向日葵种群中植株个体大小对其氮素利用策略的影响

我们利用Berendse和Aerts提出的氮素利用效率(NUE)概念及原理研究了高密度一年生草本植物向日葵(Helianthus annuus L.)种群中植株个体大小对其氮素吸收利用的影响,并对种内竞争进行了分析.结果表明,植株对氮素的吸收与其个体大小不成线性关系,说明种群内不同植株个体对土壤氮素的竞争属于非对称竞争.植株的氮素损失随着个体大小的增加而增加.个体较大的植株具有较高的氮素输入率和较低的氮素输出率,因而具有较高的氮素净增加值.植株的氮素生产力(NP)和氮素平均滞留时间(MRT)均与植株个体大小呈正相关.较大的植物个体具有较高的NP和较长的MRT,由于NUE为NP和MRT二者的乘积,因而较大个体植株的NUE高于个体较小的植株.同种植物的不同个体的NP和MRT之间不存在协衡关系.氮素回收效率(NRE)与植株个体大小密切相关.在个体水平上,较大的植株个体具有较高的NUE与其较高的NRE有关.种群内植株个体对土壤氮素的非对称竞争主要由于植株对氮素的吸收和利用效率不同所致.因此,Berendse和Aerts提出的氮素利用效率概念不仅适用于研究种间的养分利用策略,对于种内不同植株的养分策略研究也同样适用.

Effects of Plant Sizes on the Nitrogen Use Strategy in an Annual Herb,Helianthus annuus (Sunflower)

We analyzed the effects of plant sizes on nitrogen (N) uptake and use in a dense monospecific stand of an annual herb, Helianthus annuus L. (sunflower) and evaluated the consequences of intraspecific competition. Larger individuals obtained more N disproportionately to their sizes, suggesting that the competition for soil N was asymmetric (one-sided) among individual plants in the stand. Nitrogen loss of individuals also increased with plant size. N influx was greater in larger individuals, while N efflux was lower in small individuals. Therefore, the relative rate of N increment was greater in larger individuals, while it was around zero in the smallest individuals. N use efficiency (NUE) was separated into the N productivity (NP) and the mean residence time of N (MRT). Both NP and MRT were positively related to plant size. Larger individuals showed a higher NP and a longer MRT, while smaller ones displayed the reverse pattern. Consequently, NUE (i.e. the product of NP and MRT), was higher for larger individuals. No trade-off between NP and MRT was found among individuals. N resorption efficiency (NRE) was closely related to plant size. The higher NUE at individual-level was partly a result of greater N resorption during senescence. Asymmetric competition among individuals in this stand resulted mainly from lower efficiency in both N uptake and N use by smaller individuals. This study shows that the concept of NUE defined by Berendse and Aerts offers a powerful tool in studying plant strategies within species as well as among species.