Biomass, Carbon and Nitrogen Partitioning and Water Use Efficiency Differences of Five Types of Alpine Grasslands in the Northern Tibetan Plateau

Abstract

(1) Background: Grassland covers most areas of the northern Tibetan Plateau along with important global terrestrial carbon (C) and nitrogen (N) pools, so there is a need to better understand the different alpine grassland growth associated with ecosystem C, N storage and water use efficiency (WUE). (2) Methods: The plant biomass and C, N concentrations, stocks and vegetation WUE of five kinds of alpine grassland types were investigated in northern Tibetan Plateau. (3) Results: The results showed that there were significant differences among five types of alpine grasslands in aboveground biomass (AGB), belowground biomass (BGB), total biomass (TB) and root:shoot (R/S) ratio while the highest value of different indices was shown in alpine meadow type (AM). The AGB and BGB partitioning results significantly satisfied the allometric biomass partitioning theory. The C, N concentrations and C/N of the vegetation in AGB and BGB showed significant grassland type differences. The highest C, N stocks of BGB were in AM which was almost six or seven times more than the C, N stocks of AGB in alpine desert type (AD). There were significant differences in δ¹³C and intrinsic water use efficiency (WUEi) under five alpine grassland types while the highest mean values of foliar δ¹³C and WUEi were in AD. Significant negative correlations were found between WUEi and C, N concentrations, C/N of AGB and soil water content (SWC) while the correlation with BGB C/N was not significant. For AGB, BGB, TB and R/S, there were positive correlations with C, N concentrations of AGB, BGB and SWC while it had significant negative correlations with C/N of BGB. (4) Conclusions: With regard to its types, it is suggested that the AM or AS may be an actively growing grassland type in the northern Tibetan Plateau.

Keywords: C stocks; N stocks; alpine grassland; biomass; water use efficiency.

Figure 1

Research areas and sampling sites.

Figure 2

The Empirical regression model between belowground biomass (BGB) and aboveground biomass (AGB) in 5 alpine grassland types. The α and β values presented in each figure were mean ± standard errors. (A): alpine meadow (AM), (B): alpine steppe (AS), (C): alpine meadow steppe (AMS), (D): alpine desert steppe (ADS), (E): alpine desert (AD) and (F): alpine grassland types.

Figure 3

The frequency distribution of C, N concentrations and C/N of belowground biomass (BGB) and aboveground biomass (AGB) in 5 alpine grassland types. (A): alpine meadow (AM), (B): alpine steppe (AS), (C): alpine meadow steppe (AMS), (D): alpine desert steppe (ADS), (E): alpine desert (AD) and (F): alpine grassland types.

Figure 3

The frequency distribution of C, N concentrations and C/N of belowground biomass (BGB) and aboveground biomass (AGB) in 5 alpine grassland types. (A): alpine meadow (AM), (B): alpine steppe (AS), (C): alpine meadow steppe (AMS), (D): alpine desert steppe (ADS), (E): alpine desert (AD) and (F): alpine grassland types.

Figure 4

The δ¹³C, Carbon isotope discrimination (Δ) and intrinsic water use efficiency (WUEi) of aboveground biomass (AGB) in 5 alpine grassland types. Letters are reported only for significant differences in one-way ANOVA post hoc Tukey’s HSD.

Figure 5

Redundancy analysis (RDA) of biomass, WUEi and C, N concentration, soil water content in 5 alpine grassland types. Aboveground biomass (AGB), Belowground biomass (BGB), total biomass (TB), BGB to AGB ratio (R/S), carbon concentration of AGB (AGBC), carbon concentration of BGB (BGBC), nitrogen concentration of AGB (AGBN), nitrogen concentration of BGB (BGBN), C to N ratio of AGB (AGBC/N), C to N ratio of BGB (BGBC/N), soil water content (SWC) and intrinsic water use efficiency (WUEi).