Environmental Engineering Reference
In-Depth Information
Increased demand for grain from the ethanol industry will most likely result in a consistent
increase in the land area planted to corn, which will both displace some other crops as well as bring
additional area from the 14 Mha currently held in the Conservation Reserve Program (CRP) into
cultivation (Wright et al. 2006).
16.2.5 S
tovEr
r
Emoval
and
S
oil
o
rganic
m
attEr
Corn stover left in the field after grain harvest provides soil cover and contributes to the organic
matter content. To facilitate its breakdown, the soil usually undergoes multiple tillings, which erode
the organic matter content of the soil through volatilization and loss of top soil. Up to two-thirds of
the stover may be removed on a sustainable basis under no-till conditions from some corn-growing
regions without an adverse effect on soil organic matter content (Wilhelm et al. 2004; Perlack et al.
2005; Johnson et al. 2006; Graham et al. 2007). In the United States, more than 100 million tons
of maize stover could be collected annually within the tolerant limits of soil erosion at the current
production levels (Graham et al. 2007).
Roots constitute approximately 20% of total biomass at maturity and contribute to soil organic
matter content (Amos and Walters 2006). However, root mass alone is not sufficient to maintain soil
organic matter content over time (Wilts et al. 2004). Detailed discussion on this topic can be found
in several recent publications (Allmaras et al. 2004; Wilhelm et al. 2004; Wilts et al. 2004; Perlack
et al. 2005; Johnson et al. 2006; Graham et al. 2007).
Stover removal will also entail net removal of nutrients that otherwise contribute to soil nutrition.
For example, the N harvest index, which is defined as the ratio of grain N to total aboveground
plant N, is approximately 65% in maize (Banziger et al. 1999). Thus, an appropriate portion of the
one-third of the total plant N sequestered in stover would have to be replenished by an additional
fertilizer application depending upon the amount of stover removed (Dhugga 2007).
16.3 BIomass structure and comPosItIon
Maize stalks account for more than half of the stover biomass, followed respectively by leaves, cobs,
and husks (Figure 16.7; Atchison and Hettenhaus 2003; Masoero et al. 2006). Most of the stalk
biomass is concentrated in the rind tissue, which consists of a mixture of densely packed vascular
bundles embedded in a matrix of sclerenchymatous cells on the outer periphery of the internodes.
Rind accounts for less than 20% of the cross-sectional area but more than 80% of stalk dry mass
(K.S. Dhugga, unpublished data). Most of the remaining 20% of the biomass is likely accounted for
by the vascular bundles that are embedded in the ground tissue consisting of parenchymatous cells.
Parenchymatous cells have thin primary walls and are nearly devoid of free sugars at maturity so
their contribution to biomass is minor.
The chemical composition of corn stover, rice and wheat straw, and switchgrass is relatively
similar (Figure 16.8). Corn stover is approximately 38% cellulose, 26% hemicellulose, and 19%
lignin. Rice straw stands out in ash content, which is mostly accounted for by silica deposition on
the leaf and sheath surfaces (Figure 16.8).
16.4 cell Wall BIosynthesIs
Plant cells are surrounded by viscoelastic primary cell walls, that are amenable to expansion under
turgor pressure. Upon completion of the process of cell expansion, a thick secondary cell wall
is deposited in some cell types. Cellulose microfibrils are embedded in a hemicellulosic matrix
along with pectins and structural proteins in primary walls whereas secondary walls contain little
protein or pectin, but normally contain lignin (Carpita 1996). Grass cell walls are unique in that
they contain little pectin. Apparently, the role of pectin is fulfilled by the hemicellulose matrix
consisting mainly of glucuronoarabinoxylan (GAX).
