Dynamics of Water-Soluble Phosphorus in Surface-Applied Broiler Litter.
Liliana Ines Picone, Facultad Ciencias Agrarias-Univ Nacional Mar del Plata, Ruta 226 Km 73.5-CC276, Balcarce-Buenos Aires, 7620, Argentina, Miguel Cabrera, Univ of Georgia, 3111 Plant Science Building, Athens, GA 30602, Armando S. Tasistro, Agricultural and Environmental Services Laboratories, Univ of Georgia, 2400 College Station Road, Athens, GA 30602-9105, and David E. Kissel, Agricultural and Environmental Services Laboratories, Univ of Georgia, 2400 College Station Road, Athens, GA 30602-9105.
Water-soluble phosphorus (P) in broiler litter is used as indicator of potential P contamination of surface waters when broiler litter is surface applied. This form of P is quantified by shaking a broiler litter sample with water; however the amount of soluble P that can be lost by runoff could be underestimated by this method because the pH of a water extract is similar to that of the broiler litter (around 8.5), whereas the pH of broiler litter applied to the surface of soil reaches a value near 6 within a few weeks after application. Therefore, extracting the broiler litter with a solution buffered at the pH of the soil may be better than extracting broiler litter with an unbuffered solution for estimating the amount of water-soluble P that may contaminate surface runoff. Another factor that can alter the amount of water-soluble P released is the presence of thatch on the soil surface which decreases the contact between soil and broiler litter, affecting some mechanisms such as adsorption by soil of P released from broiler litter. Thus, the objectives of this study were to evaluate the effects of broiler litter extraction method (none, water, or buffer solution at pH 6) on water-soluble P dynamics following surface application of broiler litter to a soil. A laboratory experiment was conducted using a Coastal Plain soil from Georgia (USA) that was packed in cylinders to a bulk density of 1.6 Mg m-3 and wetted to reach a water content corresponding to a matric potential of -0.033 MPa, before incubation at 25oC for 60 days. The broiler litter was subjected to the following treatments before the experiment started. One fraction of the broiler litter was left "as is", another fraction was extracted with deionized water and the third fraction was extracted with a solution of buffer-MES (beta-Morpholino-ethansulfonsaeure hydrat, 99%) at pH 5.99. The latter two broiler litters were referred to as "water" and "buffer" broiler litter, respectively. The experimental design included five treatments with three replicates each one. The treatments consisted of: 1) soil without thatch or broiler litter, 2) thatch applied on soil surface, 3) broiler litter “as is” applied on thatch , 4) "water"- broiler litter applied on thatch and 5) "buffer"- broiler litter applied on thatch. Three replicates of each treatment were removed after 1, 3, 7, 14, 21, 42, and 60 days of incubation and extracts from broiler litter with thatch, thatch or top-1-cm soil samples were analyzed separately for total dissolved P by using an Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) instrument. Results showed that in the treatment with broiler litter "as is" applied on thatch, the level of water-soluble P in the broiler litter plus thatch decreased from 709 to 582 ug P cm-2 during the first 20 days of incubation, and remained relatively constant after that. This decrease in water-soluble P was probably caused by immobilization during thatch decomposition. In the treatment with “water"-broiler litter applied on thatch, water-soluble P in the broiler litter plus thatch increased from 115 to 312 ug P cm-2 during the first 14 d, indicating release from P mineralization or increased P solubilization. In the treatment with "buffer"- broiler litter applied on thatch, the water-soluble P in broiler litter plus thatch increased from 74 to 142 ug P cm-2 during the first 21 days. Thus, the increase in water-soluble P observed with the “water”-broiler litter was greater than the increase observed with “buffer”-broiler litter (197 versus 68 ug P cm-2 ). These results suggest that extraction of broiler litter with buffer removes more of the P that will be released later as water-soluble P than extraction of broiler litter with water.