  Most biosolids contain >1% total iron.  Just had to point that out.

I’ll grant that biochar can contribute to carbon sequestration, but I offer the caveat that good old biosolids cake can do the same thing. 

How about the other soil improvements?  Here we have two review papers on char (articles #2 and #3 in the library).  One of the issues with char, as is made clear in these papers, is that you can’t be sure what type of response you will get.  In the first review, the authors note that 50% of the time you see a yield increase with char.  That is terrific!  The problem is the remainder of sentence in the abstract: “…with the remainder of the studies reporting alarming decreases to no significant differences.”  So, biochar is a product that works great half the time and goes from does nothing to sucks the other half. 

The most reliable benefits of char are seen from biochar produced from hardwoods.  The next paper has done a broad statistical analysis of results with char and reports an overall small but significant yield analysis (10%) associated with char application, but notes that the range goes from -28% to 39%.  The best results are seen with acidic to neutral pH soils and/or with a coarse or sandy textured soils,  with the best response at loading rates of 100 Mg ha-1.  In terms of feedstocks, poultry litter was the best (28% yield increase) and biosolids was the only feedstock ‘showing a statistically significant negative effect (-28%).  Although the study cited on biosolids char was limited in its scope (from New Zealand and not in a peer review journal), it doesn’t make pyrolysis sound like the green alternative to direct land application. 

For as close to a head-to-head comparison between biochar and biosolids as there is in the literature, we go to study #4.  Here the authors used char from hardwood and regular old dairy manure, both singly and in combination.  They tested the efficacy of the amendments on nutrient availability and growth of corn silage.  The study was conducted on irrigated soils in Kimberley ID.  While char increased total C and extractable Mn, the manure increased nitrate and ammonia N, extractable P, K, Mg, Cu, Mn, total C and total N.  The manure had some detrimental effects as well, increasing extractable sodium and electrical conductivity.  Char had no effect on yield in comparison to the control, while manure showed a yield increase for one of the two years of the study. 

These studies suggest that pyrolysis of biosolids will not get you much apart from high drying and capital costs and soil carbon sequestration. 

If you opt to compost your biosolids (no drying costs and low capital costs), the last article in the library gives you a valuable end use for char in your composting operation.  The authors tested the ability of char to reduce N losses during composting.  They mixed poultry litter with a pine chip based biochar at 0, 5% and 20% rates of char addition.  At the highest rate of char addition, they observed increased CO2 emissions, higher pH and temperatures and suggest that this means faster composting.  Most importantly, at the highest char rate they also observed reduced N loss through volatilization, with reduction in losses up to 52% over the no char treatment.  They also observed a decrease in H2S emissions.  Now, granted that composting poultry litter with an initial C:N ratio of 10.4 is not the best approach to conserve N, these results do suggest that biochar may have a place in composting.  The reduced N losses along with reduced H2S emissions suggest a process with reduced odors and a finished product with a higher nutrient value. 

Sally Brown, University of Washington