Impacts of warming and nitrogen addition on soil autotrophic and heterotrophic respiration in a semi-arid environment

Nitrogen addition reduces soil respiration but increases the relative contribution of heterotrophic component in an alpine meadow

“…For example, R a has been reported as a saturated response to an N enrichment gradient while R h decreased linearly with increasing N addition (Peng, Li, et al, ). Nonetheless, the majority of previous studies generally applied one or two N addition levels and revealed various influences of N on R a , R h and subsequently R s , such as increase (Fang et al, ; Jia, Shao, & Wei, ), no change (Chen et al, ; Fang et al, ; Zhu et al, ) or decrease (Wei et al, ; Zhao et al, ), which make the comparison difficult between studies. In this study, the gradual increase in N addition rates spans a large range of N conditions, reflecting a continuous response of root and soil microbial activities to different N addition rates.…”

“…This study used the deep‐collar method to distinguish R a and R h , which is widely and easily applied in grassland ecosystems (Fang et al, ; Li, Zhou, Wu, Zhou, & Luo, ; Zhang et al, , ). Although we measured soil respiration rates 1 year after deep collar installation to minimize the effect of dead root decomposition, it is acknowledged that this method still has the following uncertainties.…”

“…First, R h may be underestimated due to eliminations of roots and associated root exudates (Hanson et al, ), which are respired by soil micro‐organisms and give rise to a priming effect on microbial breakdown of SOC (Kuzyakov, ). Second, R h may be also overestimated as a result of increased SM in the deep collars (Fang et al, ), the remaining dead roots decay (Hanson et al, ) and the lateral diffusion of CO 2 (Jassal & Black, ). Third, it should be noted that our estimation of R a includes not only root respiration (sourcing from root growth and root maintenance) but also rhizosphere respiration (mycorrhizal respiration and rhizospheric priming effect) (Hopkins et al, ).…”

“…Due to different turnover times and controlling factors of plant and soil C pools (Wang, Sun, et al, ), R a and R h have showed differential responses to N enrichment (Fang et al, ; Li et al, ; Zeng, Zhang, & Wang, ). Multiple lines of evidence indicate that N application generally decreases R h when the added N exceeds microbial demand (Fang et al, ; Liu & Greaver, ; Peng, Li, et al, ), which has been explained by the decrease in soil microbial biomass (Treseder, ), inhibition of extracellular enzyme activities (Jian et al, ; Ramirez, Craine, & Fierer, ; Wang, Liu, & Bai, ), alterations in soil microbial abundance and composition (Zhang, Chen, & Ruan, ), and the increase in microbial C use efficiency (Finn et al, ; Liu, Qiao, Yang, Bai, & Liu, ). The effect of N enrichment on R a is disparate, showing a positive (Fang et al, ; Li et al, ), neutral (Ren et al, ) or negative effect (Wei et al, ).…”

“…Multiple lines of evidence indicate that N application generally decreases R h when the added N exceeds microbial demand (Fang et al, ; Liu & Greaver, ; Peng, Li, et al, ), which has been explained by the decrease in soil microbial biomass (Treseder, ), inhibition of extracellular enzyme activities (Jian et al, ; Ramirez, Craine, & Fierer, ; Wang, Liu, & Bai, ), alterations in soil microbial abundance and composition (Zhang, Chen, & Ruan, ), and the increase in microbial C use efficiency (Finn et al, ; Liu, Qiao, Yang, Bai, & Liu, ). The effect of N enrichment on R a is disparate, showing a positive (Fang et al, ; Li et al, ), neutral (Ren et al, ) or negative effect (Wei et al, ). A recent global synthesis revealed that N addition decreased R h by 8.1% and increased R a by 21.6%, resulting in no response of R s across all the terrestrial ecosystems (Zhang et al, ).…”