Nitrous Oxide Emitted from Soil Receiving Anaerobically Digested Solid Cattle Manure

Modelling soil salinity effects on salt water uptake and crop growth using a modified denitrification-decomposition model: A phytoremediation approach

Soil salinization is a widespread problem affecting global food production. Phytoremediation is emerging as a viable and cost-effective technology to reclaim salt-affected soil. However, its efficiency is not clear due to the uncertainty of plant responses in saline soils. The main objective of this paper is to propose a phytoremediation dynamic model (PDM) for salt-affected soil within the process-based biogeochemical denitrification-decomposition (DNDC) model. The PDM represents two salinity processes of phytoremediation: plant salt uptake and salt-affected biomass growth. The salt-soil-plant interaction is simulated as a coupled mass balance equation of water and salt plant uptake. The salt extraction ability by plant is a combination of salt uptake efficiency (F) and transpiration rate. For water filled pore space (WFPS), the statistical measures RMSE, MAE, and R² during the calibration period are 2.57, 2.14, and 0.49, and they are 2.67, 2.34, and 0.56 during the validation period, respectively. For soil salinity, RMSE, MAE, and R² during the calibration period are 0.02, 0.02, and 0.92, and 0.06, 0.04, and 0.68 during the validation period, respectively, which are reasonably good for further scenario analysis. Over the four years, cumulative salt uptake varied based on weather conditions. At the optimal salt uptake efficiency (F = 20), cumulative salt uptake from soil was 16-90% for alfalfa, 11-70% for barley, and 10-80% for spring wheat. While at the lowest salt uptake efficiency (F = 40), cumulative salt uptake was nearly zero for all crops. Although barley has the highest peak transpiration flux, alfalfa and spring wheat have greater cumulative salt uptake because their peak transpiration fluxes occurred more frequently than in barley. For salt-tolerant crops biomass growth depends on their threshold soil salinity which determines their ability to take up salt without affecting biomass growth. In order to phytoremediate salt-affected soil, salt-tolerant crops having longer duration of crop physiological stages should be used, but their phytoremediation effectiveness will depend on weather conditions and the soil environment.

Modeling the effect of salt-affected soil on water balance fluxes and nitrous oxide emission using modified DNDC

Soil salinity restricts plant growth, affects soil water balance and nitrous oxide (N₂O) fluxes and can contaminate surface and groundwater. In this study, the Denitrification Decomposition (DNDC) model was modified to couple salt and water balance equations (SALT-DNDC) to investigate the effect of salinity on water balance and N₂O fluxes. The model was examined against four growing seasons (2008-11) of observed data from Lethbridge, Alberta, Canada. Then, the model was used to simulate water filled pore space (WFPS), salt concentration and the N₂O flux from agricultural soils. The results show that the effects of salinity on WFPS vary in different soil layers. Within shallow soil layers (<20 cm from soil surface) the salt concentration does not affect the average WFPS when initial salt concentrations range from 5 to 20 dS/m. However, in deeper soil layers (>20 cm from soil surface), when the initial salt concentration ranges from 5 to 20 dS/m it could indirectly affect the average WFPS due to changes of osmotic potential and transpiration. When AW is greater than 40%, the average growing season N₂O emissions increase to a range of 0.6-1.0 g-N/ha/d at initial salt concentrations (5-20 dS/m) from a range of 0.5-0.7 g-N/ha/d when the salt concentrations is 0 dS/m. The newly developed SALT-DNDC model provides a unique tool to help investigate interactive effects among salt, soil, water, vegetation, and weather conditions on N₂O fluxes.

Optimizing farmyard manure and cattle slurry applications for intensively managed grasslands based on UK-DNDC model simulations

