Composition of phosphorus pool and the consumption and transformation characteristics of accumulated phosphorus in fluvo-aquic soil

Objectives Long-term application of phosphate fertilizer resulted in the accumulation of a large amount of phosphorus (P) in fluvo-aquic soil. We studied the composition and consumption characteristics of phosphorus pool after cessation of phosphorous application, to provide a theoretical base for the P nutrient management.

Methods The research was based on the “National Long-term Monitoring Station of Fluvo-aquic Soil Fertility and Fertilizer Effects”, where P was applied in different rates for 26 years and the soil P accumulation amounts in different treatments varied greatly. The top layer soils from a single treatment or a mixture of two treatments were used to prepare the test soils with different Olsen-P contents for a P exhausting micro-plot trial, under wheat-maize rotation. The soils with Olsen-P content 6.7 mg/kg was defined as P deficient (L1), 14.3 and 27.6 mg/kg as moderate (L2, and L3), and 55.4 and 72.3 mg/kg as sufficient (L4, and L5), respectively. During the five years of exhausting, the total P (TP), Olsen-P content, and P fractions were analyzed.

Results Inorganic P accounted for more than 90% of total P in fluvo-aquic soil. The Resin-P, NaHCO₃-Pt and NaOH-Pt in L5 soil were 5.0, 3.5 and 2.8 times of those in L1 soil, respectively. The proportion of labile and difficult-utilization P fractions were 10.4% and 24.0% of TP in L1 soil, and the proportion were 20.6% and 14.3% in L5 soil. The proportion of moderately labile P fraction was basically maintained at 66% of TP in all the test soils. Resin-P content would not increase until the Olsen-P level was higher than the agronomic threshold (L2 soil), and the increase contributed 17.3%–22.6% of the total increase of labile P pool. During the depletion process, crops absorbed the labile P fractions first and in order of Resin-P, NaHCO₃-Pi and NaOH-Pi. For each 1 mg/kg of Resin-P, NaHCO₃-Pi, and NaOH-Pi consumption, the soil Olsen-P content was reduced by 1.3, 0.7, and 1.0 mg/kg, respectively. The P pools with different availability converted with each other. After five years of depletion, 18.0 mg/kg of difficult-utilization P fractions converted into moderately labile P (D.HCl-Pi) in L1 soil, and 22.3 and 7.2 mg/kg of moderately labile P converted into labile P in L2 and L3 soils, showing a trend of activation of accumulated P. While in L4 and L5 soils, 29.9 and 43.1 mg/kg labile P converted into moderately labile P, showing a trend of immobilization of accumulated P.

Conclusions With the increase of Olsen-P level in fluvo-aquic soil, the proportion of labile P pool (Resin-P, NaHCO₃-Pt, and NaOH-Pt) is increased, the proportion of difficult-utilization P pool is decreased, and the proportion of moderately labile P pool keeps stable. When the Olsen-P exceeds the agronomic threshold, the Resin-P content will begin to increase. Crop absorption drive the conversion of P pools, depending on the soil Olsen-P levels. In P deficient soil, the conversion is mainly from difficult-utilization P to moderately labile P, and in moderate P level soils, the conversion is from moderately labile P to labile P, the accumulated P in phosphorous deficient and moderate soils are in procession of activation, and will be used by crops at last. In high P soil, however, more than 30% of labile P fractions will be converted to moderately labile ones, and luxury P absorption of crops is common, resulting in the waste of phosphorus fertilizer nutrients. Moderate P level in soil is the most favorable for efficient utilization of phosphorus resources and high yield of crops.