Plant-based diagnostic techniques are used to determine the level of crop N nutrition but there is limited comparative research on the diff erent methods. Our objectives were to establish the relationship between chlorophyll meter (CM) readings and N nutrition index (NNI) during the corn (Zea mays L.) growing season, and to compare both methods as diagnostic tools for predicting grain yield response to N fertilization. Th e study was established at eight site-years using four to seven N fertilization rates. Th e CM readings from the youngest collared leaf were taken on fi ve to eight sampling dates in 2004, 2005, and 2006 along with NNI determinations. Generally, CM readings and NNI increased with increasing N rates. Chlorophyll meter readings and relative CM (RCM) readings were related to NNI, but the intercepts and/or slope of the response curves varied with site-year. Because they are site-specifi c, these relationships may not be reliable indicators of corn N status. Th e relationship between CM readings and relative grain yield (RY) at stage of development ≈V12 was also site-specifi c. Relative CM readings (RY = −0.64 + 1.65 RCM if RCM ≤ 0.98 and RY = 0.97 if RCM > 0.98; R 2 = 0.60) and NNI (RY = −0.34 + 1.47 NNI if NNI ≤ 0.88 and RY = 0.96 if NNI > 0.88; R 2 = 0.79) at stage of development ≈V12 were related to RY. Th ese two relationships were stable across site-years and could be used to detect and quantify N defi ciencies of corn.

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Plant‐based diagnostic tools of N deficiency can be based on the concept of critical N dilution curves describing whole‐plant critical N concentration (Nc; g kg−1 of dry matter [DM]) as a function of shoot biomass (W; Mg DM ha−1). This has been tested for several crops, including winter wheat (Triticum aestivum L.) but has not been tested for spring wheat. Our objectives were to determine a critical N dilution curve specific to spring wheat, to compare this curve with existing critical N dilution curves for winter wheat, and to assess the plausibility of using it to estimate the level of N nutrition. The study was conducted at six site‐years (2004–2006) in Québec, Canada, with four to eight N fertilization rates (0–200 kg N ha−1). Shoot biomass and N concentration were determined on five to eight sampling dates during the growing season, and grain yield was measured at harvest. A critical N dilution curve (Nc = 38.5 W−0.57) was determined for spring wheat and was different from those reported for winter wheat. The N nutrition index (NNI = Nobserved/Nc) calculated from this spring wheat critical N dilution curve was significantly related (R2 = 0.70; P < 0.001) to relative grain yield. This critical N dilution curve and the resulting NNI adequately identified situations of limiting and nonlimiting N nutrition and could be used to establish the N nutrition status.

Both tillage and fertilizer management influence soil organic C (SOC) storage, but their interactive effects remain to be determined for various soil and climatic conditions. We evaluated the long‐term effects of tillage (no‐till, NT, and moldboard plowing, MP), and N and P fertilization on SOC stocks and concentrations in profiles of a clay loam soil (clayey, mixed, mesic Typic Humaquept). Corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] were grown in a yearly rotation for 14 yr. Our results showed that NT enhanced the SOC content in the soil surface layer, but MP resulted in greater SOC content near the bottom of the plow layer. When the entire soil profile (0–60 cm) was considered, both effects compensated each other, which resulted in statistically equivalent SOC stocks for both tillage practices. Nitrogen and P fertilization with MP increased the estimated crop C inputs to the soil but did not significantly influence SOC stocks in the whole soil profile. At the 0‐ to 20‐cm depth, however, lower C stocks were measured in the plowed soil with the highest N fertilizer level than in any other treatment, which was probably caused by a greater decomposition of crop residues and soil organic matter. Conversely, the highest SOC stocks of the 0‐ to 20‐cm soil layer were observed in the NT treatment with the highest N rates, reflecting a greater residue accumulation at the soil surface. When accounting for the whole soil profile, the variations in surface SOC due to tillage and fertilizer interactions were masked by tillage‐induced differences in the 20‐ to 30‐cm soil layer.