Relationship Between Soil Nitrate Accumulation and In-Season Corn N Nutrition Indicators.

Nitrogen management tools are required to optimize crop growth and yield while minimizing the likelihood of N losses to the environment. We previously determined that non limiting N conditions for near maximum corn grain yield are reached with the following threshold values for three in-season plant-based indicators of corn (Zea mays L.) N nutrition determined at approximately the V12 stage of development: N nutrition index (NNI) = 0.88, leaf N (N_L) concentration = 32.7 mg N g^-1 leaf DM, and relative chlorophyll meter (RCM) values = 0.95. Our objective was to study the relationship between these plant-based indicators and soil NO₃-N content in an effort to develop tools to reduce the likelihood of soil NO₃-N accumulation without affecting grain yield. This study at five site-years in Québec consisted of six N fertilizer applications (20-250 kg N ha^-1). The NNI, N_L concentrations, RCM values, and soil (0 – 0.15 m) NO₃-N content were measured weekly from July to early August while soil NO₃-N content to a 0.90-m depth was measured in late August and October. The proportion of data points above the average soil NO₃-N content was greater under non-limiting N conditions (NNI ≥ 0.88, N_L concentrations ≥ 32.7 mg N g^-1 leaf DM, or RCM values ≥ 0.95) than under limiting N conditions from mid-June to early August [54 – 59% vs. 5 – 12 % with an average soil (0 – 0.15 m) NO₃-N content of 10.5 kg ha^-1], in late August [43 – 47% vs. 18 – 25% with an average soil (0 – 0.90 m) NO₃-N content of 73.6 kg ha^-1], and at harvest [45 – 61% vs. 25 – 50% with an average soil (0 – 0.90 m) NO₃-N content of 32.0 kg ha^-1]. Furthermore, the mean soil NO₃-N content of the data points above the general average was much higher under non limiting than limiting N conditions in late August (167 vs. 78 kg NO₃-N ha^-1 for NNI and RCM; 166 vs. 112 kg NO₃-N ha^-1 for N_L concentration) and October (68 vs. 49 kg NO₃-N ha^-1). High soil NO₃-N accumulation during the season and at harvest occurs only when in-season plant-based N indicators are greater than their threshold values.