Agriculture Reference
In-Depth Information
Methane and Methanotrophs
The decrease in CH
4
consumption that accompanies conversion of forest or grass-
land to row-crop agriculture is well documented (Smith et al. 2000, Robertson et
al. 2000), but had not been tied to the diversity of CH
4
-oxidizing microbes (metha-
notrophs) in these soils. Levine et al. (2011) compared molecular surveys of bac-
teria in KBS LTER soils to
in-situ
measurements of CH
4
fluxes. These surveys
were based on
pmoA
—a gene that codes for one of the subunits of CH
4
monooxy-
genase, the first enzyme in the pathway of CH
4
oxidation. Across MCSE systems,
CH
4
consumption varied by ~7-fold and was greater in soils with a higher number
of methanotroph species (Fig. 6.4A). Additionally, the temporal stability of CH
4
oxidation throughout the year increased with methanotroph richness: in different
MCSE systems, CH
4
oxidation was less variable (there was less variance among
system replicates) in treatments harboring the highest methanotroph richness (Fig.
6.4B). Levine et al. (2011) attributed increased stability to a greater capacity for
diverse methanotroph communities to oxidize CH
4
under a broader set of environ-
mental conditions.
The MCSE also provides an opportunity to examine the recovery of CH
4
oxida-
tion and methanotroph diversity following abandonment from agriculture. The rate
of CH
4
consumption and the number of methanotroph species both increase fol-
lowing the cessation of agricultural activities. Extrapolating from the current rate at
which methanotroph richness and CH
4
consumption are being reestablished, Levine
et al. (2011) estimate that approximately 80 years from the time of abandonment
will be needed for CH
4
oxidation to return to the levels of native undisturbed soils.
This relationship also suggests that managing lands to conserve or restore methano-
troph richness (see Gelfand and Robertson 2015, Chapter 12 in this volume) could
help mitigate increasing atmospheric concentrations of this potent greenhouse gas.
Bacteria and Nitrous Oxide Production
Nitrous oxide is another potent greenhouse gas of biological origin. Approximately
half of contemporary anthropogenic N
2
O emitted to the atmosphere is from agri-
cultural soils (IPCC 2007) and its emission is accelerated by N fertilizer use (Millar
and Robertson 2015, Chapter 9 in this volume). Nitrous oxide can be produced
by both nitrifying and denitrifying bacteria (Robertson and Groffman 2015), but
stable isotope tracing indicates that in agricultural soils at KBS LTER, it is made
primarily by denitrifiers (Ostrom et al. 2010). During denitrification, microbes use
nitrate (NO
3
−
) in place of oxygen (O
2
) as a terminal electron acceptor for respiratory
metabolism. A key enzyme in denitrification is nitrite reductase (
nir
) encoded by
either
nirK
or
nirS
genes (Fig. 6.5). Huizinga (2006) found that denitrifiers in KBS
LTER soils primarily carry
nirK
, but her molecular surveys did not find any patterns
in the distribution of denitrifiers with
nirK
that could be linked to N
2
O flux.
However, there may be a pattern in the distribution of denitrifiers that carry
a gene that codes for another enzyme involved in N
2
O fluxes, N
2
O reductase
(
nos
). The net production of N
2
O from denitrification is dependent not only on
the activities of the enzymes nitrite reductase (nir) and nitric oxide reductase
