Contrasting effects of mineral, farmyard manure, and integrated fertilization on soybean productivity, seed quality, and soil biochemical properties

Abstract

Mineral and organic fertilizers differ in nutrient-release patterns and effects on soil biological processes, yet their combined influence on soybean productivity, seed composition, and soil biochemical functioning remains unclear. This study evaluated soybean and soil responses to mineral, farmyard manure (FYM), and combined fertilization over two growing seasons. A two-year field experiment was conducted in a randomized complete block design with three replicates. Eight fertilizer treatments, i.e., unfertilized control, triple superphosphate, 20–20 (20% N and 20% P₂O₅), 20–20 + urea, FYM + urea, and FYM at 40, 80, or 120 t ha⁻¹ were included in the study. Agronomic traits, chlorophyll index (SPAD), normalized difference vegetation index (NDVI), seed protein and oil contents, 100-seed weight, seed yield, soil enzyme activities, and soil NH₄⁺-N and NO₃⁻-N were recorded. Data were analyzed using linear mixed-effects models, and Tukey-adjusted pairwise comparisons were used to compare means. Fertilizer treatment significantly affected all plant and soil variables. The 20–20 + urea treatment produced the highest number of pods and nodes plant − 1 and the highest urease activity. FYM + urea and FYM at 80 t ha⁻¹ produced the highest seed yields, followed by FYM at 120 t ha⁻¹. Likewise, FYM + urea produced the highest seed protein content, whereas TSP, 20–20, and FYM-only treatments generally favored seed oil content. FYM at 40–120 t ha⁻¹ increased catalase activity, and FYM at 120 t ha⁻¹ produced the highest dehydrogenase activity and soil NO₃⁻-N concentration. Yield was strongly associated with 100-seed weight and dehydrogenase activity ( r = 0.83 for both), SPAD ( r = 0.78), and catalase activity ( r = 0.73), whereas seed protein and oil contents were negatively correlated ( r = − 0.87). PCA associated FYM at 80–120 t ha⁻¹ with yield, seed weight, mineral N, and soil enzyme activity, whereas FYM + urea and 20–20 + urea were more closely related to vegetative growth, NDVI, pod production, and seed protein content. The fertilizer treatments exhibited different responses among the measured plant and soil variables. FYM + urea and the higher FYM rates resulted in high soybean yield, greater seed weight, improved canopy performance, and enhanced soil biological activity, whereas 20–20 + urea mainly favored reproductive traits, urease activity, and soil NH₄⁺-N. Thus, integrated and organic nutrient inputs can improve both productivity and short-term soil biochemical functioning, but the responses should be interpreted as treatment-package effects, as the nutrient inputs were not equivalent.