Recovery of a clover cover crop in N2O gas fluxes: A 15N tracer study

Background/Question/Methods

Legume cover crops are important nitrogen sources, with the potential to reduce inorganic fertilizer application and the associated nitrogen pollution, as well as to increase soil fertility. However, studies on gaseous losses from legume cover crops, especially the relative contribution from aboveground and belowground residues, are very limited. We conducted a ¹⁵N crop residue exchange experiment to measure the contribution of nitrogen from a clover cover crop to N₂O emissions. Four replicate plots were established in a long-term organic grain trial in upstate New York. Each replicate plot included five 2m x 2m micro-plots. Red clover was labeled in situ with ¹⁵N-urea using the petiole-feeding technique, achieving a maximum enriched δ¹⁵N of 1200 for clover shoots and 400 for roots. The aboveground biomass in labeled and unlabeled micro-plots was swapped prior to tillage, so each micro-plot had ¹⁵N enriched clover tissues derived from only one source (roots or shoots). Continuous N₂O gas sampling with static chambers was conducted four times from August to December in 2012, weekly in May through July 2013, and bi-weekly from August to November 2013. Soil extractable N, potential net N mineralization, moisture, and temperature were measured along with gas sampling in each field campaign. 

Results/Conclusions

Preliminary results showed that N₂O fluxes ranged from 0.06 to 1.06 ug N-N₂O m^-2 min^-1. There was no significant difference in N₂O fluxes between control, root-labeled and shoot-labeled micro-plots. Temperature and soil moisture explained 27% of the temporal variation (p<0.001).

¹⁵N- N₂O fluxes ranged from 0.02 to 0.56 ug ¹⁵N-N₂O m^-2 min^-1.  In week 5 after clover incorporation, ¹⁵N- N₂O fluxes from shoot- and root-labeled micro-plots were significantly higher than that from control plots, but there was no significant difference between shoot- and root-labeled micro-plots. In weeks 6 and 7 after tillage, ¹⁵N-N₂O fluxes from root-labeled micro-plots were higher than control and shoot-labeled micro- plots.  We will complete the ¹⁵N budget for these micro-plots to quantify the relative contribution of roots and shoots to N₂O gas flux. This study provides a novel data set to understand the contribution of legume cover crops to the flux of a potent greenhouse gas in organic cash grain cropping systems, and could contribute to mechanistic model simulation of the fate of legume cover crops.