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

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.

Automated summary

This study aimed to evaluate the impact of long-term nutrient management practices on P fractions and P adsorption-desorption behaviour of an acid soil with a soybean-wheat cropping system. The findings revealed that amorphous Fe and Al, which play a significant role in P fixation, increased due to cultivation. 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. Tailored P fertilization strategies should be developed to better utilize the PFe and PAl fractions and supplementing applied P.