4.3 Effects of N and P additions on C sequestration and their implication for alpine meadows
The positive NCB under N and P additions including the Control (Figure 3b) in our study indicated that C input to soil in the form of plant residue was higher than C output from SOC decomposition. This would increase SOC accumulation and soil C content in the Tibetan meadows, as shown by Yang et al. (2008). Our data did show an increasing SOC content compared with the initial SOC concentration of 3.38% (Li et al., 2018a) at the beginning of the field experiment nine-years ago. The lower NCB with vanillin amendment than that with glucose amendment in corresponding treatments (Figure 3b) suggested that recalcitrant C input due to changes in plant composition and predominance may decrease C accumulation and thereby SOC content. However, there was no consistent effect of N and P additions on NCB pattern between glucose amendment and vanillin amendment. The NCB pattern under N and P additions was in verse with the pattern of SOC decomposition. The NCB with glucose amendment under P addition and that with vanillin amendment under P and NP addition was much lower than Control (Figure 3b). Though the NCB with glucose was higher than Control, its effect on SOC accumulation might be offset by the larger extent increase in glucose mineralization and the PE over the time because glucose is more accessible to microorganisms than recalcitrant SOC (Blagodatskaya et al., 2014; Fontaine et al., 2004). The much lower NCB with glucose or vanillin amendment under P and NP additions than the Control suggested that P and NP addition would reduce C accumulation and thus SOC content. The higher NCB with glucose or vanillin amendment under N addition than the Control suggested a potential increasing SOC accumulation and SOC content. This was contrasted with our hypothesis and the decline in SOC content and storage. The seemingly contrary result suggested that the positive NCB and the potential increase in SOC content under N addition might merely be temporary, due to the shorter period of incubation in our experiment. It needed to be pointed out that the NCB here was based on a 32-day incubation period. The added substrate, especially glucose, remaining in soil would enter into the active C pool (Hou, 2018) that was most easily utilized by microorganisms (Blagodatskaya et al., 2014) and continue to stimulate the native SOM decomposition (Fontaine et al., 2004). Over time, the added glucose might be consumed but SOM decomposition would proceed and result in a net C loss (Fontaine et al., 2004).
Our previous studies from the same experimental site showed N and P additions increased plant aboveground plant biomass and modified plant community composition of alpine meadows from grasses and forbs dominated community to E . nutans dominated community (Li et al., 2014; Li et al., 2018b). These changes increased the C inputs to soil as plant residues, however, C accumulation, especially plant derived-C would be reduced and thereby decrease SOC content because of the increased C substrate decomposition and the decreased microbial CUE in response to N and P additions. The changes in plant litter quality through altering plant species’ predominance and plant composition also affected SOC decomposition due to the PEs caused by differential chemical structure of substrates. Our data showed N and P additions had positive PEs with glucose amendment but negative PEs with vanillin amendment, the total SOC decomposition with glucose amendment under P addition was increased by 33% and that with vanillin amendment under P and NP addition was increased by 7% and 8%, respectively (Figure 3a). This also contributed to the lower SOC content under P addition (-9%) than N (-2%) and NP (-4%) addition compared with Control (Table S1) (Li et al., 2018a). Therefore, the comprehensive effects of N and P additions on decomposition of SOC and plant-derived C substrates reduced SOC sequestration and thus SOC content in alpine meadows (Figure 5).
Studies on Tibetan alpine meadows, including one of our recent studies from the same study site, also showed that N and P enrichment decreased SOC content in the surface layer by reducing recalcitrant organic C content and slow C pool and increasing labile SOC content (Hou, 2018; Li et al., 2020 submitted; Luo et al., 2019; Luo et al., 2020). Some other studies out of Tibetan Plateau also reported that nutrient additions lowered SOC pool in the Arctic tundra (Mack et al., 2004), stimulated microbial decomposition of SOM in arctic permafrost soil (Wild et al., 2014) and caused considerable soil C losses in many high-latitude Arctic and sub-Arctic regions (Crowther et al., 2019). Given that alpine meadows and high-latitude Arctic tundra had higher soil C content than other grasslands (Yang et al., 2008; Reid et al., 2012; Wen et al., 2013), anthropogenic drivers such as atmospheric deposition and nutrient additions would decrease SOC content and soil C storage. This would weaken soil functioning as C pool of alpine meadows or Arctic grasslands.