Long-term Effects of a Perennial Forage Grass in a Peanut-Cotton Rotation on Soil Properties
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Agronomy and Soils
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Peanut (Arachis hypogeae) and cotton (Gossypium hirsutum) are traditionally rotated together in southern Georgia and Alabama, and in northern Florida. Maintaining crop yields and long-term economic sustainability with this rotation is difficult. Inversion of peanuts for harvest and fallow soil in the winter causes fields to be prone to water and wind erosion. This is exacerbated by the fact that neither cotton nor peanut contribute greatly to soil organic carbon (SOC), which is known to improve soil structure and drainage and reduce erosion. Conservation practices such as reduced tillage aid in soil conservation, but may not be sufficient in these highly carbon-depleted soils. The addition of bahiagrass (Paspalum notatum) to the traditional peanut-cotton rotation is a potential cropping strategy that may improve sustainability and profitability of traditional crops. It has shown to lower the inputs of fertilizers, fuel, and pesticides; reduce pest and disease pressure; and increase productivity. However, the effect of the perennial forage grass on soil organic carbon and soil quality has not been adequately quantified. The objective of this research is to determine the effect of bahiagrass incorporated into the cotton-peanut rotation (i.e., sod-based rotation) on soil organic carbon and associated physical and chemical properties. Traditional and sod-based rotation systems that have been established for more than 8 years were evaluated at the Wiregrass Research and Extension Center (WREC) in Headland, AL, and the North Florida Research and Education Center (NFREC) in Quincy and Marianna, FL. The WREC and NFREC (Marianna) sites have large-scale plots that are in all phases of the sod-based rotation (bahiagrass-bahiagrass-peanut-cotton) with cattle grazing on the 2nd year of bahiagrass and on winter oat/rye cover crops. The WREC and NFREC (Quincy) sites have small-scale plots that have all phases of the sod-based and traditional rotations without grazing. For general comparisons of the rotations, all plots are irrigated under strip tillage, but additional treatments in the small plot experiments allow comparison of moldboard plow and strip tillage (WREC), as well as irrigation and non-irrigation (NFREC Quincy). The large-scale plots have cattle exclusion cages (15x15 m) that allows for the comparison of grazed and non-grazed conditions. Soil organic carbon (SOC) was assessed with depth, and the 13C/12C isotopic ratio of the SOC was assessed at 0-5 cm to assess the contribution of bahiagrass to the system. The effects of bahiagrass on water relationships were assessed by examining the infiltration rate, bulk density, macroporosity, and the saturated hydraulic conductivity of the soil in the crop management systems. In addition to carbon analysis, nutrient assessments included available Ca, Mg, K, P and Na, as well as nitrate-N and ammonium-N. Overall, there were few differences between the sod-based and conventional rotations. Soil organic C was found to range from 1.20-19.00 g kg-1 and decrease with depth. No difference was found between the cropping sequences or the rotation; however, SOC was typically higher following the bahiagrass sequences. Secondary treatment, plow vs strip-tillage, in the small plot experiment at the WREC location was also found to differ by tillage treatment, with the strip-tilled plots usually having a higher amount of SOC than the plowed plots. Using δ13C values, it was determined that 19.0 to 37.5% of the SOC was contributed by the bahiagrass. Macroporosity ranged from 0 to 1.1% and was generally highest after the peanut and cotton of the traditional rotation and after the cotton of the sod-based rotation. Cropping sequence, secondary treatment, and rotation provided no difference in macroporosity. Bulk density was not found to differ by cropping sequence or rotation at any location, but was generally higher in the 5-10 cm depth than in the 0-5 cm depth. No difference in saturated hydraulic conductivity or infiltration was found between the cropping sequences or by rotation at any location. Calcium, K, Mg, Na, and P at all locations differed by depth, but there was no effect of cropping sequence, except with Mg (WREC small plot experiment) and P (WREC and NFREC large experiments), or secondary treatment in any of the experiments. At both the small plot experiments, a difference in the amount of NO3-N and NH4-N was found between the different cropping sequences. All benefits previously found were following the first few years of the sod-based rotation, while this study evaluated the system after 10 years. This suggests that over time the benefits of the sod-based rotation when compared to the traditional rotation, both using conservation practices, are lost. This is likely due to the accumulation of organic C by cover crops. However, when grazing was allowed on the 2nd year of bahiagrass, there was generally less organic C than in the non-grazed portion. More research is needed to complete the assessment of why the benefits of the sod-based rotation equal those of the traditional rotation when both in conservation systems.