Biogeochemical cycles of carbon (C) and nitrogen (N) are closely coupled in terrestrial ecosystems through multiple processes including photosynthesis, tissue allocation, respiration, N fixation, N uptake, and decomposition of soil organic matter (SOM) and litter (Gärdenäs et al. 2011; Zaehle 2013). Thus, it is impossible to fully understand the turnover of soil C and N without considering their interactions. Soil plays a central role in regulating responses of the global C and N cycles to climate change because SOM contains the largest amount of organic C and N in terrestrial ecosystems (Lehmann and Kleber 2015; Paustian et al. 2016). Particularly, it has been arousing more concerns of impacts of rhizosphere processes on SOM decomposition (Cheng et al. 2014; Finzi et al. 2015; Keuper et al. 2020). Rhizosphere processes, at the global scale, may control as much as 50% of the total CO2 released from terrestrial ecosystems (Schimel 1995; Hopkins et al. 2013), and govern 25% of all mineralized N in the temperate forest ecosystems (Finzi et al. 2015). However, the linkage between soil C and N mineralization in the rhizosphere remains poorly understood.
Rhizosphere is one of the most important hotspots for soil C and N biogeochemical cycling in terrestrial ecosystems (Kuzyakov and Blagodatskaya 2015; York et al. 2016). Plant living roots release a wide variety of C compounds (i.e., rhizodeposition) into the rhizosphere soil (Jones et al. 2009), thereby leading to different chemical, physical and biological characteristics in the rhizosphere soil compared with those of the bulk soil (Phillips and Fahey 2006; Cheng et al. 2014). Numerous studies have reported that C mineralization rates (Cmin) were higher for the rhizosphere soil than for the bulk soil (Huo et al. 2017; Gan et al. 2021a). Compared with Cmin, N mineralization rates (Nmin) were less studied (Yin et al. 2018). Generally, net and gross Nmin were accelerated by the rhizosphere processes (Gan et al. 2021a; Gan et al. 2021b). However, the interactions between Cmin and Nmin in the rhizosphere soil had been rarely studied. Several studies focusing on rhizosphere priming effects, reported a positive linear relationship between Cmin and gross Nmin (Bengtson et al. 2012; Yin et al. 2018; Henneron et al. 2020). Since the above relationship was based on the data including rhizosphere soil (planted) and bulk soil (unplanted) together (Cheng et al. 2014). It is likely to mask the interaction between Cmin and Nmin in the rhizosphere soil. Therefore, the relationship between soil C and N mineralization in the rhizosphere soil needs to be further investigated.
To date, almost all our understanding of interaction between Cmin and Nmin is based on bulk soil. These studies commonly sampled root-free soil (pick out root and its adhering soil) in field, and conducted laboratory incubations to determine Cmin and Nmin (e.g., Johnson et al. 1980; Campos et al. 2020). Results from the positive (Parfitt et al. 2003; Tian et al. 2017; Zhang et al. 2021), the negative (Gao et al. 2009; Song et al. 2011), and no correlations (Savin et al. 2001; Kelliher et al. 2004), are inconsistent for the linkage of C and N mineralization. Hence, we need explore the relationship between Cmin and Nmin on a broader scale (e.g., altitudinal gradient), given that controversial results in bulk soil and site specific.
The relationship between Cmin and Nmin seems to be sensitive to many factors. Microbes are the primary factor linking Cmin with Nmin, since they directly involve the decomposition of SOM (Rousk and Bengtson 2014). Soil properties such as organic C and N pools provide substrates to microbes (Schimel and Schaeffer 2012; Mooshammer et al. 2014), thereby affecting C and N mineralization. Furthermore, the heterogeneity and bioavailability of SOM affected the decomposition process (Schmidt et al. 2011; Lehmann and Kleber 2015). These factors are likely to be different in the rhizosphere from those in the bulk soil. Extensive studies reported that soil factors such as soil pH, organic C content, soil total N, available N, MBC, and extracellular enzyme activities in rhizosphere were markedly differed in bulk soil (Phillips and Fahey 2006; Gan et al. 2021a). In addition, several studies reported that both Cmin and Nmin were stimulated in the rhizosphere, which likely resulted from the higher microbial biomass and extracellular enzyme activities (Brzostek et al. 2013; Yin et al. 2014; Chen et al. 2018). Although some factors were recognized for controlling soil C and N mineralization, the pathways of C and N interactions, especially in the rhizosphere soils, are still unclear.
The objective of this study was to identify interactions between soil Cmin and net Nmin in paired rhizosphere and bulk soil along an altitudinal gradient. We hypothesized that the interactions between Cmin and net Nmin would be stronger in the rhizosphere soil than in the bulk soil, because higher rates of Cmin and net Nmin, higher soil organic C and N content, higher microbial biomass and enzyme activities had been commonly reported in the rhizosphere compared with bulk soil (Gan et al. 2021a). To test the hypothesis, we used the “soil adhering to fine roots after shaking” method (Phillips and Fahey 2006) to collect rhizosphere soil (and paired bulk soil) along an altitudinal forest gradient. Soil Cmin and net Nmin were quantified using laboratory incubation method, and were linked with the basic soil properties (e.g., pH, total C, total N, and inorganic N), microbial biomass carbon (MBC), and extracellular enzyme activities. To our best knowledge, this study is the first to distinguish the relationship between C and N mineralization in the rhizosphere soils from in bulk soils.