Depth sequence distribution of water extractable colloidal phosphorus and its phosphorus speciation in intensively managed agricultural soils.

Legacy phosphorus (P) has accelerated the subsurface transport of colloidal P (CP) in intensively managed agricultural soils in the Midwestern U.S. Because of its high P sorption capacity and mobility, understanding the depth sequence distribution of mobile CP and its speciation in the soil profile is critical in assessing total P (TP) loss to protect the water quality of adjacent water bodies. In this study, physicochemical properties of water-extractable colloids (WECs) from the soil profile at 0-180 cm were characterized using conventional wet chemical analysis. Solution P-31 nuclear magnetic resonance spectroscopy (NMR), P and Fe K-edge X-ray absorption spectroscopy, and transmission electron microscopy were also used to understand P speciation and mineralogy of CP. Percent recovery of WECs per bulk soil increased more than three times with increasing depth. Considering mildly alkaline pH of pore water and negative zeta potential (-21 ± 4 mV) of WECs (size: 1.65 ± 0.45 µm), the transport of P rich WECs (TP: approximately 210-700 mg kg-1) were facilitated from surface to subsoils. Generally, TP in WEC decreased with increasing depth. Interestingly, WECs in subsoil contain organic P (OP) as much as 60 mg kg-1. NMR analysis clearly showed the presence of OP monoesters, OP diesters, and orthophosphate in these particles. Both orthophosphate and OP species interacted with iron oxyhydroxides, calcite, and aluminol functional groups of gibbsite and or phyllosilicates. The study showed the availability of WECs from surface to subsoils that carry orthophosphate as well as OP in legacy P impacted agricultural soils in the Midwestern U.S.

Nitrogen species specific phosphorus mineralization in temperate floodplain soils.

As an essential component of enzymes, higher N availability from agricultural runoff to forest soils may boost the activity of phosphatase, increasing the bioavailability of phosphate. The objective of this study was to evaluate P mineralization rates in temperate floodplain soils as a function of inorganic N species (i.e., ammonium and nitrate) and amendment rate (1.5-3.5 g N kg-1). Accordingly, the soil was amended with nitrate and ammonium, and P dynamics were monitored during a 40-day incubation. The addition of ammonium significantly boosted acid and alkaline phosphatase activity by 1.39 and 1.44 µmol p-nitrophenol P (pNP) g-1 h-1, respectively. The degree of increase was positively correlated with the amendment rate. Likewise, the P mineralization rate increased by 0.27 mg P kg-1 in the 3.5 g N kg-1 ammonium treatment. 31P nuclear magnetic resonance spectroscopic analysis further supported the reduction in organic orthophosphate diesters on day 30. Meanwhile, the addition of nitrate promoted P mineralization to a lesser degree but did not increase phosphatase activity. While floodplain soils have great potential to sequester anthropogenic P, high availability of inorganic N, especially ammonium, could promote P mineralization, potentially increasing P fertility and/or reducing P the sequestration capacity of floodplain soils.

High flow event induced the subsurface transport of particulate phosphorus and its speciation in agricultural tile drainage system.

Subsurface storm flow of phosphorus (P), including particulate P, has been recently discussed as an important P transport path in contrast to typical surface runoff events. However, P speciation, and P concentration during storm events has not been extensively investigated; therefore, its contribution to the water quality is not clearly understood. In this study, the physicochemical properties of particulate P in tile water samples during a high flow event were investigated in Midwestern agricultural lands using wet chemical methods, 31P Nuclear Magnetic Resonance spectroscopy and P K-edge X-ray absorptions near edge structure spectroscopy. In slightly alkaline pH tile water, total P was ranging from ∼0.06 to 0.22 mg L-1, which is significantly greater than dissolved reactive P (DRP) (∼0.02-0.08 mg L-1). The tile water contains P enriched particulate matters (∼200-660 mg L-1). Total P in the colloidal fraction was from 1013 to 2270 mg kg-1. Phosphate and organic P species, especially monoesters, are sorbed in soil colloids like calcite, and iron oxides, and colloids are effective carriers of P in the subsurface transport process during storm events. The results of this study show that storm events can accelerate the subsurface transport of P with soil particles in addition to DRP.

Effects of native leaf litter amendments on phosphorus mineralization in temperate floodplain soils.

As phosphorus (P) losses from Midwestern crop fields degrade water quality in downstream water bodies, the assessment of natural P immobilization in floodplain soils is imperative to reduce P input to the Gulf of Mexico. While the organic C:P ratio of soil is widely accepted as an important indicator of P immobilization, roles of the quality/type of C sources (i.e., foliar C composition and degradability) on soil P dynamics are not clearly understood. The objective of this laboratory incubation study was to assess the influence of leaf residue of native trees (e.g., hackberry, and silver maple) on P reaction dynamics in floodplain soils as a function of C composition (i.e., carbonyl-, alkyl- and aromatic-C) and soil organic C:P ratios. Conventional wet chemical analyses and 31P NMR spectroscopy were used to understand changes in P speciation and phosphatase activities. During the incubation, at a soil organic C:P of ∼200, residues with low aromaticity promoted P mineralization, as evidenced by a sustained increase in labile inorganic P and decrease in microbial P. Conversely, residues with high aromaticity and hydrophobicity (i.e., silver maple) caused a decrease in labile inorganic P and increase in microbial P under the same soil organic C:P, indicating the dominance of P immobilization. At a soil organic C:P of 300, both sugar maple and silver maple promoted P immobilization. Mineralization rates were of lesser magnitude in the soils amended with silver maple, which interestingly contained the largest proportions of recalcitrant C and the highest ratios of aromaticity and hydrophobicity.