Decline in fertility of paddy soils induced by paddy rice and upland soybean rotation, and measures against the decline

Crop rotation between irrigated paddy rice (Oryza sativa L.) and upland soybean [Glycine max (L.) Merr.] (paddy-upland rotation) induces a decline in soil nitrogen (N) fertility, as observed by available soil N. There was a significant negative correlation between the available soil N and the proportion of upland seasons to total crop seasons after the initiation of paddy-upland rotation (upland frequency). The decline in soil N fertility was alleviated by the application of organic materials. Soil total carbon also tended to decrease with an increase in upland frequency. Soil physical properties were affected by the paddy-upland rotation. As soil organic matter decomposed in paddy-upland rotation, the soil density increased with decreasing soil porosity. A suitable range of available soil N for paddy-upland rotation was identified between 80 and 200 mg/kg, the same as for paddy rice. The keys to controlling soil N fertility in paddy-upland rotations are the upland frequency and application of organic materials. To sustain the available soil N over the minimum suitable level of 80 mg/kg, the upland frequency should not exceed approximately 60% when only crop residues and no other organic materials are applied. The upland frequency can be increased by the repeated application of organic materials, thereby maintaining a higher level of available soil N.