The Cellulosic Myth

 The Cel­lu­losic Myth

- by Bri­an Tokar

[Excerpt­ed from “Bio­fu­els and the Glob­al Food Cri­sis,” in Fred Magd­off and Bri­an Tokar, eds., Agri­cul­ture and Food in Cri­sis: Con­flict Resis­tance, and Renew­al, New York: MONTHLY REVIEW PRESS, 2010.] 

Log­ging for bio­mass ener­gy in Vermont

As con­cerns about agro­fu­els’ impli­ca­tions for food sup­plies and the envi­ron­ment have become more wide­spread, pro­po­nents have reaf­firmed their claims that cur­rent tech­nolo­gies are mere­ly a “step­ping stone” to more sus­tain­able bio­fu­el pro­duc­tion from the cel­lu­lose in grass­es and trees, rather than from food starch­es and oilseed crops. They pre­dict that the world will soon obtain increas­ing yields of liq­uid fuel extract­ed from prairie grass­es, log­ging wastes and for­est thin­nings, as well as agri­cul­tur­al byprod­ucts such as straw and corn stover (i.e., leaves and stems). The extrac­tion of ethanol from these high-cel­lu­lose sources, how­ev­er, is a com­plex, ener­gy-con­sum­ing process involv­ing many stages of enzy­mat­ic diges­tion and purifi­ca­tion of break­down prod­ucts, fol­lowed by the fer­men­ta­tion of sug­ars into ethanol. Alter­na­tive process­es, such as the high tem­per­a­ture gasi­fi­ca­tion and dis­til­la­tion of cel­lu­losic feedstocks—technically sim­i­lar to the liq­ue­fac­tion of coal—have proven equal­ly dif­fi­cult to commercialize.

The most pop­u­lar sce­nario for fuel extrac­tion from cel­lu­losic sources relies main­ly on the use of wild or cul­ti­vat­ed grass­es, such as the vari­eties of switch­grass (Pan­icum vir­ga­tum) that briefly became syn­ony­mous with “cheap, abun­dant fuel” after Pres­i­dent George W. Bush men­tioned switch­grass in his 2006 State of the Union address. But har­vest­ing grass­es for fuel rais­es a host of new prob­lems. Grass mono­cul­tures are high­ly depen­dent on nitro­gen fer­til­iz­ers and irri­ga­tion, while diverse grass­lands, with healthy pop­u­la­tions of legu­mi­nous plants, are high­ly pro­duc­tive and far bet­ter at seques­ter­ing car­bon diox­ide as soil organ­ic mat­ter. How­ev­er, the use of mixed feed­stocks in any indus­tri­al process sig­nif­i­cant­ly com­pli­cates the enter­prise. Fur­ther, many grass species deemed suit­able for agro­fu­el pro­duc­tion are con­sid­ered high­ly inva­sive. “[T]raits deemed ide­al in a bioen­er­gy crop,” report­ed one study, “are also com­mon­ly found among inva­sive species,” traits that include lack of known pests or dis­eases, high effi­cien­cy of water use and pho­to­syn­the­sis, rapid growth, and the abil­i­ty to out-com­pete weeds in the spring.

In the Unit­ed States, the most like­ly source of grass-based agro­fu­els is from grass­lands now set aside under the Agri­cul­ture Department’s Con­ser­va­tion Reserve Pro­gram (CRP). In June 2006, rep­re­sen­ta­tives of twen­ty-two lead­ing con­ser­va­tion and hunt­ing advo­ca­cy groups wrote to Con­gress chal­leng­ing pro­pos­als to grow fuel crops on con­ser­va­tion lands, cit­ing the program’s remark­able suc­cess in reduc­ing soil ero­sion, reduc­ing weed pres­sure, and pre­serv­ing wet­lands. “Most at risk are the wildlife ben­e­fits of CRP,” the let­ter stat­ed, “which to a great extent are sim­ply not com­pat­i­ble with fre­quent har­vest­ing.” Unlike the peri­od­ic fire dis­tur­bances that are nec­es­sary to sus­tain prairie ecosys­tems, har­vest­ing grass­lands returns few nutri­ents to the soil, and har­vest­ing equip­ment would like­ly prove far more dis­rup­tive to wildlife than the spread of wildfire.

The use of crop residues for fuel also rais­es seri­ous ques­tions, as these mate­ri­als are essen­tial for soil con­ser­va­tion and play an essen­tial role in agro­nom­ic cycles. The decom­po­si­tion of crop residues tilled back into the soil after har­vest is nec­es­sary for the main­te­nance of soil health, while grow­ers who prac­tice “no till” cul­ti­va­tion rely on the same residues as a mulch and for pro­tec­tion against soil ero­sion. Col­lect­ing and sep­a­rat­ing corn stover from the grain would require redesigned, prob­a­bly heav­ier, com­bines, adding to farm­ers’ costs and to soil com­paction. A 2007 study by researchers at two Depart­ment of Ener­gy lab­o­ra­to­ries con­clud­ed that a max­i­mum of 30 per­cent of crop residues could be removed with­out increas­ing soil ero­sion and less­en­ing soil organ­ic matter.

Final­ly, the pro­posed thin­ning of forests and removal of dead trees and branch­es for fuel pro­duc­tion would reduce car­bon seques­tra­tion and also threat­en wildlife habi­tats. The expe­ri­ence of bio­mass pow­er plants in the U.S. sug­gests that har­vest­ing wood for fuel inevitably increas­es log­ging, whether in forests or on plan­ta­tions ded­i­cat­ed to fuel pro­duc­tion. Thin­ning oper­a­tions dis­turb the for­est floor, accel­er­at­ing the loss of soil car­bon as CO 2 . The push for cel­lu­losic agro­fu­els has served to jus­ti­fy the expan­sion of mono­cul­ture tree plan­ta­tions, as well as the devel­op­ment of genet­i­cal­ly engi­neered trees, most notably the vari­eties of fast-grow­ing euca­lyp­tus that have been mod­i­fied to sur­vive in cool­er cli­mates such as those found in the South­east­ern U.S. The South Car­oli­na-based Arbor­gen com­pa­ny has repeat­ed­ly cit­ed the search for appro­pri­ate bio­fu­el feed­stocks as a ratio­nale for its aggres­sive devel­op­ment of genet­i­cal­ly engi­neered tree vari­eties, and agro­fu­el devel­op­ment has also become a lead­ing ratio­nale for com­mer­cial­iz­ing the exot­ic new genet­ic inter­ven­tions known as “syn­thet­ic biology.”