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Title: PHOSPHORUS FRACTIONS AND DYNAMICS AMONG SOIL AGGREGATE SIZE CLASSES OF ORGANIC AND CONVENTIONAL CROPPING SYSTEMS

Author
item Green, V
item Dao, Thanh
item Cavigelli, Michel
item Flanagan, Dennis

Submitted to: Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/1/2006
Publication Date: 11/1/2006
Citation: Green, V.S., Dao, T.H., Cavigelli, M.A., Flanagan, D.C. 2006. Phosphorus fractions and dynamics among soil aggregate size classes of organic and conventional cropping systems. Soil Science. 171:874-885.

Interpretive Summary: Elevated concentrations of soil P in surface soils increases the risk of P transport by soil erosion and subsequent water qualtiy degradation. Soil management can play a significant role in controlling nutrient transport risk, however, not enough is known about potential bioactive P transport from erosion. To better understand the risk of water quality degradation due to sediment associated P, we quantified the bioactive P fractions in five aggregate size classes from conventional no-till (NT) and chisel till (CT) systems as well as a tilled organic system. To better understand the dynamics of these bioactive P fractions over time, we conducted an incubation study on the aggregates of the NT system and quantified the bioactive P changes over time. Cropping system did not affect the concentration of bioactive P fractions in the soil. However, aggregate size did affect bioactive P fraction concentrations. In general, larger aggregates had greater concentrations of bioactive P than did smaller aggregates in all cropping systems; the smallest aggregate size class had the lowest concentrations of bioactive P. Bioactive P concentrations increased during the incubation, mainly due to an increase in the organic P that is tightly bound to soil particles. This increase in bioactive P over time indicates that water quality degradation risk may actually increase under certain conditions after an erosion event. Using soil bioactive P fraction measurements from easily erodible aggregate sizes should aid in water quality degradation risk assessments. However, our research demonstrates the need to quantify the dynamics of bioactive P fractions in order to better predict water quality degradation from soil erosion.

Technical Abstract: Elevated concentrations of P in surface soils increases the risk of P transport by erosion. Particulate nutrient transport from agricultural fields contributes to water quality degradation; however, insufficient data exist about potential bioactive P transport from erosion. Traditional measures of soil P such as total P or soil test P (STP) do not adequately assess the risk of water quality degradation due to particulate P because these measures do not necessarily reflect soil P bioactivity. To better understand the risk of water quality degradation due to sediment associated P, we quantified the bioactive P fractions in five aggregate size classes from conventional no-till (NT) and chisel till (CT) systems as well as a tilled organic system. Cropping system did not affect the concentration of bioactive P fractions in whole soils. However, aggregate size did affect bioactive P fraction concentrations. In general, macroaggregates had greater concentrations of bioactive P than did microaggregates across all cropping systems; the <0.05 mm size class had the lowest concentrations of bioactive P. To better understand the dynamics of these bioactive P fractions over time, we conducted an incubation study on the aggregates of the NT system and quantified the bioactive P changes over a 56 d period. Water extractable P and EDTA-P concentrations did not change significantly during the incubation. However, EDTA-PHP concentrations increased up to two-fold during the 56 d incubation. Using soil bioactive P fraction measurements from easily erodible aggregate sizes should aid in water quality degradation risk assessments. However our research demonstrates the need to quantify the dynamics of bioactive P fractions in order to better predict water quality degradation from soil erosion.