Abstract
Atmospheric nitrogen (N) deposition has been found to significantly affect plant growth and physiological performance in terrestrial ecosystems. Many individual studies have investigated how N addition influences plant functional traits, however these investigations have usually been limited to a single species, and thereby do not allow derivation of general patterns or underlying mechanisms. We synthesized data from 56 papers and conducted a meta-analysis to assess the general responses of 15 variables related to leaf economics, gas exchange, and hydraulic traits to N addition among 61 woody plant species, primarily from temperate and subtropical regions. Results showed that under N addition, leaf area index (+10.3%), foliar N content (+7.3%), intrinsic water-use efficiency (+3.1%) and net photosynthetic rate (+16.1%) significantly increased, while specific leaf area, stomatal conductance, and transpiration rate did not change. For plant hydraulics, N addition significantly increased vessel diameter (+7.0%), hydraulic conductance in stems/shoots (+6.7%), and water potential corresponding to 50% loss of hydraulic conductivity (P50, +21.5%; i.e., P50 became less negative), while water potential in leaves (−6.7%) decreased (became more negative). N addition had little effect on vessel density, hydraulic conductance in leaves and roots, or water potential in stems/shoots. N addition had greater effects on gymnosperms than angiosperms and ammonium nitrate fertilization had larger effects than fertilization with urea, and high levels of N addition affected more traits than low levels. Our results demonstrate that N addition has coupled effects on both carbon and water dynamics of woody plants. Increased leaf N, likely fixed in photosynthetic enzymes and pigments leads to higher photosynthesis and water use efficiency, which may increase leaf growth, as reflected in LAI results. These changes appear to have downstream effects on hydraulic function through increases in vessel diameter, which leads to higher hydraulic conductance, but lower water potential and increased vulnerability to embolism. Overall, our results suggest that N addition will shift plant function along a tradeoff between C and hydraulic economies by enhancing C uptake while simultaneously increasing the risk of hydraulic dysfunction.
Highlights
Atmospheric N deposition has increased by three-to five-fold over the past century due to increased fossil fuel combustion and artificial fertilizer application (IPCC, 2007; Davidson, 2009) and has been predicted to increase by as much as twice the current level by 2050 (Galloway et al, 2008)
Analyzing only the data for studies that had both photosynthesis and foliar N content, we found that photosynthesis significantly increased 16.3% (n = 55) under N addition in the overlapped data, and foliar N content significantly increased 13.9% (n = 55) in the overlapped data
For overlapped data from studies that included both photosynthesis and iWUE, we found that photosynthesis significantly increased 10.6% (n = 45) and iWUE increased 3.1% (n = 45), but the effect on iWUE was not significant
Summary
Atmospheric N deposition has increased by three-to five-fold over the past century due to increased fossil fuel combustion and artificial fertilizer application (IPCC, 2007; Davidson, 2009) and has been predicted to increase by as much as twice the current level by 2050 (Galloway et al, 2008). Increased plant growth with increasing N availability brings changes in plant leaf economics, gas exchange (Hacke et al, 2010), and hydraulic architecture (Borghetti et al, 2017), which in turn impact plant water relations (Kleiner et al, 1992; Goldstein et al, 2013). Hydraulic architecture and water relations strongly influence allometric scaling in the size, form, and population density of woody plants, and impact plant growth and the terrestrial ecosystem C sink (Savage et al, 2010; Blonder and Enquist, 2014). Through understanding the effect of long-term N addition on woody plant leaf economics, gas exchange and hydraulics we can better understand changes in plant growth and terrestrial productivity
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