In the present study, the dynamics of
floodwater N and pH as
affected by agricultural practices (e.g., N fertilization and cropping
system) and climate change (simultaneous elevation of [CO2] and
temperature) was also investigated throughout the two rice
seasons. From sowing till harvesting (three weeks before crop
harvest), the paddy rice
field was intermittently
flooded, as
described in Section 2.1.
The climate treatment did not affect the soluble N and pH in the
floodwater for both seasons (data not shown). But all chemical
parameters assessed in the experiment (available NH4
+ and NO3
,
and pH) varied significantly with the study years (p < 0.001), with
higher values in 2011 compared with 2012 (Table 4), similarly to
the soil pH and the non-exchangeable NH4
+ content, but contrary
to available soil N. Immediately after basal dressing, the NH4
+ and
NO3
contents and the pH in the
floodwater were 1.8 and 3.4 mg N
l1 and 7.5, respectively, but progressively decreased thereafter till
topdressing (11 and 10 July, respectively in 2011 and 2012)
(Table 4). The decrease in NH4
+ content in
floodwater continued
after topdressing till crop maturity. At the second sampling time
after topdressing (13 July 2011 and 8 August 2012), the NO3
content and pH in water increased significantly (p < 0.001);
however, at the third sampling time after topdressing (24 August
2011 and 12 September 2012) these values decreased again.
Overall, the total inorganic N (NH4
+ + NO3
) in
floodwater was
seven times lower at crop maturity than before sowing (28 May