NH3, N2O, CO2 and H2O fluxes from green manure legume crop in a Mediterranean ecosystem

Green manure using legume crops is an agronomical practice for improving soil fertility and sustainability. Although this practice is good for recycling nutrients in the system, gaseous losses in the form of ammonia (NH3), nitrous oxide (N2O), carbon dioxide (CO2) and water vapour (H2O) have to be quantified to evaluate the overall efficiency of green manure. These fluxes have been monitored with different measurement methods during faba bean (Vicia faba spp. minor L.) incorporation in a typical cropping system (faba bean - wheat) in semi-arid Mediterranean region. NH3 emission measured with the eddy covariance method, composed of a pulsed quantum cascade laser, mainly detected white noise, while the aerodynamic gradient method, using a multi-channel wet-denuder system (ROSAA, Robust and Sensitive Ammonia Analyser), was able to quantify small loss of NH3, around 0.25% of the N incorporated with the green manure (205 kgN ha-1). NH3 volatilization started 5 days after ploughing when soil wetness started to increase following rainfall. The N2O fluxes were monitored with an automatic chamber system. N2O emissions increased 3 days following ploughing; overall, N2O fluxes were small (0.054% N loss in 2.5 weeks) as expected with the low water fill pore space under dry conditions (below 35%). N2O emissions were also 40% smaller in shallow tillage compared to ploughing. Daily values of CO2 and H2O fluxes were linearly related. The CO2 fluxes were well correlated to surface temperature and water content of the soil after the provision of fresh biomass following the green manure. Before the green manuring, the crop in active growing stage was a CO2 sink, while just after the green manure incorporation, the soil became a source of CO2.