Chemistry

Ecology

Nitrogen fixation

Biomass (ecology)

Growing season

Dry matter

Agronomy

Soil water

Sowing

Organic chemistry

Biology

Crop

Environmental science

Nitrogen

Organic matter

Field Crops Research

The rainfed USA Midwestern region has deep, fertile soils and leads the USA in soybean [Glycine max, (L.) Merr.] production. Biological nitrogen (N) fixation (BNF) contributes a portion of the soybean N requirement, but variability in BNF is poorly understood and estimates of BNF for this region are rare. We established experiments in Iowa, USA to gain a better understanding of BNF and increase its predictability. We collected in-season BNF measurements accompanied by high temporal resolution soil and plant growth measurements. Across two years, two locations and two planting dates, we found that BNF contributed 23–65% of total aboveground N accumulation in soybean. The BNF rate was maximized at the early seed-filling period and varied from 1 to 3 kg N ha−1day−1. During seed filling period, the rate of BNF was related to crop growth rate (carbon (C) supply) but not to N accumulation by the reproductive organs (N demand). We found that a minimum crop growth rate of 135 kg dry matter ha−1day-1 is required to su...

Soybean nitrogen fixation dynamics in Iowa, USA