Aggregate distribution and the associated carbon in Norfolk soils under long-term conservation tillage and short-term cover cropping

Authors: PARAJULI, BINAYA - Clemson University; LUO, MIN - Fuzhou University; YE, RONGZHONG - Clemson University; Ducey, Thomas; PARK, DARA - Clemson University; SMITH, MATTHEW - Clemson University; Sigua, Gilbert

Submitted to: Communications in Soil Science and Plant Analysis
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/23/2020
Publication Date: 1/3/2021
Citation: Parajuli, B., Luo, M., Ye, R., Ducey, T.F., Park, D., Smith, M., Sigua, G.C. 2021. Aggregate distribution and the associated carbon in Norfolk soils under long-term conservation tillage and short-term cover cropping. Communications in Soil Science and Plant Analysis. 52:859-870. https://doi.org/10.1080/00103624.2020.1869769.
DOI: https://doi.org/10.1080/00103624.2020.1869769

Interpretive Summary: Conservation tillage practices, and cover cropping management are routinely used to improve soil health. Two beneficial effects typically associated with conservation tillage are an increase in soil aggregates, which are linked to reductions in soil compaction and increases in water retention, and soil carbon which has a widespread impact on the soils physical, chemical, and biological properties. Cover cropping likewise can reduce soil compaction, while increasing water infiltration, and adding carbon into the soil, along with the added benefit of reducing erosion impacts in winter months. This study, in a weathered soil typical of the Southeastern Coastal Plain of the United States, focused on the impacts of conservation tillage over the span of 40 years on soil aggregates and soil carbon. Findings indicate that the soil was dominated by macroaggregates, and soil carbon was associated primarily with the clay-silt fraction, followed by microaggregates and then macroaggregates. There was also little impact on aggregate formation, or carbon pools by short term cover crop management. These findings indicate that soils of the Southeastern Coastal Plain require long-term management plans for their improvement. Additionally, traditional measures of soil health do not adequately address the benefits seen by common management practices (conservation tillage and cover cropping) utilized in these soils.

Technical Abstract: Conservation agriculture practices have been widely implemented to improve soils and their sustainability. In the present study, we investigated the impacts of long-term conservation tillage (CS) on the distribution and stability of soil aggregates and their associated C in a typical sandy Ultisol. We also investigated the short-term effect of integrating cover crops. Soils (0-5 and 5-15 cm) were collected from fields under 40-years CS and conventional tillage (CV), to which cover crop and fallow treatments were imbedded as a split-plot design for four years. The soils were analysed for a range of physio-chemical properties. Effects attributed to tillage and cover cropping were identified for bulk density, pH, mean weight diameter (MWD), and dry aggregate distribution at 0-5 and 5-15 cm. The CS resulted in higher total C (TC) concentrations (13.56±1.14 g kg-1) than CV (10.06±0.53 g kg-1) at 0-5 cm depth only. Similar results were observed for TC concentrations of the measured aggregate size fractions (>250 µm, macroaggregates; 250-53 µm, microaggregates; 0-53 µm, clay-silt fraction). Soils were dominated by macroaggregates (>50%) at both depths with the associated C concentrations following the order of clay-silt fraction > microaggregates > macroaggregates. Nonetheless, after accounting for soil bulk density long-term CS did not resulted in higher TC stocks than CV at both depths. Cover crops (4 years) demonstrated no effect on SOC stocks at either depth. Poor soil structure and low clay content of the tested soils (i.e. poor soil structure and hence the capacity to preserve the organic carbon) may at least partially explain these neutral impacts, while relatively lower residue return of the cover crops to cash crops may also compromise the positive management outcomes. Novel management practices to increase SOC in the bulk soils by increasing organic inputs and the soils’ capacity to preserve the inputs, are needed.