全球大气氮沉降日益加剧的背景下,研究川西亚高山地区主要阔叶树种红桦(Betual albo-sinensis)幼苗在不同水、氮供应下的形态变化和生理机制,以了解该区主要植被的生长特性,并可为森林生态系统的恢复重建提供一定的科学依据.采用两因素随机区组试验设计,设置5个水分梯度,即40%(W_1)、50%(W_2)、60%(W_3)、80%(W_4)、100%(W_5)的土壤田间持水量(FC)和3个施氮梯度,即模拟氮沉降施加N 0(N_0)、20(N_1)、40(N_2) g m~(-2) a~(-1)的硝酸铵. 结果显示:土壤水分和氮素及二者交互作用对生长和生理特征均具有显著(P<0.001)影响.表现为:1)幼苗生长(株高和基径)、叶片碳同化特征(最大净光合速率、叶绿素含量和叶片氮含量)及根系生理特征[呼吸速率、NH_4~+和NO_3~- 净吸收速率及根系硝酸还原酶(NR)活性]均随土壤水分增加而增大,且在土壤水分为W_4和W_5时达到最大.根冠比的变化呈相反趋势.2)施氮在土壤水分不足(W_1和W_2)时除对基径、NO_3~-净吸收速率和NR活性无影响外,显著抑制了红桦株高及叶片和根系生理特征.而在水分充足(W_4和W_5)时,除N_2在土壤水分W_4时对叶片生理特征和根系呼吸速率无影响外,对红桦的生长和根系生理特征有显著的促进作用,且N_1效果更显著.在W_3时两种施氮浓度效果不同:N_1对红桦的生长和相关生理特性具有促进作用而N_2呈抑制作用(除对根系NH_4~+和NO_3~-净吸收速率及NR活性无影响外).根冠比变化呈相反趋势.3)根系离子(NH_4~+和NO_3~-)净吸收速率与根系呼吸速率和叶片氮含量呈极显著正相关.本研究表明,土壤水分和氮素供应对植物的影响存在显著的耦合效应,当土壤水分不足时施氮不利于红桦幼苗生长,而适量的施氮在水分充足时有利于生长.对于供试土壤以及本试验设置的水、氮梯度而言,土壤含水量为80%田间持水量,施加N20 g m~(-2) a~(-1)更适宜红桦幼苗生长.
英文摘要:
Soil water and nitrogen availabilities have significant effects on plant morphological and physiological characteristics, especially on the root physiological functions. The subalpine coniferous forest of western Sichuan, China, is sensitive to climate changes, including precipitation and atmospheric nitrogen deposition. In this research we exposed seedlings to five water regimes (field water holding capacity (FC) as 40% (W_1), 50% (W_2), 60% (W_3), 80% (W_4) and 100% (W_5)) and three nitrogen (N) regimes (with NH_4NO_3 content as 0 g N m~(-2) a~(-1) (N_0), 20 g N m~(-2) a~(-1) (N_1) and 40 g N m~(-2) a~(-1) (N_2)), and determined how N-fertilization affected the growth, carbon assimilation and root physiological activity of the dominant broadleaf Betual albo-sinensis seedling under different soil water conditions. The results showed the growth and physiological characteristics of Betual albo-sinensis significantly affected by water, N-fertilization and their interaction. 1) Plant growth (plant height and basal diameter), leaf carbon assimilation characteristics (maximum net photosynthetic rate, chlorophyll and nitrogen content) and root physiological characteristics (respiration rate, NH_4~+ and NO_3~- net influxes and nitrate reductase (NR) activity) increased with the increase of soil water content, with the highest values at W_4 and W_5, respectively. However the response of root/shoot ratio was opposite. 2) Except basal diameter, NO_3~- net influx and NR activity, nitrogen applications had a significant negative effect on plant growth, leaf carbon assimilation and root physiological characteristics when soil moisture was insufficient (W_1 and W_2). Whereas nitrogen applications under the superior soil water conditions (W_4 and W_5) gave a remarkable promotion on growth and root physiological characteristics, except that N_2 had no effect on leaf carbon assimilation characteristics or root respiration rate under the soil water condition of W_4. Nitrogen applications at 60% FC (W_3) had different effects. Namely, N, promoted but N_2 restrained seedling growth and physiological properties, except that N_2 had no effect on root NH_4~+ and NO_3 net influxes or NR activity. 3) Root NH_4~+ and NO_3 net influxes were positively correlated with root respiration rate and leaf nitrogen content. The results confirmed that nitrogen applications are beneficial to plant under sufficient soil water condition but has negative effect under insufficient soil water conditions. For the tested soil, Betual albo-sinensis seedling grows better under the condition of 80% FC with 20 gN m~(-2) a~(-1) nitrogen application.