Linking mass balances and thermodynamic energy balances at different water contents, temperature and nutrient supply in simplified model systems with artificial soils

Microbial communities in soil play a pivotal role in nutrient cycling and organic matter decomposition, relying on carbon (C) and energy sources for growth. The allocation of these substrates, crucial for their metabolic activities, is influenced by environmental conditions. We hypothesize a close linkage between carbon and energy fluxes in soil, with environmental conditions shaping substrate requirements and activities, thereby influencing Carbon Use Efficiency (CUE) and Energy Use Efficiency (EUE).

To establish the link between matter and energy fluxes, we employed artificial soil consisting of a sand, clay, and silt mixture as a simplified system, with cellulose as the only added substrate. This model system allowed us to investigate the relationship between C and energy fluxes without a large background of soil organic matter background (SOM). Experimental conditions included three different water contents (10%, 14.4% and 19%), two ratios of added carbon (C) to nitrogen (N) (C/N = 18 and C/N = 9), and two temperature regimes (7  and 20 ). Mineralization (measured by GC-TCD) and residual cellulose (measured by phenol sulphuric acid assay) were quantified on sampling days, while continuous monitoring of heat production rate (P) was monitored using isothermal microcalorimeter (i.e. TAM Air).

Results revealed a clear correlation between environmental conditions and microbial activities. Higher moisture levels led to increased CO₂ production, heat generation, and cellulose degradation. Similarly, lower N supply (higher C/N ratio) exhibited the same trend. Decreased temperatures resulted in minimal CO₂ evolution and heat production rates and diminished cellulose degradation.

Analysis of CUE over time indicated a decline, possibly due to biomass recycling and additional respiration. Surprisingly, little apparent effect of water content or N supply on CUE was observed. CUE in the two temperature treatments show similar decreasing trends, but CUE is at an overall higher level at 7°C. EUE remained relatively stable over time but tended to decrease under conditions of environmental stress, such as extreme water content or N limitation. However, high variability is observed, and no statistical significance can be found.

An energy balance framework is being developed and will be used to calculate CUE from a theoretical perspective. Comparisons between experimentally derived CUE and theoretically calculated values will prompt a re-evaluation of the underlying assumptions and a call for refined theoretical protocols. In summary, our findings suggest that environmental conditions significantly influence cellulose degradation. However, a clear correlation between CUE and EUE requires further analyses and experimental improvements. This study contributes to our understanding of the intricate relationships between carbon and energy fluxes in soil microbial systems and emphasizes the need for nuanced analyses in future research.