Simulated nitrogen deposition influences soil greenhouse gas fluxes in a Mediterranean dryland.

Soil nitrogen (N) availability is a key driver of soil-atmosphere greenhouse gas (GHG) exchange, yet we are far from understanding how increases in N deposition due to human activities will influence the net soil-atmosphere fluxes of the three most important GHGs: nitrous oxide (N2O), methane (CH4) and carbon dioxide (CO2). We simulated four levels of N deposition (10, 20 and 50 kg N ha-1 yr-1, plus unfertilised control) to evaluate their effects on N2O, CH4 and CO2 soil fluxes in a semiarid shrubland in central Spain. After 8 years of experimental fertilisation, increasing N availability led to a consistent increase in N2O emissions, likely due to simultaneous increases in soil microbial nitrification and/or denitrification processes. However, only intermediate levels of N fertilisation reduced CH4 uptake, while increasing N fertilisation had no effects on CO2 fluxes, suggesting complex interactions between N deposition loads and GHG fluxes. Our study provides novel insight into the responses of GHGs to N deposition in drylands, forecasting increases in N2O emissions, and decreases in CH4 uptake rates, with likely consequences to the on-going climate change.

Nitrification inhibitor DMPSA mitigated N2O emission and promoted NO sink in rainfed wheat.

Fertilized cropping systems are important sources of nitrous oxide (N2O) and nitric oxide (NO) to the atmosphere, and biotic and abiotic processes control the production and consumption of these gases in the soil. In fact, the inhibition of nitrification after application of urea or an ammonium-based fertilizer to agricultural soils has resulted in an efficient strategy to mitigate both N2O and NO in aerated agricultural soils. Therefore, the NO and N2O mitigation capacity of a novel nitrification inhibitor (NI), 2-(3,4-dimethyl-1H-pyrazol-1-yl) succinic acid isomeric mixture (DMPSA), has been studied in a winter wheat crop. A high temporal resolution of fluxes of NO and NO2, obtained by using automatic chambers for urea (U) and urea with DMPSA, allowed a better understanding of the temporal net emissions of these gases under field conditions. Seventy-five days after fertilization, the effective reduction of nitrification by DMPSA significantly decreased the production of NO with respect to the treatment without it, giving net consumption of NO in the soil (-61.72 g-N ha-1) for U + DMPSA in comparison to net production (227.44 g-N ha-1) for U. The explanation of NO deposition after NI application, due to biotic and abiotic processes in the soil-plant system, supposes a challenge that needs to be studied in the future. In the case of N2O, the addition of DMPSA significantly mitigated the emissions of this gas by 71%, though the total N2O emissions in both fertilized treatments were significantly greater than those of the control (43.69 g-N ha-1). Regarding the fertilized treatments, no significant effect of DMPSA in comparison to urea alone was observed on grain yield nor bread-making wheat quality. To sum up, we got a significant reduction of N2O and NO with the addition of DMPSA, without a loss in yield and quality parameters in wheat.

The effect of nitrification inhibitors on NH3 and N2O emissions in highly N fertilized irrigated Mediterranean cropping systems.

There is an increasing concern about the negative impacts associated to the release of reactive nitrogen (N) from highly fertilized agro-ecosystems. Ammonia (NH3) and nitrous oxide (N2O) are harmful N pollutants that may contribute both directly and indirectly to global warming. Surface applied manure, urea and ammonium (NH4+) based fertilizers are important anthropogenic sources of these emissions. Nitrification inhibitors (NIs) have been proposed as a useful technological approach to reduce N2O emission although they can lead to large NH3 losses due to increasing NH4+ pool in soils. In this context, a field experiment was carried out in a maize field with aiming to simultaneously quantify NH3 volatilization and N2O emission, assessing the effect of two NIs 3,4­dimethilpyrazol phosphate (DMPP) and 3,4­dimethylpyrazole succinic acid (DMPSA). The first treatment was pig slurry (PS) before seeding (50 kg N ha-1) and calcium ammonium nitrate (CAN) at top-dressing (150 kg N ha-1), and the second was DMPP diluted in PS (PS + DMPP) (50 kg N ha-1) and CAN + DMPSA (150 kg N ha-1) also before seeding and at top-dressing, respectively. Ammonia emissions were quantified by a micrometeorological method during 20 days after fertilization and N2O emissions were assessed using manual static chambers during all crop period. The treatment with NIs was effective in reducing c. 30% cumulative N2O losses. However, considering only direct N2O emissions after second fertilization event, a significant reduction was not observed using CAN+DMPSA, probably because high WFPS of soil, driven by irrigation, favored denitrification. Cumulative NH3 losses were not significantly affected by NIs. Indeed, NH3 volatilization accounted 14% and 10% of N applied in PS + DMPP and PS plots, respectively and c. 2% of total N applied in CAN+DMPSA and CAN plots. Since important NH3 losses still exist even although abating strategies are implemented, structural and political initiatives are needed to face this issue.