Digestate: Scope and application as nitrogen fertilizer for rice

Purpose: Rice production in paddy soil is important for food security, and nitrogen fertilisation is important to achieve high yields. Digestate, the organic rest from biogas production can be a good fertilizer, but relatively little is known about its use in paddy soil, which is investigated here. Method: Nitrogen transformations and rice growth in soil waterlogged and at field capacity after application of digestate and digestate products were assessed in a pot experiment. Nitrogen transformations and nitrous oxide (N2O) emissions with the same digestates were also assessed in an incubation. Results: One of the tested digestates had as good fertilizer effect as urea. Nitrogen transformations went mostly as expected under given water status, but digestates were more affected by waterlogging than urea. Some instability appeared about 10 days after fertilizer application. Then nitrite levels were high, responsive microbial populations peaked and N2O emissions started. Whilst emissions after mineral fertilizer application (urea) were not affected by waterlogging, emission after application of one of the examined digestate was high at field capacity but almost zero under waterlogging. N2O emissions from all other digestate and water treatments were also low. Conclusion: Digestates have great potential as organic fertilizers in rice production, but N2O emissions under aerobic soil conditions are of concern and warrants further investigation and mitigation strategy. Highlights: · Digestates are potentially good fertilisers for rice · Nitrogen uptake from one of the tested digestates was comparable with urea · Ammonium was released quickly from digestate, but more nitrate was formed with urea treatment · Some instability appeared about 10 days after fertiliser application, with high nitrite levels · Water-saving irrigation increases N2O emission more with digestate than with mineral fertilisers