| 其他摘要 | 湿地氮的生物地球化学过程不但可影响到湿地系统自身的调节机制，而且其在环境介质中的特殊动力学过程也与一系列全球环境问题息息相关。全球环境问题的产生反过来又会对湿地演化、物种分布以及生物多样性等产生深远影响。为了更深入的理解湿地氮的生物地球化学过程及其关键驱动机制，本论文以三江平原小叶章湿地及垦后农田为研究对象，通过野外定位观测、微区试验和室内模拟，研究了湿地土壤氮的时空分布特征，探讨了湿地大气-植物-土壤系统氮的迁移、转化及其作用机制，揭示了人类活动对湿地系统的生态影响，建立了氮分室模式并评估了其平衡状况。主要得出如下结论：（1）湿地土壤氮含量具有明显的空间结构和分布格局，其向洼地倾斜方向一般形成斑块高值区，边缘形成斑块低值区。结构因素对空间异质性的贡献至少占50~60％。微地貌特征是导致空间异质性的重要随机因素，水分条件和土壤类型是引起空间异质性的重要结构因素。湿地土壤氮含量具有明显的垂直分布特征和季节变化特征，这与不同时期不同土层影响氮分布的主导因素差异有关；（2）湿地土壤无机氮的水平运移浓度和速率与距离符合指数衰减模型，其运移受浓度梯度、水势梯度、土壤基质势和吸附饱和度的控制。无机氮垂直运移的穿透曲线符合Gauss单峰模型，但不同土壤不同土层的穿透曲线峰形特征差别较大，其主要受土壤粘粒含量、水分构成、溶质运移方式和硝化-反硝化作用等因素的影响。湿地表层土壤的无机氮迁移能力较强，当水分增加后，无机氮迁移能力的增强将不利于有效氮保持；（3）湿地土壤的净矿化/硝化速率均呈明显波动变化，并受生物固持、反硝化作用、温度、降水和C/N等因素的影响。湿地0~15cm土壤的年净矿化量为5.51~19.41kg•hm-2，年净硝化量为0.28~4.27kg•hm-2。湿地土壤氮的年净矿化量低于草地和森林生态系统，因而更有利于有效氮保持。湿地土壤0~30cm土层的反硝化活性和反硝化速率较高，并与土壤理化性质密切相关，其对反硝化氮损失的贡献率高达52.39~66.40％；（4）湿地植物地上器官及枯落物的TN含量均呈指数衰减变化，而根则呈波动变化。植物不同器官的NH4+-N和NO3--N含量变化较大且NH4+-N/NO3--N＞1。根是氮的重要储库，其在生长高峰前的15~30d存在一个明显的养分蓄积过程。植物的N/P＜14，氮是影响其生长的限制性养分；（5）湿地植物残体（枯落物和根系）在不同水分带上的失重率和分解速率呈相反规律变化，其相对分解速率受温度条件、水分状况、基质质量（C/N、C/P）和养分供给的影响。枯落物的氮在不同分解小区表现出释放-累积的交替变化特征，但仍以释放为主。根系的氮表现为一直释放，但释放模式在不同分解小区差别较大。C/N对枯落物及根系分解过程中氮的调控作用更为重要；（6）湿地土壤的氨挥发速率呈波动变化并受大气温度、水分状况和土壤理化性质的影响，生长季湿地土壤的氨挥发量为6.35~6.87kgN•hm-2。湿地土壤在生长季为N2O的重要释放“源”，非生长季为N2O的弱“汇”。无积水土壤的N2O释放及氮损失以硝化作用为主，季节积水土壤的氮损失以反硝化作用为主。积水土壤反硝化产物的N2O/N2（3.76）明显低于无积水土壤（5.49），说明可通过控制湿地水分状况来改变N2O/N2，进而降低N2O释放量；（7）大气氮湿沉降的季节变化主要与人类活动、降水强度及频次、风向、地理位置以及NOX释放有关。全年氮沉降量为7.57kgN•hm-2，TIN为主体。生长季的氮沉降对促进植物生长的直接生态意义重大，非生长季的氮沉降对大量补充植物生长初期所需养分的间接生态意义明显。近年来氮沉降量的降低可能是导致湿地退化的重要原因，其生态影响不容忽视；（8）外源氮输入对植物生物量和全氮含量具有促进作用。输入到湿地系统的氮，有0.09~0.13％溶解于水中，15.33~17.02％被土壤固定，23.55~23.70％被植物吸收，59.15~61.01％以气态损失。外源氮输入量的倍增并未提高植物吸收、土壤固定和水体溶解的比例，但会增强气态损失；（9）湿地开垦初期（3~5年），土壤氮损失较快，8~21年，土壤氮损失趋于平缓。连续火烧5年可导致土壤全氮含量及储量显著降低，而弃耕7年导致其值有所增加，但增加量较小，说明土壤氮库耗竭易、恢复难。湿地开垦后，耕地利用的“重用轻养”方式虽然会导致N2O释放量的明显降低，但对粮食生产影响较大；（10）建立了湿地大气-植物-土壤系统的氮循环分室模式，确定了不同分室的氮储量以及分室间的氮流通量。小叶章湿地系统可能处于氮亏缺状态（最大亏缺值可能为0.335~0.495g•m-2•a-1），严重时可能引起湿地退化。; The nitrogen (N) biogeochemical process of wetland not only affected its regulation mechanism, but also the special kinetics process occurred in environmental medium was correlated with a series of global environmental problems. The global environmental problems, in reverse, would have significant effect on the evolution, species distribution and biodiversity of wetland. In this paper, the Calamagrostis angustifolia wetland and a series of farmlands in the Sanjiang Plain were selected as study objects. The observations in situ, microcosm experiments and laboratory simulations were performed from May 2003 to September 2006 to study the temporal and spatial distribution characteristics of N in wetland soils, the movement, transformation and action mechanism of N in the atmosphere-plant-soil system of wetland, and the ecological effects on wetland ecosystem caused by human activities. Finally, the N cycling compartment model of atmosphere-plant-soil system was established and the status of N balance was evaluated. The main results were drawn as follows: (1) The N contents in wetland soil had significant spatial structure and distribution pattern, and the contribution of structure factors to spatial heterogeneity was larger than 50~60％. Micro-morphological characteristic was an important random factor to induce spatial heterogeneity, while water condition and soil type were two important structure factors. The N contents in wetland soil had significantly vertical characteristics and seasonal characteristics, which were correlated with the differences of the main factors that affected N distribution in soil layers in different periods. (2) Both the relationships between the horizontal movement concentrations or velocities of inorganic N and movement distances accorded with exponential decay models. The movement of inorganic N was mainly controlled by concentration gradient, water potential gradient, soil matrix potential and adsorption saturation. The breakthrough curves of inorganic N accorded with Gauss single peak models, but the peak characteristics of different soil layers were significantly different, which were mainly affected by soil clay content, water composition, solute movement pattern and nitrification-denitrification etc. (3) The soil N net mineralization/nitrification rates in wetland presented significant fluctuations, which were affected by biological immobilization, denitrification, temperatures, precipitation and C/N ratios