Insufficient and excessive N fertilizer input reduces maize root mass across soil types

Abstract Quantifying maize root response to nitrogen (N) fertilizer, soil texture, and weather is crucial to understand complex soil-root-plant processes. We performed a 2-year x 4 locations (sand content range: 5–95%) x N treatments (range: 0 to 336 kg N ha−1) field experiment in Iowa, U.S. to (1) determine the response of root traits to N fertilizer, and (2) develop generalized functions to aid understanding and prediction of root mass and root to shoot (R:S) ratio. Deep root samples (0−210 cm, increments of 30 cm) were collected using the soil core approach at early to middle grain fill period and quantified root mass, length, and N and C concentrations. In addition, yield and shoot biomass was measured. Root traits and yield had different responses to N fertilizer input. Root mass was maximized at 168 kg N ha−1; zero and excessive N fertilization decreased root mass by 33 and 17 %, respectively. Nitrogen fertilizer significantly affected root traits only in the top 30 cm soil layer. Soil texture affected root traits in a dry year (root mass was positively associated with silt and clay), but not in a wet year, suggesting that soil moisture overwhelms the effect of texture. The combined data (N rates x locations x years) revealed a negative relationship between R:S ratio and yield. This resulted in a new set of equations (e.g., upper bound R:S = e(–1.5 – 0.04*yield)) that can replace the constant R:S approach used in the literature. Yield, which is commonly measured, integrates the effects of environment, management, and genetic variation; hence the proposed equations can be widely applied. This study provides evidence that different plant traits are maximized at different levels of mineral N nutrition. Results can enhance biophysical models and prediction of R:S ratio.