Soybean Nitrogen Fixation : The Nitrogen Budget.

Cereal-legume crop rotations can benefit production, profitability and environmental performance. One benefit of these cropping systems is that legumes are capable of biological N fixation (BNF). However, BNF provides only a portion of total crop N demand and the remaining N requirement must be met from the soil N pool. The portion of total crop N demand met by BNF is variable across years, cultivars, locations, and management practices. If the amount of BNF does not meet or exceed the amount of N harvested, there is risk that the crop will reduce the soil N stock. There is growing concern that soybean is reducing soil N stocks because BNF is consistently less than the amount of N harvested in soybean grain. The aim of our study is to understand the variability in BNF in different environments (i.e., planting date x site), and years. This study was conducted in two sites located in Central and North West Iowa at two planting dates (May 1^st and 25^th). We used a ¹⁵N isotope dilution method to measure BNF throughout the growing seasons of 2015 and 2016 within different plant parts including above and below-ground tissues. Preliminary results have shown that in both sites and years soybean is a net user of soil N rather than a contributor, resulting in a net negative soil N balance.