etc. The annual N net mineralization and nitrification amounts in wetland soils (0~15cm) were 5.51~19.41kg•hm-2 and 0.28~4.27kg•hm-2, respectively. The denitrification activities and rates in wetland soils (0~30cm) were much higher, which were correlated with soil physical and chemical properties. The contribution of 0~30cm soil layers to denitrification N loss was 52.39~66.40％. (4) The total nitrogen (TN) contents in different organs of aboveground part and litter changed in exponential decay curves, while the values of root changed in fluctuations. The ammonium nitrogen (NH4+-N) and nitrate nitrogen (NO3--N) contents in different organs varied widely, with NH4+-N/NO3--N＞1. Root was the important N storage, and a significant N accumulation period (15~30d) in it was observed before the coming of growth midseason. The N/P ratio of plant was less than 14, which implied that N was the limiting nutrient. (5) The change rules of weightlessness rates and decomposition rates of plant debris (litter and root) in different water gradients were just adverse, and the relative decomposition rates were affected by temperatures, water conditions, substrate quality (C/N, C/P) and nutrient supplement. The N in litter in different decomposition sub-zones showed alternant change of release and accumulation, while the N in root showed release at all times. The C/N ratios had important regulation functions to the changes of N in litter and root in decomposition process. (6) The ammonia (NH3) volatilization rates of wetland soils had significant fluctuations, which were affected by atmospheric temperature, water condition, and soil physical and chemical properties. In growing season, the total NH3 volatilization amount of wetland soils was 6.35~6.87kgN•hm-2. The wetland soil was an important emission source of nitrous oxide (N2O) in growing season, but in non-growing season, the wetland soil was a weak sink. In meadow marsh soil, nitrification played an important role in N2O emission and N loss, while in humus marsh soil, denitrification was the main process inducing N loss. The N2O/N2 ratio (3.76) in denitrification productions of humus marsh soil was lower than that (5.49) of meadow marsh soil, which indicated that the N2O/N2 ratio could be altered through regulating the water conditions of wetland. (7) The seasonal changes of atmospheric N wet deposition were mainly correlated with human activities, precipitation intensity and frequency, wind direction, geographical location and NOx emission. The TN wet deposition amount in a year was 7.57kgN•hm-2, and total inorganic nitrogen (TIN) was the main body. The N deposition in growing season had direct ecological signification which might stimulate the growth of plant, but in non-growing season, the N deposition had indirect ecological signification which could greatly supplement the nutrient at initial growth stage of plant in the second year. (8) The input of anthropogenic N has positive effects on the biomass and TN content of plant. After the anthropogenic N was imported into wetland ecosystem, only little proportion was water-dissolved (0.09~0.13％), a considerable proportion was soil-immobilized (15.33~17.02％), or plant-assimilated (23.55~23.70％), and most was lost by gaseous forms (59.15~61.01％). The double input of anthropogenic N could not elevate the proportions of plant-assimilation, soil-immobilization and water-dissolution, but it could enhance gaseous losses. (9) At initial stage of wetland reclamation (3~5a), the N in soil lost greatly, after a long period (8~21a), the N lost amount inclined smoothly. The sequential fire for 5 years induced the TN content and its storage in soils to be decreased significantly, while the desertion under “Grain for Green” Project for 7 years induced the values increased, but the increase was very limited, which indicated that the soil N pool, once depleted, was hard to restore. Although the mode of intensive utilization with little maintenance might induce N2O emission amount to be decreased, it had great effects on foodstuff production. (10) The N cycling compartment model of atmosphere-plant-soil system in wetland was established. The wetland ecosystem might be situated in the status of lacking N (the maximum N deficit might be 0.335~0.495g•m-2•a-1), and the status might induce the degradation of C. angustifolia wetland. |
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