【作者】 涂利华；

【导师】 胡庭兴；

【作者基本信息】 四川农业大学 ， 森林培育， 2011， 博士

【摘要】 从工业革命以来,人类活动制造的活性氮急剧增加并超过了自然陆地过程制造的活性氮,大量活性氮通过一系列途径释放至大气,其中绝大部分又沉降回陆地或海洋生态系统(氮沉降)。由于绝大多数的陆地和海洋生态系统的初级生产力均受到氮元素的限制,因此,氮沉降的全球化及其在未来持续增加的趋势,势必会对全球碳循环过程造成重大影响。竹林是世界重要的森林类型之一,在世界广泛分布,中国现有竹林面积居全球首位,为5.712×106hm-2,占全国森林总面积的2.94%,竹林生物量蓄积占全国森林总生物量蓄积的10%,竹林生态系统碳储量占我国森林碳储量的5.1%。因此,竹林生态系统在中国乃至全球碳循环过程中具有举足轻重的作用。我国竹林主要分布在南方各省,同时这些地区也是氮沉降十分严重的地区。通过定位模拟氮沉降试验的方式,研究了华西雨屏区苦竹人工林生态系统碳循环关键过程和特征对几种氮沉降量梯度的短期响应。模拟氮沉降水平分别为对照(不施氮)、低氮(50kg·N·hm-2·a-1)、中氮(150kg·N·hm-2·a-1)和高氮(300kg·N·hm-2·a-1)。主要结果如下。(1)位于四川洪雅柳江镇的试验地,2008年全年总氮湿沉降量为8.241 g·m-2。苦竹林凋落量平均为691 g·m-2·yr-1,凋落叶、凋落枝和凋落箨分别占总凋落量的79%、16%和5%(2008年1月至2010年12月),季节变化明显,凋落高峰期为4至6月。苦竹林土壤呼吸速率具有明显的季节变化,仲夏最高、深冬最低：自然状态下年土壤呼吸速率为529.8±15.1 g·CO2-C·m-2(即5.30±0.15 Mg·C·hm-2);凋落物层、无根土壤和植物根系对土壤呼吸的贡献率分别为30.9%、20.8%和48.3%,各呼吸组分均具有较明显的季节动态,并与土壤温度和凋落量等因素相关；自然状态下土壤呼吸、凋落物层C02排放、无根土壤C02排放和植物根系呼吸的Q10值分别为2.90、2.28、3.09和3.19,凋落物层CO2排放的温度敏感性显著低于总呼吸和其它各组分；土壤呼吸的昼夜变化有小幅波动,但夜间呼吸速率与白天呼吸速率之间无显著差异；土壤呼吸速率以及土壤呼吸各组分速率与土壤温度呈极显著指数正相关关系,土壤呼吸速率与土壤微生物生物量碳、苦竹细根生物量(细根密度)和苦竹细根氮含量呈极显著线性正相关关系,年土壤呼吸速率与净初级生产力(net primary productivity, NPP)呈极显著线性正相关关系。模拟氮沉降通过显著促进凋落物层CO2排放、无根土壤CO2排放和植物根系呼吸,进而显著促进了苦竹林土壤呼吸作用,并且各氮处理与对照之间的差异均达显著水平；模拟氮沉降显著降低了无根土壤CO2排放和根呼吸温度敏感性(或Q10值),对总呼吸和凋落物层CO2排放温度敏感性无显著影响。自然状态下苦竹人工林生态系统NPP和净生态系统生产力(net ecosystem productivity,NEP)分别为10.95±0.58 Mg.C·hm-2·yr-1和5.09±0.58 Mg·c.hm-2·yr-1,土壤呼吸约占NPP的50%左右；受氮沉降影响NPP和NEP分别增加了8%～37%和5%～45%,其中NPP和NEP的差异在高氮处理与对照之间达显著水平。苦竹人工林生态系统目前为碳汇状态,氮元素调控着树木的生长,虽然模拟氮沉降促进了通过土壤呼吸的CO2排放,但由于氮沉降对植被生长的作用更强,使得该系统碳汇增加；虽然华西雨屏区氮沉降量较高,但由于低龄人工林生长迅速、碳吸存增长潜力巨大,一定范围增加的氮沉降可能会促进该地区这类生态系统的碳吸存能力。(2)苦竹林土壤表层细根(<2mm)密度较高,自然状态下为533±89 g·m-2,氮沉降显著促进了细根密度,其中高氮处理达到820±161 g·m-2。