Effects of green manure storage and incorporation methods on nitrogen release and N2O emissions after soil application

More efficient use of green manure-derived nitrogen (N) may improve crop yields and reduce environmental impacts in stockless organic arable farming. In this 3-month incubation study, we tested a new strategy where green manure leys are harvested and preserved until the following spring either as compost mixed with straw or as silage of harvested ley biomass.

Grass-clover compost or silage was soil-incorporated by either simulated ploughing (green manure placed at 15 cm depth) or harrowing (green manure mixed into the upper 5-cm soil horizon) in order to assess treatment effects on net release of plant-available N, nitrous oxide (N2O) fluxes, and soil respiration.

Grass-clover silage provided the highest net N release with similar results for the two incorporation methods. Up to one third of the total N content in silage became plant-available during the three months. In contrast, no net N release was observed for the composted grass-clover and straw mixture.

In fact, soil incorporation of compost by harrowing caused temporal immobilization of soil mineral N. Silage incorporated by ploughing gave rise to largest N2O effluxes with silage-induced emissions corresponding to 0.3 % of applied total N. Possibly N2O production via denitrification was stimulated by oxygen-limited conditions near the decomposing silage.

In contrast, compost incorporated by harrowing caused net N2O uptake, presumably an effect of reduced mineral N availability in this treatment. Overall, our study revealed that ensiled grass-clover was the best fertilizer product, and that the method chosen for incorporation of green manure is likely to influence N2O emissions.