Global leaf nitrogen and phosphorus stoichiometry and their scaling exponent

: Leaf nitrogen (N) and phosphorus (P) concentrations constrain photosynthetic and metabolic processes, growth and the productivity of plants. Their stoichiometry and scaling relationships regulate the allocation of N and P from subcellular to organism, and even ecosystem levels, and are crucial to the modelling of plant growth and nutrient cycles in terrestrial ecosystems. Prior work has revealed a general biogeographic pattern of leaf N and P stoichiometric relationships and shown that leaf N scales roughly as two-thirds the power of P. However, determining whether and how leaf N and P stoichiometries, especially their scaling exponents, change with functional groups and environmental conditions requires further verification. In this study, we compiled a global data set and documented the global leaf N and P concentrations and the N:P ratios by functional group, climate zone and continent. The global overall mean leaf N and P concentrations were 18.9 mg g−1 and 1.2 mg g−1, respectively, with significantly higher concentrations in herbaceous than woody plants (21.72 mg g−1 vs. 18.22 mg g−1 for N; and 1.64 mg g−1 vs. 1.10 mg g−1 for P). Both leaf N and P showed higher concentrations at high latitudes than low latitudes. Among six continents, Europe had the highest N and P concentrations (20.79 and 1.54 mg g−1) and Oceania had the smallest values (10.01 and 0.46 mg g−1). These numerical values may be used as a basis for the comparison of other individual studies. Further, we found that the scaling exponent varied significantly across different functional groups, latitudinal zones, ecoregions and sites. The exponents of herbaceous ABSTRACT Leaf nitrogen (N) and phosphorus (P) concentration constrain photosynthetic and metabolic processes, growth, and productivity of plants. Their stoichiometry and scaling relationship regulate allocation of N and P from subcellular to organism even ecosystem levels, and are crucial to modelling plant growth and nutrient cycles in terrestrial ecosystems. Prior work has 35 revealed a general biogeographic pattern of leaf N and P stoichiometric relationships and 36 shown that leaf N scales roughly as 2/3 power of P. However, determining whether and how 37 leaf N and P stoichiometry, especially their scaling exponent, change with functional groups 38 and environmental conditions requires further verification. In this study we compiled a global 39 data set and documented the global leaf N and P concentrations and the N:P ratios by 40 functional type, climate zone, and continent. The global overall mean leaf N and P 41 concentrations were 18.9 mg g -1 and 1.2 mg g -1 , respectively, with significantly higher 42 concentrations in herbaceous than woody plants (21.72 mg g -1 vs. 18.22 mg g -1 for N; and 43 1.64 mg g -1 vs. 1.10 mg g -1 for P). Both leaf N and P showed higher concentrations at high 44 than low latitudes. Among six continents, Europe had the highest N and P concentrations 45 (20.79 and 1.54 mg g -1 ) and Oceania had the smallest values (10.01 and 0.46 mg g -1 ). These 46 numerical values may be used as a base for the comparison of other individual studies. 47 Further, we found that the scaling exponent varied significantly across different functional 48 groups, latitudinal zones, ecoregions, and sites. The exponents of herbaceous and woody 49 plants were 0.659 and 0.705, respectively, with significant latitudinal patterns decreasing from 50 tropical to temperate to boreal zones. At sites with a sample size ≥ 10, the values fluctuated 51 from 0.366 to 1.928, with an average of 0.841. Several factors including the intrinsic 52 attributes of different life-forms, P-related growth rates and relative nutrient availability of 53 soils likely account for the inconstant exponents of leaf N vs. P scaling relationships.

