Global trends and uncertainties in terrestrial denitrification and N2O emissions.

Soil nitrogen (N) budgets are used in a global, distributed flow-path model with 0.5° × 0.5° resolution, representing denitrification and N2O emissions from soils, groundwater and riparian zones for the period 1900-2000 and scenarios for the period 2000-2050 based on the Millennium Ecosystem Assessment. Total agricultural and natural N inputs from N fertilizers, animal manure, biological N2 fixation and atmospheric N deposition increased from 155 to 345 Tg N yr(-1) (Tg = teragram; 1 Tg = 10(12) g) between 1900 and 2000. Depending on the scenario, inputs are estimated to further increase to 408-510 Tg N yr(-1) by 2050. In the period 1900-2000, the soil N budget surplus (inputs minus withdrawal by plants) increased from 118 to 202 Tg yr(-1), and this may remain stable or further increase to 275 Tg yr(-1) by 2050, depending on the scenario. N2 production from denitrification increased from 52 to 96 Tg yr(-1) between 1900 and 2000, and N2O-N emissions from 10 to 12 Tg N yr(-1). The scenarios foresee a further increase to 142 Tg N2-N and 16 Tg N2O-N yr(-1) by 2050. Our results indicate that riparian buffer zones are an important source of N2O contributing an estimated 0.9 Tg N2O-N yr(-1) in 2000. Soils are key sites for denitrification and are much more important than groundwater and riparian zones in controlling the N flow to rivers and the oceans.

Decreased N2O reduction by low soil pH causes high N2O emissions in a riparian ecosystem.

Quantification of harmful nitrous oxide (N(2)O) emissions from soils is essential for mitigation measures. An important N(2)O producing and reducing process in soils is denitrification, which shows deceased rates at low pH. No clear relationship between N(2)O emissions and soil pH has yet been established because also the relative contribution of N(2)O as the denitrification end product decreases with pH. Our aim was to show the net effect of soil pH on N(2)O production and emission. Therefore, experiments were designed to investigate the effects of pH on NO(3)(-) reduction, N(2)O production and reduction and N(2) production in incubations with pH values set between 4 and 7. Furthermore, field measurements of soil pH and N(2)O emissions were carried out. In incubations, NO(3)(-) reduction and N(2) production rates increased with pH and net N(2)O production rate was highest at pH 5. N(2)O reduction to N(2) was halted until NO(3)(-) was depleted at low pH values, resulting in a built up of N(2)O. As a consequence, N(2)O:N(2) production ratio decreased exponentially with pH. N(2)O reduction appeared therefore more important than N(2)O production in explaining net N(2)O production rates. In the field, a negative exponential relationship for soil pH against N(2)O emissions was observed. Soil pH could therefore be used as a predictive tool for average N(2)O emissions in the studied ecosystem. The occurrence of low pH spots may explain N(2)O emission hotspot occurrence. Future studies should focus on the mechanism behind small scale soil pH variability and the effect of manipulating the pH of soils.

Denitrification at pH 4 by a soil-derived Rhodanobacter-dominated community.

Soil denitrification is a major source of nitrous oxide emission that causes ozone depletion and global warming. Low soil pH influences the relative amount of N2O produced and consumed by denitrification. Furthermore, denitrification is strongly inhibited in pure cultures of denitrifying microorganisms below pH 5. Soils, however, have been shown to denitrify at pH values as low as pH 3. Here we used a continuous bioreactor to investigate the possibility of significant denitrification at low pH under controlled conditions with soil microorganisms and naturally available electron donors. Significant NO3⁻ and N2O reduction were observed for 3 months without the addition of any external electron donor. Batch incubations with the enriched biomass showed that low pH as well as low electron donor availability promoted the relative abundance of N2O as denitrification end-product. Molecular analysis of the enriched biomass revealed that a Rhodanobacter-like bacterium dominated the community in 16S rRNA gene libraries as well as in FISH microscopy during the highest denitrification activity in the reactor. We conclude that denitrification at pH 4 with natural electron donors is possible and that a Rhodanobacter species may be one of the microorganisms involved in acidic denitrification in soils.

N2O emission hotspots at different spatial scales and governing factors for small scale hotspots.

Chronically nitrate-loaded riparian buffer zones show high N(2)O emissions. Often, a large part of the N(2)O is emitted from small surface areas, resulting in high spatial variability in these buffer zones. These small surface areas with high N(2)O emissions (hotspots) need to be investigated to generate knowledge on the factors governing N(2)O emissions. In this study the N(2)O emission variability was investigated at different spatial scales. Therefore N(2)O emissions from three 32 m(2) grids were determined in summer and winter. Spatial variation and total emission were determined on three different scales (0.3 m(2), 0.018 m(2) and 0.0013 m(2)) at plots with different levels of N(2)O emissions. Spatial variation was high at all scales determined and highest at the smallest scale. To test possible factors inducing small scale hotspots, soil samples were collected for slurry incubation to determine responses to increased electron donor/acceptor availability. Acetate addition did increase N(2)O production, but nitrate addition failed to increase total denitrification or net N(2)O production. N(2)O production was similar in all soil slurries, independent of their origin from high or low emission soils, indicating that environmental conditions (including physical factors like gas diffusion) rather than microbial community composition governed N(2)O emission rates.