Abstract
Agricultural practices affect the status of legacy phosphorus (P) in soils and consequently the P bioavailability and susceptibility of losses to water resources. Previous studies have primarily assessed P status within agro-ecosystems and rarely have these results been compared to virgin conditions. We evaluated the effects of long-term (45 years) consistent cropping (continuous corn, CC; corn-oats-alfalfa-alfalfa rotation, CR,; and continuous bluegrass sod, CB) with and without P fertilization on changes in P fractions of different bioavailability in a Brookston clay loam, as compared to an adjacent virgin forest. Soil P was separated into various inorganic P (Pi) and organic P (Po) fractions using a modified sequential fractionation method. Phosphorus in virgin soil was predominated by moderately labile Po (NaOH-Po), 44%, followed by moderately stable Pi (HCl-P), 26%. Compared to the virgin soil, consistent cropping without P fertilization significantly decreased all P fractions except for water extractable Po, with the largest decrease in labile Pi (water-Pi + NaHCO3-Pi) and moderately labile Po of 65 and 73 mg kg-1, respectively, over 45 years. Consistent cropping with fertilization retained comparable amount of total P in CC and RC, but increased total P in CB, relative to the virgin soil. Averaged over cropping systems, labile Pi, NaOH-Pi, and HCl-P increased by 129, 74 and 20 mg kg-1, respectively, whereas, labile Po and moderately labile Po decreased by 8 and 60 mg kg-1, respectively, compared to the virgin soil. This study indicates that long-term cropping significantly enhanced the rate of moderately labile Po mineralization, irrespective of fertilization. The increased total P and Po in the fertilized CB plots suggests that P accumulation in long-term grass fields is a concern as far as potential P contamination in surface waters.
Highlights
Soil P status plays important roles in P bioavailability and mobility from soil to water (Simard et al, 1995; Zheng and Zhang, 2012)
We evaluated the effects of long-term (45 years) consistent cropping [continuous corn (CC), corn-oats-alfalfa-alfalfa rotation (CR), and continuous bluegrass sod (CB)] with and without P fertilization on changes in P fractions of different bioavailability in a Brookston clay loam, as compared to an adjacent forest native soil
The P deficits were expectedly observed after 45-year consistent cropping without P fertilization in both CC and CR plots, whereas no changes were anticipated in the CB plots as the grasses were not removed from the plots (Table 1)
Summary
Soil P status plays important roles in P bioavailability and mobility from soil to water (Simard et al, 1995; Zheng and Zhang, 2012). P decline in soils under agricultural production has been observed after long-term cessation of fertilization relative to native soils due to P removal in the harvested crops. The supply of plant availability P was regulated by the rate of mineralization of Po (Cole et al, 1977) Those studies, as well as the ones reported recently (e.g., Tiecher et al, 2018), indicated that the mineralization of Po in cultivated soil plays important roles in both plant available P and P transformation in soil. Hedley et al (1982) considered that P loss from cultivated soil resulted mainly from crop removal, which would account for the 75% of the decrease in total P loss from soil Those studies, as well as the ones reported recently (e.g., Tiecher et al, 2018), indicated that the mineralization of Po in cultivated soil plays important roles in both plant available P and P transformation in soil. Hedley et al (1982) considered that P loss from cultivated soil resulted mainly from crop removal, which would account for the 75% of the decrease in total P loss from soil
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