Modelling nutrient dynamics and leaching losses in intensively managed rice-wheat systems

Introduction

Feeding a growing population demands more from our croplands, but the ecological cost of nitrogen surplus is mounting fast [1] . In intensive rice-wheat systems covering over 24 million hectares across South and East Asia nitrogen application rates often exceed 250 kg N/ha/year, yet crop recovery rarely exceeds 40% [2] . The remainder enters groundwater as nitrate, escapes as N₂O (a potent greenhouse gas), or volatilizes as ammonia [3] . Process-based models like DNDC simulate the biogeochemical transformations that govern N fate in agricultural soils: mineralization, nitrification, denitrification, leaching, and plant uptake [4, 5] . Unlike empirical N-balance approaches, DNDC can capture the temporal dynamics of N loss events critical for rice-wheat systems where anaerobic-aerobic transitions during paddy flooding and drainage create unique biogeochemical windows [6] . This research aimed to: (i) calibrate and validate DNDC for a rice-wheat rotation at Michigan State University; (ii) quantify the partitioning of applied N among crop uptake, leaching, denitrification, volatilization, and soil storage; and (iii) test whether reduced N rates combined with nitrification inhibitors can lower leaching without major yield penalty.

Theoretical background

The DNDC model (v9.5) simulates soil carbon and nitrogen biogeochemistry through four interconnected sub-models: thermal-hydraulic, decomposition, denitrification, and plant growth [4] . The thermal-hydraulic module calculates soil temperature and moisture profiles hourly. Decomposition tracks organic matter through labile, resistant, and humus pools. Denitrification simulates anaerobic microsites where NO₃⁻ is reduced to N₂O and N₂. Plant growth follows a degree-day-driven phenology with N uptake governed by root density and soil solution NO₃⁻ and NH₄⁺ concentrations [5] . Key inputs include daily weather, soil texture, organic carbon, pH, bulk density, crop management (sowing date, fertilizer rate and timing, irrigation schedule), and tillage practices [7] .

Materials and Methods

Materials

Lysimeter experiments were conducted at the Michigan State University W.K. Kellogg Biological Station, East Lansing, Michigan, USA (42°24'N, 85°24'W) during 2021-2023. Twelve weighing lysimeters (1.2 m diameter, 1.5 m depth) were used. Daily weather data came from the on-site automated station. Soil was a Kalamazoo loam (Typic Hapludalf), pH 6.4, organic carbon 1.86%. Ethics: MSU Agricultural Research Compliance (Protocol MSU-ARC/2021-062, January 2021).

Methods

DNDC was calibrated using 2021 data by adjusting soil hydraulic parameters and crop genetic coefficients to match observed drainage volume (±10%), nitrate concentration (±15%), and grain yield (±8%). Validation used 2022-2023 data. Five N management scenarios were tested: S1 current recommended rate (150 kg N for rice, 120 for wheat), S2 20% reduced rate, S3 recommended rate + nitrification inhibitor (DCD at 10 kg/ha), S4 20% reduced + DCD, S5 split application (4 splits vs standard 2) [8] . Model performance was assessed by RMSE, d-index, and model efficiency (EF) [9] . Soil and Hydrological Characteristics Pre-season soil analysis showed available N of 186 kg/ha, Olsen-P 22.4 mg/kg, and exchangeable K 164 kg/ha. Groundwater table depth fluctuated between 0.8 m (rice season) and 1.4 m (wheat season). Lysimeter drainage averaged 312 mm during rice (May-October) and 184 mm during wheat (November-April). Soil saturated hydraulic conductivity was 18.6 mm/h at 0-30 cm depth [10] .

Results

Table Figure 1 shows the non-linear relationship between N application rate and N leaching for both rice and wheat seasons leaching accelerates sharply above 160 kg N/ha. Figure 2 presents the N fate partitioning pie chart under current management, illustrating that over half of applied N is lost.

Discussion

The 42.3% N recovery rate confirms that current rice-wheat N management wastes over half of applied fertilizer. This aligns with Ladha et al.'s [2] field-measured recovery of 30-45% across South Asian sites. The non-linear leaching response (Figure 1) shows that each additional kilogram of N above 160 kg/ha produces diminishing yield returns but accelerating environmental loss a classic case of diminishing marginal productivity meeting increasing marginal pollution [3,6] . The nitrification inhibitor DCD reduced leaching by 38.4% by slowing the conversion of NH₄⁺ to mobile NO₃⁻, keeping N in the ammonium form longer where it is less prone to leaching and denitrification [8] . Combined with a 20% rate reduction (S4), this achieved 93.8% of baseline yield a 6.2% penalty that may be economically acceptable given reduced fertilizer costs and environmental benefits [5,7] .

Limitations

Lysimeter conditions may not fully represent field-scale hydrology where lateral flow and deep percolation patterns differ. DNDC's denitrification estimates showed higher uncertainty (nRMSE 18.6%) than other outputs, reflecting the model's difficulty capturing episodic anaerobic events. Two years of validation data limit assessment of interannual variability.

Conclusion

Current N management in intensive rice-wheat systems recovers only 42.3% of applied nitrogen, with 22.8% lost through leaching. DNDC modelling showed that combining a 20% N rate reduction with nitrification inhibitor (DCD) can cut leaching by 38.4% while sacrificing only 6.2% of yield. The non-linear relationship between N application and leaching provides a clear target: keeping rates below 160 kg N/ha per season dramatically reduces environmental losses. These findings support transitioning from blanket high-N recommendations to precision N management in rice-wheat systems.

References

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