氮沉降条件下苦竹细根氮、钾、镁含量显著增加。氮沉降对土壤呼吸的促进作用在很大程度上可以解释为氮沉降促进了细根代谢强度,以及通过增加根际沉降物质和土壤酶活性促进了根际微生物呼吸。氮沉降处理两年后苦竹地上部分生物量显著增加,而苦竹林地下部分生物量也受氮沉降的促进作用,氮沉降的增加会通过促进苦竹的生长增加碳固定,同时,也在一定程度上通过促进土壤呼吸的方式增加碳排放。另外,细根氮含量的增加可能预示着细根周转速率的加快。(3)利用原位分解袋法研究了华西雨屏区苦竹人工林三种凋落物组分在模拟氮沉降下分解过程和养分释放状态,试验周期为2 a。结果表明,模拟氮沉降通过抑制木质素和纤维素分解显著抑制了叶种枝后期分解,对箨的分解无显著影响；各凋落物组分在分解过程中养分元素释放差异主要与初始养分元素含量和凋落物物理性质有关；凋落物养分元素初始含量对元素释放模式和最终净释放率的大小具有重要的决定作用；目前,苦竹林生态系统土壤氮输入主要以大气氮沉降为主同时凋落物氮输入也是一个重要途径；模拟氮沉降对磷(phosphorus,P).钾(potassium,K).钙(calcium.Ca)和镁(Magnesium,Mg)元素释放影响不明显,但模拟氮沉降显著增加了凋落物分解后期氮残留率。(4)自然状态下,凋落叶是最主要的凋落物组分,占七成以上,模拟氮沉降对这一比例无显著影响。苦竹林通过地上部分凋落物输入的碳、氮和磷分别为261±11、2.72±0.18和0.86±0.03 g·m-2·yr-1。模拟氮沉降显著增加了通过凋落物的碳、氮和磷输入量。从量上来说,氮沉降是苦竹林土壤目前最主要的氮输入途径(8.24g N·m-2·a-1)。苦竹林两种凋落物组分氮和磷元素含量呈极显著正线性关系。模拟氮沉降显著增加了苦竹凋落叶、凋落枝和总凋落物量,并显著降低了两种凋落物组分C/N和凋落叶N/P。氮沉降对苦竹凋落物基质特性的影响,可能会加速其前期分解速率,使得各种养分元素的循环速度加快。(5)苦竹人工林样地中测定的6种土壤酶活性均具有较明显的季节变化,其中,蔗糖酶、纤维素酶和酸性磷酸酶活性高峰期为春季,脲酶活性高峰期为秋季,过氧化物酶和多酚氧化酶活性高峰期为冬季；氮沉降增加了苦竹林土壤中木质素分解酶和碳、氮、磷分解相关酶(多酚氧化酶、蔗糖酶、酸性磷酸酶和脲酶)活性,抑制了纤维素酶活性,而对过氧化物酶的影响不显著；所研究的6种土壤酶对氮沉降总体上呈正响应；模拟氮沉降主要通过刺激细根生长、活化根际土壤环境,促进了根际微生物及其胞外酶活泩。(6)模拟氮沉降增加了土壤总有机碳(total organic carbon, TOC)、微生物生物量碳(microbial biomass carbon, MBC)、总氮(total nitrogen, TN)、微生物生物量氮(microbial biomass nitrogen, MBN)、铵态氮(NH4+-N)和有效磷(available phosphorus, AP)含量,对土壤浸提性溶解有机碳(extractable dissolved organic carbon, EDOC)、活性碳(liable carbon, LC;或称易氧化碳,readily oxidizable carbon,ROC)、硝态氮(N03--N)和速效钾(available potassium, AK)无显著影响。土壤MBC和MBN的季节动态明显,并与气温极显著正相关。土壤AP、AK与MBC、MBN呈极显著负相关关系。模拟氮沉降提高了土壤中碳、氮、磷元素的活性,并通过微生物的转化固定作用使得碳、氮、磷元素在土壤中的含量增加。土壤有机碳和养分对模拟氮沉降呈正响应,氮沉降的增加可能会提高土壤肥力并促进植被的生长,进而促进生态系统对碳的固定。更多还原

【Abstract】 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.更多还原