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Fig. 3. Foliar 
13C of seedlings grown for 18 wk in co-composted drilling wastes or in soil growth media. Error bars are standard errors of the means (n = 4). The same lowercase letters indicate statistically nonsignificant differences (P > 0.05). Foliar 13C was significantly affected by compost age and N fertilization (P < 0.01). Treatment codes indicate different ages (1, 2, 3, and 4 yr old) of compost without N addition (1YN0, 2YN0, 3YN0, and 4YN0, respectively) or with N addition (1YN1, 2YN1, 3YN1, and 4YN1, respectively), and soil represents the soil-based growth media.
