Submitted to: Communications in Soil Science and Plant Analysis
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/25/2005
Publication Date: 8/1/2006
Citation: Schroeder, P.D., Kovar, J.L. 2006. Comparison of organic and inorganic phosphorus fractions in an established buffer and adjacent production field. Communications in Soil Science and Plant Analysis. 37:1219-1232. Interpretive Summary: Nearly two decades ago, the United States Department of Agriculture established the Conservation Reserve Program (CRP). During this time, thousands of acres of cropland near streams and rivers have been planted with native grasses and trees that act as buffers or filters for runoff water. The ability of these buffers to capture phosphorus (P) is related to the chemical forms of P in the buffer soils. Following laboratory work that allowed us to separate the P forms, we found that the amount of plant available P forms was higher in the surface soil (0-5, 0-10, and 0-15 cm depths) of a crop production field than in an adjacent 13-year-old buffer. However, there were few differences between the buffer and the crop field when the P forms associated with the organic matter were compared. We also found that soil sampling depth had a significant impact on the distribution of P among the various forms, with differences between the cropped area and the buffer decreasing as sampling depth increased. Therefore, care should be taken in choosing soil sampling depths when relating soil P forms to the potential for P loss. Lower available P levels and coarser soil texture in the buffer suggested either minimal P retention by the buffer, or minimal P losses from the production area. The results of this research will benefit both commercial growers and conservation groups by providing information on the effectiveness of buffer areas to minimize potential P losses.
Technical Abstract: Since the United States Department of Agriculture's Conservation Reserve Program (CRP) was established by the Food Security Act of 1985, thousands of acres of cropland in stream riparian zones have been converted to conservation buffers through the planting of native grasses and trees. The capacity of these buffers to retain phosphorus (P) is related not only to the P sorbing capacity of the soils, but also to the distribution of P forms in the soils. The distribution of soil P in various organic (Po) and inorganic (Pi) fractions, including labile-Pi (1 M NH4Cl), Al-Pi (0.5 M NH4F), Fe-Pi (0.1 M NaOH), reductant-soluble Pi (dithionite-citrate-bicarbonate), Ca-Pi (0.25 M H2SO4), labile Po (0.5M NaHCO3), microbial biomass Po, moderately labile Po (1.0 M HCl), humic acid (HA) and fulvic acid (FA) Po (0.5 M NaOH), and non-labile Po(550 degrees C, 1.0 M H2SO4), in the surface soil (0-5, 0-10, and 0-15 cm depths) of a buffer and an adjacent crop production field in central Iowa were compared. Most of the extractable Pi (32 to 39%) in both the crop field and the buffer was present in the Ca-Pi fraction. Levels of the most labile Pi fractions were higher in the cropped area; however, more P was found in the Fe-Pi fraction in the buffer than in the cropped area (23 vs. 18%, respectively). There were few differences among the Po fractions between the buffer and the crop field. Soil sampling depth had a significant impact on the distribution of P among the Pi and Po fractions, with differences between the cropped area and the buffer becoming less significant as sampling depth increased. These results confirm that care should be taken in choosing soil sampling depths when relating the distribution of P fractions to the potential for P loss. Lower P levels and coarser soil texture in the buffer suggest either minimal P retention by the buffer, or minimal P losses from the production area.