Low‐temperature pyrolysis of biomass produces a product known as biochar The incorporation of this material into the soil has been advocated as a C sequestration method. Biochar also has the potential to influence the soil N cycle by altering nitrification rates and by adsorbing or NH3 Biochar can be incorporated into the soil during renovation of intensively managed pasture soils. These managed pastures are a significant source of N2O, a greenhouse gas, produced in ruminant urine patches. We hypothesized that biochar effects on the N cycle could reduce the soil inorganic‐N pool available for N2O‐producing mechanisms. A laboratory study was performed to examine the effect of biochar incorporation into soil (20 Mg ha−1) on N2O‐N and NH3–N fluxes, and inorganic‐N transformations, following the application of bovine urine (760 kg N ha−1). Treatments included controls (soil only and soil plus biochar), and two urine treatments (soil plus urine and soil plus biochar plus urine). Fluxes of N2O from the biochar plus urine treatment were generally higher than from urine alone during the first 30 d, but after 50 d there was no significant difference (P = 0.11) in terms of cumulative N2O‐N emitted as a percentage of the urine N applied during the 53‐d period; however, NH3–N fluxes were enhanced by approximately 3% of the N applied in the biochar plus urine treatment compared with the urine‐only treatment after 17 d. Soil inorganic‐N pools differed between treatments, with higher concentrations in the presence of biochar, indicative of lower rates of nitrification. The inorganic‐N pool available for N2O‐producing mechanisms was not reduced, however, by adding biochar.

Intensifying agricultural production and coastal urbanization are increasing nitrogen (N) loads to estuaries, potentially increasing emissions of the greenhouse gas nitrous oxide (N2O). Here we present a first assessment of how changes in land use intensity affect estuarine N2O fluxes. We measured N2O concentrations over marine‐freshwater transects in the wet and dry seasons in eight subtropical estuaries selected for differences in land use intensity. Daily estuary N loads ranged from 0.5 ± 0.4 kg N km−2 d−1 (minimally impacted) to 51 ± 30 kg N km−2 d−1 (highly impacted), corresponding to higher concentrations of all inorganic N species (nitrate, ammonium, and N2O) in the highly impacted estuaries. Net N2O fluxes from the eight estuaries ranged from −20 μg N2O‐N m−2 d−1 (sink) to +300 μg N2O‐N m−2 d−1 (source). However, neither N concentrations nor N loads explained the variations in N2O fluxes. Instead, seasonal differences in freshwater flushing times increased either N2O uptake (minimally impacted systems) or N2O efflux (moderately impacted systems) relative to N load. The lack of relationship between freshwater flushing times (kinetics) and N2O fluxes from the highly impacted estuaries, combined with evidence for both low carbon quality and phosphorous limitation in those systems, suggests that N2O emissions from highly impacted estuaries were controlled by stoichiometry rather than kinetics. This study shows that estuaries can shift from net sinks to sources of N2O as land use intensity increases but that the magnitude of this switch cannot be predicted based on N loads alone.

A Bayesian network model was developed to assess the combined influence of nutrient conditions and climate on the occurrence of cyanobacterial blooms within lakes of diverse hydrology and nutrient supply. Physicochemical, biological, and meteorological observations were collated from 20 lakes located at different latitudes and characterized by a range of sizes and trophic states. Using these data, we built a Bayesian network to (1) analyze the sensitivity of cyanobacterial bloom development to different environmental factors and (2) determine the probability that cyanobacterial blooms would occur. Blooms were classified in three categories of hazard (low, moderate, and high) based on cell abundances. The most important factors determining cyanobacterial bloom occurrence were water temperature, nutrient availability, and the ratio of mixing depth to euphotic depth. The probability of cyanobacterial blooms was evaluated under different combinations of total phosphorus and water temperature. The Bayesian network was then applied to quantify the probability of blooms under a future climate warming scenario. The probability of the "high hazardous" category of cyanobacterial blooms increased 5% in response to either an increase in water temperature of 0.8°C (initial water temperature above 24°C) or an increase in total phosphorus from 0.01 mg/L to 0.02 mg/L. Mesotrophic lakes were particularly vulnerable to warming. Reducing nutrient concentrations counteracts the increased cyanobacterial risk associated with higher temperatures.

Anthropogenic nitrogen inputs cause major negative environmental impacts, including emissions of the important greenhouse gas N2O. Despite their importance, shifts in terrestrial N loss pathways driven by global change are highly uncertain. Here we present a coupled soil-atmosphere isotope model (IsoTONE) to quantify terrestrial N losses and N2O emission factors from 1850-2020. We find that N inputs from atmospheric deposition caused 51% of anthropogenic N2O emissions from soils in 2020. The mean effective global emission factor for N2O was 4.3 ± 0.3% in 2020 (weighted by N inputs), much higher than the surface area-weighted mean (1.1 ± 0.1%). Climate change and spatial redistribution of fertilisation N inputs have driven an increase in global emission factor over the past century, which accounts for 18% of the anthropogenic soil flux in 2020. Predicted increases in fertilisation in emerging economies will accelerate N2O-driven climate warming in coming decades, unless targeted mitigation measures are introduced.