No abstract

Phosphorus (P) availability can affect nitrogen (N) dynamics in forest soil, and this effect might depend largely on the soil N status of forest ecosystems. So far, however, this view has not been well tested among forests with contrasting N status. Here, we used a 6‐year experiment with additions of N and P to evaluate the effects of P availability and its interaction with N availability on soil N dynamics in one N‐saturated and two N‐limited tropical forests in southern China. Soil inorganic N concentrations and rates of N mineralization, nitrification, nitrous oxide (N2O) emission and nitrate leaching were measured. Our results showed that addition of P alone changed soil N dynamics in the N‐saturated forest only; it accelerated rates of soil N transformation and decreased rates of N2O emission and nitrate leaching, but had no significant effects on N dynamics in the two N‐limited forests. Furthermore, compared with the addition of N alone, addition of both N and P caused significant increases in the rates of net N mineralization and nitrification and a significant decrease in N2O emission in the two N‐limited forests. Our results suggest that P availability stimulates soil N dynamics only when the ecosystem is saturated with N or there is considerable N deposition. Highlights We compared the effects of P addition on soil N dynamics among forests with different N status. Addition of P alone changed N dynamics in the N‐saturated forest, but not in N‐limited forests. Combined N and P additions had a larger effect on N dynamics than N addition alone. Phosphorus addition affects N dynamics only when an ecosystem has considerable N status.

Although it has been established that electron mediators substantially promote extracellular electron transfer (EET), electron shuttling pathways are not fully understood. Here, a new electron shuttling pathway was found in the EET process by Shewanella oneidensis MR-1 with resazurin, a lipophilic electron mediator. With resazurin, the genes encoding outer-membrane cytochromes (mtrCBA and omcA) were downregulated. Although cytochrome deletion substantially reduced biocurrent generation to 1−12% of that of wild-type (WT) cells, the presence of resazurin restored biocurrent generation to 168 μA•cm −2 (ΔmtrA/ omcA/mtrC), nearly equivalent to that of WT cells (194 μA•cm −2 ), indicating that resazurin-mediated electron transfer was not dependent on the Mtr pathway. Biocurrent generation by resazurin was much lower in ΔcymA and ΔmtrA/omcA/mtrC/fccA/cctA mutants (4 and 6 μA•cm −2 ) than in WT cells, indicating a key role of FccA, CctA, and CymA in this process. The effectiveness of resazurin in EET of Mtr cytochrome mutants is also supported by cyclic voltammetry, resazurin reduction kinetics, and in situ c-type cytochrome spectroscopy results. The findings demonstrated that low molecular weight, lipophilic electron acceptors, such as phenoxazine and phenazine, may facilitate electron transfer directly from periplasmic and inner membrane proteins, thus providing new insight into the roles of exogenous electron mediators in electron shuttling in natural and engineered biogeochemical systems.