No abstract

No abstract

The natural soil N supply in volcanic soils (Andisols) can be a significant source of plant-available N for agro-ecosystems. Nevertheless, intensive farming systems in south Chile apply high fertilization rates, which lead to high production costs and involve a risk for adverse ecosystem effects. In order to achieve sustainable land management, a better understanding of the processes that govern soil N availability and loss, and their external drivers, is required. In this study, we selected a winter-cropland, a summer crop-winter fallow rotation, and a forest, used as a reference ecosystem. Gross N transformations ((15)N isotope dilution) and microbial community structure (phospho-lipid fatty acid analysis) in the topsoil were determined. Gross N mineralization was about ten times lower in the agro-ecosystems than in the forest, while gross nitrification was low in all sites. Gross N immobilization equalized or exceeded the gross inorganic N production in all sites. Microbial biomass was 3-5 times more abundant in the forest than in the agro-ecosystems. A positive relationship between the ratio fungi/bacteria and total microbial biomass was observed in these Andisols. We suggest that the reduction in fungal biomass induced a lower extracellular enzyme production and limited soil organic matter depolymerisation in the agro-ecosystems. We conclude that soil N cycling was unable to provide a significant N input for the croplands, but also the risk for ecosystem N losses was low, even under fallow soil conditions. Current fertilization practices appropriately anticipated the soil N cycling processes, but further research should indicate the potential of alternative land management to reduce fertilizer cost

Gaseous nitrogen exchanges from undrained peat soils and the effect of external nitrate input are poorly understood. This paper reports net N-2, and N2O exchanges as well as the short-term responses to nitrate addition from contrasting tussock and hollow soil habitats in an undrained monolith fen ecosystem located in northeast Poland. Gaseous N exchange rates were quantified by means of a He substitution technique. The net N-2 production in hollows (2.53 mg N m(-2) hour(-1)) was significantly higher than in tussocks (1.04 mg N m(-2) hour(-1)). Hollows also constituted a marked N2O sink (-3.04 mu g N m(-2) hour(-1)), while tussocks were a source for atmospheric N2O (2.08 mu g N m(-2) hour(-1)). Following amendment by NO3-, at a rate similar to atmospheric NO3- deposition (wet + dry), hollows showed a drastic shift to net production of N2O but a non-significant increase in N-2 production. In tussocks only a minor increase of N-2 and N2O production was observed after NO3- addition. This study emphasizes the influence of physico-chemical conditions and biotic resource competition on the rates and responses of microbial denitrification in undrained fen ecosystems. Inferring a simple field-scale estimation of gaseous N emission suggests that undrained, monolith fen ecosystems constitute substantial sources of N-2 (1.79 mg N m(-2) hour(-1)) and considerable sinks for N2O (-0.96 mu g N m(-2) hour(-1)). These findings are of importance to further improve greenhouse gas budgeting and assess the impacts of global change on undrained fen ecosystems

The microbial consumption of nitrous oxide (N2O) has gained great interest since it was revealed that this process could mitigate the greenhouse effect of N2O. The consumption of N2O results from its reduction to dinitrogen gas (N2) as part of the denitrification process. However, there is ongoing debate regarding an alternative pathway, namely reduction of N2O to NH4(+), or assimilatory N2O consumption. To date, this pathway is poorly investigated and lacks unambiguous evidence. Enrichment of denitrifying activated sludge using a mineral nitrogen-free medium rendered a mixed culture capable of anoxic and oxic N2O consumption. Dilution plating, isolation and deoxyribonucleic acid fingerprinting identified a collection of Pseudomonas stutzeri strains as dominant N2O consumers in both anaerobic and aerobic enrichments. A detailed isotope tracing experiment with a Pseudomonas stutzeri isolate showed that consumption of N2O via assimilatory reduction to NH4(+) was absent. Conversely, respiratory N2O reduction was directly coupled to N2 fixation.