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.