Sustainability of a Rainfed Wheat Production System in Relation to Water and Nitrogen Dynamics in the Soil in the Eyre Peninsula, South Australia

Rainfed wheat production systems are usually characterized by low-fertility soils and frequent droughts, creating an unfavorable environment for sustainable crop production. In this study, we used a processed-based biophysical numerical model to evaluate the water balance and nitrogen (N) dynamics in soils under rainfed wheat cultivation at low (219 mm, Pygery) and medium rainfall (392 mm, Yeelanna) sites in south Australia over the two seasons. Estimated evapotranspiration components and N partitioning data were used to calibrate and validate the model and to compute wheat’s water and N use efficiency. There was a large disparity in the estimated water balance components at the two sites. Plant water uptake accounted for 40–50% of rainfall, more at the low rainfall site. In contrast, leaching losses of up to 25% of seasonal rainfall at the medium rainfall site (Yeelanna) indicate a significant amount of water evading the root zone. The model-predicted N partitioning revealed that ammonia–nitrogen (NH 4 –N) contributed little to plant N nutrition, and its concentration in the soil remained below 2 ppm throughout the crop season except immediately after the NH 4 –N-based fertilizer application. Nitrate–nitrogen (NO 3 –N) contributed to most N uptake during both seasons at both locations. The N losses from the soil at the medium rainfall site (3.5–20.5 kg ha −1 ) were mainly attributed to NH 4 –N volatilization (N v ) and NO 3 –N leaching (N L ) below the crop root zone. Water productivity (8–40 kg ha −1 mm −1 ) and N use efficiency (31–41 kg kg −1 ) showed immense variability induced by climate, water availability, and N dynamics in the soil. These results suggest that combining water balance and N modeling can help manage N applications to optimize wheat production and minimize N losses in rainfed agriculture.