we explore the patterns of scaling exponent numerical variation across different sites to test 132 whether the "global" N vs. P scaling relationship obscures site-related significant differences. 133 We hypothesize that the scaling exponent at each site should reflect site-specific N vs. P  In order to carry out the aforementioned studies, we compiled a large and geographically 141 comprehensive global dataset of pairwise leaf N and P concentration distributions, including 142 global, regional, and site-level records for as many of the variables of interest as possible. We 143 adopted only those records reporting paired N and P concentrations of green leaves with 144 detailed location information, and excluded all records without site information or with 145 unpaired N-P records. Using a detailed review of the literature, our own field sampling, and 146 the open TRY data set (Table S1;     We used the above-described global dataset of 12,055 pairwise leaf N and P concentration 183 records to characterize large-scale leaf N and P stoichiometry by functional group, latitudinal 184 zone, eco-region, and site. The geometric mean values of leaf N and P concentrations and N:P 185 mass ratios of the pooled data were 18.9 mg g -1 and 1.2 mg g -1 , and 15.8, respectively, but 186 these numerical values differed significantly among the contrasting functional groups (Table   187 1). Compared to woody plants, herbaceous plants showed significantly higher N and P 188 concentrations (21.72 mg N g -1 vs. 18.22 mg N g -1 ; 1.64 mg P g -1 vs. 1.10 mg P g -1 ), and  The leaf N and P stoichiometry also changed significantly with latitude. For the pooled 194 data, the leaf N and P concentrations significantly increased from the tropical to boreal 195 regions, but N:P ratios decreased (Table 2). Specifically, the geometric mean value of leaf N 196 concentration was 17.41 mg g -1 in the tropical region, 19.24 mg g -1 in the temperate region, 197 and 19.83 mg g -1 in the boreal region. The geometric mean value of leaf P concentration was i.e. leaf N and P concentrations increased, but leaf N:P ratios decreased from tropical, to 203 temperate, to boreal regions (Table 2). 204 Furthermore, there are remarkably differences in leaf N-P stoichiometry across different 205 ecoregions (six continents) ( Table 3). The geometric mean values of leaf N concentrations 206 were 18.48 mg g -1 , 20.79 mg g -1 , 19.33 mg g -1 , 10.01 mg g -1 , 10.32 mg g -1 , and 18.51 mg g -1

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At different study sites, the leaf N-P stoichiometry also showed statistically significant 212 differences. The site-level geometric mean values of leaf N and P concentrations and N:P 213 mass ratios exhibited large variations among 142 sites with records of n ≥ 10, ranging 214 between 4.6 and 30.5 mg g -1 for N, 0.16 and 2.83 mg g -1 for P, and 6.3 and 35.4 for N:P, with 215 respective geometric mean of 17.8 mg g -1 , 1.1 mg g -1 , and 15.8 (Fig. 1).

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In short, in this section we documented the numerical values of the global leaf N and P 217 stoichiometry by functional group, latitudinal zone, eco-region, and local site, which reveals a 218 large variation in the leaf N and P concentrations and N:P ratios biologically and ecologically.  In this section, we examine whether the leaf N-P scaling exponent varies among plant 227 functional groups, latitudes, ecoregions, and local sites. As shown in Figure 2 and Table 1 Table 1).

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The leaf N-P scaling exponents also showed significant latitudinal differences. For the 235 pooled data, the scaling exponents decreased from 0.747 in the tropical region, to 0.715 in the 236 temperate region, and to 0.603 in the boreal region (Table 2 and Table 3).

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The data of species growing in different sites manifested statistically significant   The data presented here also shed some light as to why variations in N and P scaling 276 relationships exist, although a detailed study of causalities is out of the scope of this paper.

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For example, the scaling exponents governing the N vs. P relationship are negatively 278 correlated with leaf P concentration and positively correlated with N:P ratios but not 279 significantly related to leaf N concentration (Fig. 5). We speculate that leaf P concentration 280 play a pivotal role in "shaping" the numerical values of the N vs. P scaling exponent, i.e., 281 metabolic requirements for P mainly account for scaling differences across different  (Table 1).

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Species composition, P-related growth rates, and soil relative nutrient availabilities  (Fig. 6a). In accordance with the 315 aforementioned differences in the N-P scaling exponent across functional groups (Table 1) Table 1. Summary of reduced major axis (RMA) regression results between leaf N and leaf P 565 concentrations, e.g., log10 leaf N = α log10 leaf P + log10 β , the statistics of leaf N and P 566 concentrations, and the N:P ratios in terrestrial plants for different functional groups. Note: 567 woody plants were divided into three groups: coniferous gymnosperms, and two angiosperms 568 groups, deciduous broad-leaved and evergreen broad-leaved woody. Mean indicates the 569 geometric mean, and n is the number of observations. Each regression relationships were 570 statistically significant with p<0.05. Different letters denote significant difference (p<0.05) 571 among latitude zones based on a likelihood-ratio test.            Figure S1. Relationships between leaf N and leaf P concentrations in terrestrial plants  Figure S3. Relationship between the leaf N and P scaling exponents and the total P 751 density in the top 50cm soil at different scales of globe, latitude range, and continent.

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Estimates of soil total P density were extracted from the Global Gridded Soil