(4) (25%) Atmospheric nitrogen (N) deposition is considered to be a driver of increasing carbon (C) storage in woody biomass, particularly in N-limited temperate and boreal forests in North America and Europe (1-3). However, there is still debate on the impact of N deposition on the greenhouse gas (GHG) balance of tropical forests, which have abundant available soil N and a much leakier N cycle than temperate forests (4, 5). The tree growth and C storage in woody biomass were not found to significantly respond atmospheric N deposition in tcmperate forests across Europe and northeastern United States (6, 7). Similar results were also observed in P-limited Hawaiian tropical forests growing on soil of 4. 1 million years (8). However, Frey et al. (9) demonstrated that N addition increased soil C accum mulation largely due to a suppression of organic matter decomposition rather than enhanced C inputs via litterfall and root production. These results suggest that the responses of forest C ration to N addition are variable depending on background limitation, changes in the nutrient use efficiency of plants and decon mposers, and Iimitation of other nutrients such as phosphorus (P) (10, 11). Few st tudies on N deposition effects bave focused on tropical and subtropical regions compared with boreal and temperate regions (12).
Methane (CH4) and nitrous oxide (N20) are the important GHGs that contribute to more than one-quarter of anthropogenic global warming, and their emissions partly offset terrestrial CO2 uptake at the global scale (13, 14). It has been reported that the emissions of CH4 and N20 from terrestrial ecosystems offset 73% of the land CO2 sink over the North American continent (15). Forest soils are a sink for CH4, but this sink may have decreased by 77% from 1988 to 2015 (16). Humid tropical forest soils have higher background N2O Iosses than temperate forest soils and are the largest source of N2O across natural ccosystems (12). Nitrogen addition was reported to increase CH4 and N20 production and inhibit CH4 consumption in recent studies, which may shift the net GHG balance toward a source of CO2 equivalents (CO2eqs) and could offset 53 to 76% of increased C sequestration induced by N deposition across terrestrial ecosystems (14).
This study focuses on Moso bamboo (Phyllostachys edulis), a widely planted species in East and Southeast Asia, covering 4.43 Mha in subtropical China (17). Moso bamboo has extremely fast growth rates (completing height and diameter growth within 2 months after shoot emergence) and a strong regeneration ability (alternating high and low new bamboo recruitment years). The site studied is intensively managed with one-third of the aboveground biomass being rem oved by thinning each 2 years, thus leaving tree cohorts younger than 4 years. The residence time of most harvested bamboo products, such as bamboo flooring and bamboo furniture, is larger than 20 years (18, 19). Management practices include fertilization, ploughing, and weeding. Therefore, Moso bamboo forests can al ways grow and remove CO2 from the atmosphere; at steady state, their net ecosystem cxchange is equal to barvest, and the standing biomass carbon stock remains constant. The net CO2 sink of the system formed by bamboo forests and their wood products thus depends mainly on the lifetime and accumulation in wood products and waste. Our 5-year observation using eddy covariance measurements found that the mean annual net C sequestration in both trees and soil was 6.03 Mg C ha-1 (17). Over a 30-year time period, the carbon stock of the system including plantation and its products pool is 246 metric tons (t) C ha-1, higher than the 200 t C ha-1 of Chinese fir (Cunninghamia lanceolata), another fast growing tree in China (19).
The regions where Moso bamboo mainly distributes are today subject to much higher N deposition rates of 30 kg N ha-1 year-1 (20) than Western European (8 to lI kg) and United States (4 to 5 kg) (21). Moreover, N deposition is predicted to increase by 50 to 100% by 2030 relative to the year 2000 and could reach up to 50 kg N ha-I year-1 by 2050 in China (22, 23). It was shown that N deposition significantly increased P uptake (24), photosynthetic capacity (25), decomposition rates of leaf litter (26) and fine roots (27), loss of dissolved organic carbon (DOC) (28), and soil microbial biomass (29) in bamboo forests. However, how N deposition affects ecosystem productivity, soil carbon storage, soil uptake of CH4 and soil N20 emissions, and thus the net GHG balance remains to be quantified. In this study, we performed a 4-year field experiment at three N addition levels of 30, 60, and 90 kg N ha-I year-1 upon a control (N-free addition with an ambient N deposition rate of 30 kg N ha-1 year-1) in Moso bamboo forests composed of 1 - and 3-year-old bamboo cohorts since January 2013.
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