Environmental Systems Analysis Group, Wageningen University and Research, Wageningen, the Netherlands.
Environmental Research (Alterra), Wageningen University and Research, Wageningen, the Netherlands.
Glob Chang Biol. 2018 Feb;24(2):e416-e431. doi: 10.1111/gcb.13862. Epub 2017 Oct 16.
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).
氮（N）沉降增加可能会提高氮限制陆地生态系统的净初级生产力，从而增强陆地碳（C）汇。为了评估这种 N 诱导的 C 汇的幅度，我们对森林施肥实验的数据进行了荟萃分析，以估计地上树木木质生物质中 N 诱导的 C 封存，这是一个具有长周转时间的稳定 C 库。我们的结果表明，北方森林和温带森林对 N 添加的反应强烈，分别在地上木质生物质中额外封存了平均 14 和 13 kg C/kg N。然而，热带森林对 N 添加没有显著响应。热带森林对 N 没有反应的常见假设是因为它们受到磷的限制，但我们发现热带森林对磷添加没有显著响应，因此这一假设无法得到证实。在不同气候带，我们发现年轻森林对 N 添加的反应更为强烈，这很重要，因为许多以前的 N 添加实验的荟萃分析严重依赖于幼苗和幼树实验的数据。此外，C-N 响应（定义为每增加单位 N 添加量额外封存的质量单位数）受到森林生产力、实验 N 添加率和环境 N 沉积率的影响。我们的荟萃分析估计的 C-N 响应通常低于化学计量比例、动态全球植被模型以及沿 N 沉积梯度的森林生长清查得出的响应。我们通过将荟萃分析中获得的 C-N 响应乘以每个生物群落的 N 沉积估计值，来估计全球树木地上木质生物质中的 N 诱导的 C 封存。因此，我们得出，全球每年通过 N 诱导的地上和地下木质生物质中的 C 封存约为 177（112-243）Tg C/年，占森林生物质 C 汇（1400 Tg C/年）的约 12%。
