purpurea and a sedge Carex rigescens. Other species with high importance value in
the community include Poa crymophila Keng, Agropyron cristatum, and
Heteropappus altaicus. The set of multilevel N addition gradient aims to make
sure such a gradient is adequate to simulate both N limitation and N saturation of
the ecosystem. Their experiment site was fenced to exclude livestock grazing. The
experiment followed a randomized complete block design with five replications
(blocks). Each treatment was applied to one plot (with an area of 6 Â 6 m for each) in
each of the five blocks. The distance between any two neighbour blocks was 2 m and
that between the plots within a block was 1 m. In each year, N fertilizer (NH 4 NO 3 )
was dissolved in water and then applied to each plot using a backpack sprayer at the
beginning of each month during May to September. The control plot received no N
fertilizer but the same amount of water as in other plots.
In the alpine grassland of Bayinbuluk in southern Tian Shan mountains of
Xinjiang Uygur Autonomous Region, China, Dr. Kai-Hui Li established a N addition experiment since 2009 (Li et al. 2015). The mean altitude of this area is 2500 m
a.s.l., the annual precipitation is 282 mm, and the mean annual temperature is
À4.8
C. The plant community is dominated by perennial grasses (Stipa purpurea,
Festuca ovina, Agropyron cristatum, and Koeleria cristata) and perennial forbs
(Oxytropis glabra, Potentilla multifida, and Potentilla bifurca). There were four
different levels of N addition, 0, 10, 30, and 90 kg N ha
À1 year
À1 . Each treatment
was located in four different blocks. Nitrogen fertilizer (NH 4 NO 3 ) was added twice
each year with a sprayer after being dissolved in 8 L water, in late May and June.
Given the complex composition of N compounds in the deposited N and the fact
that different N compounds are being used in different controlled experiments, we
are still not sure whether different N compounds would have the same impacts on
ecological processes and ecosystem functioning. To address this knowledge gap, a
large-scale experiment was established in a meadow steppe near the Erguna ForestSteppe Ecotone Research Station (50
2
0 N, 119
30
0 E, 523 m a.s.l.) since 2014. The
mean annual precipitation is 352 mm, with 66% falling during June to August.
Nitrogen forms, addition rates, and mowing are the three main factors in the
experimental design. There are five different kinds of N compounds, including
urea, slow-release urea, NH 4 HCO 3 , (NH 4 ) 2 SO 4 , and NH 4 NO 3 . There are six different addition levels for each fertilizer, including 0, 10, 20, 50, 100, 200, and
500 kg N ha
À1 year
À1 . There are two treatments of mowing (mown vs. unmown).
In total, there are 60 different treatments, with each treatment being randomly
replicated in each of the eight blocks. A total of 480 plots with the area of
10 Â 10 m
2 for each were established (Fig. 10.1).
10.3 Impacts on Soil Eutrophication and Nutrient Cycling
Soil N cycling in terrestrial ecosystems is characterized by a variety of N transformation processes and fluxes mediated by both plants and soil microorganisms.
Microbial and biological processes that drive soil nutrient cycling operate at different
220
X. Lü et al.
the community include Poa crymophila Keng, Agropyron cristatum, and
Heteropappus altaicus. The set of multilevel N addition gradient aims to make
sure such a gradient is adequate to simulate both N limitation and N saturation of
the ecosystem. Their experiment site was fenced to exclude livestock grazing. The
experiment followed a randomized complete block design with five replications
(blocks). Each treatment was applied to one plot (with an area of 6 Â 6 m for each) in
each of the five blocks. The distance between any two neighbour blocks was 2 m and
that between the plots within a block was 1 m. In each year, N fertilizer (NH 4 NO 3 )
was dissolved in water and then applied to each plot using a backpack sprayer at the
beginning of each month during May to September. The control plot received no N
fertilizer but the same amount of water as in other plots.
In the alpine grassland of Bayinbuluk in southern Tian Shan mountains of
Xinjiang Uygur Autonomous Region, China, Dr. Kai-Hui Li established a N addition experiment since 2009 (Li et al. 2015). The mean altitude of this area is 2500 m
a.s.l., the annual precipitation is 282 mm, and the mean annual temperature is
À4.8
C. The plant community is dominated by perennial grasses (Stipa purpurea,
Festuca ovina, Agropyron cristatum, and Koeleria cristata) and perennial forbs
(Oxytropis glabra, Potentilla multifida, and Potentilla bifurca). There were four
different levels of N addition, 0, 10, 30, and 90 kg N ha
À1 year
À1 . Each treatment
was located in four different blocks. Nitrogen fertilizer (NH 4 NO 3 ) was added twice
each year with a sprayer after being dissolved in 8 L water, in late May and June.
Given the complex composition of N compounds in the deposited N and the fact
that different N compounds are being used in different controlled experiments, we
are still not sure whether different N compounds would have the same impacts on
ecological processes and ecosystem functioning. To address this knowledge gap, a
large-scale experiment was established in a meadow steppe near the Erguna ForestSteppe Ecotone Research Station (50
2
0 N, 119
30
0 E, 523 m a.s.l.) since 2014. The
mean annual precipitation is 352 mm, with 66% falling during June to August.
Nitrogen forms, addition rates, and mowing are the three main factors in the
experimental design. There are five different kinds of N compounds, including
urea, slow-release urea, NH 4 HCO 3 , (NH 4 ) 2 SO 4 , and NH 4 NO 3 . There are six different addition levels for each fertilizer, including 0, 10, 20, 50, 100, 200, and
500 kg N ha
À1 year
À1 . There are two treatments of mowing (mown vs. unmown).
In total, there are 60 different treatments, with each treatment being randomly
replicated in each of the eight blocks. A total of 480 plots with the area of
10 Â 10 m
2 for each were established (Fig. 10.1).
10.3 Impacts on Soil Eutrophication and Nutrient Cycling
Soil N cycling in terrestrial ecosystems is characterized by a variety of N transformation processes and fluxes mediated by both plants and soil microorganisms.
Microbial and biological processes that drive soil nutrient cycling operate at different
220
X. Lü et al.