title: Nitrogen:phosphorous supply ratio and allometry in five alpine plant species.
abstract: In terrestrial ecosystems, atmospheric nitrogen (N) deposition has greatly increased N availability relative to other elements, particularly phosphorus (P). Alterations in the availability of N relative to P can affect plant growth rate and functional traits, as well as resource allocation to above- versus belowground biomass (MAandMB). Biomass allocation among individual plants is broadly size-dependent, and this can often be described as an allometric relationship betweenMAandMB, as represented by the equation MA=αMBβ, or logMA= logα + βlogMB. Here, we investigated whether the scaling exponent or regression slope may be affected by the N:P supply ratio. We hypothesized that the regression slope betweenMAandMBshould be steeper under a high N:P supply ratio due to P limitation, and shallower under a low N:P supply ratio due to N limitation. To test these hypotheses, we experimentally altered the levels of N, P, and the N:P supply ratio (from 1.7:1 to 135:1) provided to five alpine species representing two functional groups (grasses and composite forbs) under greenhouse conditions; we then measured the effects of these treatments on plant morphology and tissue content (SLA, leaf area, and leaf and root N/P concentrations) and on the scaling relationship betweenMAandMB. Unbalanced N:P supply ratios generally negatively affected plant biomass, leaf area, and tissue nutrient concentration in both grasses and composite forbs. High N:P ratios increased tissue N:P ratios in both functional groups, but more in the two composite forbs than in the grasses. The positive regression slopes between logMAand logMBexhibited by plants raised under a N:P supply ratio of 135:1 were significantly steeper than those observed under the N:P ratio of 1.7:1 and 15:1.
  
  
  
  Plant biomass allocation is highly plastic in response to variation in the N:P supply ratio. Studies of resource allocation of individual plants should focus on the effects of nutrient ratios as well as the availability of individual elements. The two forb species were more sensitive than grasses to unbalanced N:P supplies. To evaluate the adaptive significance of this plasticity, the effects of unbalanced N:P supply ratio on individual lifetime fitness must be measured.
authors.affiliation: College of Resources and Environmental Sciences Nanjing Agricultural University Nanjing Jiangsu China.
authors.forename: Xi
authors.initials: X
authors.lastname: Luo
authors.affiliation: Department of Ecology Evolution & Marine Biology, University of California, Santa Barbara Santa Barbara CA USA.
authors.forename: Susan J
authors.initials: SJ
authors.lastname: Mazer
authors.affiliation: College of Resources and Environmental Sciences Nanjing Agricultural University Nanjing Jiangsu China.
authors.forename: Hui
authors.initials: H
authors.lastname: Guo
authors.affiliation: College of Resources and Environmental Sciences Nanjing Agricultural University Nanjing Jiangsu China.
authors.forename: Nan
authors.initials: N
authors.lastname: Zhang
authors.affiliation: Department of Plant and Environmental Sciences University of Copenhagen Frederiksberg Denmark.
authors.forename: Jacob
authors.initials: J
authors.lastname: Weiner
authors.affiliation: College of Resources and Environmental Sciences Nanjing Agricultural University Nanjing Jiangsu China; Department of Plant Pathology North Carolina State University Raleigh NC USA.
authors.forename: Shuijin
authors.initials: S
authors.lastname: Hu
keywords: N:P ratio; P limitation; Tibetan Plateau; above‐ and belowground biomass; allocation; allometry; nutrient balance