杭州西湖北里湖荷叶枯落物分解及其对水环境的影响

湖泊水生植物枯落物的分解过程影响着枯落物的淤积以及营养元素向水体和底泥的释放,进而影响湖泊水环境。用分解袋法研究了杭州西湖北里湖荷叶枯落物的分解速率和营养动态。荷叶枯落物的分解速率表现快慢交替的阶段性特点,分解速率常数峰值出现在6月,其余时段呈现波动状态。枯落物氮、磷含量变化趋势基本一致,呈现下降-逐渐上升-渐趋稳定的变化特点。氮、磷积累指数(NAI)呈现释放-积累-释放,整体以释放为主的变化特征。在此基础上推算了北里湖荷叶枯落物一个分解周期残留量的变化及氮、磷释放情况,探讨枯落物分解对水环境的影响。荷叶枯落物入湖量以立枯体总量的20%估算,残留量从11月至翌年1月急剧增加,在1月底达到峰值,随后持续下降,至10月底尚残留1.675 t干重,合30.45 kg/m²干重(按荷塘面积计算),与残留量峰值相比减少了74.39%。枯落物分解导致的氮、磷释放在11月至翌年1月持续增高,2、3月份释放量趋于下降,4、5月份出现净积累,6-10月保持净释放。整个分解周期氮、磷总释放分别为92.247 kg和6.421 kg,相当于北里湖水中氮、磷含量分别增加0.143 mg/L和0.010 mg/L。由于挺水植物生长过程吸收的主要是沉积物中的氮、磷营养盐,因此,荷花的生长和枯落分解过程具有促进氮、磷从沉积物迁移到水体,增加水中氮、磷含量的作用。

Decomposition of lotus leaf litter and its effect on the aquatic environment of the Beili Lake in the Hangzhou West Lake

The decomposition of lake aquatic plant litter affects litter deposition and nutrient release into sediments and water. Consequently, the aquatic environment of the lake is affected. In the current article, the emergent aquatic plant lotus (Nelumbo nucifera) in the Beili Lake of the Hangzhou West Lake was chosen as the experimental material. The mesh bag method was used to study the decomposition rate and nutrient dynamics of lotus leaf litter. The effect of the decomposition of lotus leaf litter on the aquatic environment of the Beili Lake and even of the whole West Lake was also examined. The results revealed that the decomposition process of lotus leaf litter has alternating fast and slow stages. The peak of the decomposition rate constant occurred in June and fluctuated during other periods. For the lotus leaf, the average decomposition rate constant was 5.441 × 10^-3/d, and the annual residual rate was 13.72%. For the lotus petiole, the average decomposition rate constant was 4.477 × 10^-3/d, and the annual residual rate was 19.51%. Both nitrogen and phosphorus contents in the litter initially decreased, and then gradually increased until stabilizing. Although the nitrogen and phosphorus accumulation indices presented a release-accumulation-release trend, the overall variation characteristic was release. The initial decomposition stage featured the net release of nitrogen and phosphorus. In contrast, the net accumulation of the two elements occurred in April and May, and the net release continued after June. The nitrogen release rates were 78.98% (after 260 d) for leaves and 31.15% (after 180 d) for petioles. The phosphorus release rates were 57.01% (after 260 d) for leaves and 34.69% (after 180 d) for petioles. Changes in the residues as well as in the release amounts of nitrogen and phosphorus of lotus leaf litter in the Beili Lake within a decomposition cycle were quantified. The calculations were based on the variation characteristics of the litter decomposition rate constant, the nitrogen and phosphorus contents, as well as the lotus leaf biomass. Lotus leaf litter falling into the lake was calculated as 20% of the total amount of standing dead litter. The residues drastically increased from November to January of the following year, and reached the peak dry weight value of 6.541 t in late January. This peak value is equivalent to 118.93 kg m^-2 dry weight (calculated based on the area of the lotus pond). The dry weight of the residues continued to decrease to 1.675 t (30.45 kg m^-2) in late October, which was 74.39% less than the peak value. Litter decomposition led to the continued increase in nitrogen and phosphorus release from November to January of the following year. The release decreased in February and March. Net accumulation occurred in April and May, and net release continued from June to October. During the decomposition cycle in the Beili Lake, the nitrogen and phosphorus contents respectively increased by 0.143 and 0.010 mg L^-1. These values correspond to total nitrogen and phosphorus release amounts of 92.247 and 6.421 kg, respectively. Emergent aquatic plants absorb nitrogen and phosphorus salts from sediments throughout their growth. Therefore, lotus growth and decomposition play important roles in transferring nitrogen and phosphorus from sediments into water bodies. Consequently, aquatic nitrogen and phosphorus contents increase.