Abstract
Abstract. Chinese grasslands are extensive natural ecosystems that comprise 40 % of the total land area of the country and are sensitive to N deposition. A field experiment with six N rates (0, 30, 60, 120, 240, and 480 kg N ha−1 yr−1) was conducted at Duolun, Inner Mongolia, during 2005 and 2010 to identify some effects of N addition on a temperate steppe ecosystem. The dominant plant species in the plots were divided into two categories, grasses and forbs, on the basis of species life forms. Enhanced N deposition, even as little as 30 kg N ha−1 yr−1 above ambient N deposition (16 kg N ha−1 yr−1), led to a decline in species richness. The cover of grasses increased with N addition rate but their species richness showed a weak change across N treatments. Both species richness and cover of forbs declined strongly with increasing N deposition as shown by linear regression analysis (p < 0.05). Increasing N deposition elevated aboveground production of grasses but lowered aboveground biomass of forbs. Plant N concentration, plant δ15N and soil mineral N increased with N addition, showing positive relationships between plant δ15N and N concentration, soil mineral N and/or applied N rate. The cessation of N application in the 480 kg N ha−1 yr−1 treatment in 2009 and 2010 led to a slight recovery of the forb species richness relative to total cover and aboveground biomass, coinciding with reduced plant N concentration and soil mineral N. The results show N deposition-induced changes in soil N transformations and plant N assimilation that are closely related to changes in species composition and biomass accumulation in this temperate steppe ecosystem.
Highlights
Atmospheric nitrogen (N) deposition has become a global concern because of its potential influence on ecosystem productivity, biodiversity and function, especially in many N deposition “hotspots” worldwide (Phoenix et al, 2006)
Species richness is here expressed as the average number of plant species in the 5 replicates of each treatment determined using a 1.0 m2 quadrat (Fig. 2)
There was a negative relationship between N deposition and species richness in general, especially for forbs which decreased significantly with N addition rate and over the years, but there was no obvious influence on grasses whose species richness ranged from 3 to 4 and did not show any change over time
Summary
Atmospheric nitrogen (N) deposition has become a global concern because of its potential influence on ecosystem productivity, biodiversity and function, especially in many N deposition “hotspots” worldwide (Phoenix et al, 2006). Many terrestrial ecosystems worldwide are facing an N-enriched environment which may lead to changes in plant production and species richness induced by acidification and eutrophication (Vitousek et al, 1997). Nitrogen deposition can stimulate plant growth and enhance the soil N pool (Storm and Suss, 2008) and long-term continuous N inputs may lead to loss of species diversity and changes in species composition (DeFalco et al, 2003). Compared with NO3-N addition, Goulding et al (1998) reported a much larger decrease in species richness of grasses in plots receiving NH4-N applications in the Park Grass long-term field experiment, suggesting a larger impact on species richness by acidification than by eutrophication. Compared with NO3-N addition, Goulding et al (1998) reported a much larger decrease in species richness of grasses in plots receiving NH4-N applications in the Park Grass long-term field experiment, suggesting a larger impact on species richness by acidification than by eutrophication. Stevens et al (2004) showed that grass species richness in acidic soils decreased linearly with increasing N deposition (ranging from 5 to 35 kg N ha−1 yr−1) in the UK and each additional increment in long-term N deposition of approximately 2.5 kg N ha−1 yr−1 is associated with a reduction in species richness of one species. Clark and Tilman (2008) further found that long-term low level N addition (10 kg N ha−1 yr−1) has reduced plant species numbers by 17 % relative to controls receiving ambient N deposition
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