Rice Yield and the Fate of Fertilizer Nitrogen as Affected by Addition of Earthworm Casts Collected from Oilseed Rape Fields: A Pot Experiment

The mechanism associated with improvement of soil nutritional status by oilseed rape crop, leading to better performance of rice crop, in rice-oilseed rape cropping systems is little known. The present study was aimed to test the hypothesis that earthworm casts produced during oilseed rape-growing season have positive effects on grain yield and fertilizer nitrogen (N) utilization in the subsequent flooded rice crop. A (15)N-tracing pot experiment was conducted to determine the effects of earthworm casts collected from oilseed rape fields on yield attributes in rice and the fate of fertilizer N. Soil treated with earthworm casts (soil: earthworm casts = 4: 1, w/w) (EC(1)) produced 39% higher grain yield than soil only (EC(0)). EC(1) had 18% more panicle number and 10% higher spikelet filling percentage than EC(0). Aboveground biomass and harvest index were higher in EC(1) than in EC(0) by 20% and 15%, respectively. SPAD values in flag leaves were 10% and 22% higher under EC(1) than EC(0) at 15 and 20 days after heading, respectively. EC(1) had 19% higher total N uptake and 18% higher physiological N-use efficiency than EC(0). These positive effects of earthworm casts on yield attributes offset negative effects of decreasing N rate from 0.74 g pot(–1) (equivalent to the recommended field rate of 150 kg ha(–1)) to 0.44 g pot(–1) (equivalent to 60% of the recommended rate). Fertilizer N retention rate was 7% higher while fertilizer N loss rate was 6% lower in EC(1) than in EC(0). Our study suggests that earthworm casts produced during oilseed rape-growing season are expected to have the following benefits on the subsequent flooded rice system: (1) improving growth and physiological processes in rice plants and consequently increasing rice grain yield, and (2) increasing fertilizer N retention rate and hence decreasing fertilizer N loss rate and reducing environmental risk.