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Aims The scaling relationship between nitrogen (N) and phosphorus (P) concentrations ([N] and [P], respectively) in leaves manifests plants’ relative investment between the two nutrients. However, the variation in this relationship among taxa as well as its causes was seldom described. Methods The analysis was based on a dataset including 2483 leaf samples from 46 genera of angiosperm woody plants from 1733 sites across China. We calculated the leaf N–P scaling exponent (βL) with an allometric equation ([N] = α[P]β), for each genus, respectively. We then performed phylogenetic path analyses to test how the climate and soil niche conditions of these genera contributed to the inter-genus variation in βL. Important Findings The genera living with lower soil P availability presented a more favoured P uptake relative to N, as shown by the higher βL, suggesting a resistant trend to P limitation. Additionally, genus-wise βL was positively correlated with soil N–P scaling exponents (βS), implying that the variation in leaf nutrients is constrained by the variability in their sources from soil. Finally, climatic factors including temperature and moisture did not affect βL directly, but could have an indirect influence by mediating soil nutrients. Phylogeny did not affect the inter-genus variation in βL along environmental gradients. These results reveal that the trade-off between N and P uptake is remarkably shaped by genus niches, especially soil nutrient conditions, suggesting that the βL could be considered as a functional trait reflecting characteristics of nutrient utilization of plant taxa in response to niche differentiation.

Carbon (C), nitrogen (N) and phosphorus (P) in soil are characterized by decreasing patterns with soil depth. However, these patterns and their driving factors have rarely been investigated in shrublands. We conducted extensive sampling of the top 100 cm of soil in 1120 shrublands across China to measure the soil organic C (SOC), total N (STN) and total P (STP) concentrations and densities. We found that in shrublands, the geometric means (and geometric standard errors) of SOC, STN and STP concentrations were 5.62 (0.09), 0.66 (0.07), and 0.31 (0.07) mg g−1, respectively, and those of their densities were 5.46 (0.08), 0.67 (0.08), and 0.30 (0.08) kg m−3, respectively. The decrease along soil depth for nutrients could be parameterized by a power function. The rates of decrease with depth differed between shrubland types and were negatively correlated with temperature but positively correlated with biomass. Climatic factors tended to have a relatively stronger effect than vegetation factors on the vertical distribution patterns of soil nutrients. Our findings reveal nutrient limitations in shrublands in terms of the total pools, suggest the necessity of soil protection for vegetation conservation and restoration, and provide an important supplement for the accurate prediction of terrestrial element cycles.

Root exudation stimulates microbial decomposition and enhances nutrient availability to plants. It remains difficult to measure and predict this carbon flux in natural conditions, especially for mature woody plants. Based on a known conceptual framework of root functional traits coordination, we proposed that root functional traits may predict root exudation. We measured root exudation and other seven root morphological/chemical/physiological traits for 18 coexisting woody species in a deciduous-evergreen mixed forest in subtropical China. Root exudation, respiration, diameter and nitrogen (N) concentration all exhibited significant phylogenetic signals. We found that root exudation positively correlated with competitive traits (root respiration, N concentration) and negatively with a conservative trait (root tissue density). Furthermore, these relationships were independent of phylogenetic signals. A principal component analysis showed that root exudation and morphological traits loaded on two perpendicular axes. Root exudation is a competitive trait in a multidimensional fine-root functional coordination. The metabolic dimension on which root exudation loaded was relatively independent of the morphological dimension, indicating that increasing nutrient availability by root exudation might be a complementary strategy for plant nutrient acquisition. The positive relationship between root exudation and root respiration and N concentration is a promising approach for the future prediction of root exudation.

Frontiers in Plant Science, 8, ISSN:1664-462X