2.1 Study site and sampling
The study was conducted at Haibei Alpine Meadow Ecosystem Research
Station, belonging to the Northwest Institute of Plateau Biology of the
Chinese Academy of Sciences. The site is situated within a large valley
surrounded by the Qilian Mountains, in the northeast of the Tibetan
Plateau (37°29’-37°45’ N, 101°12’-101°23’ E, altitude of 3200-3500 m
a.s.l.). The area has a typical plateau continental climate, with mean
annual temperatures and precipitation of –1.7 °C and 582 mm,
respectively. More than 80% of precipitation falls from May to
September during the plant growing season. The site has Mat Cry-gelic
Cambisols and silty clay loam soils, based on Inceptisols of the U.S.
soil taxonomic system.
The dominant population of alpine
grassland gradually changes from Gramineae-Kobresia humiliscommunity to Kobresia humilis and Kobresia pygmaeacommunities under long-term overgrazing. Furthermore, long-term
overgrazing increases root accumulation and decreases surface soil bulk
density and the available soil nutrients, resulting in soil nutrient
imbalance between demand and supply. Moreover, the thickening of the
mattic epipedon reduces the water infiltration capacity. It forms dark
spots and local death in Kobresia pygmaea community, which
eventually evolve into ”Black-soil-type” degraded land. Therefore, the
experimental sites with different grazing intensities were defined as
Gramineae-Kobresia humilis community (GK), Kobresia
humilis community (K), thickening-in-mattic epipedon of Kobresia
pygmaea community (KT), cracks-in-mattic epipedon of Kobresia
pygmaea community (KC), and Forb-black soil type grassland (FB) (Table
1). The sites were named after the succession of the plant community
after a long period of grazing lambs and sheep. The grazing intensities
of the GK, K, KT, KC sites were 3.65, 7.50, 11.25, and 13.00 sheep
ha-1, respectively. FB caused substantial degradation
and could not be used for additional grazing. The sites were used as
winter pastures with a grazing period from 20th September to 20th May.
Each site was approximately 30-40 ha, and was fenced. The alpine meadow
has experienced different grazing intensities since 1995. The
aboveground plant tissues were not removed or consumed by grazing in the
summer since the meadow was used as winter pasture.
The nutrient addition experiment was conducted in the
Gramineae-Kobresia humilis community in Haibei station to
determine the effect of nutrients on the grassland. The above experiment
was a completely randomized block group experimental design, with four
treatments: nitrogen addition (N), phosphorus addition (P), nitrogen and
phosphorus mixed addition (NP), and control (CK). The treatments were
randomly arranged in six plots (six repeats) (24 plots), measuring 6 m×6
m, with protection lines between all the plots. N fertilizer (100 kg/
(hm2·year)) and P fertilizer (50
kg/(hm2·year) were added in the form of urea, and
heavy superphosphate, respectively. NP was added as a mixture of N and P
fertilizers, and the control treatment had no nutrients. Fertilizing was
conducted in the middle of the planting season, rainy or sunny evenings.
Fertilizer was evenly distributed in the corresponding area by hand.
Fertilizer was applied twice in each district to ensure the uniformity
of fertilization. Nutrients were added from 2011 to 2015, on 1st June,
1st July, and 1st August of each year.
Five meadow sites under different grazing intensities and nutrient
addition experimental sites were selected on 23rd July 2015 to determine
GRSP and soil properties. Each site was divided into six plots. Soil was
collected from five randomized points in each plot at depths of 0-10 cm
and 10-20 cm. The soil samples of each soil depth in every plot were
mixed into one sample, yielding 32 (five succession stages) and 48
(nutrient addition) samples, then kept on ice before being stored at 4
ºC in the laboratory as quickly as possible. Fresh soil was passed
through a 2-mm mesh sieve after removing visible litter, roots, and
stones. The soil sample was thoroughly mixed and stored at 4 °C for the
determination of the biogeochemical properties.