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Water-Use Efficiency and Carbon Isotopic Composition in Reduced Tillage Systems

Dep. of Soil Science, 51 Campus Drive, Univ. of Saskatchewan, Saskatoon, SK S7N 5A8, Canada

G. P. Lafond

Indian Head Research Farm, P.O. Box 760, Indian Head, SK S0G 2K0, Canada

C. van Kessel

Dep. of Agronomy and Range Science, Univ. of California-Davis, Davis, CA 95616

*Corresponding author (walley{at}sask.usask.ca).

ABSTRACT

Cropping systems that influence soil water availability are expected to influence the C isotopic composition (¹³C) of crop residues and consequently, the ¹³C of soil organic matter. We hypothesized that the ¹³C 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 ¹³C 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 ¹³C 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 ¹³C 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.

Contribution no. R836 of the Saskatchewan Centre for Soil Research.

Received for publication January 7, 1998.