Quantifying the nitrogen balance of high-yielding soybean with reduced yield gaps

Abstract

Context: Nitrogen (N) is the most extracted nutrient and exported by soybean crops, yet estimates of N balance in commercial farming systems with reduced yield gaps (i.e. difference between water-limited yield potential and farmers´yield) remain limited. Objective: This study quantified the N balance and its components of high-yielding soybean systems at the farm scale in the Rolling Pampas region of Argentina. Methods: Twenty-three on-farm trials compared conventional management practices with a high-input strategy incorporating additional inputs not typically used by local farmers. We measured total N uptake, seed N removal, seed protein concentration, and the contributions of N derived from atmosphere (%Ndfa), using the ¹⁵N natural abundance method, and from the soil (%Nsoil). We estimated the N-gap as the difference between N uptake and N derived from the atmosphere; and apparent N balance as the difference between N derived from the atmosphere and seed N removal. Results: High-input management increased total N uptake (3.5 %) and seed N removal (4 %), driven by 17 % increase in %Nsoil. However, %Ndfa remained unchanged. A strong positive relationship was found between % Ndfa and apparent N balance, with 72 % %Ndfa required to achieve neutral balance. Fields with low yield gaps (0–5 % yield gap) exhibited higher N uptake (14 %), %Ndfa (11 %), seed N removal (17 %), and improved N balances of − 32 kg N ha − 1 compared to − 40 kg N ha − 1 of fields with high yield gaps (15–20 % yield gap). Conclusions: Input intensification enhanced N uptake and removal but did not improve %Ndfa, exacerbating N balances. Fields with low yield gaps showed more favorable N balance than fields with high gaps. Implications: Sustainable improvements in N balance will require integrated, long-term management beyond single season input intensification.