Modeling nitrous oxide emissions from tile-drained winter wheat fields in Central France

Abstract

Modeling nitrous oxide (N₂O) emissions from agricultural soils is still a challenge due to influences of artificial management practices on the complex interactions between soil factors and microbial activities. The aims of this study were to evaluate the process-based DeNitrification-DeComposition (DNDC, version 9.5) model and modified non-linear empirical Nitrous Oxide Emission (NOE_V2) model with weekly N₂O flux measurements at eight sites cropped with winter wheat across a tile-drained landscape (around 30-km²) in Central France. Adjustments of the model default field capacity and wilting point and the optimum crop production were necessary for DNDC95 to better match soil water content and crop biomass yields, respectively. Multiple effects of varying soil water and nitrate contents on the fraction of N₂O emitted through denitrification were added in NOE_V2. DNDC95 and NOE_V2 successfully predicted background N₂O emissions and fertilizer-induced emission peaks at all sites during the experimental period but overestimated the daily fluxes on the sampling dates by 54 and 25 % on average, respectively. Cumulative emissions were slightly overestimated by DNDC95 (4 %) and underestimated by NOE_V2 (15 %). The differences between evaluations of both models for daily and cumulative emissions indicate that low frequency measurements induced uncertainty in model validation. Nonetheless, our validations for soil water content with daily resolution suggest that DNDC95 well represented the effect of tile drainage on soil hydrology. The model overestimated soil ammonium and nitrate contents mostly due to incorrect nitrogen partitioning when urea ammonium nitrate solution was applied. The performance of the model would be improved if DNDC included the canopy interception and foliar nitrogen uptake when liquid fertilizer was sprayed over the crops.

Appendix: Equations in NOE and NOEV2

Actual denitrification rate (D_a, kg N ha⁻¹ day⁻¹) is calculated by:

$$ D_{a} = D_{p} \cdot F_{N} \cdot F_{W} \cdot F_{T} $$

where D_p is the potential denitrification rate (kg N ha⁻¹ day⁻¹); F_N, F_W and F_T are the effects of soil NO₃ ⁻ content ([NO₃ ⁻], mg N kg⁻¹), water-filled pore space (WFPS) and soil temperature (T,  °C), respectively.

$$ F_{N} = \frac{{\left[ {NO_{3}^{ - } } \right]}}{{{\text{km}}_{1} + \left[ {NO_{3}^{ - } } \right]}} $$

where km₁ denotes the half-saturation constant (mg N kg⁻¹). km₁ is calculated at each gravimetric soil water content (GSWC), corresponding to 22 mg N kg⁻¹ at GSWC = 27 % (Hénault and Germon 2000).

$$ F_{W} = 0,WFPS < 0.69 $$

$$ F_{W} = \left[ {\frac{WFPS - 0.69}{0.31}} \right]^{1.53} ,WFPS \ge 0.69 $$

$$ F_{T} = \exp \left[ {\frac{(T - 11)\ln (89) - 9\ln (2.1)}{10}} \right],T < 11 $$

$$ F_{T} = \exp \left[ {\frac{(T - 20)\ln (2.1)}{10}} \right],T \ge 11 $$

N₂O emitted through denitrification is calculated by:

$$ N_{2} O_{denit} = r_{a} \cdot D_{a} $$

where r_a is the actual fraction of N₂O emitted through denitrification.

$$ r_{a} = r_{\hbox{max} } \cdot r_{W} \cdot r_{{NO_{3} }} $$

where r_max denotes the maximum fraction of N₂O emitted through denitrification; r_W and r_NO3 are the effects of soil WFPS and NO₃ ⁻ content, respectively.

$$ r_{W} = 1 - F_{W} $$

$$ r_{{NO_{3} }} = F_{N} $$

Actual nitrification rate (N_a, kg N ha⁻¹ day⁻¹) is calculated by:

$$ N_{a} = 0,WFPS \ge 0.8 $$

$$ N_{a} = N_{W} \cdot N_{{NH_{4} }} \cdot N_{T} ,WFPS < 0.8 $$

where N_W, N_NH4 and N_T are the effects of GSWC, NH₄ ⁺ content ([NH₄ ⁺], mg N kg⁻¹) and soil temperature (°C), respectively.

$$ N_{W} = a \cdot GSWC + b $$

where a and b (kg N ha⁻¹ day⁻¹) are the slope and intercept of the linear regression between incubated nitrification rate (kg N ha⁻¹ day⁻¹) and GSWC, respectively.

$$ N_{{NH_{4} }} = \frac{{\left[ {NH_{4}^{ + } } \right]}}{{km_{2} + \left[ {NH_{4}^{ + } } \right]}} $$

where km₂ denotes the half-saturation constant (mg N kg⁻¹). km₂ is calculated at each GSWC, corresponding to 2.6 mg N kg⁻¹ at GSWC = 27 % (Hénault et al. 2005).

$$ N_{T} = F_{T} $$

N₂O emitted through nitrification is calculated by:

$$ N_{2} O_{nit} = z \cdot N_{a} ,WFPS < 0.69 $$

$$ N_{2} O_{nit} = r_{a} \cdot z \cdot N_{a} ,0.69 \le WFPS < 0.8 $$

where z is the fraction of N₂O emitted through nitrification.

Soil N₂O flux (kg N ha⁻¹ day⁻¹) is the sum of N₂O emitted through denitrification and nitrification:

$$ N_{2} O = N_{2} O_{denit} + N_{2} O_{nit} $$