Since
rotation of paddy rice-upland crop cultivations is a major land use management
widely conducted in Japan, effect of change in land use on greenhouse gas
emissions from drained paddy fields are required to be clarified. In this
study, methane (CH4) and nitrous oxide (N2O) emissions in both paddy
fields (PR) and drained fields under upland crop cultivations (UR: single
cropping of upland rice, SW: double cropping of soybean/wheat) were measured
over two and a half years in the experimental fields with an automated flux monitoring
system.
CH4 emission in the PR plots in
2003 was 5.8 times higher than that in 2002, mainly because of a prolonged
period of continuous submergence in 2003. N2O emissions in the PR plots
were low during the paddy rice cultivation periods.
In the
UR and SW plots, CH4 emissions were stopped. On the other hand, significantly
higher N2O emissions were often observed during the flowering to
ripening stages of the cultivated upland crops, and other temporal increases in
N2O
fluxes were also occasionally found after heavy rainfall or crop harvesting.
Cumulative N2O emissions in the UR and SW plots were three- to fivefold
higher than that in the PR plots.
The
combined net global warming potentials (net GWP) by the CH4 and N2O emissions were 100 to 442,
102 to 110, and 79 to 146 g CO2 m–2 in the PR, UR, and SW plots,
respectively. Net GWP in the PR plots in 2003 was extremely high compared with
the other values, owing to the high levels of CH4 emission.
These
results suggest that drainage of paddy soils for upland crop cultivation is
effective for reducing net GWP in paddy fields with high potential for CH4 emission, such as those that
are poorly drained or have low soil iron content.