Water-use efficiency and carbon isotopic composition in reduced tillage systems

Cropping systems that influence soil water availability are expected to inf luence the C isotopic composition (delta(13)C) of crop residues and consequ ently, the delta(13)C of soil organic matter. We hypothesized that the delt a(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 cro p rotations would vary and would relate to water-use efficiency (WUE). The study was conducted on an Indian Head heavy clay (Udic Haploboroll) in Sask atchewan, Canada. The three crop rotations were fallow-spring wheat (Tritic um aestivum L.)-spring wheat-winter wheat, spring wheat-spring wheat-flax ( Linum usitatissimum L.)-winter wheat, and spring wheat-flax-winter wheat-fi eld pea (Pisum sativum L.). Water use was estimated yearly using mass balan ce. The delta(13)C of the standing crop residue, roots, and soil organic ma tter 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 delta(13)C of plant t issue and soil organic matter were not detected. In one instance, within th e most diversified rotation, WUE was reduced in ZT compared with CT; howeve r, observed variations in WUE did not conform to theoretical expectations o f the delta(13)C of plant residues and soil organic matter. Factors other t han WUE, including soil fertility and timing of moisture deficits, may have influenced the degree to which C isotope discrimination was expressed in t he plant residues and soil organic matter.