national averages over the surface rather than waters (26.0 mg km
À2 s
À1 ) (Xu et al.
2015) and European countries (EANET. 2000) and mean values across the United
States (Li et al. 2016), but less for the rest forms of dry deposition of N.
Our results also indicate that NH 3 dominated the fluxes of dry deposition over
Lake Dianchi with intensive agricultural activities and fuel combustions in its
watershed. Specifically, NH 3 accounted for 82.8 Æ 3.4% of dry deposition fluxes
in later spring and summer (i.e., from May to August), while N oxidations (i.e., NO 2 ,
HNO 3 /HNO 2 ) contributed more than half in the other seasons. Seasonality of NH 3
deposition can be explained by the distributions of fertilizer uses and mean air
temperature. NH 3 is emitted primarily from croplands (Zhou et al. 2016) and then
deposited locally close to the emission source (<1 km; Asman et al. 1998). Croplands were mainly distributed surrounding the Lake Dianchi, where N fertilizer
application rate (including synthetic fertilizer, manure, and crop residue return
back to cropland) is larger than that of the national average. To determine the
seasonality of fertilizer use in Lake Dianchi watershed, in-house surveys of farmers
were conducted by Peking University in 2011. Three hundred representative farmers
were selected for a face-to-face, questionnaire-based household survey to collect
information on fertilizer use in different towns (Fig. 12.5). Figure 12.2a showed that
high rates of N fertilizer use occur in later May, early June, and early July (>
146.6 kg N hm
À2 biweek
À1 ), partly explaining the abrupt increase of NH 3 deposition
flux since May. Additionally, NH 3 volatilizations are expected to be extremely
temperature-sensitive, that is, the emission response to temperature is exponential
rather than linear (Sommer et al. 2004). This climate-dependent paradigm implies
that NH 3 deposition fluxes are kept at more than 95.2 mg km
À2 s
À1 in the following
months (i.e., later July and August). Third, the concentrations of N oxidations
decreased in summer compared to those in other seasons in five sites, which agree
with previous observations in South China (Yang et al. 2010a). Such seasonal
pattern could be explained by the discrepancies in atmospheric mixing and photochemical reaction between cold and warm seasons (Yang et al. 2010a).
12.3.2 Wet N Deposition
Wet deposition over the Lake Dianchi has a N flux of 46.5 Æ 13.2 mg km
À2 s
À1 ,
10.5% higher than dry deposition (Fig. 12.2c). NO 3
À -N dominated the total flux of
wet deposition (24.3 Æ 7.5 mg km
À2 s
À1 , 52.2%), followed by ON (11.9 Æ 5.4 mg
km
À2 s
À1 , 25.6%) and NH 4
+
-N (10.3 Æ 3.9 mg km
À2 s
À1 , 22.2%). Peak N fluxes of
wet deposition occurred in the wet season (i.e., from later July to early October;
Fig. 12.2d), following the dry deposition peaks. The N fluxes during this period were
up to 172.9 Æ 72.2 mg km
À2 s
À1 , sixfold greater than the mean value of the rest of
the year. The temporal variation of wet deposition was primarily driven by precipitation, not N concentrations in rainfall. Correlation relationship in Fig. 12.4
12 Impacts of Nitrogen Deposition on China’s Lake Ecosystems. . .
275
À2 s
À1 ) (Xu et al.
2015) and European countries (EANET. 2000) and mean values across the United
States (Li et al. 2016), but less for the rest forms of dry deposition of N.
Our results also indicate that NH 3 dominated the fluxes of dry deposition over
Lake Dianchi with intensive agricultural activities and fuel combustions in its
watershed. Specifically, NH 3 accounted for 82.8 Æ 3.4% of dry deposition fluxes
in later spring and summer (i.e., from May to August), while N oxidations (i.e., NO 2 ,
HNO 3 /HNO 2 ) contributed more than half in the other seasons. Seasonality of NH 3
deposition can be explained by the distributions of fertilizer uses and mean air
temperature. NH 3 is emitted primarily from croplands (Zhou et al. 2016) and then
deposited locally close to the emission source (<1 km; Asman et al. 1998). Croplands were mainly distributed surrounding the Lake Dianchi, where N fertilizer
application rate (including synthetic fertilizer, manure, and crop residue return
back to cropland) is larger than that of the national average. To determine the
seasonality of fertilizer use in Lake Dianchi watershed, in-house surveys of farmers
were conducted by Peking University in 2011. Three hundred representative farmers
were selected for a face-to-face, questionnaire-based household survey to collect
information on fertilizer use in different towns (Fig. 12.5). Figure 12.2a showed that
high rates of N fertilizer use occur in later May, early June, and early July (>
146.6 kg N hm
À2 biweek
À1 ), partly explaining the abrupt increase of NH 3 deposition
flux since May. Additionally, NH 3 volatilizations are expected to be extremely
temperature-sensitive, that is, the emission response to temperature is exponential
rather than linear (Sommer et al. 2004). This climate-dependent paradigm implies
that NH 3 deposition fluxes are kept at more than 95.2 mg km
À2 s
À1 in the following
months (i.e., later July and August). Third, the concentrations of N oxidations
decreased in summer compared to those in other seasons in five sites, which agree
with previous observations in South China (Yang et al. 2010a). Such seasonal
pattern could be explained by the discrepancies in atmospheric mixing and photochemical reaction between cold and warm seasons (Yang et al. 2010a).
12.3.2 Wet N Deposition
Wet deposition over the Lake Dianchi has a N flux of 46.5 Æ 13.2 mg km
À2 s
À1 ,
10.5% higher than dry deposition (Fig. 12.2c). NO 3
À -N dominated the total flux of
wet deposition (24.3 Æ 7.5 mg km
À2 s
À1 , 52.2%), followed by ON (11.9 Æ 5.4 mg
km
À2 s
À1 , 25.6%) and NH 4
+
-N (10.3 Æ 3.9 mg km
À2 s
À1 , 22.2%). Peak N fluxes of
wet deposition occurred in the wet season (i.e., from later July to early October;
Fig. 12.2d), following the dry deposition peaks. The N fluxes during this period were
up to 172.9 Æ 72.2 mg km
À2 s
À1 , sixfold greater than the mean value of the rest of
the year. The temporal variation of wet deposition was primarily driven by precipitation, not N concentrations in rainfall. Correlation relationship in Fig. 12.4
12 Impacts of Nitrogen Deposition on China’s Lake Ecosystems. . .
275