Comparative study of carbon and nitrogen characteristics in three common herbaceous plant species in southwestern Guizhou

Carbon (C) and nitrogen (N) are essential elements of plant tissues, with nitrogen playing a critical role in plant growth, development, and metabolism. However, comprehensive studies comparing the C and N characteristics across species and plant organs of herbaceous plants in southern China remain limited. In this study, based on a transect survey method, 12 sampling sites in southwestern Guizhou were selected for field sampling and subsequent laboratory analysis. The aim was to investigate the differences in C and N contents and C:N ratios among species and plant organs in Trifolium repens, Cynodon dactylon, and Dactylis glomerata, and to assess the relationships between C and N characteristics, climatic variables, and plant functional traits. Significant interspecific differences were observed in C and N contents and C:N ratios among the three species (P < 0.05). Specifically, the stem C content of C. dactylon was significantly higher than that of T. repens and D. glomerata. In contrast, the C:N ratios in the roots, stems, and leaves of C. dactylon were significantly lower than those of the other two species. Significant differences in C and N contents and C:N ratios were also observed among different plant organs (P < 0.001). C content exhibited lower coefficients of variation than N content across organs, and the C:N ratios of roots and stems were generally higher than those of leaves. Average annual temperature was significantly correlated with the C and N characteristics of T. repens. Canonical Correspondence Analysis (CCA) revealed that variation in C and N characteristics of T. repensand C. dactylon could be explained by average annual precipitation. Moreover, leaf length and leaf area of C. dactylon were significantly associated with the soil C:N ratio (P < 0.05). The effects of future climate change on the C:N ratios of these herbaceous species appear to be species-specific. These findings may support the informed management and sustainable utilization of grasslands in southwestern Guizhou under changing climatic conditions.