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

Introduction

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).