**1. Introduction**

Rice feeds more than half of the global population [1], and many consumers of rice are among the world's poorest, with diets that are mainly restricted to rice as it is filling, accessible, and affordable [2]. However, rice is deficient in many nutrients [3], and thus malnutrition is common in countries where rice is the main food [4]. Therefore, it is important to enhance the nutritional value of rice to improve the health of rice consumers, especially those living in poverty.

Amino acid content is an important nutritional component on which many rice breeding programs have focused [5]. Grain amino acid content in rice is determined not only by genetic factors but also by environmental factors and management practices [6,7]. Previous studies demonstrated that there is a positive linear relationship between amino acid and nitrogen (N) contents in grains of rice [8], and that amino acid content in the rice grain can be increased by applying N fertilizer at the heading stage [6].

China is the largest producer and consumer of rice in the world, accounting for about a third of the global rice economy [9]. In the past several decades, the large input of N fertilizer across wide areas of China has caused considerable environmental costs, including soil acidification, water eutrophication, and enhanced N deposition [10–12]. Because of these effects, methods other than an artificial increase of N input are sought to increase rice grain amino acid content.

Earthworm castings, or vermicompost, are a form of fertilizer produced from earthworms as they digest soil organic matter. This excrement is enriched in mineral N [13]. Our recent study showed that application of earthworm castings could improve N uptake and utilization in rice [14]. The objective of the present study was to determine the effects of applying earthworm castings to soil on amino

acid and N contents in the rice grain. We hypothesized that application of earthworm castings may increase grain amino acid content in rice.
