Quantifying the Contribution of Above- and Below-Ground Residues of Chickpea, Faba Bean, Lentil, Field Pea and Wheat to the Nitrogen Nutrition of a Subsequent Wheat Crop

Context or problem: The individual nitrogen (N) contribution from above-ground plant residue (AGRP) P ) and total below-ground residuals (BGRT) T ) to a subsequent wheat crop remains poorly explored. The need to understand this dynamic is crucial for optimizing crop yield and soil nutrient management. Objective or research question: This study aimed to discern the individual N contributions from AGRP P and BGRT T to a succeeding wheat crop and understand the implications of various grain legume residues on these contributions. Methods: A four-year field study consisting of two 2-year grain legume-cereal cropping sequences was conducted in Saskatoon, SK, Canada. The grain legumes were chickpea, faba bean, lentil, and field pea, with spring wheat grown as the reference crop in 2014 and 2016. Each plot was split into quadrants with one quadrant receiving 15 N-urea and the other three receiving non-labeled (natural abundance, NA) urea in spring of the pulse phase. After pulse harvest, 15 N-AGR P was swapped with NA-AGRP P when returned to the field, resulting in sub-plots with 15 N-AGR P /NA-BGR T and NA-AGRP/15N-BGRT P / 15 N-BGR T combinations. In 2015 and 2017 all plots were planted with spring wheat and were fertilized based on soil mineral N measurements and target yields. One NA quadrant received 15 N-urea during the cereal phase to track the fate of fertilizer N versus legume N. Results: Grain legume residue species did not affect the seed yield of the subsequent wheat crop. However, wheat grown on lentil and pea residue required less N fertilizer than other residues. Over the five tested residues, BGRT T (roots, rhizodeposits, residual fertilizer, and soil) was the largest N source, accounting for 70-91% of wheat N uptake, which surpassed the combination of the contribution from AGRP P (1-11% of wheat N uptake) and fertilizer (5-21% of the wheat N uptake). Among BGR components, soil was the main N contributor to subsequent wheat. The N recovery rates for the wheat crop were below 9% for AGRP, P , 18% for BGRT, T , and 26% for fertilizer N. Conclusions: Below-ground residue, particularly the soil, plays a pivotal role in regulating N supply to the succeeding wheat crop, overshadowing contributions from AGR and fertilizer. Implications or significance: This research underscores the significance of BGR, especially the soil, in N management for subsequent crops. Recognizing these dynamics can help tailor crop residue management strategies and optimize soil nutrient supply for subsequent crops.