Going where no grains have gone before: The N economy of a later successional agroecosystem

Background/Question/Methods

Disturbance regimes in terrestrial ecosystems can be evaluated in terms of duration, extent, and magnitude.  When all three measures of disturbance are considered, we propose that the agroecosystems that are maintained to produce grains and other annual crops are the most highly disturbed of all terrestrial ecosystems.  One consequence of arresting agroecosystems in such an early stage of succession is a very open and leaky nitrogen economy.   In fall of 2012, we initiated an experiment to explore how the nitrogen economy of grain production would change if we were to move the ecosystem along a successional gradient.  We sowed replicated plots of a perennial intermediate wheatgrass (IWG) (Thinopyrum intermedium--which produces Kernza^TM grain), in single species stands (SS) at two densities.  The plots were fertilized with 0, 50 (low density) and 100 (high density) kg urea-N ha^-1 yr^-1.  We also sowed plots of wheatgrass that were intercropped with the N-fixing legume alfalfa (Medicago sativa).  The intercrop was unfertilized.  For three years we measured N-fixation by alfalfa using the natural abundance δ¹⁵N, root biomass and aboveground net primary production (ANPP), N₂O fluxes with static chambers, and in year 3, net N mineralization with field and lab incubations from fertilized, unfertilized and intercropped arrangements. 

Results/Conclusions

IWG ANPP of the different N treatments did not diverge until year three, when the intercrop maintained equal ANPP as the fertilized, equal-density SS plots, higher than the unfertilized SS plot (low and high density), but lower than fertilized high-density SS plots. Total ANPP of the intercrop (IWG + alfalfa) was ~6 t ha^-1 which equaled ANPP of high-density fertilized SS. Also in year three, fertilized IWG SS plots maintained N₂O fluxes ranging between 15-50 μL N₂O m^-2 h^-1 for several weeks after fertilizer was applied, while fluxes from alfalfa-IWG intercrops and unfertilized IWG never exceeded 8 μL N₂O m^-2 h^-1 throughout the growing season. Net N mineralization ranged between 0.05 – 72.0 mg N kg^-1 soil d^-1, with low rates in all treatments in April and August, and rates several fold higher in June and July.  The fertilized, high density SS plots maintained the highest net N mineralization rates followed by the intercrop plots, and then the unfertilized SS plots.  The IWG grown with alfalfa appears to maintain a comparable N economy as the fertilized IWG plots planted at the same IWG density.