Effects of simulated precipitation changes and nitrogen deposition on biomass allocation patterns of Reaumuria soongarica and Salsola passerina seedlings under different growth modes

Water and nitrogen are important resources for plant biomass accumulation, as well as the main factors that regulate biomass allocation in different plants and their growth patterns under adverse conditions. In this study, the seedlings of two dominant desert plants, Reaumuria soongarica and Salsola passerina, were used as test materials to study the synergistic effect of precipitation –30% (low water), 0 (medium water), +30% (high water) and nitrogen 0 (N₀, no nitrogen, 4 g·(m²·a)⁻¹ (N₁, low nitrogen), and 8 g·(m²·a)⁻¹ (N₂, high nitrogen) on biomass distribution and related growth relationships under different growth modes (R. soongarica grown separately, S. passerina grown separately, and R. soongarica and S. passerina grown together). The results showed that: 1) R. soongarica and S. passerina seedlings were significantly affected by water and nitrogen (P < 0.01). With separate R. soongarica, the root biomass increased under low nitrogen and high water conditions and the biomass of stems and leaves increased under high nitrogen and medium water, while the distribution of the biomass between organs was root > stem > leaf. The root, stem, and leaf biomass of mixed R. soongarica increased as the conditions went from no nitrogen and low water, to no nitrogen and medium water, and then to low nitrogen and high water, and the biomass distribution was leaf > stem > root. The organ biomass of both separate and mixed S. passerina increased under low nitrogen and high water, and its distribution remained as leaf > stem > root. 2) The leaf-root weight ratio, leaf weight ratio, and source to sink weight ratio of R. soongarica under the mixed mode were higher than those under the separate mode, whereas, the root-shoot ratios under the mixed mode were reduced compared with the separate mode by 52.63% (R. soongarica) and 37.45% (S. passerina), indicating that both species could allocate more biomass to the above-ground parts when grown combinedly. In the future, under changes in precipitation patterns and nitrogen deposition, mixed communities of R. soongarica and S. passerina may be more conducive to adapting to global climate changes than those grown separately, which will lead to the mixed state having an advantage in the desert ecosystems.