| Human-made reactive nitrogen (N) increases dramatically since Industrial Revolution, and the amount of human-made reactive N is more than the natural production. A large amount of reactive N emissions to the atmosphere through several channels, most of which deposits back to terrestrial or marine ecosystems. N limits net primary production much of the time in most terrestrial biomes and many marine ecosystems. Thus, continuous increasing N deposition will inevitably have a significant impact on global carbon (C) cycle. Bamboo forest is one of the most important forest types in the world, accounting for 3% of the forest area in China. In the last five decades, C stocks in bamboo forests increased, and it contributes about 10% of the C stock in the living biomass of forests in China. To our knowledge there has not yet been a study of the effect of N deposition on the C balance in bamboo ecosystems. Therefore, understanding the response of the C status of bamboo ecosystems to an elevated N deposition level can provide an important scientific basis for evaluation of C response of forest ecosystems. Furthermore, bamboo forests/plantations mainly distribute in the southern provinces in China, where the level of atmospheric N deposition is very high. N deposition in southern China would likely increase continuously in the following decades. Therefore, it is critical to address the effects of increased N deposition on the C cycling in the forest ecosystem, especially the bamboo forest/plantation ecosystems in this region. To evaluate the effects of N deposition on ecosystem processes regarding C cycling and relate characteristics, we simulated N deposition in a Pleioblastus amarus (Keng) Keng f. plantation during a three year field experiment. Four levels of N treatments:control (no N added), low-N (50 kg·N·hm-2·yr-1), medium-N (150 kg·N·hm-2·yr-1), and high-N (300 kg·N·hm-2·yr-1) were set in the present study. The main results are as follows.(1) The wet N deposition in the experimental site (Liujiang, Hongya, Sichuan, PR China) was 8.241 g·m-2. The mean total litterfall in the P. amarus plantation was 691 g·m-2·yr-1 during the experimental period, with the maximum rates from April to June. The contribution of leaves, twigs, and sheaths to total litterfall were 79%,16%, and 5% during January 2008 to December 2010, respectively. Soil respiration followed a clear seasonal pattern, with the maximum rates in mid-summer and the minimum in late winter. The annual cumulative soil respiration was 529.8±15.1 g·CO2-C·m-2 (5.30±0.15 Mg·C·hm-2) in the control plots. The contribution of forest litter, root-free soil, and plant roots to the total soil respiration were 30.9%,20.8%, and 48.3%, respectively. The three fractions of soil respiration also followed a seasonal pattern similar to total soil respiration. The Q10 values of total soil respiration, litter layer CO2 efflux, and root-free soil CO2 efflux were 2.90,2.28,3.09, and 3.19, respectively. The Q10 value of litter layer CO2 efflux was significantly lower than that of other soil respiration fractions and total soil respiration. There was no significant difference of soil respiration in daily and night. Soil respirations exhibited a positive exponential relationship with soil temperature, and positive linear relationships with MBC, fine root density, and fine root N concentration. Annual soil respiration rate exhibited a significant positive linear relationship with NPP.Simulated N deposition significantly increased the rates of total soil respiration, litter layer CO2 efflux, root-free soil CO2 efflux, and plant root respiration. Simulated N deposition significantly decreased Q10 value of root-free soil CO2 efflux, and plant root respiration, and have no significant effect on temperature sensitivity of total soil respiration and litter layer CO2 efflux. The net primary productivity (NPP) ranged from 10.95 to 15.01 Mg C·hm-2·yr-1and was higher than the annual soil respiration (5.85 to 7.62 Mg C·hm-2·yr-1) in all treatments. Simulated N deposition increased the net ecosystem productivity (NEP), and there was a significant difference between the control and high N treatment NEP, whereas, the difference of NEP among control, low-N, and medium-N was not significant. Results suggest that N controlled the primary production in this bamboo plantation ecosystem. Simulated N deposition increased the C sequestration of the P. amarus plantation ecosystem through increasing the plant C pool, though CO2 emission through soil respiration was also enhanced.