Nitrogen effects on net ecosystem carbon exchange in a temperate steppe

Abstract

It has widely been documented that nitrogen (N) enrichment stimulates plant growth and net primary production. However, there is still dispute on how N addition affects net ecosystem CO 2 exchange (NEE), which represents the balance between ecosystem carbon (C) uptake and release. We conducted an experimental study to examine effects of N addition on NEE in a temperate steppe in northern China from 2005 to 2008. N was added at a rate of 10 g N m−2 yr−1 with NH 4 NO 3 alone or in combination with phosphorous (P, 5 g P 2 O 5 m−2 yr−1) in both clipped and unclipped plots. Over the 4 years, N addition significantly stimulated growing‐season NEE, on average, by 27%. Neither the main effects of P addition or clipping nor their interactions with N addition were statistically significant on NEE in any of the 4 years. However, the magnitude of N stimulation on NEE declined over time. N addition significantly increased NEE by 60% in 2005 and 21% in 2006, but its effect was not significant in 2007 and 2008. N‐induced shift in species composition was primarily responsible for the declined N stimulation over time. The gradually increasing coverage of the upper canopy species (Stipa krylovii) and standing litter accumulation induced light limitation on the lower canopy species (Artemisia frigida). Thus, N‐induced shifts in plant species composition strongly regulated the direct effects of N addition on C sequestration in the temperate steppe.