Effect of fertilizer-N application and seed coating with rhizobial inoculants on soybean yield in eastern Paraguay.

摘要

Nitrogen removal in soyabean grains at harvest may exceed biological N2 fixation, particularly if grain yields are as high as typically achieved on 'Terra Rossa' soils of Eastern Paraguay. Applying N fertilizer or coating seeds with rhizobial inoculants that enhance nodulation may represent a way of balancing the N budget. However, the effects of such treatments appear to be highly site-specific. This study was conducted to examine the effects of N application (N) and rhizobial inoculation (I) on nodulation, N accumulation and soyabean yields in Eastern Paraguay. Field experiments were conducted in two consecutive soyabean seasons. Dry conditions in the first year delayed sowing and reduced plant number m-2 and pod number plant-1. Grain yields were generally below 2 tonnes ha-1 but the +N+I treatment increased yields by approximately 75%. In the second year, favourable conditions resulted in yields of approximately 4 tonnes ha-1 and the treatments had no effect. Nitrogen accumulation was higher in the first year and could therefore not explain the observed yield differences between years and treatment combinations. The positive effect of the +N+I treatment in year one was associated with a more rapid root growth, which could have reduced susceptibility to intermittent drought stress. Nodule biomass decreased between flowering and pod setting stages in the +I treatment whereas further increases in nodule biomass in the -I treatment may have led to competition for assimilates between nodules and developing pods. Based on these preliminary results we conclude that N application and seed inoculation can offer short-term benefits in unfavourable years without negative effects on yield in favourable years.