Changes in degree of phosphorus saturation and risk of P loss upon twelve years of manuring and reduced tillage

Abstract

Abstract Adoption of minimum tillage and application of fertilizer integrated with farmyard manure in the long-term has been envisaged to sustain rice-based cropping systems in the Indo-Gangetic plains of India. However, substitution of mineral fertilizer based on crop N requirement results not only in increase of the total P load but also in the distribution of P fractions and soil properties that influence P adsorption. A twelve year old experiment consisting of two tillage (conventional, CT and minimum, MT tillage) and three fertilization treatments (100% inorganic fertilization, IF, 100 and 50% organic fertilization, OF) was examined to unravel the relationship between tillage and manuring effects on distribution of soil P forms vis-a-vis soil test P and how it relates to degree of P saturation (DPS). Reduced tillage intensity and organic fertilization resulted in increase of all P fractions and soluble P. Olsen-P varied from 3.4 to 59.4\xa0mg\xa0kg−1 and was exponentially (NH4F-P/NaOH-P) or linearly (Occ-P/H2SO4-P) related to P fractions by direct or indirect effects. Increase in pH and total organic carbon coupled with increase in P load especially in OF and MT treatments resulted in decrease in the P maximum adsorption capacity (PMAC) and P bonding energy of the soil. Consequently, the degree of phosphorus saturation increased with decrease in PMAC and a change point was noted at 13.62% DPS above which soluble P increased more rapidly. Olsen P, which is used as an index of P availability, increased with reduction of tillage and organic manure addition and was closely associated with all the P fractions and DPS. The Olsen P at the change point DPS was 50.4\xa0mg\xa0kg−1 which indicated risk of P losses and hence could be used as an index for risk assessment and identify soils that need to be managed agronomically and environmentally to avoid P losses to the environment.