Predominant role of soil moisture in regulating the response of ecosystem carbon fluxes to global change factors in a semi-arid grassland on the Loess Plateau.

Climate warming, altered precipitation and nitrogen deposition may critically affect plant growth and ecosystem carbon fluxes. However, the underlying mechanisms are not fully understood. We conducted a 2-yr, multi-factor experiment (warming (W), altered precipitation (+30% and - 30%) and nitrogen addition (N)) in a semi-arid grassland on the Loess Plateau to study how these factors affect ecosystem carbon fluxes. Surprisingly, no interactive effects of warming, altered precipitation and nitrogen addition were detected on parameters of ecosystem carbon fluxes, including net ecosystem CO2 exchange (NEE), ecosystem respiration (ER), gross ecosystem productivity (GEP) and soil respiration (SR). Warming marginally reduced NEE and GEP mainly due to its negative effects on them in July and August. Altered precipitation significantly affected all parameters of carbon fluxes with precipitation reduction decreasing NEE, ER and GEP, whereas precipitation addition increasing SR. In contrast, nitrogen addition had little effect on any parameters of carbon fluxes. Soil moisture was the most important driver and positively correlated with ecosystem carbon fluxes and warming impacted ecosystem carbon fluxes indirectly by decreasing soil moisture. While plant community cover did not show significant association with carbon fluxes, semi-shrubs cover was positively related to NEE, ER and GEP. Together, these results suggest that soil water availability, rather than soil temperature and nitrogen availability, may dominate the effect of the future multi-faceted global changes on semi-arid grassland carbon fluxes on the Loess Plateau.

Su F ，Wang F ，Li Z ，Wei Y ，Li S ，Bai T ，Wang Y ，Guo H ，Hu S ... -  《-》

Reduction in precipitation amount, precipitation events, and nitrogen addition change ecosystem carbon fluxes differently in a semi-arid grassland.

The increases in extent and frequency of extreme drought events and increased nitrogen (N) deposition due to global change are expected to have profound impacts on carbon cycling in semi-arid grasslands. However, how ecosystem CO2 exchange processes respond to different drought scenarios individually and interactively with N addition remains uncertain. In this study, we experimentally explored the effects of different drought scenarios (early season extreme drought, 50 % reduction in precipitation amount, and 50 % reduction in precipitation events) and N addition on net ecosystem CO2 exchange (NEE), ecosystem respiration (ER), and gross ecosystem productivity (GEP) over three growing seasons (2019-2021) in a semi-arid grassland in northern China. The growing-season ecosystem carbon fluxes in response to drought and N addition were influenced by inter-annual precipitation changes, with 2019 as a normal precipitation year, and 2020 and 2021 as wet years. Early season extreme drought stimulated NEE by reducing ER. 50 % reduction in precipitation amount decreased ER and GEP consistently in three years, but only significantly suppressed NEE in 2019. 50 % reduction in precipitation events stimulated NEE. Nitrogen addition stimulated NEE, ER, and GEP, but only significantly in wet years. The structural equation models showed that changes in carbon fluxes were regulated by soil moisture, soil temperature, microbial biomass nitrogen (MBN), and the key plant functional traits. Decreased community-weighted means of specific leaf area (CWMSLA) was closely related to the reduced ER and GEP under early season extreme drought and 50 % reduction in precipitation amount. While increased community-weighted means of plant height (CWMPH) largely accounted for the stimulated ER and GEP under 50 % reduction in precipitation events. Our study stresses the distinct effects of different drought scenarios and N enrichment on carbon fluxes, and highlights the importance of soil traits and the key plant traits in determining carbon exchange in this water-limited ecosystem.

Du L ，Luo Y ，Zhang J ，Shen Y ，Zhang J ，Tian R ，Shao W ，Xu Z ... -  《-》

Water- and plant-mediated responses of ecosystem carbon fluxes to warming and nitrogen addition on the Songnen grassland in northeast China.

Jiang L ，Guo R ，Zhu T ，Niu X ，Guo J ，Sun W ... -  《PLoS One》

Effects of Warming on CO2 Fluxes in an Alpine Meadow Ecosystem on the Central Qinghai-Tibetan Plateau.

Ganjurjav H ，Gao Q ，Zhang W ，Liang Y ，Li Y ，Cao X ，Wan Y ，Li Y ，Danjiu L ... -  《PLoS One》

Effects of warming and clipping on ecosystem carbon fluxes across two hydrologically contrasting years in an alpine meadow of the Qinghai-Tibet Plateau.

Peng F ，You Q ，Xu M ，Guo J ，Wang T ，Xue X ... -  《PLoS One》