Microbial efficiency of crop residue decomposition
Quantify residue decomposition efficiencies of different crop residues (canola, flax, wheat, pea) in different soils and water regimes.
The study consisted of a series of bench-top incubation undertaken to measure nitrous oxide (N2O) and carbon dioxide (CO2) emissions from soils with canola, flax, pea or wheat residues dually labeled with 15N and 13C. Additionally the efficiency of decomposition of the different crop residues was calculated to determine if GHG emissions were related to the resilience of the residue. By labeling the plant materials we can determine if N2O and CO2 originate from soil or residues alone. Soils from the Brown, Dark Brown, Black and Black/Gray transition were maintained at two water levels – representing a consistently dryish soil and a consistently wet soil. N2O is produced by soil microorganisms – under aerobic (low water) conditions nitrifying organisms dominate and in anaerobic (high water) denitrifying organisms dominate. Denitrification normally produces far more N2O than nitrification. As expected, N2O production was much higher from the wetter soils than from the drier soils and the residue type that caused the highest emissions differed between the dry and wet soils. In all of the dry soils, flax residue resulted in the highest total emissions, whereas in the wet soil either pea (Brown & Dark Brown) or wheat (Black & Black/Gray) had the highest emissions. Flax residue increased soil-derived emissions rather than residue-derived emissions. CO2 emissions arises from all soil organisms during respiration and is much less specialized than N2O. Unlike N2O, canola residue always had the highest CO2 emissions. CO2 production was directly correlated with amounts of C added with the residue, whereas N2O was not correlated with the amount of N added. When we investigated if the efficiency of degradation was affected by residue type we found that flax was less efficiently degraded, but canola, wheat, and pea were very similar. Reduced efficiency means that the microbial populations have to work harder (expend more energy) to degrade residues. We thought that hard to degrade (less efficient) residues would have lower N2O and CO2 emissions, because the nutrients would stay tied up in the residues longer. However, this was not the case as flax had the lowest microbial efficiency (hardest to decompose) but emitted the highest immediate N2O levels. N2O emissions from flax amended soils leveled off quickly in most soils so that by the end of the incubation cumulative emissions were not necessarily high – this was the case when water levels were high and all of the residues were stimulating N2O production.
These types of bench top studies are import to reveal what factors affect greenhouse gas emissions and how. But they cannot be extended directly to field situations. They represent “best-case” – prolonged dryish soils and “worst-case” – prolonged wet soils for greenhouse gas production. In the incubations soils are mixed with the residues in such a way that destroys the inherent soil structure. Certainly encouraging soil structure development through reduced tillage and organic matter development will encourage water infiltration and drainage in soils that will help to minimize greenhouse gases. A crop growing on decomposing residues will reduce nitrate levels that act as substrates for nitrification and denitrification and the presence of roots will further regulate water in the soils. Arguably, the biggest potential benefit will be from fertilizer management – not over applying N fertilizers – and possibly the use of enhanced efficiency fertilizer products.