Labile soil organic matter as influenced by cropping practices in an arid environment

The dynamics of organic matter (OM) in prairie soils play an important role in long-term soil productivity and in the global carbon cycle. Temporal fluctuations in OM occur primarily in the readily-decomposable, labile fractions. We analyzed soils from a long-term study on a Brown Chernozem (Aridic Haploboroll) in southwestern Saskatchewan to determine the effect of cropping practices on OM content and composition under arid conditions. Soils (0-7.5 and 7.5-15 cm layers) from various rotations of spring wheat (Triticum aestivum L.), winter wheat, flax (Linum usitatissimum L.), lentil (Lens culinaris Medikus) and fallow were analyzed for organic C, total N and selected indicators of labile OM [light fraction(LF)-C, LF-N, mineralizable-C, mineralizable-C, microbial biomass(MB)-C and MB-N]. The indicators of labile OM were all more sensitive than total OM to agronomic variables. Treatment effects on all characteristics were usually similar in the 0-7.5 and 7.5-15 cm layers, but effects in the surface layer were often more highly significant. Frequency of fallow in the rotation was the dominant factor influencing labile OM. For example, LF-C in the 0-7.5 cm layer of well-fertilized continuously-cropped spring wheat, bare fallow-wheat-wheat and bare fallow-wheat was 3.15, 1.55 and 1.17 mg C kg-1 soil, respectively. The corresponding values for CO2-C mineralized at 21 degrees C in 30 days were 371, 184 and 158 mg C kg-1 soil and those for cumulative net N mineralized in 16 weeks at 35 degrees C were 126, 96 and 80 mg N kg-1 soil. MB-C and MB-N exhibited similar trends (e.g. MB-C was 368, 256 and 257 mg C kg-1 soil for the three treatments, respectively). Application of N fertilizer and substitution of winter wheat (with chemical fallow) for spring wheat (with tilled bare fallow) tended to increase labile OM, while substitution of flax or lentil for spring wheat had little effect or reduced labile OM. Differences in labile OM appeared to be related, not only to residue inputs, but also to moisture and temperature conditions. Our findings suggest that it may be possible to manipulate the timing of residue inputs and moisture through cropping practices and thereby maintain adequate labile OM concentrations and improve the synchrony of mineralization with crop requirements. Our results also imply that LF and mineralizable fractions may be useful as early indicators of OM change.