**Featured Application: Enhancing earthworm activity in the field may be a feasible way to increase grain amino acid content in rice.**

**Abstract:** Enhancing the nutritional value of rice can improve the health of rice consumers. Grain amino acid content is an important nutritional component. This study was conducted to test the hypothesis that the application of earthworm castings could increase the grain amino acid content in rice. Results showed that total amino acid content in the grain of rice was significantly elevated by applying earthworm castings (17 kg m−2), with an average increase of 8% across four tested rice cultivars. Application of earthworm castings had no significant effect on total nitrogen (N) content but significantly increased the ratio of amino acid to N (total amino acid content/total N content) in rice grains. The results of the present study suggest that application of earthworm castings can increase grain amino acid content in rice by improving the efficiency of the N to amino acid conversion, and highlight that further studies are required to assess the effects of earthworm castings on the amino acid metabolism in rice grains.

**Keywords:** amino acid content; earthworm castings; rice
