3.3 Soil enzymes characteristics
Soil enzymes decomposed complex
organic compounds into smaller organic compounds and inorganic nutrients
(Grandy et al., 2007). As the most active and sensitive components in
soil, soil enzymes played an important role in the nutrient cycle of
soil and the supply of nutrients needed for plant growth, and were an
important index for evaluating soil quality of different grassland types
(Verónica et al., 2007). Its activity and mainly affected by soil
physical and chemical properties, soil microorganisms and vegetation
litters (Cui et al., 2019). In ours research, with the increase of soil
depth, urease, alkaline phosphatase and sucrase enzyme activity
decreased and catalase enzyme activity increased. From TS, TSD to TD,
the alkaline phosphatase, catalase and sucrase enzyme activity were
decreased. Because the soil surface layer accumulates more litter and
humus than the depth soil layer, the organic matter content was high and
the nutrient source was sufficient, which was conducive to the growth
and reproduction of microorganisms. meanwhile water and heat in topsoil
better and ventilation conditions, plant hair root and litter turnover
rapid, which further accelerated the microbial growth and metabolism,
making the soil Urease, Alkaline phosphatase and Sucrase enzyme activity
decreased with the increase of soil depth, but the Catalase enzyme
activity increased.
3.4 The evolution
mechanism of grassland desertification succession
This research used space instead of time to analyze the evolution of
grassland vegetation and soil in the process of grassland
desertification, in the western Qilian Mountains- eastern Qinghai-Tibet
Plateau during the global warming process. Based on the steppe spatial
distribution pattern, the evolution from temperate steppe to temperate
desert, rainfall decreased and temperature raised, that was, climate
change was ”warming and drying” (Rong, 2019). The complex of altitude as
a regional hydrothermal factor also showed a decreasing trend (from
temperate steppe to temperate desert was from 2800m to 1500m) (Xu et
al., 2021). In the grassland ecosystem, above-ground vegetation was a
sensitive indicator of climate change (Zhou et al., 2020), and
prioritized the perception of climate change and adjusted its growth
characteristics to adapt to environmental changes. For example, in the
process of grassland desertification, grassland vegetation coverage,
biomass and diversity index decreases, and the height of the grass layer
increased (Fig.2), and plant species evolved from herbaceous plants to
lignified shrubs to reduce water evaporation and adapt to rising
temperatures, which was a survival strategy for plants (Zhou et al.,
2020). When vegetation changed, soil, as a substrate and nutrient donor
for plants, would also change, such as physical structure (increased
bulk density, decreased porosity), decreased chemical nutrients (carbon,
nitrogen, and phosphorus), and decreased soil enzyme activities (Fig2)
(Wang et al., 2018). Meanwhile, changes in soil physical structure,
nutrient content and enzyme activity will adversely affect plants and
affect their growth. The ”coordination” and ”antagonism” of grassland
plants and soil promote the flow of material and energy in the grassland
ecosystem to adapt to changes in the environment (He et al., 2021). The
results could provide theoretical
basis and data support for curbing grassland desertification and
inducing forward succession of desert grassland.