Site description and experimental design
The
study area is a typical alpine
steppe, which is situated in the northeastern Qinghai - Tibetan Plateau
(QTP), China (37°18′N, 100°15′E). The study site exhibits a plateau
continental climate. The average annual temperature is
~0.1°C. The mean annual precipitation is
~390 mm, most of which occurs from June to August. The
vegetation is mainly dominated by grasses, such as Stipa purpureaGrisebach, Leymus secalinus (Georgi) Tzvel, and Poa
crymophila Keng (Xiao et al., 2020). The growing season is from May to
October, and peaks in August. At the beginning of the experiments, the
chemical properties of the soil (0–30 cm depth) were as follows: total
N, 2.5 g kg−1;
NO3−-N, 11.5
mg kg−1;
NH4+-N, 5.1 mg kg−1.
The topsoil was mainly composed of chestnut soil with a pH of 8.3.
The experimental platform was established in 2013 (Figure 1). The
experiments consisted of six different treatments
(NP: ambient nitrogen with ambient
precipitation; NP−: ambient nitrogen with
50% reduced precipitation; NP+:
ambient nitrogen with 50% increased
precipitation; N+P: nitrogen addition
with ambient precipitation;
N+P−: nitrogen addition with 50%
reduced precipitation; N+P+:
nitrogen addition with 50% increased precipitation). These 30 plots
(2.7 m × 3.3 m each) were randomly established in a 5×6 block design and
were each separated by a buffer zone (2 m wide).
Concave sunlight-pervious polyvinyl
chloride (PVC) boards without slots were placed above the 50% reduced
precipitation treatments to intercept rainfall. The collected water
(50% ambient precipitation) was immediately transferred to the 50%
increased precipitation treatments after the rain (sprinkling evenly).
To avoid light differences between the treatment plots, the same PVC
boards with slots were also installed on the ambient precipitation and
50% precipitation increment treatment plots.
N
fertilizer
(NH4NO3:
10 g m−2yr−1) was dissolved in 1 L water and was evenly
applied two times to the N supply plots (June and July every year).
Identical amounts of water without N fertilizer were sprayed evenly on
the ambient treatment plots. A previous study indicated that the N
saturation level was 8 g m−2 yr−1 in
this study area (Peng et al., 2017). Therefore, the current N input
level (10 g m−2 yr−1) should be
sufficient to simulate N saturation of the alpine grasslands.