Field-aged rice hull biochar stimulated the methylation of mercury and altered the microbial community in a paddy soil under controlled redox condition changes.

Mercury (Hg) contaminated paddy soils are hot spots for methylmercury (MeHg) which can enter the food chain via rice plants causing high risks for human health. Biochar can immobilize Hg and reduce plant uptake of MeHg. However, the effects of biochar on the microbial community and Hg (de)methylation under dynamic redox conditions in paddy soils are unclear. Therefore, we determined the microbial community in an Hg contaminated paddy soil non-treated and treated with rice hull biochar under controlled redox conditions (< 0 mV to 600 mV) using a biogeochemical microcosm system. Hg methylation exceeded demethylation in the biochar-treated soil. The aromatic hydrocarbon degraders Phenylobacterium and Novosphingobium provided electron donors stimulating Hg methylation. MeHg demethylation exceeded methylation in the non-treated soil and was associated with lower available organic matter. Actinobacteria were involved in MeHg demethylation and interlinked with nitrifying bacteria and nitrogen-fixing genus Hyphomicrobium. Microbial assemblages seem more important than single species in Hg transformation. For future directions, the demethylation potential of Hyphomicrobium assemblages and other nitrogen-fixing bacteria should be elucidated. Additionally, different organic matter inputs on paddy soils under constant and dynamic redox conditions could unravel the relationship between Hg (de)methylation, microbial carbon utilization and nitrogen cycling.

Transferring network analysis to the study of potential biogeochemical interactions of phosphorus-relevant elements in floodplain subsoils - A new use case for the Soilscape Network Approach (SNAp).

Subsurface phosphorus (P) loss from deep P stocks in floodplain subsoils can contribute to eutrophication of freshwaters. To date, knowledge on the complex biogeochemical interactions of P in floodplain subsoils is too scarce to enable targeted P management to mitigate subsurface P loss from deep P stocks. We propose using graph theory and the Soilscape Network Approach (SNAp) based on correlations between P-relevant elements to study these complex biogeochemical interactions in the soilscape. Complex interactions of several elements in soils are difficult to investigate from a holistic perspective with conventional data analysis. We translated soil element data from topsoils and subsoils of terrestrial sites, proximal and distal floodplain sites into relational data and analyzed network structure, centrality, and modularity. The results indicate that a higher frequency of groundwater level fluctuations in distal subsoils and proximal topsoils could result in 24-44 % less biogeochemical interaction compared to sites with stable conditions. Impeded microbial processes on the frequently disturbed sites may explain this finding. Our analyses suggest biogeochemical differences between floodplain topsoils and subsoils expressed in 24 % lower and 75 % higher network connectivity in distal and proximal subsoils (respectively). We also found 22 % lower network connectivity in distal than proximal floodplain subsoils, suggesting biogeochemical differences between both soil sections. These findings imply that floodplain P management should not take a whole-floodplain approach but a 3D-approach, which differentiates laterally between floodplain zones and vertically between soil sections. In addition, SNAp indicated that Fe(II) oxides are important in P biogeochemistry of floodplain subsoils but are not the key element. Instead, labile P forms are suggested to have different major associations in distal (Alox, Feox) versus proximal deep P stocks (Alox, Mn, Ca). Our study provides new insights into the biogeochemistry of deep P stocks in floodplain subsoils which require targeted validation by other methods.