I apologize. I accidentally posted my reply to your comment as a separate comment. I don’t know if this effects whether or not a notification is sent to you regarding my reply, so I thought a would re-post my comment as a reply:
“Hi Dr. Gehring. Thanks for your question. Nitrous oxide emissions tend to increase with increased nitrogen N inputs, regardless of whether they come from inorganic fertilizers or through organic sources, such as residues of nitrogen-fixing crops. If we are talking about reducing inorganic fertilizer emissions by incorporating nitrogen-fixing legumes or cover crops into rotation with corn and sorghum, it is debatable whether or not there would be reduced N2O emissions. Some studies report lower N2O emissions when legumes are incorporated into rotations, but some also report high N2O emissions from these systems. However, decreasing the amount of inorganic N applied to these crops would reduce the emissions originating from the production of inorganic fertilizer, which is a major source of greenhouse gas emissions in the life cycles of these crops.
There is also some evidence that miscanthus might associate with nitrogen fixing bacteria (Anderson-Teixeira, et al. 2009, GCB Bioenergy 1: 75-96). However more studies are needed to confirm these claims. If this proves true then it could mean many studies are applying too much nitrogen fertilizer to miscanthus and that both N2O emissions and greenhouse gas emissions from nitrogen fertilizer production could potentially be reduced.”

Thank you, Andrew. I was thinking not only of nitrogen-fixing associations, but also mycorrhizal associations that can improve crop access to soil resources.
Catherine

Yes, decreasing the nitrogen applied could potentially help encourage mycorrhizal associations with these crops and improve plant-access to soil nutrients. Switchgrass and miscanthus are both nitrogen thrifty crops – partly because they have more extensive root systems and translocate nutrients to their roots at the end of the growing season, but they also seem to support more mycorrhizae than either of the annual crops. We have been using phospholipid fatty acid analysis to monitor the microbial community structure in the soils of these crops and have observed a higher occurrence of the 16:1w5c biomarker in the perennial crops, which is commonly attributed to arbuscular mycorrhizae. Other studies have noted that both switchgrass and miscanthus do not always have a strong response to nitrogen fertilizer. Perhaps mycorrhizal associations are part of the reason. If nitrogen rates in these crops could be decreased while maintaining high productivity we would expect to see big decreases in the nitrous oxide emissions. Currently, I am also monitoring nitrous oxide emissions in an N rate study in switchgrass in order to investigate this specific issue.

Good question. There was very high temporal variability in the nitrous oxide emissions. Approximately 70% of the total annual emissions occurred over a 3 week period in late May and early June. This period of high emissions occurred shortly after nitrogen fertilizer application and was also an extremely wet period with several large rainfall events. We observed very high soil water contents and very high soil nitrogen levels in the research plots during this period, which are ideal conditions for nitrous oxide production by denitrifying microbes. In July and August there were smaller emission events occurring after rainfall, but the soil nitrogen concentrations were much lower, presumably from plant uptake. This may have prevented major fluxes such as those observed in May and June. We did not observe substantial fluxes in late fall, winter or early spring, presumably from low soil temperatures and low soil nitrogen levels.

I apologize. I accidentally posted my reply to your comment as a separate comment. I don’t know if this effects whether or not a notification is sent to you regarding my reply, so I thought a would re-post my comment as a reply:
“Thanks. You make a great point. The results from this study are from only one location. The study site included Ivan, Kennebec, and Kahola silt loams (fine-silty, mixed, superactive, mesic Cumulic Hapludolls). If the same study were conducted on a different soil type with a different texture we would likely get a substantially different N2O response. Unfortunately, because of great amount of time involved in measuring N2O emissions, we were only able to collect data from one study site. However, in future work we hope to modify existing biogeochemical models, such as the DNDC (DeNitrification-DeComposition) model so that we can estimate N2O emissions from the crops in this study on larger spatial scales while accounting for factors like soil type.

Regarding your question about accounting for other soil properties and microbial communities: we have been monitoring changes in selected soil properties such soil structure (water-stable aggregates), soil organic matter content, as well as changes in soil microbial community structure using phospholipid fatty acid analysis. We have seen greater water-stable macro aggregates and increases in the relative abundance of fungi in the soils planted in miscanthus and switchgrass, which are likely impacting the way that elements such as nitrogen and carbon cycle through these agroecosystems. However, it is difficult to say specifically what impacts that these changes could be having on nitrous oxide fluxes."

Thank you for the nice feedback. The variability in N2O estimates was quite high within crops, especially on days where high fluxes were occurring. This was especially an issue in plots with switchgrass, which as a “clump grass”, had high spatial variability in vegetation distribution within the plots (and probably in nitrogen uptake and N2O emissions as well) that might have made it difficult to get consistent N2O measurements using our PVC chambers. The standard error on the annual flux estimates was 0.49 kg N2O-N per hectare per year, which can be seen on the error bars in the chart from the results section of the poster under “nitrous oxide emissions”. Pairwise comparisons using Tukey-Kramer post-hoc tests with an alpha of 0.05 showed that the annual flux of the corn was significantly higher than that of miscanthus. However, the differences in the annual flux between miscanthus, photoperiod sensitive sorghum and switchgrass were not significant, nor were those between corn, photoperiod sensitive sorghum and switchgrass.
As a side note, the life cycle assessment results show corn stover as having no N2O emissions. This is not because soils in the corn plots did not emit N2O, but rather because the greenhouse gas emissions from fertilizer production and direct N2O were attributed to corn grain. This decision was based on the assumption that the value of corn grain drives the decision of whether or not to plant corn, regardless of whether or not stover is harvested. If we changed this assumption and attributed half of the direct N2O and fertilizer production emissions from corn to the corn stover, ethanol from corn stover would have the highest greenhouse gas emissions among the 4 feedstocks. The bottom line: the model is very sensitive to assumptions regarding direct N2O and fertilizer production emissions. As one of the next steps of this research, I plan on conducting a sensitively analysis with the GREET life cycle assessment model so that we can better understand the uncertainties in our life cycle estimates.

Thanks Sarah!

Thank you!

Thanks John. We are not monitoring nitrate leaching on these plots. However, a previous study done on similar soils (silt loam/silty clay loam) on a nearby experiment suggested that leaching should be minimal on these soils. I don’t think we will be able to do a complete mass balance since we are not measuring NH3 volatilization, which can be a potentially large nitrogen loss pathway with urea fertilizer. However, we are tracking changes in total soil nitrogen, N2O production and nitrogen taken up in plant biomass, which should give us a rough idea of where the nitrogen is going.

There is some evidence that of nitrogen
fixing bacteria associated with miscanthus. However, the nitrogenase enzyme responsible for nitrogen fixation is typically not synthesized by microorganisms when ammonium or nitrate are present in large concentrations. Since we were applying large amounts of urea to the miscanthus in this study, there was probably not much N fixation taking place. However, this does bring up the question of how much, if any nitrogen should be applied to miscanthus. Most of the studies I have seen show that there is a limited yield response of miscanthus to nitrogen fertilizer. Obviously, cutting the amount of nitrogen applied could have a big impact on the greenhouse gas emissions from the production of miscanthus-based cellulosic ethanol. In subsequent years in this study we have actually cut back the nitrogen rate for both miscanthus and switchgrass because of this.