Turnover of soil organic matter and microbial biomass under C3-C4 vegetation change: implications for carbon sequestration in Mediterranean agricultural soils.

 Carbon isotopic composition of soils subjected to C3–C4 vegetation change can be used to estimate C turnover in bulk soil, but more specifically in soil organic matter (SOM) pools with fast and intermediate turnover rates. Analysis of phospholipid fatty acids (PLFA) has been widely used to evaluate rapid changes in soil microbial populations. In this study we investigated the effect a C3–C4 vegetation change experiment, along with a sustainable practice versus tillage soil microbial community composition as well as their isotopic C composition by compound-specific PLFA 13C analysis.Soils (Calcaric Cambisol) from an agricultural trial located in Southern Spain were sampled, which are characterized by high carbonate content (~27%) low fertility and low organic matter contents. The experimental trial consisted in replacing former C3 vegetation by maize crop (C4 plant) since February 2017, comprising two different treatments: A) after harvesting, maize surpluses were chopped and applied to surface soil, hereafter known as aboveground biomass “A” treatment; B) the total part of maize plant was left out after harvesting, including the roots, known as belowground biomass “B” treatment. Moreover, untreated soil was taken as control plots, “C”, where soil was tillaged and kept the same isotopic signature as the former land use. Composite soil samples (0-5 cm) were taken.PLFA profiles revealed a great abundance of bacterial activity, comprising gram-positive and gram-negative, along with branched (i-14:0, i-&a- 15:0, i:16:0, i-&a- 17:0) and mono- and polyunsaturated groups (16:1n7, 18:2n6, 18:1w9c and 18:1w7c). Significant increase of fungal abundance in “B” treatment may indicate decrease of litter decomposability, which facilitates fungal development. The “A” treatment also indicated a greater microbial activity, though intermediate in most of the groups compared to control. Lastly, in control plots, it is observed a significant decrease of G- bacteria, which correlates well with lower C content. indicates the low amount of easily available root exudates (Gütlein et al., 2017), which are the preferred C source for this microbial group. On the other hand, significant 13C enrichment of PLFAs varied across microbial groups. “B” plots showed greater 13C contribution for fungi, whereas the application of aboveground biomass contributes greatly to the gram-positive and gram-negative bacteria. PLFA 13C mean residence times were much longer for bacteria compared to the rest of microbial groups.Our results indicate that the addition of biomass in SOM-depleted agricultural soils resulted an increase of microbial biomass, denoting a predominant bacterial activity. Over 5 years of C3-C4 vegetation change, fungi and actinobacteria showed the fastest turnover rates compared to bacteria, which appeared to play a major role in the rapid acquisition of C into the soil microbial community. Fungi and actinobacteria appeared to have a delayed utilization of C or to prefer other C sources upon application of grounded biomass. Further discussion will be made on the implications of sustainable practices for enhancing C sequestration under Mediterranean climate.