Subject: [SANET-MG] bioenergy from a C mass balance perspective

From: Joel Gruver <[email-masked]>
Date: Mon, 11 Apr 2005 15:19:30 -0400

Hello folks,

As we discuss the impacts of using crop residues or other organic materials
for energy purposes (as compared to soil amendment), it is important to
evaluate possible scenarios from a C mass balance perspective.

The functional benefits of SOM (nutrient storage, structural stabilization,
water holding capacity, nourishing of soil organisms...) are well documented
and widely appreciated... in contrast, the factors which constrain SOM
accumulation (e.g. climate, texture, mineralogy, landscape position...) and
interactions between these factors and management (e.g. tillage,
fertilization, organic additions, irrigation...) are rather poorly
understood.

Current research (including my own) is evaluating the concept of saturatable
pools of SOM...  an example of a potentially saturatable pool of OM is the
OM that gets adsorbed on the surfaces of fine mineral particles. There is
considerable evidence that adsorption on the surfaces of clay and fine silt
particles can confer protection against decomposition.

The amount of OM that can be protected by adsorption on mineral surfaces is
likely to be a finite quantity related to the chemical and physical
properties of a soil's mineral particles  (e.g. external surface area,
internal surface area, CEC...).

The impact of new organic inputs on soil function and the partitioning of
new organic inputs into SOM fractions (microbial biomass, particulate OM,
humus...) and CO2 is likely to depend on the quantity and quality of OM
already in the soil.

All of the above is intended to set the stage for a brief discussion of the
following question:

How will the removal of crop residue for bioenergy purposes impact C mass
balance ?

When fossil hydrocarbon energy sources (natural gas, oil and coal) are
displaced by renewable carbohydrate sources (e.g. crop residue, wood...)
less CO2 is released into the atmosphere...unless of course,  the C cost of
production, harvest, processing and transportation of the renewable
carbohydrate source is greater than the fossil C it displaces.

When crop residues are returned to soil, most of the C in the residues is
returned to the atmosphere as CO2 within a year.
 Biologically mediated soil processes (nutrient transformations,
biodrilling, detoxification...) are energized as residue C traverses the
redox continuum from carbohydrate to CO2...  Interactions between management
practices and the factors described above control whether the soil will act
as a net source, net sink or just a conduit of C.

If a soil system is at or near saturation i.e. accumulating very little SOM
because of non-management factors, use of crop residue for bioenergy
purposes will probably result in a more favorable C balance than if the
residue were returned to the soil and quickly converted back into CO2.

If a soil system has low OM relative to its potential and management
practices are adopted which promote C accumulation, return of residues will
probably result in a more favorable C balance than if the residue was
removed for bioenergy purposes.

Hope that this makes sense... looking forward to your comments.

Joel

BTW I will send some comments on composting vs. direct land application of
raw organics shortly...

Joel

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