Carbon and Nitrogen Cycling under Conservation and Conventional Tillage in Peanut and Collard Agroecosystems

Abstract

Although there has been considerable adoption of conservation tillage by agronomic and vegetable producers in the US, information on nutrient release from organic residues is lacking. Information on release of nutrients from organic residues will help producers make informed decisions regarding residue management, including adoption of conservation or conventional tillage. The objectives of this study were: 1. to assess mass loss rates and carbon (C) and nitrogen (N) release rates from organic residues (organic mulches, peanut (Arachis hypogaea), and summer cover crops) under conventional and conservation tillage, and 2. to determine changes in soil C and N, aggregate stability, and yield during no-till herbicide-free collard (Brassica oleracea L. var. Champion) production using high biomass cover crops and organic mulches over a three year period. The collard study was conducted during 2005-2008 in eastern central Alabama, USA. A summer cover crop of forage soybean (Glycine max (L.) Merr. cv. Derry) or no summer cover crop control were established into killed winter rye (Secale cereale L. cv. Elbon) residue. Collards were transplanted into killed summer residue in the fall, followed by mulching with in situ organic residues three weeks later and fertilized with 202 kg N ha-1. Mulches applied at 6.7 Mg ha-1 yr-1 did not mineralize nutrients in sufficient quantities to meet collard demands after three years, although the crop appeared healthy. All treatments, including controls, improved soil organic C in the 0-5 cm soil depth over three years. At the end of three years, treatments did not affect collard yield or aggregate stability compared to the control. Mulch decomposition studies of mimosa (Albizia julibrissin Durazz.), lespedeza (Lespedeza cuneata (Dum. Cours.) G. Don), oat (Avena sativa) straw, and soybean (Glycine max var. Stonewall) residues were conducted using litterbag methodology and applied at a rate equivalent to 6.7 Mg ha-1 during 2007-2008 in eastern central Alabama, USA. Buried residues decomposed faster than surface residues, particularly in the labile portion. More N was potentially available to spring crops from surface residues, which may act as a slow release fertilizer, compared to incorporated residues. At spring planting, mimosa residue contained 78 kg N ha-1 when buried the previous fall, compared to 123 kg N ha-1 when left on the soil surface. Surface placed mimosa mineralized 33% of initial N content after one year, compared to 71% when buried. Similarly, C was sequestered for longer periods when residue was placed on the surface compared to incorporation. Aboveground soybean residue decomposed too quickly to warrant a N credit to subsequent crops. However, organic residues with an intermediate C:N ratio may be utilized under conservation tillage for the enhancement of soil organic matter (SOM), C sequestration, and soil N status. Peanut residue decomposition studies were conducted at Rocky Mount, NC and Headland, AL, USA using litterbag methodology at a rate equivalent to 3.5 Mg ha-1 during 2004-2005. Residues of three peanut varieties were buried and surface-placed at both locations. In NC, buried residues mineralized N at higher rates than surface residues during the initial 50 days of decomposition. After the initial rapid phase of decomposition, there was no difference in rates of N release at either location. No treatment differences were found at the Wiregrass Experiment Station. These data suggest that N is released too quickly from peanut residue to warrant N credits to subsequent crops. This conclusion was supported by a laboratory microlysimeter incubation study conducted on the same three peanut varieties on a Dothan soil.