Abstract
Water use efficiency (WUE) is an important indicator of ecosystem functioning but how ecosystem WUE responds to climate change including precipitation and nitrogen (N) deposition increases is still unknown. To investigate such responses, an experiment with a randomized block design with water (spring snowfall or summer water addition) and nitrogen addition was conducted in a temperate steppe of northern China. We investigated net ecosystem CO2 production (NEP), gross ecosystem production (GEP) and evapotranspiration (ET) to calculate ecosystem WUE (WUEnep = NEP/ET or WUEgep = GEP/ET) under spring snow and summer water addition with or without N addition from 2011 to 2013. The results showed that spring snow addition only had significant effect on ecosystem WUE in 2013 and summer water addition showed positive effect on ecosystem WUE in 2011 and 2013, as their effects on NEP and GEP is stronger than ET. N addition increased ecosystem WUE in 2012 and 2013 both in spring snow addition and summer water addition for its increasing effects on NEP and GEP but no effect on ET. Summer water addition had less but N addition had greater increasing effects on ecosystem WUE as natural precipitation increase indicating that natural precipitation regulates ecosystem WUE responses to water and N addition. Moreover, WUE was tightly related with atmospheric vapor-pressure deficit (VPD), photosynthetic active radiation (PAR), precipitation and soil moisture indicating the regulation of climate drivers on ecosystem WUE. In addition, it also was affected by aboveground net primary production (ANPP). The study suggests that ecosystem WUE responses to water and N addition is determined by the change in carbon process rather than that in water process, which are regulated by climate change in the temperate steppe of northern China.
Highlights
In terrestrial regions, water is the most important factor limiting plant growth and ecosystem processes, so plants have evolved various adaptive water-related strategies to survive, such as reducing water loss and increasing water absorption [1]
The results showed that WUEnep and WUEgep increased linearly with precipitation and aboveground net primary production (ANPP) upon both spring snow addition and summer water addition with or without N addition
This study showed that variation of vapor-pressure deficit (VPD) and photosynthetic active radiation (PAR) could best account for changes in both WUEnep and WUEgep, indicating that they are the main drivers of the carbon and water cycles, directly or indirectly influencing ecosystem Water use efficiency (WUE)
Summary
Water is the most important factor limiting plant growth and ecosystem processes, so plants have evolved various adaptive water-related strategies to survive, such as reducing water loss and increasing water absorption [1]. As plant carbon cycle processes following with water use like transpiration, so the balance between them are the key points to study ecosystem responses to global climate change, which remain gaps in global change studies [3, 4]. Ecosystem water use efficiency (WUE) is an effective tool for assessing ecosystem responses to global climate change linking carbon and water cycles [5] It is calculated by the rate of carbon uptake per unit of water lost, liking the ratio of net ecosystem CO2 production (NEP) to evapotranspiration (ET) [6,7,8] or gross ecosystem photosynthesis (GEP) to evapotranspiration (ET) [9,10,11]. WUE is useful to make projections about potential changes in project global carbon and water cycles, which control the responses of ecosystem processes to global climate change
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