How does simulated climate change affect the susceptibility of SOM to priming by LMWOS in the Subarctic?

Soil organic matter (SOM) stabilization plays an important role in the long-term storage of carbon (C). However, many ecosystems are undergoing climate change, which will change the soil C balance via altered plant communities and productivity that change C inputs, and altered C losses via changes in SOM decomposition. The ongoing change of aboveground plant communities in the Subarctic (“greening”) will increase rhizosphere inputs containing low molecular weight organic substances (LMWOS), which will likely affect C-starved microbial decomposers and their subsequent contribution to SOM mineralization (priming effect).In the present study, we simulated the effects of climate change with N fertilization (simulating warming enhanced nutrient cycling) and litter additions (simulating arctic greening) in Abisko, Sweden. The 6 sampled field-treatments included a full factorial combination of 3-years of chronic N addition and litter additions, as well as, a single year of extreme climate change (3x N fertilizer or litter additions in one growth season). We found that N treatments changed plant community composition and productivityand that the associated shift in belowground LMWOS induced shifts in the soil microbial community. In the chronic N fertilization treatments, plant productivity, and therefore belowground LMWOS input, increased. This coincided with a tendency for more bacterial dominated decomposition (lower fungi/bacterial growth ratio). However, N treatments had no effect on soil C mineralization, but increased gross N mineralization.

These responses in belowground communities and processes driven by rhizosphere input prompted the next question: how does simulated climate change affect the susceptibility of SOM to priming by LMWOS? To assess this question and determine the microbial mechanisms underpinning priming of SOM mineralization, we added a factorial set of additions including ¹³C-glucose with and without mineral N, and ¹³C-alanine semi-continuously (every 48 hours) to simulate the effect of rhizosphere LMWOS on SOM mineralization and microbial activity. We incubated these samples for 2 weeks and assessed the priming of soil C and gross N mineralization, bacterial and fungal growth rates, PLFAs, enzyme activities, and microbial C use efficiency (CUE). We found that alanine addition primed soil C mineralization by 34%, which was higher than soil C priming induced by glucose and glucose with N. Furthermore, glucose primed fungal growth, whereas the alanine primed bacterial growth, but microbial PLFAs did not respond to either treatment. The C enzyme acquisition activity was higher than N enzyme acquisition activity in all the treatments, while P enzyme acquisition activity was higher than C for all the treatments. Surprisingly, this suggested a chronic microbial limitation by P, which was unaffected by field and lab treatments. LMWOS additions generally reduced microbial CUE. Responses of microbial mineralization of N from SOM to LMWOS suggested a directed microbial effort towards targeting resources that limited bacterial or fungal growth, suggesting that microbial SOM-use shifted to N-rich components (selective microbial “N-mining”), in contrast with enzyme results. Surprisingly, alanine primed the highest N mineralization compared other additions indicating that there was strong N-mining even if N was sufficient.