Purpose

Green manure is an efficient nitrogen (N) source when used as an alternative to chemical fertilizer. However, the N taken up by rice derived from green manure, chemical fertilizers or soil native N in complex nutrient systems is unclear.

Materials and methods

A pot experiment with partial substitution of urea with Chinese milk vetch (a green manure) implemented with 15N-labeled urea and Chinese milk vetch was set up to study the sources of N in rice and the fate of the fertilizers. The objectives of this study were (1) to investigate the characteristics of urea or fresh CMV N supply for different amounts of urea; (2) to evaluate N uptake by rice from the 15N-labeled urea or CMV under different managements and (3) to determine the fates of 15N-labeled urea and CMV and their use efficiencies in a rice system.

Results and discussion

The rice dry weights, rice N contents, rice N uptake and urea N use efficiency were notably higher (by 15-16%, 4-13%, 22-30% and 182%-203%, respectively) in the Chinese milk vetch applied with urea treatment than in the urea alone treatment. The uptake of N from Chinese milk vetch and the use efficiency of Chinese milk vetch N were increased with reductions in the urea input amount. The application of Chinese milk vetch substantially changed the fate of urea: higher amounts of urea N were taken up by rice (approximately 29%) and remained as residue in the soil (approximately 15%) in the related treatments than in the treatment with urea alone (10% and 9%). More urea N than Chinese milk vetch N was taken up by rice (29% vs 20%, respectively) and lost (56% vs 14%, respectively), but less urea N than Chinese milk vetch N remained as residue in the soil (15% vs 66%, respectively).

Conclusion

The partial substitution of chemical fertilizer with green manure is an effective method of promoting rice growth by supplying N for rice uptake and promoting more efficient N use.