Soil nitrogen dynamics and leaching under conservation tillage in the Atlantic Coastal Plain, Georgia, United States

Abstract

Conservation tillage (CsT) involves management that reduces soil erosion by maintaining crop residue cover on farm fields. Typically, both infiltration and soil organic matter increase over time with CsT practices. We compared the impact of a commonly used CsT practice, strip tillage (ST), to conventional tillage (CT) management on soil nitrogen (N) dynamics and leaching and examined associations to soil N availability and microbial biomass. A winter cover crop was used in both tillage treatments. The study was conducted over a five-year period during rotational cotton (Gossypium hirsutum L.) and peanut (Arachis hypogaea L.) production in the Atlantic Coastal Plain region in Georgia, United States. Fertilizer and poultry litter were applied ahead of the cotton crops. Sets of PVC cylinders were filled with soil from each of six plots, three in ST and three in CT, and maintained in situ in their respective plots for 16 intervals of about 90 days. After retrieval, the soil in each cylinder was analyzed for inorganic N (ammonium and nitrates [NH₄⁺ and NO₃⁻]), total N, total carbon (C), and microbial biomass. Leached NO₃⁻-N was captured on anion exchange resin-filled bags attached to the bottom of each cylinder. After the five-year study period, the ST and CT soil C content increased by 22% and 23%, respectively. Total soil N content increased 27% with ST compared to 22% with CT. Temporal patterns in NO₃⁻-N leaching were not different between CT and ST treatments, and a high amount of NO₃⁻-N leaching was observed after the application of poultry litter. The cumulative amount of NO₃⁻-N leached from soils throughout the five-year study was 141 and 122 kg N ha⁻¹ (126 and 109 lb N ac⁻¹) with CT and ST practices, respectively. Results suggest that leaching from the top 15 cm (6 in) of soil may be an important pathway of N loss from both CT and ST cropping systems in the region. Regardless of tillage, soil microbial biomass N was equal to or higher than the total inorganic N, but still represented a small percentage (up to 9%) of the total soil N. Overall, microbial biomass N was higher in ST compared to CT. Minimizing NO₃⁻-N in the soil from reaching ground and surface waters while increasing crop productivity represents a major challenge. The use of ST in conjunction with winter cover crops may improve plant N availability by more than 27 kg ha⁻¹ y⁻¹ (24 lb ac⁻¹ yr⁻¹) in the sandy landscapes of the southeastern Coastal Plain region through microbial cycling of organic N while reducing subsurface NO₃⁻-N losses.