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A general occurrence of the phenomenon of priming effect (PE) across varying land use and soil types has not been established so far, particularly on a large geographical scale. Moreover, the impacts of soil properties and soil organic matter (SOM) distribution among physical fractions on the magnitude of PE are still unclear. We addressed these knowledge gaps by incubating thirty five soils with or without addition of 13C labeled cellulose for 262 days. The soils were sampled from different land uses and depths of contrasting soil types from five continents (Asia, Europe, America, Australia and Africa). Results showed positive PE in all soils including grassland, cropland, forest, savannah and orchard. On average, the cumulative PE represented 27.0 ± 28.7% of the CO2 efflux in control soils and 28.48 ± 21.08% of the remaining/unrespired cellulose-C. The PE was 72.1% higher in surface than deep soils suggesting that surface soils are more prone to PE induced by cellulose addition. Variations in PE were mainly explained by soil characteristics and not by land use. We found that the PE increased with the relative abundance of SOM not associated with minerals and rich in nitrogen (N). The observation of systematic positive PE in all soils suggested that microbial co-metabolism to decompose SOM is a widespread microbial strategy. Our results also support the idea that microorganisms use co-metabolism to mine nutrients in SOM since they target N-rich fractions. However, other mechanisms are also at play since positive PE was maintained despite the high availability of mineral nutrients. Overall, PE is a worldwide process playing a major role for soil C dynamics, especially in N-rich soils

A general occurrence of the phenomenon of priming effect (PE) across varying land use and soil types has not been established so far, particularly on a large geographical scale. Moreover, the impacts of soil properties and soil organic matter (SOM) distribution among physical fractions on the magnitude of PE are still unclear. We addressed these knowledge gaps by incubating thirty five soils with or without addition of C-13 labeled cellulose for 262 days. The soils were sampled from different land uses and depths of contrasting soil types from five continents (Asia, Europe, America, Australia and Africa). Results showed positive PE in all soils including grassland, cropland, forest, savannah and orchard. On average, the cumulative PE represented 27.0 +/- 28.7% of the CO2 efflux in control soils and 28.48 +/- 21.08% of the remaining/unrespired cellulose-C. The PE was 72.1% higher in surface than deep soils suggesting that surface soils are more prone to PE induced by cellulose addition. Variations in PE were mainly explained by soil characteristics and not by land use. We found that the PE increased with the relative abundance of SOM not associated with minerals and rich in nitrogen (N). The observation of systematic positive PE in all soils suggested that microbial co-metabolism to decompose SOM is a widespread microbial strategy. Our results also support the idea that microorganisms use co-metabolism to mine nutrients in SOM since they target N-rich fractions. However, other mechanisms are also at play since positive PE was maintained despite the high availability of mineral nutrients. Overall, PE is a worldwide process playing a major role for soil C dynamics, especially in N-rich soils.

Extracellular enzymes catalyze biogeochemical reactions in soil, cycling carbon and nutrients in agricultural systems. Enzymes respond quickly to soil management, including organic amendment inputs, such as biochar, a charcoal-like solid byproduct of bioenergy production. In a previous agricultural field trial, a pine biochar amendment caused an approximately 40% decrease in the enzyme activities of β-glucosidase (BG) and phosphatase (PHOS). The large surface area of the pine biochar has the potential to sorb nutrients and other organic molecules. To test if sorption caused decreased enzyme activity, we used a laboratory assay to quantify the activity of two sorbed enzymes: BG and acid PHOS, involved in the cycling of carbon and phosphorous. The enzymes were incubated with three solid phases: (1) the high surface area pine biochar, (2) the agricultural soil, and (3) a low surface area grass biochar, for an additional comparison. We quantified the sorbed enzymes at pH 6, 7, and 8, using a Bradford protein assay, and measured the immobilized enzyme activities via high-throughput fluorometric analysis. After sorption onto pine biochar, detectable BG and PHOS activity levels dropped by over 95% relative to the soil, supporting direct sorption as one mechanism that reduces enzyme activity in biochar amended soil. This laboratory assay demonstrated that sorption could account for the lack of priming of native soil organic matter and changes in soil phosphorous cycling after pine biochar addition.

Temporal dynamics of C isotopic composition (δ13C) of CO2 and leaf litter was monitored during a litter decomposition experiment using Arbutus unedo L., as a slow decomposing model substrate. This allowed us (1) to quantify isotopic discrimination variation during litter decomposition, and (2) to test whether selective substrate use or kinetic fractionation could explain the observed isotopic discrimination. Total cumulative CO2-C loss (CL) comprised 27% of initial litter C. Temporal evolution of CL was simulated following a three-C-pool model. Isotopic composition of respired CO2 (δRL) was higher with respect to that of the bulk litter. The isotopic discrimination Δ(L/R) varied from −2‰ to 0‰ and it is mostly attributed to the variations of δRL. A three-pool model, with the three pools differing in their δ13C, described well the dynamic of Δ(L/R), in the intermediate stage of the process. This suggests that the observed isotopic discrimination between respired CO2 and bulk litter is in good agreement with the hypothesis of successive consumption of C compounds differing in δ13C during decomposition. However, to explain also 13C-CO2 dynamics at the beginning and end of the incubation the model had to be modified, with discrimination factors ranging from −1‰ to −4.6‰ attributed to the labile and the recalcitrance pool, respectively. We propose that this discrimination is also the result of further selective use of specific substrates within the two pools, likely being both the labile and recalcitrant pool of composite nature. In fact, the 2‰ 13C enrichment of the α-cellulose observed by the end of the experiment, and potentially attributable to kinetic fractionation, could not explain the measured Δ(L/R) dynamics.