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 (M A and M B). Biomass allocation among individual plants is broadly size‐dependent, and this can often be described as an allometric relationship between M A and M B, as represented by the equation MA=αMBβ, or log M A = logα + βlog M B. 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 between M A and M B should 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 between M A and M B. 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 log M A and log M B exhibited 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. Synthesis: 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.
Highlights
In recent decades, atmospheric nitrogen (N) deposition has greatly increased the availability of N relative to other elements in terrestrial ecosystems (Bobbink et al, 2010)
While N input generally has positive effects on aboveground biomass (MA) (DiTommaso & Aarssen, 1989; Gough, Osenberg, Gross, & Collins, 2000), it is possible that the ratio of N to P is more important than N alone for determining above- versus belowground biomass (MB) allocation (Graham & Mendelssohn, 2016)
Based on a few previous studies that report that N or P limitation may affect plant belowground growth (Andrews, Sprent, Raven, & Eady, 1999; Carroll, Caporn, Johnson, Morecroft, & Lee, 2003; Güeswell, 2004; Müller et al, 2000; Yang, Fang, Ji, & Han, 2009), we hypothesize that the scaling exponent or slope of the relationship between MA and MB should change in response to a change in the N:P supply ratio, rather than the absolute level of N or P available
Summary
Atmospheric nitrogen (N) deposition has greatly increased the availability of N relative to other elements in terrestrial ecosystems (Bobbink et al, 2010). While N input generally has positive effects on aboveground biomass (MA) (DiTommaso & Aarssen, 1989; Gough, Osenberg, Gross, & Collins, 2000), it is possible that the ratio of N to P is more important than N alone for determining above- versus belowground biomass (MB) allocation (Graham & Mendelssohn, 2016) Consistent with this conjecture, root allocation decreased and litter production increased with increasing N:P supply ratio in five wetland Carex species (Güeswell, 2005). Based on a few previous studies that report that N or P limitation may affect plant belowground growth (Andrews, Sprent, Raven, & Eady, 1999; Carroll, Caporn, Johnson, Morecroft, & Lee, 2003; Güeswell, 2004; Müller et al, 2000; Yang, Fang, Ji, & Han, 2009), we hypothesize that the scaling exponent or slope of the relationship between MA and MB should change in response to a change in the N:P supply ratio, rather than the absolute level of N or P available. The light provided in the greenhouse was 12-hr day length and 900 Par, and the temperature was 25–30°C.The positions of the pots were randomized once every 2 weeks to control for spatial heterogeneity in light and temperature
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