Nitrogen economy of alpine plants on the north Tibetan Plateau: Nitrogen conservation by resorption rather than open sources through biological symbiotic fixation

Abstract

Nitrogen (N) is one of the most important factors limiting plant productivity, and N fixation by legume species is an important source of N input into ecosystems. Meanwhile, N resorption from senescent plant tissues conserves nutrients taken up in the current season, which may alleviate ecosystem N limitation. N fixation was assessed by the 15N dilution technique in four types of alpine grasslands along the precipitation and soil nutrient gradients. The N resorption efficiency (NRE) was also measured in these alpine grasslands. The aboveground biomass in the alpine meadow was 4–6 times higher than in the alpine meadow steppe, alpine steppe, and alpine desert steppe. However, the proportion of legume species to community biomass in the alpine steppe and the alpine desert steppe was significantly higher than the proportion in the alpine meadow. N fixation by the legume plants in the alpine meadow was 0.236 g N/m2, which was significantly higher than N fixation in other alpine grasslands (0.041 to 0.089 g N/m2). The NRE in the alpine meadows was lower than in the other three alpine grasslands. Both the aboveground biomass and N fixation of the legume plants showed decreasing trends with the decline of precipitation and soil N gradients from east to west, while the NRE of alpine plants showed increasing trends along the gradients, which indicates that alpine plants enhance the NRE to adapt to the increasing droughts and nutrient‐poor environments. The opposite trends of N fixation and NRE along the precipitation and soil nutrient gradients indicate that alpine plants adapt to precipitation and soil nutrient limitation by promoting NRE (conservative nutrient use by alpine plants) rather than biological N fixation (open sources by legume plants) on the north Tibetan Plateau.

Describing the vertical root distribution of alpine plants with simple climate, soil, and plant attributes

“…In addition, soil nutrient limitations (e.g. nitrogen; Zong et al, 2020) and plant-specific aspects related to growth and survival strategies of alpine plants (e.g. Cooper et al, 2003;Germino, 2014) can be considered in future versions of the VRD model to portray more realistically the characteristic allocation of plant biomass above-and belowground in alpine ecosystems (e.g.…”

“…In addition, the growth and development of alpine plants is also constrained by the low availability of nutrients in the soil, particularly nitrogen (e.g. Freppaz et al, 2019;Zong et al, 2020). Still, alpine vegetation may play a crucial role in cycling carbon and nutrients in alpine ecosystems (e.g.…”

Resorption Efficiency of Four Cations in Different Tree Species in a Subtropical Common Garden

“…Cation elements, such as potassium (K), calcium (Ca), sodium (Na) and magnesium (Mg), are nutrients that play essential roles in the growth of plants [1,2]. Like other nutrients, these elements can be retransferred to new tissues during senescence [3]; therefore, resorption is considered to be one of the most crucial nutrient conservation strategies, helping to maintain the balance of nutrients in plant tissues [4,5] and reducing the dependence of plants on soil nutrient supplies [6,7]. Therefore, it is essential to investigate nutrient resorption to better understand the nutrient use strategies of different tree species [3].…”

The symbiotic nitrogen fixation by legumes in a legume‐companion and a legume‐dominant alpine steppe on the central Tibetan Plateau

“…The legumes species, Oxytropis glacialis , Oxytropis microphylla , and Astragalus arnoldii , are widespread across the alpine steppe, even at 4700 m above sea level, either as companion or dominant species. Previous studies have investigated BNF in alpine legume companion communities in which legumes constituted 0–25% of the total aboveground plant biomass (Yang et al, 2011; Zong et al, 2020), but no studies on BNF have yet been conducted in alpine communities dominated by legumes, where legume abundance might exceed 60% of the total aboveground biomass.…”

“…Initially, this approach was used in cropland ecosystems (Chalk, 1991). More recently, it has been used in several alpine ecosystems (Jacot et al, 2000a; Jacot et al, 2000b; Lonati et al, 2015; Zong et al, 2020).…”