论文摘要：

Aims

Shifts of plant community composition with enhanced atmospheric nitrogen (N) deposition in grasslands have occurred globally. Despite extensive studies on the effects of enhanced N deposition on plant species composition of grassland community, few studies have focused on belowground ecological processes and duration of N addition.

Methods

In situ long-term (14-year: 2004–2017) and short-term (4-year: 2014–2017) N-addition experiments in the same sites were conducted in a temperate steppe of northern China. We investigated the effects of N-addition on plant community composition and root traits by comparing results of short-term and long-term N addition.

Results

Nitrogen-evoked changes in plant community composition were primarily dependent on N-addition duration and functional groups. Short-term N addition favored growth of grasses, and depressed growth of forbs, while long-term N addition favored growth of sedges, and concurrently depressed growth of grasses and forbs, ultimately leading to a time-dependent shift of plant community composition from co-dominance by grasses and forbs to dominance by grasses, then to dominance by sedges. Similar to the observed aboveground biomass, short-term N addition led to an increase in the relative belowground biomass (the proportion of belowground biomass of individual plant to community belowground biomass) of the grass Stipa krylovii and a decrease in the relative belowground biomass of the forb Artemisia frigida, respectively. In contrast, long-term N addition enhanced the relative belowground biomass of the sedge Carex korshinskyi, and reduced the relative belowground biomass of the grass S. krylovii and the forb A. frigida. Moreover, traits of absorptive fine roots (first-two orders) in the three species (grass: S. krylovii; forb: A. frigida; sedge: C. korshinskyi) that are representatives of three functional groups differed in their responses to short-term and long-term N addition, thus rendering them differentially competitive for acquisition of belowground resource. The changes in root traits of the three species in responses to N addition of varying duration were associated with their relative aboveground biomass.

Conclusions