Soil-test N recommendations augmented with PEST-optimized RZWQM simulations.

Area of Science:

• Agricultural Science
• Environmental Science
• Soil Science

Background:

• Late-spring soil nitrate tests (LSNT) offer potential for optimizing nitrogen (N) fertilizer management.
• Variability in LSNT results can impact producer confidence and adoption rates.
• Understanding watershed-scale N dynamics is crucial for water quality management.

Purpose of the Study:

• To evaluate the Root Zone Water Quality Model (RZWQM) for simulating watershed-scale LSNT variability.
• To analyze long-term N dynamics under LSNT and conventional N application strategies.
• To identify key environmental factors influencing interannual LSNT-based N rate variations.

Main Methods:

• Utilized autoregressive techniques and the PEST calibration program to optimize RZWQM.
• Simulated watershed-scale LSNT effects in the tile-drained Walnut Creek, Iowa watershed.
• Developed a statistical model using RZWQM simulations (1970-2005) to assess weather impacts on N rates.

Main Results:

• RZWQM accurately simulated lower nitrate concentrations in discharge from LSNT treatments compared to fall N applications.
• Early-season precipitation and temperature explained 90% of interannual variation in LSNT-based N rates.
• Long-term simulations indicated lower average N loss in tile flow for LSNT (20.4 kg N ha⁻¹) versus spring (22.2 kg N ha⁻¹) and fall (27.3 kg N ha⁻¹) applications.

Conclusions:

• RZWQM is a valuable tool for assessing the water quality impacts of LSNT.
• LSNT enhances N retention in the root zone for crop uptake, reducing N loss.
• Interannual LSNT N rate adjustments are primarily driven by early-season weather patterns.