Change in phosphorus availability, fractions, and adsorption-desorption by 46-years of long-term nutrient management in an Alfisol of eastern India

Abstract

In acid soils, majority of the applied phosphorus (P) transforms to sparingly soluble Fe (PFe) and Al phosphates (PAl), triggering rampant P deficiency. This study aimed to evaluate the impact of long-term (46-years) nutrient management practices on P fractions and P adsorption-desorption behaviour of an acid soil with a soybean-wheat cropping system. Out of ten treatments employed in this experiment, eight most pertinent to the study, viz., control, N (100 % of recommended N fertilizer), NP (100 % of recommended N and P fertilizers), 50 % NPK (50 % of recommended N, P, and K fertilizers), NPK, 150 % NPK, NPK+FYM (100 % of recommended N, P, and K fertilizers with farmyard manure), and NPK+Lime (100 % of recommended N, P, and K fertilizers with lime) were selected. The findings revealed that amorphous Fe and Al, which play a significant role in P fixation, increased due to cultivation. The treatment 150 % NPK recorded the highest available P (104 mg kg-1), followed by NPK+FYM (84.1 mg kg-1) at 0–15 cm depth. The PFe was the dominant inorganic P fraction (25–46 %) while soluble and loosely bound P (the easiest plant available fraction) was the lowest (0.9–1.9 %). The long-term cultivation led to an accumulation of PFe and PAl fractions while caused dissolution of calcium bound P (PCa). The highest content of organic P (Po) was recorded with NPK+FYM (234 kg ha-1). The analysis of Langmuir and Freundlich isotherms indicated that NPK+FYM and 150 % NPK reduced the P adsorption maxima to 341 and 231 mg kg-1, respectively, compared to 406 mg kg-1 for NPK. Moreover, these treatments increased the P sorption saturation to 33 % and 37 %, respectively, as compared to 12 % for NPK, and significantly enhanced P desorption, raising concerns about eutrophication. A path analysis was carried out to elucidate the effects of P dosage, liming, and manuring on soil properties, available P, P adsorption-desorption parameters, yield, and P uptake. In summary, the NPK+Lime treatment offered the most balanced approach, improving both crop yield and P uptake while effectively managing P dynamics in the soil. On the contrary, long-term application of NPK+FYM in an acid soil may result in faster P saturation of adsorption sites and increase the chances of leaching and eutrophication. Furthermore, tailored P fertilization strategies should be developed to better utilize the PFe and PAl fractions and supplementing applied P.