Search

Societal Impact Statement: Increasing atmospheric nitrogen (N) deposition represents a major global change factor, but its long‐term effect on tree growth and carbon (C) sequestration remains uncertain. Our manipulation experiment and meta‐analysis reveal that N deposition in temperate and boreal forests promoted tree growth and the allocation of more C into wood than into leaves in China and worldwide. Thus, N deposition may increase forest C sequestration through enhanced wood production and distribution of C into stable sinks. In the context of achieving "Carbon Neutrality," understanding how N deposition affect long‐term forest C sinks will help us with mitigation strategies under climate change. Summary: Increased nitrogen (N) deposition is driving many temperate and boreal forests in the Northern Hemisphere towards N saturation. However, it is uncertain how long‐term N deposition affects tree growth and carbon (C) allocation in these forests.To investigate this, we treated temperate larch and mixed forests in northeastern China with N additions for 8 years. In addition, we collected data from 25 N‐addition experiments in temperate and boreal forests worldwide to reveal the overall effects of N on tree growth and C allocation.Nitrogen additions significantly promoted total biomass increment by 24% in both study forests, with on average additional 8 kg C per kg N gain into woody biomass over the study period. Nitrogen additions increased the ratio of woody biomass increment to foliage litterfall production in the larch forest (by 34%). Literature data analysis also revealed greater N promotion on wood (24%) over foliage (9%) production. However, the positive effect on foliage diminished over time.These results combined imply that N deposition may promote tree growth in temperate and boreal regions and drive proportionally more photosynthate allocation into wood than leaves, thus may enhance forest C sequestration in the long run. [ABSTRACT FROM AUTHOR]

The impacts of increasing nitrogen (N) deposition on forest ecosystems, including on carbon (C) sequestration, largely depend on the extent to which forests are N-limited and so whether and where deposited N is retained within the ecosystem. The 15 N tracer method can provide excellent insight into the ecosystem fates of N, but while it has been extensively used in temperate forests it has yet to be sufficiently employed in tropical forests, which are often thought not to be N-limited. Here, we used stable isotope 15 NH 4 + and 15 NO 3 − tracers applied as solutions to the forest floor to examine the fates of different forms of N in a tropical montane primary forest with low background atmospheric N deposition (6 kg N ha −1  yr −1 ) in China. We found that a substantial amount of 15 N was assimilated by plants over time and significantly more 15 N was recovered following 15 NO 3 − addition than following 15 NH 4 + addition: 7% and 16% of 15 N were recovered three months after the respective 15 NH 4 + and 15 NO 3 − tracer additions and 11% and 29% respectively after one year. In contrast to plants, the organic layer was only an important short-term sink for deposited N: while 21% and 12% of the 15 N from 15 NH 4 + and 15 NO 3 − additions were accumulated in the organic layer after three months, more than half of the retained 15 N was lost after one year. Mineral soil was the largest sink for deposited N, and the 15 N retained in soil was relatively stable over time for both N forms, with 39% and 32% of the initial 15 N input recovered after one year for 15 NH 4 + and 15 NO 3 − tracer additions, respectively. Overall, the total ecosystem 15 N recovery one year after the 15 NH 4 + and 15 NO 3 − tracer additions was large (60% and 66% respectively), and not significantly different from total recovery after three months, suggesting that a large proportion of deposited N could be retained in the longer term. Based on the measured fate of 15 N one year after labeling and the C:N ratios of different plant components, this tropical forest’s carbon sequestration efficiency is calculated to be 17 kg C per kg N added, comparable to the values reported for temperate and boreal forests in Europe and North America and indicating substantial N limitation of this tropical forest. Our results suggest that anthropogenic N input in moderate levels may contribute to enhance C sequestration in some tropical forests, without significant long-term loss of N to the environment. [ABSTRACT FROM AUTHOR]

Background and aims: The impacts of atmospheric nitrogen (N) deposition on terrestrial ecosystem processes remain controversial, mostly because of the uncertainty regarding the fates of deposited N. We conducted a 16-week simulated deposition study to experimentally trace N in a greenhouse plant-soil system. Methods: Using a two-way factorial design, we added (NH)SO solution twice a week to pots containing different soil organic matter (SOM) content and with or without a live plant ( Salix dasyclados). The recoveries of N in soil, plant biomass, and leaching solution were quantified. Results: We found most N was retained in soil (18.0-59.2%), with significantly more N recovered from high-SOM soils than from low-SOM soils. Plant presence significantly increased N retention in soil. Plant biomass accounted for 10-20% of the N input, with proportionally more N assimilated when plants were grown in low-SOM soils. Leaching loss of N was relatively low (10-17%). Conclusion: Our study suggests that SOM content and plant presence significantly affect the fates of deposited N. Indeed, N would be preferentially retained in soils with high SOM content and live plant, while plants would assimilate more deposited N when grown in low SOM soils. Global biogeochemical models thus need to incorporate such soil-specific N retention and plant N assimilation. [ABSTRACT FROM AUTHOR]

Abstract: Dissolved inorganic nitrogen (DIN) (as ammonium nitrate) was applied monthly onto the forest floor of one old-growth forest (>400 years old, at levels of 50, 100 and 150kgNha−1 yr−1) and two young forests (both about 70 years old, at levels of 50 and 100kgNha−1 yr−1) over 3 years (2004–2006), to investigate how nitrogen (N) input influenced N leaching output, and if there were differences in N retention between the old-growth and the young forests in the subtropical monsoon region of southern China. The ambient throughfall inputs were 23–27kgNha−1 yr−1 in the young forests and 29–35kgNha−1 yr−1 in the old-growth forest. In the control plots without experimental N addition, a net N retention was observed in the young forests (on average 6–11kgNha−1 yr−1), but a net N loss occurred in the old-growth forest (−13kgNha−1 yr−1). Experimental N addition immediately increased DIN leaching in all three forests, with 25–66% of added N leached over the 3-year experiment. At the lowest level of N addition (50kgNha−1 yr−1), the percentage N loss was higher in the old-growth forest (66% of added N) than in the two young forests (38% and 26%). However, at higher levels of N addition (100 and 150kgNha−1 yr−1), the old-growth forest exhibited similar N losses (25–43%) to those in the young forests (28–43%). These results indicate that N retention is largely determined by the forest successional stages and the levels of N addition. Compared to most temperate forests studied in Europe and North America, N leaching loss in these seasonal monsoon subtropical forests occurred mainly in the rainy growing season, with measured N loss in leaching substantially higher under both ambient deposition and experimental N additions. [Copyright &y& Elsevier]