Abstract:

The growth and survival rates of plants are greatly affected by rainfall in desert steppe.This research investigated the soil water fluctuations after rainfall events for four consecutive years from 2017 to 2020 in an Artemisia scoparia population in an Eastern Ningxia desert steppe. We determined the effects of several simulated rainfall patterns （decreased by 30%， unchanged， increased by 30%） on the photosynthetic characteristics and water use efficiency of A. scoparia. The photosynthetic characteristics of A. scoparia including net photosynthetic rate， transpiration rate， stomatal conductance and intercellular CO₂ concentration were measured， and the intrinsic water use efficiency and instantaneous water use efficiency at leaf level were analyzed. It was found that trivial rainfall events （<5 mm） occurred frequently， accounting for 60.00%-82.54% of the rainfall frequency from 2017 to 2020. Heavy rain events （≥25 mm and <50.0 mm） and rainstorm events （≥50 mm） were few， and the average rainfall in a single rainfall event over the four years was only 3.3-6.5 mm. There were large differences in soil water content in different soil horizons under different treatments， and the variation of soil water content in the surface horizon （0-10 cm） was larger than that in the subsurface horizon （10-20 cm） and deep layer （20-60 cm）. The average soil water content in the subsurface layer （20 cm） was lower than that in the surface layer （10 cm）， and this reflected the high frequency of small rainfall events， the inability of water to infiltrate and penetrate， and the absorption and consumption of water by plants. The water content of the deep soil horizon changed little over time. The net photosynthetic rate increased with increase in photosynthetically active radiation. At photosynthetically active radiation levels of 400-1600 μmol·m^-2·s^-1， increase in soil water had a great influence on the net photosynthetic rate of A. scoparia. At photosynthetically active radiation levels higher than 1600 μmol·m^-2·s^-1， photosynthesis tended to be suppressed by increase in soil water and the rainfall treatment exhibited the largest decline. At photosynthetically active radiation levels above 400 μmol·m^-2·s^-1， the intercellular CO₂ concentration decreased linearly with increase in radiation， and at 1600 μmol·m^-2·s^-1， the intercellular CO₂ concentration reached its minimum. Transpiration rate and stomatal conductance did not change significantly with increase in light intensity， but the transpiration rate and stomatal conductance of A. scoparia under the rainfall enhancement treatment were higher than those under other treatments. Stomatal limitation increased with increase in photosynthetically active radiation， and decreased with increase in rainfall. Intrinsic water use efficiency and instantaneous water use efficiency of A. scoparia increased rapidly with increase in light intensity at light levels less than 400 μmol·m^-2·s^-1， but at higher light levels the growth rate slowed down. The intrinsic water use efficiency and instantaneous water use efficiency of A. scoparia reached their maxima at photosynthetically active radiation levels of 1600-2000 μmol·m^-2·s^-1. With increase in drought severity， the light intensity at which the maximum water use efficiency was observed decreased from 2000 to 1600 μmol·m^-2·s^-1， and when the light intensity was above 400 μmol·m^-2·s^-1， moderate drought improved the water use efficiency of A. scoparia.

Key words: desert steppe, Artemisia scoparia, changes in rainfall patterns, photosynthesis, water use efficiency