Abstract
Long-term contrasts in agricultural management can shift soil resource availability with potential consequences to microbial carbon (C) use efficiency (CUE) and the fate of C in soils. Isothermal calorimetry was combined with 13C-labeled glucose stable isotope probing (SIP) of 16S rRNA genes to test the hypothesis that organically managed soils would support microbial communities with greater thermodynamic efficiency compared to conventional soils due to a legacy of lower resource availability and a resultant shift toward communities supportive of more oligotrophic taxa. Resource availability was greater in conventionally managed soils, with 3.5 times higher available phosphorus, 5% more nitrate, and 36% more dissolved organic C. The two management systems harbored distinct glucose-utilizing populations of Proteobacteria and Actinobacteria, with a higher Proteobacteria:Actinobacteria ratio (2.4 vs. 0.7) in conventional soils. Organically managed soils also harbored notable activity of Firmicutes. Thermodynamic efficiency indices were similar between soils, indicating that glucose was metabolized at similar energetic cost. However, differentially abundant glucose utilizers in organically managed soils were positively correlated with soil organic matter (SOM) priming and negatively correlated to soil nutrient and carbon availability, respiration, and heat production. These correlation patterns were strongly reversed in the conventionally managed soils indicating clear differentiation of microbial functioning related to soil resource availability. Fresh C addition caused proportionally more priming of SOM decomposition (57 vs. 51%) in organically managed soils likely due to mineralization of organic nutrients to satisfy microbial demands during glucose utilization in these more resource deprived soils. The additional heat released from SOM oxidation may explain the similar community level thermodynamic efficiencies between management systems. Restoring fertility to soils with a legacy of nutrient limitation requires a balanced supply of both nutrients and energy to protect stable SOM from microbial degradation. These results highlight the need to consider managing C for the energy it provides to ıcritical biological processes that underpin soil health.
Highlights
Microorganisms are central drivers of soil organic matter (SOM) dynamics and are critical to controlling the flow of carbon (C) through terrestrial ecosystems
Available P was 3.5 times greater, NO−3 -N was 5% greater, and Dissolved organic C (DOC) was 36% greater in soils under CON compared to ORG management regardless of cropping history (Table 1), reflecting different nutrient input regimes and crop residue returns between the two systems over the course of the 20 year field study
The effect of management system on Microbial biomass C (MBC) depended on cropping history
Summary
Microorganisms are central drivers of soil organic matter (SOM) dynamics and are critical to controlling the flow of carbon (C) through terrestrial ecosystems. Microbial carbon use efficiency (CUE) is receiving increased attention as an important factor governing the fate of metabolized C and SOM formation, nutrient dynamics, and release of C to the atmosphere (Manzoni et al, 2012; Schimel and Schaeffer, 2012; Cotrufo et al, 2013; Blagodatskaya et al, 2014). Soil microbial community structure (i.e., abundance and composition), resource availability, and substrate quality (nutrient and energy content) are important factors that govern CUE (Manzoni et al, 2012; Sinsabaugh et al, 2013; Herrmann et al, 2014; Lee and Schmidt, 2014; Bölscher et al, 2016). Microbial CUE at the community level reflects the complex interplay of both microbial community structure and the chemical and energetic properties of the utilized substrates
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