Microbial efficiency of crop residue decomposition
By Donna Fleury
Quantifying decomposition efficiencies of canola, wheat, pea and flax crop residues in different soils and water regimes.
Managing and optimizing nitrogen (N) is important for maximizing crop production during the growing season. It is also important to understand and manage the fate of N in crop residues to mitigate greenhouse gas emissions and optimize N transfer to subsequent crops. Researchers are interested in finding out what factors affect greenhouse gas emissions and how they do that, as well as the impact of microbial efficiencies on crop residue decomposition.
“In an earlier study looking at greenhouse gas emissions from different crop residues, we realized there were additional factors that were likely influencing our results,” explains Diane Knight, professor of soil science, University of Saskatchewan. “In the first study, we found the residues of canola and flax had stimulated N2O production quite significantly, while pea and wheat were lower than expected. In this next expanded study, we wanted to look at ways to estimate the efficiency of microorganisms and greenhouse gas emissions in different soils and water regimes.”
In the first study, 13C- and 15N-labelled crop residues from canola, wheat, flax and field pea were compared to track the fate of residue carbon (C) and N during decomposition in four different soils from Brown, Dark Brown, Black and Black/Gray transition soil zones. The study included controlled environment incubation trials and stable isotope tracing to measure soil N pools, gaseous emissions and microbial dynamics. The trials helped to determine the drivers and dominant source of greenhouse gases. The results showed that most of the N2O flux originated from soil N pools rather than residue N. However, the source of C mostly came from the crop residues. Overall, the total N2O, as well as residue-derived and residue-induced N2O emissions were highest for canola, intermediate for wheat and pea and lowest for flax.
“This next study also used 13C- and 15N-labelled crop residues from canola, wheat, flax and field pea to measure N2O and CO2 emissions and determine their source,” says Knight. “We also added two different water levels to the four different soils, one representing a consistently dryish soil at about 50 per cent field capacity and the other a consistently optimal moisture condition of about 70 per cent field capacity. We were interested in measuring the efficiency of the microorganisms under different conditions. N2O is produced by soil microorganisms, and under aerobic or low water conditions, nitrifying organisms dominate, and in anaerobic or high water conditions, denitrifying organisms dominate. Denitrification normally produces far more N2O than nitrification. Efficiency is a measure of N in the crop residue at the start, and then the amount that gets emitted as N2O during decomposition. Reduced efficiency means the microbial populations have to work harder, or expend more energy to degrade residues.”
The results from this second study indicated that water is a main driver of emissions, but there are still some other factors around the microorganisms and their activity that are unclear. 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. Under low moisture soils, the efficiency factors were really low for all crop residues, except for flax. In all of the dry soils, flax residue resulted in the highest total emissions, whereas in the wet soil either pea in Brown and Dark Brown soils or wheat in Black and Black/Gray had the highest emissions.
“It was interesting to note that canola was no different than the other crops for N2O production, even though we expected it to be different,” says Knight. “Surprisingly, canola, pea and wheat residues were quite similar in the efficiency of degradation. However, flax was the most surprising one in this study, where in some cases the efficiency factors were quite significant, even in drier conditions. By looking at the N15 tag, we were able to identify that for the most part the microorganisms were stimulating the soil to release the N, not the flax residue. Sometimes we see priming effects with microorganisms, where rather than directly decomposing the residues, they are stimulated and start working on natural sources of nutrients in the soil. In this study, the flax residue increased soil-derived emissions rather than residue-derived emissions in dry soils, but was more similar to other crop residues under wetter soil conditions.”
In terms of the CO2 emissions, canola residue always had the highest CO2 emissions, even though it had lower N2O emissions. CO2 emissions arise from all soil organisms during respiration and is much less specialized than N2O. In this study, CO2 production was directly correlated with amounts of C added with the residue, whereas N2O was not correlated with the amount of N added.
“Overall, our controlled environment studies have been very valuable in helping to reveal what factors affect greenhouse gas emissions and how. However, they cannot be extended directly to field situations,” adds Knight. “The results do show that water is a major driver, how dry and wet soils may affect greenhouse gas production, and whether the emissions are soil-derived or residue-derived. There is potential for future research in this area to extend into the field. From our collective research work in this area, what growers can focus on right now is using best management practices such as getting crops into the ground as early as possible, depending on the crop being seeded, to draw down those N supplies in the soil. A crop growing on decomposing residues will reduce nitrate levels that act as substrates for nitrification and denitrification. The presence of crop roots will further regulate water in the soils. The largest potential benefit will be from best fertilizer management practices including implementing the 4Rs of Nutrient Stewardship to optimize the efficiency of fertilizer use.”
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