9.3 Impacts of Nitrogen Deposition on Soil Nitrogen
Transformation and Soil Chemistry
9.3.1 Impacts of N Deposition on Soil Nitrogen
Transformation
Soil N transformation processes, such as N mineralization, immobilization, nitrification and denitrification, determine soil N availability and respond promptly to
external N inputs. Nitrogen mineralization, converting organic N to inorganic N, is a
key process that supplies available N for plants and regulates leaching losses,
denitrification and consequent N 2 O emissions. Conversely, N immobilization is
the transformation of N from inorganic (i.e. NH 4
+
, NO 3
À ) to organic forms and
assimilation by microbes. Soil nitrification is a process of oxidizing organic N or
ammonium into nitrite and nitrate. Nitrogen deposition increases external N inputs to
forest soils and thus significantly alters N transformation progresses. The N-induced
alteration of soil N transformation varies with the chemical composition of external
N inputs. For instance, root uptake of nitrate is rapid due to its high mobility, while
most of ammonium should be nitrified before taken up by plants, because it is tightly
bound to soil colloidal particles. The N-induced alteration also varies with the level
of external N inputs, resulting in more nitrate leaching and N 2 O emissions at highlevel N deposition.
Nitrogen addition experiments indicate positive, neutral or negative responses of
net N mineralization in forest ecosystems. The responses of net N mineralization to
N additions are likely dose-dependent. In N-limited forests, N deposition generally
stimulates N mineralization by reducing soil C:N ratio, increasing microbial activity
and C availability (Sun et al. 2016; Zhang et al. 2014). However, excessive N inputs
may decrease soil N mineralization by enhancing stability of soil organic matter and
inhibiting activity of humus-degrading enzymes (Gao et al. 2016a, b). If added N is
insufficient to cause significant changes in soil C, soil N, soil C:N ratio and microbial
abundance, soil N mineralization may show neutral responses to N additions (Tian
et al. 2017a). For instance, Hu (2009) found that low-level N additions (20–25 kg N
ha
À1 year
À1 ) had insignificant effects on net N mineralization in a broad-leaved
Korean pine forest in Northeast China, while high-level N additions (40–50 kg N
ha
À1 year
À1 ) increased net N mineralization. Similar results were observed in a
subtropical Acacia auriculiformis plantation and a Eucalyptus urophylla plantation
(Zhang et al. 2014). Responses of net N mineralization to N additions also depend on
the duration of N treatments. In a subtropical mature forest, short-term N additions
(50–150 kg N ha
À1 year
À1 ) had no significant effects on net N mineralization, while
6-year high-level N additions (150 kg N ha
À1 year
À1 ) significantly decreased net N
mineralization (Chen et al. 2017). As for gross N mineralization, N additions tend to
exert positive (Sun et al. 2016) or neutral (Gao et al. 2016c; Tian et al. 2017a) effects
in temperate and boreal forests, while in most cases N additions exerted neutral to
negative effects on gross N mineralization in tropical and subtropical forests (Gao
et al. 2016a, b).
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E. Du et al.
Transformation and Soil Chemistry
9.3.1 Impacts of N Deposition on Soil Nitrogen
Transformation
Soil N transformation processes, such as N mineralization, immobilization, nitrification and denitrification, determine soil N availability and respond promptly to
external N inputs. Nitrogen mineralization, converting organic N to inorganic N, is a
key process that supplies available N for plants and regulates leaching losses,
denitrification and consequent N 2 O emissions. Conversely, N immobilization is
the transformation of N from inorganic (i.e. NH 4
+
, NO 3
À ) to organic forms and
assimilation by microbes. Soil nitrification is a process of oxidizing organic N or
ammonium into nitrite and nitrate. Nitrogen deposition increases external N inputs to
forest soils and thus significantly alters N transformation progresses. The N-induced
alteration of soil N transformation varies with the chemical composition of external
N inputs. For instance, root uptake of nitrate is rapid due to its high mobility, while
most of ammonium should be nitrified before taken up by plants, because it is tightly
bound to soil colloidal particles. The N-induced alteration also varies with the level
of external N inputs, resulting in more nitrate leaching and N 2 O emissions at highlevel N deposition.
Nitrogen addition experiments indicate positive, neutral or negative responses of
net N mineralization in forest ecosystems. The responses of net N mineralization to
N additions are likely dose-dependent. In N-limited forests, N deposition generally
stimulates N mineralization by reducing soil C:N ratio, increasing microbial activity
and C availability (Sun et al. 2016; Zhang et al. 2014). However, excessive N inputs
may decrease soil N mineralization by enhancing stability of soil organic matter and
inhibiting activity of humus-degrading enzymes (Gao et al. 2016a, b). If added N is
insufficient to cause significant changes in soil C, soil N, soil C:N ratio and microbial
abundance, soil N mineralization may show neutral responses to N additions (Tian
et al. 2017a). For instance, Hu (2009) found that low-level N additions (20–25 kg N
ha
À1 year
À1 ) had insignificant effects on net N mineralization in a broad-leaved
Korean pine forest in Northeast China, while high-level N additions (40–50 kg N
ha
À1 year
À1 ) increased net N mineralization. Similar results were observed in a
subtropical Acacia auriculiformis plantation and a Eucalyptus urophylla plantation
(Zhang et al. 2014). Responses of net N mineralization to N additions also depend on
the duration of N treatments. In a subtropical mature forest, short-term N additions
(50–150 kg N ha
À1 year
À1 ) had no significant effects on net N mineralization, while
6-year high-level N additions (150 kg N ha
À1 year
À1 ) significantly decreased net N
mineralization (Chen et al. 2017). As for gross N mineralization, N additions tend to
exert positive (Sun et al. 2016) or neutral (Gao et al. 2016c; Tian et al. 2017a) effects
in temperate and boreal forests, while in most cases N additions exerted neutral to
negative effects on gross N mineralization in tropical and subtropical forests (Gao
et al. 2016a, b).
190
E. Du et al.