Seasonal Trade-Off between Water- and Nitrogen-Use Efficiency of Constructive Plants in Desert Riparian Forest in Hyperarid Region of China

Seasonal Trade-Off between Water- and Nitrogen-Use Efficiency of Constructive Plants in Desert Riparian Forest in Hyperarid Region of China

Shengkui Cao (Qinghai Normal University, China and Chinese Academy of Sciences, China), Qi Feng (Chinese Academy of Sciences, China), Jianhua Si (Chinese Academy of Sciences, China), Yonghong Su (Chinese Academy of Sciences, China), Zongqiang Chang (Chinese Academy of Sciences, China) and Haiyang Xi (Chinese Academy of Sciences, China)
DOI: 10.4018/978-1-60960-064-8.ch024
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Foliar d13C values are often used to denote the long-term water use efficiency (WUE) of plants whereas long-term nitrogen use efficiency (NUE) are usually estimated by the ratio of C to N in the leaves. Seasonal variations of d13C values, foliar nitrogen concentration and C/N ratios of Populus euphratica and Tamarix ramosissima grown under five different microhabitats of Ejina desert riparian oasis of northwestern arid regions in China were studied. The results indicated that T. ramosissima had higher d13C value compared with that of P. euphratica. The N concentration and C/N ratios of two species were not significantly different. The seasonal pattern of three indexes in two species was different. The d13C values and N concentration decreased during the plant’s growth period. However, the change of C/N ratios was increased. Among microhabitats, there were higher d13C values and N concentration as well as lower C/N ratios in the Dune and Gobi habitats. Foliar d13C values significantly and positively correlated with N concentration in P. euphratica and T. ramosissima, whereas a significantly negative correlation between d13C values and C/N ratios was found for P. euphratica. This relation in T. ramosissima was weak, but there was a significant quadratic curve relationship between d13C values and C/N ratios, which revealed that there was a trade-off between WUE and NUE for P. euphratica and in natural condition, P. euphratica could not improve WUE and NUE simultaneously. T. ramosissima could simultaneously enhance WUE and NUE. The above characters of WUE and NUE in two plants reflected the different adaptations of desert species to environmental condition.
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The adaptive use of limited resources by plants is a central topic of plant ecology, as the amount of resources required to support plant growth is one of the key factors that determines species persistence in a community (Aerts & Chapin, 2000). Plant adaptation to resource availability is often reflected as resource-use efficiency (Binkley et al., 2004). Water and nitrogen (N) are two key resources for plant life and associated physiological processes. The supplies of water and N in most natural arid and semi-arid ecosystems cannot meet the demand of plants. Therefore, water and N limitations may serve as evolutionary pressures on plants to utilize water and N more efficiently (Yasumura et al., 2002).

Water-use efficiency (WUE) is one important trait that can contribute to growth, survival, and distribution of plant species in water-limited habitats. WUE is traditionally defined either as the ratio of dry matter accumulation to water consumption over a season (WUET, T means longer period of time, for example, a growing season) or as the ratio of net photosynthesis (A) to transpiration (E) over a period of seconds or minutes (WUEi, where i means a short period of time) (Sinclair et al., 1984). During the past several decades, carbon isotope composition (δ13C) of plant tissues has been substantially studied (Farquhar & Richards, 1984; Martin & Thorstenson, 1988; Leroux et al., 1996; Amdt et al., 2001). Because a strong positive correlation was found between δ13C and WUE (Farquhar et al., 1989), the δ13C has been developed as an indirect indicator of WUE of C3 plants. C3 refers to the firstly formed compounds consisting of three carbon atoms after CO2 is fixed.

Nitrogen use efficiency (NUE) has also been widely studied (Vitousek, 1982; Bridgham et al., 1995; Aerts & Chapin, 2000; Binkley et al., 2004). Nitrogen is an essential component of photosynthetic proteins. Because of its limited availability, nitrogen is one of the primary factors that limit plant growth in many ecosystems (Kachi & Hirose, 1983). Berendse and Aerts (1987) defined nitrogen-use efficiency (NUE) as the amount of dry matter produced per g or kg of nitrogen. Hiremath (2000) developed a concept to study the different components that determine leaf life time NUE in a detailed and integrated manner. She defined cumulative NUE as the ratio of total carbon assimilation by a leaf to total nitrogen investment in that leaf over its lifetime. Recently, long-term NUE has been estimated by plant C/N ratios in several studies (Patterson et al., 1997; Livingston et al., 1999; Chen et al., 2005). So far, almost all the studies on NUE have exclusively addressed the species in subarctic tundra or tropical rain forests, while few such studies have ever been conducted of the plants in the arid regions of the world (Yuan et al., 2006).

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