Plant photosynthesis has a non-negligible influence on forage quality and ecosystem carbon sequestration. However, the influence of long-term warming, increasing precipitation, and their interactions on the photosynthesis of dominant species in desert steppe remains unclear, and the main factors regulating plant photosynthesis in desert steppes have remained unrevealed. Therefore, we measured the photosynthetic parameters and specific leaf area of the dominant species and calculated the water and nitrogen content of leaves and soil in a desert steppe after long-term warming and increasing precipitation (air temperature, W0, air temperature increases of 2 °C and 4 °C, W1 and W2; natural precipitation, P0, natural precipitation increases of 25% and 50%, P1 and P2). Results showed that warming and increasing precipitation significantly enhanced photosynthesis in C₃ and C₄ species (p < 0.05). Compared to W0P0, the net photosynthetic rate of C₃ and C₄ species in W2P2 increased by 159.46% and 178.88%, respectively. Redundancy analysis showed that soil water content significantly explained the photosynthesis of C₃ and C₄ plants (the degree of explanation was 48% and 67.7%), followed by soil-available nitrogen content (the degree of explanation was 19.6% and 5.3%). Therefore, our study found that climate change enhanced photosynthesis in C₃ and C₄ plants, and soil water content plays a critical role in regulating photosynthesis in desert steppes.

Differences in leaf structure determine the intense competition for resources between C3 and C4 species under climate warming. Competitive advantage is not only reflected in photosynthetic rate, but also in the water and nitrogen content of the leaves. However, the competitive mechanisms between C3 and C4 species in temperate desert steppe under long-term warming remain unclear. Therefore, after six to eight years of warming (ambient temperature, the control treatment, warming 2 ℃ and 4 ℃, T1 and T2 treatments), we estimated the importance values of C3 and C4 species in the plant community, and monitored the photosynthetic rate, nitrogen and water content of plants and soil. Results showed that long-term warming increased the importance values of C4 species in temperate desert steppe. Compared to the control treatments, the importance value of C4 species increased by 29.30% and 26.97% in T1 and T2 treatments, respectively. Variation partitioning analysis and structural equation model showed that leaf photosynthetic rate had a non-significant effect on the distribution of C3 and C4 species. The proportion of C3 and C4 species in plant communities was mainly influenced by soil water and nitrogen content. In summary, this study suggests that the competitive advantage of C3 species will be replaced by C4 species under long-term warming. The nitrogen and water content in soils explain the distributional of C3 and C4 species in temperate desert steppe.

Soil heterotrophic respiration ( R _h ) and its temperature sensitivity ( Q ₁₀ ) are important variables reflecting the intensity of feedback between the grassland carbon cycle and climate change. However, the factors regulating R _h and Q ₁₀ in desert grasslands under climate change remain unclear, and the asymmetric response of R _h during warming and cooling remains unquantified. We collected and measured the physicochemical properties of soils after 8 years of elevated temperature and precipitation (ambient temperature, T0, +2°C, T1, +4°C, T2, ambient precipitation, P0, +25%, P1, and +50%, P2). To quantify the asymmetric response of R _h , we continuously monitored R _h during warming and cooling at six incubation temperature levels in the laboratory (5°C, 10°C, 15°C, 20°C, 25°C, and 30°C). Results indicated that elevated temperature and precipitation significantly affected soil moisture, soil organic carbon, R _h , and Q ₁₀ . Compared with the T0P0 treatment, the T2P2 treatment enhanced R _h and Q ₁₀ by 11.72% and 1.56%, respectively. In addition, R _h was significantly affected by periodic changes in incubation temperature during both warming (5°C–30°C) and cooling (30°C–5°C), and R _h was 15.05% higher under warming than cooling. Variation partitioning analysis showed that R _h was primarily governed by soil carbon content, whereas Q ₁₀ was largely driven by soil water content. In summary, soil carbon and water content are limiting factors for R _h and Q ₁₀ in the desert steppe, and periodic temperature changes should be taken into account when quantifying R _h and Q ₁₀ .