Agriculture Reference
In-Depth Information
Crop Carbon Dioxide Capture
Crop production goals, that is, crop productivity and how its biomass is used, have
great influence on the GWI of agricultural systems. Net Ecosystem Productivity
(NEP; Fig. 12.1) is the net annual uptake of CO
2
from the atmosphere by the
plant-soil system, defined as gross primary production (total CO
2
uptake) less eco-
system respiration (total CO
2
produced) (Randerson et al. 2002). Net Ecosystem
Productivity thus represents the overall C balance, with a few caveats (Chapin et
al. 2006). In long-established annual cropping systems, NEP is typically zero—as
much CO
2
is respired as is captured annually. Although not all the biomass may
be consumed in the year produced—a portion of the crop residue, for example,
may persist as soil organic matter (SOM) C for decades or centuries (see Paul et
al. 2015, Chapter 5 in this volume)—for ecosystems at C-balance equilibrium an
equivalent amount of older SOM C may be respired. Thus on an annual basis, as
much CO
2
will leave the ecosystem as enters.
Cropping systems recently converted from forests or grasslands have a nega-
tive NEP, annually releasing more CO
2
to the atmosphere than they capture.
More CO
2
is respired than fixed because the original vegetation left on-site,
including roots, will decompose and long-stored SOM will be rapidly oxidized
when tillage breaks up soil aggregates and exposes protected C to microbial
attack (Grandy and Robertson 2006, 2007; Paul et al. 2015, Chapter 5 in this
volume). In most temperate regions, the SOM content of recently converted soils
will approach a new steady-state equilibrium at 40-60% of original levels in
40-60 years (Paul et al. 1997).
Conversely, cropping systems that are gaining C have a positive NEP. In
annual cropping systems, this occurs when SOM accumulates with the adoption
of no-till cultivation or cover crops. When left untilled, soil aggregates that form
around small particles of organic matter are more stable and protect the organic
matter from microbial oxidation (Six et al. 2000, Grandy et al. 2006)—thereby
allowing soil C pools to rebuild to some proportion of their original C content
(West and Post 2002). At KBS, rates of soil C gain in the No-till system are typi-
cal of gains elsewhere in the Midwest (West and Post 2002): ~33 g C m
−2
yr
−1
in the Ap horizon, with no significant change in deeper layers to 1 m (Syswerda
et al. 2011).
Even in tilled soils, cover crops can build SOM quickly—in the unfertilized
Biologically Based system, C was sequestered in the surface soil (A/Ap horizon)
at ~50 g C m
−2
yr
−1
over the first 12 years of establishment (Syswerda et al. 2011).
The mechanisms underlying cover crop gains are not yet clear, but may be related
to the greater polyphenolic content in legume residue that could slow its decom-
position (Palm and Sánchez 1991). Chemical protection may also be occurring in
the Early Successional community, where in addition to the cessation of tillage,
plant residue diversity and perennial roots help to explain C sequestration rates
in the surface soil of >100 g C m
−2
yr
−1
over the first 12 years of abandonment
(Syswerda et al. 2011).
Perennial crops provide an additional soil C advantage by having permanent
deep roots, which both sequester C in long-lived belowground tissue and produce
