Fig. 2. (a) Soil NO₃^– concentration in till and no-till treatments between 1989 and 2002. Letters indicate the crop harvested in that year: c, corn; s, soybean; w, wheat. * Indicates statistically different responses within a year (P < 0.05) determined by slicing of the treatment by year interaction. (b) Difference between conventional till (Ct) and no-till (Nt) normalized by the overall mean NO₃^– concentration for that year. Points greater than zero indicate a till response greater than a no-till response. Year means were significantly different (P < 0.05) and separated using Tukey's test. Linear regression analysis showed no trend in NO₃^– concentration (P < 0.05) over across all crops and years; NO₃^– concentrations in wheat significantly increased in till relative to no-till over time [normalized NO₃^– difference = –15 650 + 7.84 (year); r² = 0.44; n = 3 yr]. Mean ± SE.