The Innovation Team of Fertilizer and Fertilization Technology of the Institute of Agricultural Resources and Regional Planning (IARRP) of the Chinese Academy of Agricultural Sciences (CAAS), has constructed a low nitrogen footprint green manure-rice production system by optimizing the best nitrogen application rate based on different biomass levels of green manure. This achievement has been published under the title "Exploring site-specific N application rate to reduce N footprint and increase crop production for green manure-rice rotation system in southern China" in the international Journal of Environmental Management.

Milk vetch is the most important green manure (GM) crop in the rice-growing areas of Southern China. Cultivating milk vetch in rice fields during winter fallow can provide a large amount of organic N for the subsequent rice crop, reducing the use of chemical N fertilizer and lowering active N loss. Due to spatial and temporal differences in soil, climate, and field management conditions, there is significant variation in GM biomass between different regions and years. It is difficult to determine the N fertilizer application rate for the subsequent rice crop after incorporating GM, often relying on expert experience. Therefore, developing a way to determine the best N fertilizer input for the subsequent rice crop based on GM biomass is the key to building a low-N footprint GM-rice production system.

Based on field surveys, multi-year and multi-location experiments, and model simulations, this study establishes a calculation framework, in which several simple regression equations and linear regression equations are respectively used to describe the relationship between the optimal N application rate and the amount of GM incorporated in for single-season rice and early-season rice systems (R²>0.9). These equations are used to optimize the best N application rate under different GM biomass levels. As the amount of GM incorporated in increases from 0.0 to 45.0 t ha^-1, the optimal N application rate for single-season rice decreases from 180-280 kg N ha^-1 to 100-120 kg N ha^-1, and for early-season rice it decreases from 140-180 kg N ha^-1 to 0-20 kg N ha^-1. Results from nine southern provinces indicate that, compared with the N fertilizer application rate of farmers, optimizing N application rate can reduce N fertilizer application by 40.8% (71.9 kg ha^-1) and reduce N footprint by 33.5% (1.741 kg N-eq t^-1) while ensuring stable yields.

Specifically, the regional N fertilizer input for single-season rice and early-season rice is reduced by 29.6% and 65.3% respectively, and the N footprint is reduced by 23.4% and 49.3%, respectively. This research is of great significance for optimizing N fertilizer management in the GM-rice production system in Southern China and reducing Nn footprint.

The optimal nitrogen fertilizer application rate under different green manure incorporating levels for single-season rice and early-season rice.

A comparison of rice yield, nitrogen fertilizer application, and nitrogen footprint under optimized N and farmer's N application rates in the study area.

Dr. Liang Hao from Hohai University is the first author of this article, and Dr. Cao Weidong from the IARRP, and Dr. Gao Songjuan from Nanjing Agricultural University, are the corresponding authors. This research was supported by the "14th Five-Year Plan" National Key Research and Development Program (2021YFD170020) and the National Green Manure Industry Technology System (CARS-22).

Paper  link:

https://www.sciencedirect.com/science/article/pii/S0301479723018212