Carbon limitations to nitrous oxide emissions in a humid tropical forest of the Brazilian Amazon

The availability of labile organic C for microbial metabolic processes could be an important factor regulating N₂O emissions from tropical soils. We explored the effects of labile C on the emissions of N₂O from a forest soil in the State of Rondônia in the southwestern quadrant of the Brazilian Amazon. We measured emissions of N₂O from a forest soil after amendments with solutions containing glucose, water only or NO₃^–. Addition of glucose to the forest soil resulted in very large increases in N₂O emissions whereas the water only and NO₃^– additions did not. These results suggest a strong C limitation on N₂O production in this forest soil in the southwestern Amazon.     

Emissions of CO2, N2O,and NO in conventional and no-till management practices in Rondônia,Brazil

Efforts to restore productivity of pastures often employ agricultural management regimes involving either tillage or no-tillage options combined with various combinations of fertilizer application, herbicide use and the planting of a cash crop prior to the planting of forage grasses. Here we report on the emissions of CO₂, N₂O and NO from the initial phases (first 6 months) of three treatments in central Rondônia. The treatments were (1) control; (2) conventional tillage followed by planting of forage grass (Brachiaria brizantha) and fertilizer additions; (3) no-tillage/herbicide treatment followed by two plantings, the first being a cash crop of rice followed by forage grass. In treatment 3, the rice was fertilized. Relative to the control, tillage increased CO₂ emission by 37% over the first 2 months, while the no-tillage/herbicide regime decreased CO₂ emissions by 7% over the same period. The cumulative N₂O emissions over the first 2 months from the tillage regime (0.94 kg N ha^–1) were much higher than the N₂O releases from either the no-tillage/herbicide regime (0.64 kg N ha^–1) or the control treatment (0.04 kg N ha^–1). The highest levels of N₂O fluxes from both management regimes were observed following N fertilizations. The cumulative NO releases over the first 2 months were largest in the tillage treatment (0.98 kg N ha^–1), intermediate in the no-tillage treatment (0.72 kg N ha^–1), and smallest in the control treatment (0.12 kg N ha^–1). For the first week following fertilization the percentage of fertilizer N lost as N₂O plus NO was 1.0% for the tillage treatment and 3.0% for the no-tillage treatment.     

Soil warming and carbon-cycle feedbacks to the climate system

In a decade-long soil warming experiment in a mid-latitude hardwood forest, we documented changes in soil carbon and nitrogen cycling in order to investigate the consequences of these changes for the climate system. Here we show that whereas soil warming accelerates soil organic matter decay and carbon dioxide fluxes to the atmosphere, this response is small and short-lived for a mid-latitude forest, because of the limited size of the labile soil carbon pool. We also show that warming increases the availability of mineral nitrogen to plants. Because plant growth in many mid-latitude forests is nitrogen-limited, warming has the potential to indirectly stimulate enough carbon storage in plants to at least compensate for the carbon losses from soils. Our results challenge assumptions made in some climate models that lead to projections of large long-term releases of soil carbon in response to warming of forest ecosystems.     

Consequences of nitrogen fertilization on soil methane consumption in a productive temperate deciduous forest

To investigate the consequences of long-term N additions on soil CH₄ dynamics, we measured in situ CH₄ uptake rates, soil profiles and kinetics parameters during the growing season in a temperate deciduous forest in northwestern Pennsylvania (Allegheny College Bousson Environmental Forest). Measurements were made in control and adjacent plots amended with 100 kg N ha^–1 year^–1 for 8 years. We found that the in situ consumption rates were 0.19±0.02 (mean±SE) for the control and 0.12±0.01 mg CH₄–C m^–2 h^–1 for the N treatment, indicating that consumption had been reduced by 35% after 8 years of N amendments. Despite the large difference in rates of consumption, there were no differences in the CH₄ concentration profiles between the control and N-amended plots. Laboratory incubations of CH₄ consumption throughout the soil column (organic horizon and mineral soil depths) showed that rates were greatest in the organic horizon of both control and N-amended soils, although consumption was reduced by 42% in the N-amended plot. However, the rate in the organic horizon was only about 50% the rate measured in organic horizons at other temperate forests. The apparent K_m [K_m(app)] value in the organic horizon of the control plot was fourfold less than the K_m(app) value in the organic horizon of another temperate forest, but similar to the K_m(app) values in adjacent plots amended with N for a decade. Unlike results for other temperate forests, K_m(app) values at Bousson generally did not decrease with soil depth. These results indicate that N cycling strongly controls the CH₄-consuming community, and suggest that alterations of the N cycle due to N deposition or addition may alter rates and the location of CH₄ consumption by soils, even in soils with high N content and cycling rates.