Leaf N/P ratio and nutrient reuse between dominant species and stands: predicting phosphorus deficiencies in Karst ecosystems, southwestern China

Variation of vegetation coverage and canopy height may reflect the complex spatial heterogeneity of nutrient storage and supply capacity, soil moisture, and surface hydrology in the karst terrains suffering from severe land degradation. To assess the patterns of nutrient limitation under different vegetation covers in the subtropical karst ecosystems from Guizhou province, southwestern China, topsoil and leaf samples of dominant tree species were collected in forest stand (FO), shrub stand (SH) and shrub-grass stand (SG), respectively. Nutrient concentrations of both soil and leaf were determined, and ratios of N to P and vegetation nutrient reuse capacity (VNR) calculated as well as vegetation coverage, vegetation canopy height and tree density measured across the three stands. Mean leaf N/P ratio was lowest (16.1 ± 1.4) in FO and highest (33.5 ± 3.2) in SG. Vegetation nutrient reuse increased with the decline in N and P availability in soils for these three stands. VNR of N and P ranged from 8.5 to 25.2 mg N g⁻¹ and from 0.4 to 1.1 mg P g⁻¹, respectively, and appeared lowest in SG (10.4 mg N g⁻¹ and 0.5 P mg g⁻¹ on average, respectively) and highest in FO (22.4 mg N g⁻¹ and 0.9 mg P g⁻¹ on average, respectively). Although there was no substantial difference in phosphorus reuse efficiencies between plant species and vegetation stands, concentrations of N and P of senesced leaves (SLs) were, respectively, found in positive correlation with the concentrations of mature leaves. The variation of VNR with elements indicated that P is cycled within vegetation much more efficiently than N across the stands. This study demonstrated that the karst vegetations were generally at P-limited or N- and P- co-limited stresses and that N/P ratio could be an effective indictor for nutrient limitation in the karst ecosystems at vegetation community level rather than at tree species level. It is proposed that phosphorus reuse by mature leaves could be an adaptation strategy by the dominant species to the low P availability in the karst soil.