Micronutrient Transformations in Soil after 12 Years of Phosphorus and Farmyard Manure Application Under Maize-Wheat System
摘要
Continuous phosphorus (P) fertilization influences soil micronutrient dynamics, but excessive P can reduce their availability, thereby limiting plant uptake. This study evaluated the effects of P and farmyard manure (FYM) on zinc (Zn), copper (Cu), iron (Fe), and manganese (Mn) availability and their transformations in soil. A long-term field experiment was conducted under maize-wheat system with five treatments involving varying P levels (0, 13, 26, 39, and 52 kg ha− 1), three treatments combining FYM with P (0, 13, and 26 kg ha− 1), and control. Phosphorus application up to 26 kg ha⁻¹ improved grain yield, whereas higher doses did not provide additional benefit in wheat and reduced maize yield by 4.4% at 52 kg P ha− 1 compared to 26 kg P ha− 1. Among all treatments, 26 kg P ha− 1 + FYM recorded highest maize and wheat yields (6.25 and 5.36 t ha− 1, respectively). Micronutrient uptake followed a similar trend. In soils, DTPA-extractable Zn, Cu, Fe, and Mn increased by 32.8, 36.9, 38.5, and 43.4%, respectively, under 26 kg P ha− 1 + FYM compared with control. Fractionation analysis revealed that higher P shifted micronutrients into oxide-bound fractions, reducing availability, whereas FYM enhanced water-soluble + exchangeable and organically bound fractions. Path analysis indicated that water-soluble plus exchangeable, specifically adsorbed, and organically bound fractions contributed more to plant uptake, while oxide-bound fractions had minimal impact. These findings indicate a critical P threshold beyond which micronutrient availability declines, emphasizing balanced P fertilization with FYM to sustain long-term soil fertility in intensive cropping systems.