Global‐scale impacts of nitrogen deposition on tree carbon sequestration in tropical, temperate, and boreal forests: A meta‐analysis

Abstract

Elevated nitrogen (N) deposition may increase net primary productivity in N-limited terrestrial ecosystems and thus enhance the terrestrial carbon (C) sink. To assess the magnitude of this N-induced C sink, we performed a meta-analysis on data from forest fertilization experiments to estimate N-induced C sequestration in aboveground tree woody biomass, a stable C pool with long turnover times. Our results show that boreal and temperate forests responded strongly to N addition and sequestered on average an additional 14 and 13 kg C per kg N in aboveground woody biomass, respectively. Tropical forests, however, did not respond significantly to N addition. The common hypothesis that tropical forests do not respond to N because they are phosphorus-limited could not be confirmed, as we found no significant response to phosphorus addition in tropical forests. Across climate zones, we found that young forests responded more strongly to N addition, which is important as many previous meta-analyses of N addition experiments rely heavily on data from experiments on seedlings and young trees. Furthermore, the C-N response (defined as additional mass unit of C sequestered per additional mass unit of N addition) was affected by forest productivity, experimental N addition rate, and rate of ambient N deposition. The estimated C-N responses from our meta-analysis were generally lower that those derived with stoichiometric scaling, dynamic global vegetation models, and forest growth inventories along N deposition gradients. We estimated N-induced global C sequestration in tree aboveground woody biomass by multiplying the C-N responses obtained from the meta-analysis with N deposition estimates per biome. We thus derived an N-induced global C sink of about 177 (112-243) Tg C/year in aboveground and belowground woody biomass, which would account for about 12% of the forest biomass C sink (1,400 Tg C/year).

A threefold difference in plant growth response to nitrogen addition between the laboratory and field experiments

“…). However, negative even no significant relationships of plant growth with the changes in N dose or treatment duration have also been reported (Schulte‐Uebbing and De Vries , Wu et al. ).…”

“…). As a consequence, a reducing rate of the leaf photosynthesis and an increasing carbon consumption with age are approved even under N fertilization (Ryan and Yoder , Schulte‐Uebbing and De Vries , Wu et al. ).…”

“…Results from those experimental studies have dramatically improved our understanding of the roles of plants in sustaining terrestrial carbon sequestration especially under the increasing atmospheric N deposition (Lu et al. , Schulte‐Uebbing and De Vries ).…”

Effects of nitrogen enrichment on tree carbon allocation: A global synthesis

“…This indicates that deciduous species have relatively higher C requirements for foliar production and root respiration than evergreen species with increasing N availability, and thereby reduced carbohydrate availability for reserves in deciduous species, particularly in leaves and roots. LeBauer & Treseder, 2008;Newton & Amponsah, 2006;Xia & Wan, 2008) of N addition experiments might overestimate N fertilization effects due to heavily relying on data from experiments on saplings/seedlings in pots (Schulte-Uebbing & de Vries, 2018;Vadeboncoeur, 2010). It should be noted that the existing two observations on above-ground woody respiration from deciduous trees were also from experiments in boreal forests that were fertilized with NH 3 NO 3 ; these confounding effects limit our inference.…”

“…We also considered the studies cited by several recent related syntheses (Li et al, , 2015Peng, Guo, & Yang, 2017;Schulte-Uebbing & de Vries, 2018;Zhang et al, 2018). We also considered the studies cited by several recent related syntheses (Li et al, , 2015Peng, Guo, & Yang, 2017;Schulte-Uebbing & de Vries, 2018;Zhang et al, 2018).…”