Efficient microbial growth has been a central focus of determining how microbial activity in soil helps to build stable organic matter. Efficiency has often been examined through the lens of substrate uptake and mineralization rates, yet processes like exudation and mineral stabilization of carbon substrates may be equally important for influencing microbial growth dynamics. We explored these potential effects by measuring the partitioning of 13C-glucose in a forest soil among pools of respired CO2, dissolved organic carbon, microbial biomass, and unextractable 13C (e.g., total soil 13C – (13MBC + 13DOC)). Labeled glucose was tracked with high temporal resolution by destructively harvesting mesocosms 9 times over 72 hours of total incubation. We compared our results to other published tracer experiments (N = 23) via meta-analysis.
Results/Conclusions
Carbon use efficiency (= 13MBC/(13MBC + 13CO2)) was ~0.7 within 6 h of amendment and minimal tracer remained in DOC indicating rapid microbial uptake of substrate. Focusing on 13MBC and 13CO2 alone ignored the largest pool of tracer; ~45% was unextractable from soil and presumably in the form of stabilized microbial exudates. Meta-analysis confirmed that a wide variety of soils incubated with glucose had significant accumulation of tracer in an unextractable form after both short (≤2 days; 29% of tracer C; p<0.0001) and long (>24 days; 27% of tracer C; p<0.0001) incubations. The percent of tracer C found in the unextractable pool was positively associated percent clay in these studies (r2 = 0.27; p<0.001), indicating an important role for mineral stabilization of tracer C. This large pool of generally unaccounted for tracer C has the potential to dramatically change estimates of CUE in soil that consider only two pools as microbial uptake (i.e., uptake = 13MBC + 13CO2). Future work is necessary to ascertain the precise mechanism of stabilization in order to better understand the source and fate of C substrates in soil, particularly as climate change alters the net flux of C into the atmosphere from terrestrial habitats.