Background Desert steppe is an important natural barrier in Ningxia, but its soil is barren, vegetation composition is simple, and most of them are in nitrogen restricted state, thus its response to various environmental disturbance factors is obvious. In recent years, abundant nitrogen has entered grassland ecosystems through nitrogen settlement. Therefore, the method of simulating nitrogen settlement is adopted to study the effect of nitrogen addition on desert steppe vegetation characteristics, soil nutrients and enzyme activity, which may provide a theoretical basis for the dynamic changes of desert steppe ecosystem structure under nitrogen settlement.

Methods Taking grassland as the research object in desert steppe of Ningxia, urea as nitrogen source, setting four pure nitrogen added treatment: CK (0), LN (5 g/(m²·a)), MN (10 g/(m²·a)) and HN (15 g/(m²·a)). The short-term effects of nitrogen addition on vegetation and soil characteristics in the desert steppe were studied from three aspects: Vegetation characteristics, soil nutrients and enzyme activities. In August 2020, soil samples of 0-20 cm were collected to determine the physical and chemical properties of the soil, and vegetation survey was carried out.

Results 1) With the increase of nitrogen addition, the vegetation species composition of the desert steppe changed, important values for grasses and legumes increased and cruciferous plants decreased. The coverage and aboveground biomass increased first and then decreased. The grassland coverage of MN treatment was significantly higher than that of CK treatment (P<0.05). The aboveground biomass of LN and MN treatments was significantly higher than that of CK and HN treatments (P<0.05). 2) With the increase of nitrogen addition, the height and density of herbaceous plants in the desert steppe gradually increased, Simpson index and Pielou index decreased first and then increased, Simpson index in LN treatment was significantly lower than that in CK, MN and HN treatment(P<0.05). Pielou index of MN treatment was significantly lower than that in CK treatment (P<0.05). 3) Nitrogen addition had no significant effect on soil total nutrients(P>0.05), but significantly reduced SAK in desert steppe(P<0.05). Compared with CK, SAK (soil available potassium) in LN, MN and HN treatments decreased 30.82%, 31.35% and 32.13%, respectively. 4) Redundancy analysis showed that pH value, urease and catalase were the main factors affecting vegetation characteristics in the desert steppe.

Conclusions LN and MN treatment could increase aboveground biomass in the desert steppe in the short term. Their soil moisture content was higher, nutrient content and enzyme activity increased to varying degrees. Under HN treatment, soil nutrient and water content in the desert steppe were generally lower, and above ground biomass of vegetation decreased, but the growth of dominant species was promoted. The results showed that low and medium nitrogen addition levels were beneficial to vegetation restoration and soil environment improvement in the desert steppe, but high nitrogen addition levels weakened the favorable effects.