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Cropping systems that influence soil water availability are expected to influence the C isotopic composition (δ 13 C) of crop residues and consequently, the δ 13 C of soil organic matter. We hypothesized that the δ 13 C of plant residues and soil organic matter in three tillage systems (zero [ZT], minimum [MT], and conventional tillage [CT]) and three, 4-yr crop rotations would vary and would relate to water-use efficiency (WUE). The study was conducted on an Indian Head heavy clay (Udic Haploboroll) in Saskatchewan, Canada. The three crop rotations were fallow-spring wheat (Triticum aestivum L.)-spring wheat-winter wheat, spring wheat-spring wheat-flax (Linum usitatissimum L.)-winter wheat, and spring wheat-flax-winter wheat-field pea (Pisum sativum L.). Water use was estimated yearly using mass balance. The δ 13 C of the standing crop residue, roots, and soil organic matter was determined 9 yr after the study was initiated. Although crop water use was higher in ZT (31.2 cm yr -1 ) and MT (30.9 cm yr -1 ) than in CT (28.8 cm yr -1 ), corresponding differences in WUE and δ 13 C of plant tissue and soil organic matter were not detected. In one instance, within the most diversified rotation, WUE was reduced in ZT compared with CT; however, observed variations in WUE did not conform to theoretical expectations of the δ 13 C of plant residues and soil organic matter. Factors other than WUE. including soil fertility and timing of moisture deficits, may have influenced the degree to which C isotope discrimination was expressed in the plant residues and soil organic matter. |
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