Agriculture Reference
In-Depth Information
(nor) that make N
2
O, but also on the activity of N
2
O reductase (nos) that reduces
N
2
O to the innocuous gas N
2
(Fig. 6.5). A tRFLP analysis of cloned
nosZ
genes
(Stres et al. 2004) revealed greater diversity in the denitrifying community in the
MCSE Conventional system than in the Mown Grassland (never tilled) treatment.
Furthermore, even though nirS was less abundant than nirK, quantitative PCR
revealed that this agricultural community had a higher ratio of
nirS
genes to
nosZ
genes and so had greater potential to produce N
2
O (rather than N
2
) from the same
amount of NO
3
−
(Morales et al. 2010). This represents the intriguing possibility
that agricultural land use has selected for a subset of denitrifiers that may acceler-
ate N
2
O production.
Another possible functional difference between denitrifying bacteria from agri-
cultural and successional soils at KBS LTER was suggested by a study of N
2
O
reductases from cultured representatives. Even though these cultured species rep-
resent only a small fraction of the denitrifying bacteria, the average O
2
-sensitivity
of their N
2
O reductases varied significantly between the two soils (Cavigelli and
Robertson 2001). Next-generation sequencing technologies will enable analysis of
all the bacterial
nosZ
genes that code for N
2
O reductase in agricultural and succes-
sional communities.
Microbial Respiration and Carbon Dioxide Flux
Increasing worldwide demand for agriculture to produce food, fuel, and fiber
affects reservoirs of soil organic matter (SOM), which are highly responsive to both
changing land use and shifts in climate (Paul et al. 2015, Chapter 5 in this volume;
Robertson et al. 2015, Chapter 2 in this volume). Microorganisms play a crucial
role in determining the turnover of SOM: they rapidly assimilate and respire labile
fractions to CO
2
or transform organic matter to more recalcitrant compounds that
are critical to C sequestration and long-term soil productivity. Approximately half
of the C lost as CO
2
from soils is due to the metabolism of heterotrophic microbes,
with the remainder ascribed primarily to plant root respiration (Hanson et al. 2000).
Predicting the fate of microbially processed C (i.e., assimilation into biomass vs.
respiratory loss) is thus critical to the development of robust models that accurately
predict terrestrial C transformations.
Carbon dioxide emission from soils varies across MCSE systems (Paul et al.
1999), but unlike the specialized functions of CH
4
consumption or N
2
O produc-
tion, no direct relationship exists between CO
2
emission and bacterial rich-
ness in KBS LTER soils (Levine et al. 2011). Although the number of bacterial
species does not vary dramatically with land management, the composition of
the microbial heterotroph communities does. And because C cycling involves a
broad diversity of microbes, it may be in the changing composition of hetero-
trophic microbes that we find explanations for the variation in respiratory CO
2
production. Most biogeochemical models of C cycling assume that microbial
communities assimilate C at a fixed rate. For example, there are a number of
transformations of C in the widely used CENTURY model (Fig. 6.6; Parton
et al. 1987), and it is assumed that in each transformation, 55% of the C con-
sumed by microbes is oxidized to CO
2
, with the remainder incorporated into
