Different sensitivity and threshold in response to nitrogen addition in four alpine grasslands along a precipitation transect on the Northern Tibetan Plateau.

Ning Zong,Guangshuai Zhao,Peili Shi

doi:10.1002/ece3.5514

Abstract

The increase in atmospheric nitrogen (N) deposition has resulted in some terrestrial ecological changes. In order to identify the response of sensitive indicators to N input and estimate the sensitivity and saturation thresholds in alpine grasslands, we set up a series of multilevel N addition experiments in four types of alpine grasslands (alpine meadow [AM], alpine meadow‐steppe [AMS], alpine steppe [AS], and alpine desert‐steppe [ADS]) along with a decreasing precipitation gradient from east to west on the Northern Tibetan Plateau. N addition only had significant effects on species diversity in AMS, while had no effects on the other three alpine grasslands. Aboveground biomass of grasses and overall community in ADS were enhanced with increasing N addition, but such effects did not occur in AS. Legume biomass in ADS and AS showed similar unimodal patterns and exhibited a decreasing tend in AM. Regression fitting showed that the most sensitive functional groups were grasses, and the N saturation thresholds were 103, 115, 136, and 156 kg N hm−2 year−1 in AM, AMS, AS, and ADS, respectively. This suggests that alpine grasslands become more and more insensitive to N input with precipitation decrease. N saturation thresholds also negatively correlated with soil N availability. N sensitivity differences caused by precipitation and nutrient availability suggest that alpine grasslands along the precipitation gradient will respond differently to atmospheric N deposition in the future global change scenario. This different sensitivity should also be taken into consideration when using N fertilization to restore degraded grasslands.

Highlights

In the last century, human activities have resulted in more than threefold increase in atmospheric nitrogen (N) deposition all over the world (Galloway et al, 2004, 2008; Lamarque et al, 2005)
| 9783 structure will alter under the increasing N deposition (Bobbink, Hornung, & Roelofs, 1998; Bragazza et al, 2004; Sala et al, 2000), which is generally associated with the loss in plant species richness and biodiversity (Bobbink et al, 2010; Stevens et al, 2010)
One‐way analysis of variance (ANOVA) followed by Tukey's multiple comparisons was used to examine the effects of N addition on plant species richness, diversity index, community coverage, and aboveground biomass of total community and each functional group in every sampling year

Summary

| INTRODUCTION

Human activities have resulted in more than threefold increase in atmospheric nitrogen (N) deposition all over the world (Galloway et al, 2004, 2008; Lamarque et al, 2005). A meta‐analysis on N enrich‐ ment experiments found greater plant species loss in cold regions and larger increases in plant biomass (Clark & Tilman, 2008) These studies show that besides community composition, the responses of different ecosystems to N enrichment depend on the climatic factors. N saturation, defined by Aber, Nadelhoffer, Steudler, and Melillo (1989), is the point when the availability of soil ammonium and nitrate exceeds ecosystem N demand During this process, plant production shifts from an N‐limited, N intermediate, to N saturation stages with the increasing of N addition (Song et al, 2017). We used multilevel N addition experiments in four alpine grasslands to determine the saturation thresholds in the heartland of the QTP; we investigated the controlling factors for the sat‐ uration thresholds to N addition along the precipitation transect on the QTP

| MATERIALS AND METHODS

Findings

| DISCUSSION