of herbivores, but abundance-based mechanisms are the dominant driver for the
reduction of species richness in the presence of herbivores. Their work extends past
related works on diversity loss due to nutrient enrichment by demonstrating that the
importance of functional- and abundance-based mechanisms of diversity loss would
be dependent on the presence or absence of herbivores. The results from Yang et al.
(2015) highlight the importance of ecosystem management strategies in mediating
the impacts of N enrichment on biodiversity changes in grassland ecosystems.
While all the above-mentioned studies focused on the effects of the rates of N
addition or deposition on biodiversity loss, it remains largely unknown whether
biodiversity loss would vary or not under different frequencies of N addition. To
answer this question, Zhang et al. (2014) independently manipulated the rate and the
frequency of N inputs and disentangled those two potentially contrasting effect in a
grassland of northern China (Fig. 10.6). They found that plant species richness was
negatively correlated with N addition rates. Such relationship became much stronger
with the duration of experiment. Plant species richness and their frequency in the
community were declined more under the treatment of lower frequency of N
addition in the whole experimental period. At plant functional group level, higher
frequency of N addition reduced the rate of species losses of grasses, annuals and
biennials, but had neutral effects on perennial forbs. Their results indicate that both
the rate and the frequency of N addition would affect plant species loss. At a
particular rate of N addition, plant species richness loss will be much greater and
more rapid under the treatment with lower frequency of N addition. Such difference
in the response of species richness loss could be explained by ammonium toxicity
due to less soil ammonia loss. The traditional studies using one time or a few times
addition of N might thus overestimate the effects N deposition on plant species loss.
Furthermore, they found that species loss following N enrichment was mainly
caused by the extinction of acid- and ammonium-sensitive species. Consequently,
ecosystem management strategies that could reduce soil acidification and ammonium toxicity, such as mowing or hay harvest, will help alleviate the negative effects
of N deposition on plant species richness in grasslands (Yang et al. 2019).
Metal toxicity would be an important abiotic pathway through which N deposition reduces biodiversity (Stevens et al. 2006), as N deposition can acidify the soil
and thus potentially release metal ions that are phytotoxic, including aluminium,
manganese, and iron. However, the role of metal toxicity in the commonly observed
shift in grass-forb ratios that parallel biodiversity loss had never been empirically
tested till the study of Tian et al. (2016). Based on a 9-year N addition experiment in
the temperate steppe, they examined the relationship between plant dominant status,
species richness, and plant Mn concentrations. They showed that plant community
changes from grass and forb co-dominated under ambient N conditions to monodominated by grass in the N-enriched treatments, paralleled with substantial decline
in species richness. The reduction of abundance of forb species was correlated with
soil acidification that enhanced the concentrations of soil extractable Mn. Foliar Mn
concentrations were ten times higher in forbs than in grasses, which caused reduction
of the photosynthetic rates and growth of forbs. By linking soil nutrient processes,
232
X. Lü et al.
reduction of species richness in the presence of herbivores. Their work extends past
related works on diversity loss due to nutrient enrichment by demonstrating that the
importance of functional- and abundance-based mechanisms of diversity loss would
be dependent on the presence or absence of herbivores. The results from Yang et al.
(2015) highlight the importance of ecosystem management strategies in mediating
the impacts of N enrichment on biodiversity changes in grassland ecosystems.
While all the above-mentioned studies focused on the effects of the rates of N
addition or deposition on biodiversity loss, it remains largely unknown whether
biodiversity loss would vary or not under different frequencies of N addition. To
answer this question, Zhang et al. (2014) independently manipulated the rate and the
frequency of N inputs and disentangled those two potentially contrasting effect in a
grassland of northern China (Fig. 10.6). They found that plant species richness was
negatively correlated with N addition rates. Such relationship became much stronger
with the duration of experiment. Plant species richness and their frequency in the
community were declined more under the treatment of lower frequency of N
addition in the whole experimental period. At plant functional group level, higher
frequency of N addition reduced the rate of species losses of grasses, annuals and
biennials, but had neutral effects on perennial forbs. Their results indicate that both
the rate and the frequency of N addition would affect plant species loss. At a
particular rate of N addition, plant species richness loss will be much greater and
more rapid under the treatment with lower frequency of N addition. Such difference
in the response of species richness loss could be explained by ammonium toxicity
due to less soil ammonia loss. The traditional studies using one time or a few times
addition of N might thus overestimate the effects N deposition on plant species loss.
Furthermore, they found that species loss following N enrichment was mainly
caused by the extinction of acid- and ammonium-sensitive species. Consequently,
ecosystem management strategies that could reduce soil acidification and ammonium toxicity, such as mowing or hay harvest, will help alleviate the negative effects
of N deposition on plant species richness in grasslands (Yang et al. 2019).
Metal toxicity would be an important abiotic pathway through which N deposition reduces biodiversity (Stevens et al. 2006), as N deposition can acidify the soil
and thus potentially release metal ions that are phytotoxic, including aluminium,
manganese, and iron. However, the role of metal toxicity in the commonly observed
shift in grass-forb ratios that parallel biodiversity loss had never been empirically
tested till the study of Tian et al. (2016). Based on a 9-year N addition experiment in
the temperate steppe, they examined the relationship between plant dominant status,
species richness, and plant Mn concentrations. They showed that plant community
changes from grass and forb co-dominated under ambient N conditions to monodominated by grass in the N-enriched treatments, paralleled with substantial decline
in species richness. The reduction of abundance of forb species was correlated with
soil acidification that enhanced the concentrations of soil extractable Mn. Foliar Mn
concentrations were ten times higher in forbs than in grasses, which caused reduction
of the photosynthetic rates and growth of forbs. By linking soil nutrient processes,
232
X. Lü et al.