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Title: SPATIAL DISTRIBUTION OF SOIL CARBON AND NITROGEN POOLS UNDER GRAZED TALL FESCUE

Author
item Franzluebbers, Alan
item Stuedemann, John
item Schomberg, Harry

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/19/1999
Publication Date: N/A
Citation: N/A

Interpretive Summary: Cattle grazing on permanent pastures spend extended periods of time near shade and water sources during nonforaging or camping times of the day. Because of this animal behavior pattern, these areas tend to receive more excreta, which are sources of recyclable nutrients in organic and inorganic forms. We sampled a Cecil sandy loam soil that was planted to several tall fescue paddocks (0.7 to 0.8 ha each) and grazed regularly for 8 and 15 years. Soil organic matter and microbial biomass were enhanced in a zone up to 30 m radiating from shade and water sources compared with the remainder of the paddock. Inorganic N was concentrated in soil immediately adjacent to shade and water sources where frequent urination occurred and grass stand was deteriorated due to excessive trampling. To minimize the probability of nitrate contamination of surface and ground water supplies, shade or water sources should be either (1) moved periodically to avoid point accumulation of inorganic N, (2) positioned on the landscape to minimize the flow of percolate or runoff directly from these areas to water supplies, or (3) avoided during routine fertilization.

Technical Abstract: Animal behavior may be an important variable controlling the spatial distribution of soil C and N pools in long-term, grazed pastures. Shade and water sources are more frequented areas of a pasture that can also serve as camping areas where excreta are deposited. We sampled a Cecil sandy loam (fine, kaolinitic, thermic Typic Kanhapludult) under tall fescue eat distances of 1, 10, 30, 50, and 80 m from permanent shade or water sources at the end of 8 and 15 years of grazing. To a depth of 300 mm, soil organic C was 46 Mg/ha at 1 m, was 43 Mg/ha at 10 m, and averaged 40 Mg/ha at distances of 30, 50, and 80 m from shade or water. Particulate organic C averaged 15.3 Mg/ha at distances of 1, 10, and 30 m and 13.0 Mg/ha at distances of 50 and 80 m from shade or water. Soil microbial biomass C, basal soil respiration, and net potential N mineralization were also greater nearer shade or water than farther away. Although lateral distribution effects were most dramatic at a depth of 0 to 25 mm, similar effects were observed even at a depth of 150 to 300 mm. Long-term cattle grazing in relatively small paddocks (0.7 to 0.8 ha) with permanent shade and water sources resulted in significant lateral and vertical changes in soil organic C and N pools.