S13 - Session O4 - Extending the growth period of green manure can reduce the risk of soil organic matter loss due to priming effects

Authors: Kae Miyazawa *, Yu-Chau Chen, Shunto Ishiguro

Green manure is often grown and incorporated into the soil to increase soil organic matter (SOM) and is considered to be one of the sustainable agricultural practices. However, contrary to expectations, this method often results in a decrease in SOM due to priming effects, which in turn leads to the release of CO2 into the atmosphere. In this study, we tested whether increasing the carbon/nitrogen (C/N) ratio of green manure in a realistic cropping system can increase soil organic matter and also increase the yield of subsequently planted vegetables. Field experiments were conducted using Sorghum ( Sorghum bicolor ) as a summer green manure. Three green manure treatments (NGM: no green manure, 2mGM: standard green manure practice of growing green manure for two months and leaving the field one month after application, 3mGM: growing green manure for three months) were tested in combination with two chemical fertilization treatments (CF: conventional fertilization, NF: no fertilization). After each green manure treatment, cabbage or lettuce was grown as fall and spring crops. Biomass of sorghum, C/N ratio, vegetable yield, and soil chemical properties were investigated during 3 years. The biomass of sorghum was 2-5 times higher and the C/N ratio 2-3 times higher under 3mGM compared to those under 2mGM; SOM increased under 3mGM but decreased under the combination of 2mGM and CF, which is the standard practice for green manure application. Vegetable yields increased after two years of sorghum incorporation. Increasing the C/N ratio by extending the growing period of green manure seems to be a promising method to increase both vegetable yield and SOM.