Root‐zone enrichment of carbon, nitrogen, and soil‐test biological activity under cotton systems in North Carolina

Abstract

AbstractSoils of the southeastern United States are generally low in organic matter. Root‐zone enrichment of organic carbon (C) and nitrogen (N) may be possible with conservation agricultural management, but little is known of how this might be distributed across private farms. A stratified random survey of 120 cotton (Gossypium hirsutum L.) fields from throughout North Carolina characterized root‐zone contents of soil organic C (SOC), total soil N (TSN), and soil‐test biological activity (STBA). Soils were mostly udults and aquults, but included some udepts, aquepts, aqualfs, and psamments. Depth distribution of soil properties at 0–10, 10–30, and 30–60 cm was fitted to a nonlinear regression model for each soil profile to calculate root‐zone enrichment over that of a baseline content estimated from concentration at 30‐cm depth. Continuous conservation tillage was practiced on 23% of fields. Root‐zone enrichment of TSN and STBA was greater with conservation than with disk tillage (1.30 ± 0.10 vs 0.82 ± 0.04 Mg N ha–1, respectively, p < .001; 209 ± 16 vs 144 ± 6 kg CO2–C ha–1 3 d–1, respectively; p < .001) (mean ± standard error). Difference in root‐zone enrichment of SOC between continuous conservation tillage and frequent disk tillage was 8.5 ± 2.5 Mg C ha–1 among eight subregions and was not affected by variations in baseline SOC content, which was greater in the Flatwoods than the Coastal Plain region due to inherent soil formation factors. These on‐farm results confirm that continuous conservation tillage is needed to enrich root‐zone SOC, TSN, and STBA under cotton production.