Long-term nitrogen addition of different types and levels altered the relationship between species diversity and ecosystem multifunctionality in alpine grasslands

Abstract

In alpine grasslands, nitrogen limitation constrains plant growth, and nitrogen fertilization is a common strategy for rehabilitating degraded lands. However, the effects of different nitrogen types, levels, and durations on plant diversity and ecosystem functionality remain unclear. This study investigates slightly degraded alpine grasslands in the Three Rivers Source Region of the Qinghai-Tibet Plateau. We applied ammonium sulfate, potassium nitrate, and urea at varying concentrations (20 g m^–2, 10 g m^–2, and 0 g m^–2) and assessed plant biodiversity, biomass, and multifunctionality. Under long-term nitrogen addition, different nitrogen types influenced species loss and gain rates, thereby affecting species richness; increasing nitrogen levels elevated species loss rates, while ecosystem multifunctionality remained unaffected by environmental constraints. In contrast, under short-term nitrogen addition, nitrogen type primarily influenced species gain rates, which altered species richness and, in turn, ecosystem multifunctionality; nitrogen levels affected both species loss and gain rates, jointly shaping richness and multifunctionality. Overall, short-term nitrogen addition significantly increased species diversity and biomass, whereas long-term addition reduced diversity without affecting multifunctionality. These findings underscore the contrasting impacts of short- and long-term nitrogen inputs and the combined regulatory roles of nitrogen type, addition level, and application duration in the ecological restoration of degraded alpine grasslands.

Graphical abstract

Keywords

nitrogen type and application rate / species turnover / community productivity / grassland restoration

Highlight

	● The ecological effects of nitrogen addition depend on the duration, as well as nitrogen levels and types.
	● Long-term nitrogen addition reduces species diversity by favoring grasses and excluding non-dominant species.
	● Nitrogen addition alters biodiversity–multifunctionality relationships via species turnover.

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