淹水-落干与季节性温度升高耦合过程对消落带沉积物氮矿化影响

为揭示淹水-落干循环及季节性温度升高耦合过程对三峡支流消落带沉积物氮矿化的影响,根据野外调查,选取三峡支流澎溪河上游和下游两个水文断面,150、160和170 m这3个水位高程的沉积物样品,根据库区水文和气温特征,进行淹水-落干控温培养,分析沉积物氮矿化速率和累积量变化.结果表明与低水位高程相比,高水位高程(170 m)消落带总氮和铵态氮含量相对较低,而硝态氮含量较高.沉积物净氮矿化累积量和矿化速率均表现为落干期高于淹水期,且不同水位高程净氮矿化速率均随时间延长而下降.落干期净氮矿化累积量与沉积物总碳含量和碳氮比显著正相关,而淹水期与之负相关(P0.001).沉积物净氮矿化速率在落干期对温度升高敏感(Q_(10)1),而淹水期低水位高程对温度升高不敏感(Q_(10)1).可见,冬季淹水期温度升高对氮矿化影响较小,氮累积且释放缓慢.夏季落干期温度升高加速了氮矿化过程,增加了二次淹水后无机氮素输入水体和水体富营养化的风险.

Effect of Coupling Process of Wetting-Drying Cycles and Seasonal Temperature Increasing on Sediment Nitrogen Minerization in the Water Level Fluctuating Zone

To reveal the effect of coupling process of wetting-drying and seasonal temperature on sediment nitrogen (N) minerization, surface sediment samples were collected from the water level fluctuating zone(WLFZ) of Pengxi River crossing two hydrological sections. The sediment samples were incubated under drying and submerging conditions at the controlled temperature. The result showed that NO₃^--N and sand% in the sediment of higher altitude of water level (170 m) were higher than those in low altitudes (150 and 160 m), whereas contents of TN, NH₄⁺-N and clay% and silt% in low altitudes were much higher. Generally, Net N mineralization rate and cumulation were lower in higher altitude of water level during the drying period and submerging period. The ammonification rate decreased rapidly at the initial stage of incubation (0-7 d), and then had no obvious change, and no significant differences among altitudes was observed. The nitrification rate at low altitude decreased with incubation time, while it had only a little change at higher altitude; The nitrification contributed a higher fraction of net N mineralization than ammonification. Net N mineralization rate and its cumulation were significantly higher in the drying period than in the submerging period, while net N mineralization rate decreased with incubation time at all altitudes. Net N mineralization cumulation tended to rise first and then declined at all altitudes of the drying period, whereas it was continuously decreasing at the low water level altitude during the submerging period. Net N nitrogen mineralization rate of the drying period was positively correlate to both the sediment organic matter content and its C:N ratio, while it showed a negative correlation in the submerging period(P<0.001). Net N mineralization was sensitive to temperature increase (Q₁₀>1) in the drying period, while it was insensitive during the submerging period of low altitude (Q₁₀<1). Thus, the impact of temperature on Net N mineralization was relatively low in submerging period of winter and N was accumulated with low releasing rate. In contrast to winter, summer exhibited warmer and drying period, this two factors would lead to higher N mineralization rate and further induce the potential risk of eutrophication as N releasing into water body.