Properties of Phosphorus Forms in Leachate from Soils with Long-Term Manure Additions.
Charles Hyland1, Shinjiro Sato2, Zhongdong Lan3, Quirine Ketterings1, and Johannes Lehmann1. (1) Cornell Univ, 918 Bradfield Hall, Ithaca, NY 14853, (2) SWFREC/Univ of Florida, 2686 SR 29 N, Immokalee, FL 34142, (3) Chinese Academy of Sciences, Xinjiang Institute of Ecology and Geography, Xinjiang, China
Phosphorus (P) runoff from agroecosystems in the northeast United States is a well known environmental concern for surface waters, particularly in areas where poultry and dairy manure are abundant. More recent studies have concluded that P leaching can also significantly contribute to environmental degradation under P-saturated soil conditions. Laboratory studies were conducted to gain a greater understanding of P forms in leachate extracted from soils subjected to heavy annual dairy and poultry manure applications for between 1 and 40 years. The soils ranged from 60 to 2167 mg Mehlich-3 extractable P per kg soil (7.5 to 1146 mg Morgan extractable P/kg soil). Intact soil cores (0-15 cm) were taken in stainless steel cylinders and fitted with airtight stainless steel caps. Three cores were taken per field. The stainless steel units were saturated with water and attached to a peristaltic pump using chemically inert tubing. Water was slowly pumped through the soil profile and recycled until equilibrium concentrations were reached after many cycles. The concentration of P in the soil water was measured at daily intervals. Initial concentrations of P in leachate differed greatly from P concentrations in leachate that was recycled until equilibrium was achieved in terms of both inorganic P and organic P content and proportion, yielding valuable information concerning P mobility in agroecosystems. Equilibrium concentrations of total dissolved P in leachate ranged from 0.11 mg L-1 in soil subjected to short-term manure additions and to 7.64 mg L-1 in soil subjected to long-term manure additions. The percentage of total P in leachate that was organic P ranged from 90% in soil subjected to short-term manure additions and 2% in soil subjected to long-term manure additions. While the proportion of organic P to inorganic P decreased, the organic P content in leachate remained unchanged. Manure additions increased inorganic P leachate concentrations, but did not affect organic P concentrations. After determining this, the hydrophobic and hydrophilic properties of the organic P were investigated to see if the leaching process affected the properties of organic P in soil, which could have implications for the mobility of organic P in the subsoil. A resin fractionation procedure was developed and employed to distinguish between leachate hydrophobic and hydrophilic organic P forms using Amberlite XAD-7 and MSC-1H. It was determined that there was no systematic difference in the proportion of hydrophobic and hydrophilic organic P as a function of duration of manure amendments. The property and therefore most likely the mobility of organic P in the subsoil remained the same regardless of duration of manure amendments.