Soybean as a receiver crop for nitrogen in swine lagoon effluent

Soybean (Glycine max L. Merr.) has been approved by the Natural Resource Conservation Service as a receiver crop for N in swine lagoon effluent applied to spray fields. However, its efficacy as a N receiver crop depends on the sensitivity of symbiotic N2 fixation to inhibition by N in the applied effluent. The objectives of this study were to use a N natural abundance method to (i) assess the degree of inhibition of symbiotic N2 fixation in soybean by applied effluent N and (ii) determine the quantity of effluent N removed from the soil in harvested seed of nodulating soybean. Two field experiments were conducted to evaluate seed yield; seed N accumulation and whole shoot N accumulation of nodulating and nonnodulating soybean cultivars supplied a range of N levels as either lagoon effluent or ammonium nitrate. Measurement of N natural abundances indicated that, on average, 27% of the N in seed of nodulating cultivars supplied 200 kg of plant-available N (PAN)/ha as swine lagoon effluent was derived from symbiotic N2 fixation. While this effluent N rate did not inhibit N2 fixation completely, seed N recovery of 100 kg effluent N/ha (1997 trial) and 64 kg effluent N/ha (1998 trial) was not different between the nodulating and nonnodulating cultivars. Similar recovery of effluent N in nodulating and nonnodulating cultivars, even though inhibition of N2 fixation by the nodulating cultivar was incomplete, resulted from the nodulating cultivars having higher yields and harvest indices than the nonnodulating cultivar. Subtraction of seed N at maturity from whole shoot N at the R6.5 growth stage (between full seed stage and physiological maturity) was used to estimate crop N returned to the soil. This estimate indicated that nodulating and nonnodulating cultivars returned similar amounts of N to the soil. Our results show that nodulating soybeans can recover as much applied effluent N in seed as a 6.3 Mg/ha (100 bu/ac) corn crop.