Water and nitrogen availability co-control ecosystem CO2 exchange in a semiarid temperate steppe.

Xiaolin Zhang,Shiping Chen,Tingting Ren,Yulian Tan,Jianhui Huang,Ang Li,Lixin Wang

doi:10.1038/srep15549

Abstract

Both water and nitrogen (N) availability have significant effects on ecosystem CO2 exchange (ECE), which includes net ecosystem productivity (NEP), ecosystem respiration (ER) and gross ecosystem photosynthesis (GEP). How water and N availability influence ECE in arid and semiarid grasslands is still uncertain. A manipulative experiment with additions of rainfall, snow and N was conducted to test their effects on ECE in a semiarid temperate steppe of northern China for three consecutive years with contrasting natural precipitation. ECE increased with annual precipitation but approached peak values at different precipitation amount. Water addition, especially summer water addition, had significantly positive effects on ECE in years when the natural precipitation was normal or below normal, but showed trivial effect on GEP when the natural precipitation was above normal as effects on ER and NEP offset one another. Nitrogen addition exerted non-significant or negative effects on ECE when precipitation was low but switched to a positive effect when precipitation was high, indicating N effect triggered by water availability. Our results indicate that both water and N availability control ECE and the effects of future precipitation changes and increasing N deposition will depend on how they can change collaboratively in this semiarid steppe ecosystem.

Highlights

Melting of snow can take away nutrients from soil, which often happens during plants’ initial growth under the continental climate[13,14]
Our results showed that ECE were all significantly enhanced by summer water addition across the three growing seasons, but the effect was related to natural precipitation amount
Our results indicate that the three ecosystem processes (NEP, ecosystem respiration (ER), and gross ecosystem photosynthesis (GEP)) are controlled by different factors, which are influenced by water availability to different extents

Summary

Introduction

Melting of snow can take away nutrients from soil, which often happens during plants’ initial growth under the continental climate[13,14]. There is no consensus on the NEP responses to N deposition either, and clearly more studies are needed, especially in the semiarid steppe ecosystems As both water and N are important limiting factors in the semiarid temperate steppe, changes in precipitation and N deposition at the same time may potentially induce complex interactions on ecosystem structure and functioning. To address the uncertainties of interactive water and N effects on ECE, we conducted a manipulative experiment including water addition (either through spring snow or through summer rainfall) and N addition in a semiarid temperate steppe to examine how ECE will respond to changing precipitation and increasing N deposition. Interactions between spring snow or summer water and N addition on ECE are expected since N will become more available with increase of water supply

Methods

Results

Discussion

Conclusion