Paired land-use assessment of soil organic C and N in the root zone of agricultural systems

Authors: Franzluebbers, Alan

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/24/2022
Publication Date: 6/12/2021
Citation: Franzluebbers, A.J. 2021. Paired land-use assessment of soil organic C and N in the root zone of agricultural systems. Soil Science Society of America Journal. 85:1785-1798. https://doi.org/10.1002/saj2.20290.
DOI: https://doi.org/10.1002/saj2.20290

Interpretive Summary: Soils 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.

Technical Abstract: Depth stratification of soil organic carbon (SOC) and total soil nitrogen (TSN) is a natural consequence of historical organic resource inputs to soil, i.e. plant roots and surface deposition of detritus. Decades and centuries of agricultural cultivation with inversion tillage can erode, redistribute, and enhance decomposition to create a more uniform distribution of SOC and TSN concentrations, albeit typically at low concentrations throughout. Contemporary changes in SOC and TSN storage are argued as only practically possible in the upper 30 cm of the profile, i.e. the primary root zone. This investigation was conducted under paired land use of conventional-till cropland with (1) no-till cropland and (2) conservation grassland on multiple research stations throughout North Carolina, USA. Across locations, surface residue C and N were significantly greater under no-till than conventional-till cropland. Stock of SOC at 0-30-cm depth was not different between tillage systems, but stock of TSN was significantly greater under no-till than under conventional-till cropland. Baseline stock of SOC was variable within research stations due to variable soil types sampled, and this limited statistical difference in total stock calculation. However, root-zone enrichment of SOC and TSN was significantly greater under no-till than under conventional-till cropland across locations (76 and 98% greater, respectively). Soil types were more closely matched between conservation grassland management and conventional-till cropland, resulting in no difference in baseline SOC and TSN stocks and significantly greater root-zone enrichment of SOC and TSN with conservation grassland than with conventional-till cropland (120 and 138% greater, respectively). Depth distribution calculations of root-zone enrichment of SOC and TSN were highly effective in separating conservation from conventional management systems.