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Fig. 4. (a) Soil N2O emissions in till and no-till treatments between 1991 and 2002 (no data available for 1995). 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 N2O emissions for that year. Points greater than zero indicate a till response greater than a no-till response. There was no detectable difference between years based on a Tukey's test. Linear regression analysis showed no trend in N2O emissions (P < 0.05) across all crops and years; emissions from soybean significantly increased in till relative to no-till over time [normalized N2O difference = 31 210 – 15.7 (year); r2 = 0.48; n = 4 yr]. Mean ± SE.
