Search

Differences in 13C inputs between two adjacent Canadian mixed grass prairie communities were used to estimate turnover of two soil organic matter fractions with increasing depth below the surface. Partial cultivation and abandonment of a C4-dominated grassland ca. 1930, and natural recovery to a native C3-dominated grassland changed the 13C/12C ratio (reported in units of δ13C‰) of plant residue inputs to soil organic matter. After 70 years, macro-organic matter carbon (MOMC) differed up to 5.6‰, and fine soil organic carbon (FSOC) by 2.9‰ near the surface; differences declined to a depth of 1 m. Mean residence time estimates differed among fractions from a short 35 years for MOMC 0–5 cm, to a longer 413 years for FSOC 5–10 cm. The background carbon pool on this site was substantial, and the minimum change in soil carbon of 10.29 Mg C ha−1 over 70 years represented less than 7% of the carbon pool to a 1-m depth. Considerable vertical and horizontal variation in soil organic carbon mass and 13C content were observed. Variation due to small sample size was a more substantial limitation than cumulative analytical variation for calculating turnover estimates >500 years or at depths >20 cm. Corrections for bias were also necessary since imperfect physical separation of soil organic matter fractions, or CO32− contamination, were issues.

Nitrous oxide (N2O) emission from farmland is a concern for both environmental quality and agricultural productivity. Field experiments were conducted in 1996–1997 to assess soil N2O emissions as affected by timing of N fertilizer application and straw/tillage practices for crop production under irrigation in southern Alberta. The crops were soft wheat (Triticum aestivumL.) in 1996 and canola (Brassica napusL.) in 1997. Nitrous oxide flux from soil was measured using a vented chamber technique and calculated from the increase in concentration with time. Nitrous oxide fluxes for all treatments varied greatly during the year, with the greatest fluxes occurring in association with freeze-thaw events during March and April. Emissions were greater when N fertilizer (100 kg N ha−1) was applied in the fall compared to spring application. Straw removal at harvest in the fall increased N2O emissions when N fertilizer was applied in the fall, but decreased emissions when no fertilizer was applied. Fall plowing also increased N2O emissions compared to spring plowing or direct seeding. The study showed that N2O emissions may be minimized by applying N fertilizer in spring, retaining straw, and incorporating it in spring. The estimates of regional N2O emissions based on a fixed proportion of applied N may be tenuous since N2O emission varied widely depending on straw and fertilizer management practices.