Fertilizer applications can enhance soil fertility, pasture growth and thereby increase production. Nitrogen fertilizer has, however, been identified as a significant source of nitrous oxide (N₂O) emissions from agriculture if not used correctly and can thereby increase the environmental damage costs associated with agricultural production. The optimum use of organic fertilizers requires an improved understanding of nutrient cycles and their controls. Against this context, the objective of this research was to evaluate the scope for reducing N₂O emissions from grassland using a number of manure management practices including more frequent applications of smaller doses and different methods of application. We used a modified UK-DNDC model and N₂O emissions from grasslands at Pwllpeiran (PW), UK during the calibration period in autumn, were 1.35 kg N/ha/y (cattle slurry) and 0.95 kg N/ha/y (farmyard manure), and 2.31 kg N/ha/y (cattle slurry) and 1.08 kg N/ha/y (farmyard manure) during validation period in spring, compared to 1.43 kg N/ha/y (cattle slurry) and 0.29 kg N/ha/y (farmyard manure) during spring at North Wyke (NW), UK. The modelling results suggested that the time period between fertilizing and sampling (TPFA), rainfall and the daily average air temperature are key factors for N₂O emissions. Also, the emission factor (EF) varies spatio-temporally (0-2%) compared to uniform 1% EF assumption of IPCC. Predicted N₂O emissions were positively and linearly (R² ≈ 1) related with N loadings under all scenarios. During the scenario analysis, the use of high frequency, low dose fertilizer applications compared to a single one off application was predicted to reduce N₂O peak fluxes and overall emissions for cattle slurry during the autumn and spring seasons at the PW and NW experimental sites by 17% and 15%, respectively. These results demonstrated that an optimized application regime using outputs from the modelling approach is a promising tool for supporting environmentally-friendly precision agriculture.

Predicting nitrous oxide emissions after the application of solid manure to grassland in the United Kingdom

Nitrous oxide (N₂ O) emission from agricultural soils represents a significant source of greenhouse gas to the atmosphere. We evaluated the suitability of a modified Soil and Water Assessment Tool (SWAT) model to estimate the N₂ O flux from the application of solid manure at two grassland sites (North Wyke [NW] and Pwllpeiran [PW]) in the United Kingdom. The simulated N₂ O emissions were validated against field observations measured in 2011 and 2012 for model calibration and validation, respectively. The SWAT model predicts water-filled pore space (WFPS) very well with Nash-Sutcliffe efficiency (NSE), R² , RMSE, and percentage bias (PBIAS) values of 0.67, .72, 0.06, and 3.64, respectively, during the calibration period for NW site, whereas it gives 0.68, .69, 0.07, and 3.04, respectively during the validation period. At PW, the model predicted the NSE, R² , RMSE, and PBIAS of 0.55, .69, 0.04, and -4.5, respectively, during calibration and 0.63, .71, 0.05, and -2.6, respectively, during the validation period. Compared with WFPS, the model resulted in a slightly lower fit for N₂ O emissions for NW (NSE = 0.47, R²  = .63 during calibration, and NSE = 0.55, R²  = .58 during validation) and for PW (NSE = 0.54, R²  = .71 for calibration, and NSE = 0.47, R²  = .69 for validation). Results revealed that the SWAT model performed reasonably well in representing the dynamics of N₂ O emissions after solid manure application to grassland.

Modeling nitrous oxide emissions from rough fescue grassland soils subjected to long-term grazing of different intensities using the Soil and Water Assessment Tool (SWAT)

Given the rising nitrous oxide (N₂O) concentration in the atmosphere, it has become increasingly important to identify hot spots and hot moments of N₂O emissions. With field measurements often failing to capture the spatiotemporal dynamics of N₂O emissions, estimating them with modeling tools has become an attractive alternative. Therefore, we incorporated several semi-empirical equations to estimate N₂O emissions with the Soil and Water Assessment Tool from nitrification and denitrification processes in soil. We then used the model to simulate soil moisture and the N₂O flux from grassland soils subjected to long-term grazing (> 60 years) at different intensities in Alberta, Canada. Sensitivity analysis showed that parameters controlling the N₂O flux from nitrification were most sensitive. On average, the accuracy of N₂O emission simulations were found to be satisfactory, as indicated by the selected goodness-of-fit statistics and predictive uncertainty band, while the model simulated the soil moisture with slightly higher accuracy. As expected, emissions were higher from the plots with greater grazing intensity. Scenario analysis showed that the N₂O emissions with the recommended fertilizer rate would dominate the emissions from the projected wetter and warmer future. The combined effects of fertilization and wetter and warmer climate scenarios would increase the current N₂O emission levels by more than sixfold, which would be comparable to current emission levels from agricultural soils in similar regions.