Nitrogen Limitation in Soybean: Physiology and Diagnosis

In high-yield soybean environments, the seed yield is limited by nitrogen (N) supply. However, the response of soybean to N fertilizer application is inconsistent. Understanding the seasonal N uptake dynamics in soybean can provide insights on diagnosing N limitation if soybean. To understand soybean N dynamics, we assessed seasonal trends on dry matter and N accumulation in vegetative organs (leaves, stems, and petioles) from 42 field experiments. Each experiment included a ‘zero-N’ treatment, where crops relied on N supply from soil and biological N fixation (BNF), and a ‘full-N’ that received ample N via N fertilizer addition. Vegetative storage proteins (VSPs) in stems were measured via a detailed proteomic analysis at one site. Similarly, we used a large number of variables from a subset of 35 experiments to identify N-limited fields. We also included had included an 80-N treatment in this subset of experiments to identify fields with a cost-effective yield response to N fertilizer. Seed yield ranged from 4.2 to 7.3 Mg ha-1, with full-N higher by 11% relative to zero-N treatment. The full-N showed increased mobilized N from stems and leaves to seeds due to larger N stored before seed filling. These increases in mobilized N were larger in stems than leaves. Indeed, the VSPs abundance increased by 91% in the full-N relative to the zero-N in the stems. Analysis of yield response to full-N and 80-N indicated that 66% soybean fields were N-limited and amongst these fields, 65% were profitable to 80 kg N ha-1 applied around flowering. Soybean yield response to N fertilizer application can be diagnosed through stem N around flowering. Fields with stem N less than 3 kg N ha-1, between 3-6 kg N ha-1, and higher than 6 kg N ha-1 had 75%, 47%, and near zero chance of profitable yield response to 80-N. Our findings show that stem N plays an important role in meeting seed N demand and can be used to diagnose yield response to N fertilizer in soybean. Advisors: Nicolas Cafaro La Menza and Patricio Grassini