In a semiarid temperate steppe, Lan and Bai (2012) tested the mechanisms
underlying the loss of plant species richness in response to N enrichment based on
10-year data from a field experiment with six levels of N addition rate. In their
experiment, plant species richness declined by 16% even at the lowest level of N
addition (17.5 kg N ha
À1 year
À1 ), highlighting the high sensitivity of this grassland
ecosystem to N deposition with respect to biodiversity change. More than half of
species would go extinction in the treatments with high N input (105–280 kg N ha
À1
year
À1 ) after 10 years. The responses of species richness to N enrichment depended
on the variation of annual precipitation, with the responses being greater in wet years
than dry years. They found evidence for the abundance-based mechanism, in that the
critical threshold for N-induced decline in species richness differed between common species and rare species in this ecosystem. They proposed that both the low
initial abundance and low aboveground competitive ability attribute to the loss of
rare species. Further, their findings suggest that biotic factors, including recruitment
limitation and interspecific competition, are the key mechanisms for the loss of
abundant species. In contrast, soil acidification is less important. With the same
dataset, they quantified the scale dependence of N-induced species loss by analysing
the changes of increasing slope in the species-area relationship with N addition (Lan
et al. 2015). The results showed that proportional species loss (compared to control)
decreased and critical threshold for biodiversity losses increased with sampling
areas. The study of Lan et al. (2015) presented strong evidence that both the
sensitivity and magnitude of species loss in response to N deposition are scaledependent. Further, their findings highlight that the previously estimated critical
loads of N deposition should be reassessed; as such values were estimated based on
small scale (one or several square metre quadrats) but have been widely used as the
backbone of biodiversity conservation and environmental legislation on
transboundary pollution in Europe and the USA.
In an alpine meadow on eastern Tibetan Plateau, Yang et al. (2015) carried a
4-year field experiment to examine the evidence for abundance- and functionalbased mechanisms of plant species loss after fertilization. Moreover, they examined
whether the importance of both mechanisms would change with the presence and
absence of herbivores. Fertilization significantly reduced plant species richness by
an average of 7.3 species per 0.25 m
2 relative to the control. Such a decrease in
species richness was due to the increases of species loss but not due to the decline of
species gain. The likelihood of species loss following nutrient eutrophication was
negatively correlated with their mean abundance at the beginning of the experiment,
indicating the role of abundance-based mechanism for the decline of species richness
following fertilization. Meanwhile, they found substantial changes in both functional
traits of dominant species and community composition, in that the species with
higher specific leaf area and higher stature were favoured by fertilization. Such a
result indicates that the functional-based mechanism also plays a role in driving
species loss following N enrichment. More importantly, they found no evidence for
the role of functional-based mechanism with fertilization in the presence of herbivores. Together, their results demonstrated that both abundance- and functionalbased mechanisms drive species loss following nutrient enrichment with the absence
10 Impacts of Nitrogen Deposition on China’s Grassland Ecosystems
231
underlying the loss of plant species richness in response to N enrichment based on
10-year data from a field experiment with six levels of N addition rate. In their
experiment, plant species richness declined by 16% even at the lowest level of N
addition (17.5 kg N ha
À1 year
À1 ), highlighting the high sensitivity of this grassland
ecosystem to N deposition with respect to biodiversity change. More than half of
species would go extinction in the treatments with high N input (105–280 kg N ha
À1
year
À1 ) after 10 years. The responses of species richness to N enrichment depended
on the variation of annual precipitation, with the responses being greater in wet years
than dry years. They found evidence for the abundance-based mechanism, in that the
critical threshold for N-induced decline in species richness differed between common species and rare species in this ecosystem. They proposed that both the low
initial abundance and low aboveground competitive ability attribute to the loss of
rare species. Further, their findings suggest that biotic factors, including recruitment
limitation and interspecific competition, are the key mechanisms for the loss of
abundant species. In contrast, soil acidification is less important. With the same
dataset, they quantified the scale dependence of N-induced species loss by analysing
the changes of increasing slope in the species-area relationship with N addition (Lan
et al. 2015). The results showed that proportional species loss (compared to control)
decreased and critical threshold for biodiversity losses increased with sampling
areas. The study of Lan et al. (2015) presented strong evidence that both the
sensitivity and magnitude of species loss in response to N deposition are scaledependent. Further, their findings highlight that the previously estimated critical
loads of N deposition should be reassessed; as such values were estimated based on
small scale (one or several square metre quadrats) but have been widely used as the
backbone of biodiversity conservation and environmental legislation on
transboundary pollution in Europe and the USA.
In an alpine meadow on eastern Tibetan Plateau, Yang et al. (2015) carried a
4-year field experiment to examine the evidence for abundance- and functionalbased mechanisms of plant species loss after fertilization. Moreover, they examined
whether the importance of both mechanisms would change with the presence and
absence of herbivores. Fertilization significantly reduced plant species richness by
an average of 7.3 species per 0.25 m
2 relative to the control. Such a decrease in
species richness was due to the increases of species loss but not due to the decline of
species gain. The likelihood of species loss following nutrient eutrophication was
negatively correlated with their mean abundance at the beginning of the experiment,
indicating the role of abundance-based mechanism for the decline of species richness
following fertilization. Meanwhile, they found substantial changes in both functional
traits of dominant species and community composition, in that the species with
higher specific leaf area and higher stature were favoured by fertilization. Such a
result indicates that the functional-based mechanism also plays a role in driving
species loss following N enrichment. More importantly, they found no evidence for
the role of functional-based mechanism with fertilization in the presence of herbivores. Together, their results demonstrated that both abundance- and functionalbased mechanisms drive species loss following nutrient enrichment with the absence
10 Impacts of Nitrogen Deposition on China’s Grassland Ecosystems
231