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Fig. 3. (a) Soil NH4+ concentrations 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 NH4+ 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 trends in soil NH4+ concentration (P < 0.05) across all crops and years or for specific crops over time.
