Short-term N and P additions differentially alter the multiple functional traits and trait associations of a desert ephemeral plant in China

Nitrogen (N) and phosphorus (P) are the main restrictive elements in terrestrial ecosystems. In a N-deficient desert, low exogenous N input is conducive to plant growth; however. However, the influence of P and N additions on the growth and physiology of desert plants remains controversial. In this study, Alyssum linifolium, an ephemeral plant widely distributed in the Gurbantunggut Desert in China, was administered with different amounts of N (CK, N1, N2, and N3: 0, 3, 6, and 18 g N m–2 yr–1, respectively), P (CK, P1, P2, and P3: 0, 0.6, 1.2, and 3.6 g N m–2 yr–1, respectively), and N + P (CK, N1P1, N2P2, and N3P3) to systematically analyze the effect of these additions on soil nutrients and plant multiple traits. Certain N–P interaction effects were observed on soil properties (15 out of 18) and plant traits (13 out of 33). N addition markedly improved soil N availability and plant N content and reduced plant P content; meanwhile, soil P availability remained steady. P addition enhanced soil P availability and plant P concentration but severely decreased soil N availability. N + P addition improved the effectiveness of soil N and P, thereby promoting plant N content but largely reducing plant P content. N and N + P additions remarkably enhanced the photosynthetic capacity, soluble protein content, allocation proportion of leaf biomass, and even individual growth (including plant height, root length, leaf length, and organ biomass) and chlorophyll content at low concentrations. Leaf N was negatively scaled to P under each nutrient treatment, and this trend was completely different from that in natural ecosystems. The effect of N and P under N + P addition differed from those under individual N and P additions. In addition, plant trait networks differed with nutrient treatment, and the network complexity and trait association were weakened markedly under P addition. A degree of modularity was observed in the plant trait network in response to N and P additions, and biomass accumulation was identified as the hub trait. Multivariate analysis showed that available soil N and N:P levels were the primary factors influencing the variation in plant traits. Structural equation modeling revealed that soil N availability regulated the allocation of photosynthetic products by affecting plant stoichiometry, enzyme activity, and photosynthetic capacity. In conclusion, P addition alone markedly reduced soil N availability and plant performance in N-deficient habitats, and low-concentration N or N + P addition was generally conducive to soil nutrient availability and plant growth.