Soil aggregation under cotton production systems in North Carolina

Authors: Franzluebbers, Alan

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/6/2022
Publication Date: 12/23/2020
Citation: Franzluebbers, A.J. 2020. Soil aggregation under cotton production systems in North Carolina. Soil Science Society of America Journal. https://doi.org/10.1002/agj2.20573.
DOI: https://doi.org/10.1002/agj2.20573

Interpretive Summary: Soil organic carbon (SOC) and N fractions are considered important indicators of soil health due to their multifunctional roles in storing and supplying nutrients, enhancing soil physical properties, and feeding soil biological activity and plant nutrition. Soil health condition from 120 fields under cotton (Gossypium hirsutum L.) cultivation in North Carolina was assessed from soil analyses associated with a survey of historical management. Surface residue was collected, and soil was sampled at 0-to-10-cm, 10-to-30-cm, and 30-to-60-cm depths. Soil properties varied by physiographic region of Piedmont, Coastal Plain, and Flatwoods. Conservation tillage was the dominant form of cotton cultivation, but many farms used inversion tillage for other crops in the rotation sequence. Soil-test biological activity averaged 138 and 92 mg kg-1 3 d-1 under continuous conservation tillage and frequent tillage, respectively, in the surface 10-cm depth (p < .001), and the tillage effect persisted into the 10-to-30-cm depth (44 and 37 mg kg-1 3 d-1, p < .05). Cover cropping and animal manure had fewer and smaller effects on soil properties. History of animal manure application elevated (p < .05) soil-test P (229 vs. 179 g m-3), Cu (4.3 vs. 1.6 g m-3), and Zn (10.0 vs. 4.0 g m-3). Intermittent tillage in the rotation sequence was the largest impediment to sustained SOC, total soil N accumulation, and soil microbial properties. This assessment illustrated positive soil health condition from farmer adoption of conservation tillage, but further improvements will be possible only with more continuous practice of conservation approaches.

Technical Abstract: Aggregation of coarse-textured soils is typically not considered significant, and yet soil erosion and surface sealing can still be significant outcomes from frequently tilled soil in relatively flat landscapes of the Coastal Plain and Flatwoods of the southeastern US. Soil at 0-10-cm-depth was sampled from 120 randomly chosen cotton (Gossypium hirsutum L.) fields in North Carolina. This study explored mean-weight diameter (MWD) of soil aggregate distribution when sieved dry and following wet sieving. Using whole-soil analysis, and not a particular aggregate fraction, it was shown that significant aggregation occurred across a gradient of sand concentration. Water-stable aggregation was typically greater when soil was managed with continuous conservation tillage than with frequent disk tillage. Strongest association of MWD from dry-stable aggregation (r = .71, p < .001) and of MWD from water-stable aggregation (r = .65, p < .001) occurred with soil-test biological activity, suggesting that organic C resources near the soil surface with continuous conservation tillage promoted both biological activity and aggregation. Physical manipulation of soil by biota (i.e. fauna, microbes, and roots) into water-stable aggregates was a consequence of continuously undisturbed organic C resources near the soil surface, which also led to greater soil-test biological activity and net N mineralization.