(2) There was significant difference between< 1 mm and 1-2 mm fine root in tissue and element characteristics. The concentrations of lignin, P, and Mg of< 1 mm fine root were significantly higher than those of 1-2 mm fine root. The concentrations of cellulose and Ca were just the reverse. Nitrogen deposition increased fine root biomass and concentrations of N, K, and Mg. The fine root biomass were 533±89,630±140, 632±168, and 820±161 g·m-2 in control, low-N, medium-N, and high-N plots, respectively. The annual soil respiration in control, low-N, medium-N, and high-N plots were 5.85±0.43,6.48±0.71,6.84±0.57, and 7.62±0.55 Mg C-hm-2·a-1,respectively. N deposition significantly stimulated the soil respiration. There are significant linear relationships between annual soil respiration and fine root (0-2 mm) biomass and fine root N concentration. The increasing of soil respiration rate mainly due to the stimulating effects of N deposition on fine root biomass, root metabolism, and microbial activities. This study suggests that the P. amarus plantation is N limited. Nitrogen deposition is likely to stimulate rhizosphere respiration through enhancing of fine root metabolism and increasing microbial activities.(3) We observed different patterns of mass loss for the three P. amarus litter fractions (leaves, sheaths, and twigs) of varying substrate quality in the control plots. N addition did not affect the decomposition of sheaths during the study period. However, N addition slowed the decomposition of leaves and twigs in the later stages of decomposition by inhibiting the decay of lignin and cellulose. The release of C, phosphorus (P), Potassium (K), Calcium (Ca) of P. amarus litter was gently inhibited by simulated N deposition. Simulated N deposition significantly slowed down the release of N in decomposition of P. amarus litter and the rate of reduction was 19.0%-27.2%. However, the soil nutrient supply for the growth of plant was not cut down because of the direct and indirect fertility effect of N deposition on the soil. On the whole, the continuous increasing N deposition may strengthen the C sequestration in the young plantation ecosystems in Rainy Area of West China, for the enormous potential C sequestration ability in these ecosystems through fast growth of plant.(4) Leaf litter is the most important component of total litterfall, accounts for more than 70%. Simulated N deposition did not affect the ratio of litter fractions of total litterfall. C, N, and P return through litterfall (were 261±11,2.72±0.18, and 0.86±0.03 g·m-2·yr-1, respectively. Simulated N depositon significantly increased C, N, and P input to soil through litterfall. Nitrogen deposition is the most important way of N input into the soil of this ecosystem. N and P concentrations of both two litter fractions exhibited significant positive linear relationships. Simulated N depositon significantly increased the amount of leaves, twigs, and total litterfall, and significantly decreased the C/N of two litter fractions, and N/P of leaf litter. The nutrient cycling may be accelerated by the N deposition through indirect effect of litter quality.(5) There are apparent seasonal variation for all the six enzyme activities, and the peaks of sucrase, cellulase and acidic phosphatase activities occurred in Spring, urease in Autumn and peroxidase and polyphenol oxidase in Winter. Nitrogen deposition stimulated the activities of sucrase, polyphenol oxidase, acidic phosphatase and urease, restrained the activity of cellulose, but had no significant effect on peroxidase activity. Simulated N deposition mainly stimulated the activities of rhizosphere microorganism and extracellular enzymes, through stimulate the growth of fine roots and activate the rhizoshpere environment.(6) For one year from November 2008, (?) (?)ly collected 0-20 cm horizon soil samples and measured soil total organic carbon (TC), microbial biomass carbon (MBC), extractable dissolved organic carbon (EDOC), liable carbon (LC), total nitrogen (TN) microbial biomass nitrogen (MBN), NH4+-N, NO3--N, available phosphorus (AP), and available potassium (AK). Nitrogen deposition increased concentrations of TC, MBC, TN, MBN, NH4+-N, and AP in soil, and had no effect on the other indicators. MBC and MBN exhibited significantly seasonal patterns, and had significant positive relationship with temperature. There were significant negative correlation between AP, AK and MBC, MBN. Nitrogen deposition stimulated availabilities of C, N, P, and increased the accumulation of these elements in the soil of P. amurus plantation. Results suggested P. amurus plantation ecosystem is in N-limited condition, and soil organic carbon and nutrients respond positively to nitrogen deposition. The increasing of nitrogen deposition may enhance fertility of the soil, stimulate growth of plants, and increase carbon fixation of P. amurus ecosystem in the future. |
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