Carbon and nitrogen pools in aggregate size fractions as affected by sieving method and land use intensification

- Sieving method affects SOC distribution among the various aggregate size fractions.
- Dry sieving can be a reliable method of soil C separation in sandy soils.
- Alkyl C and O-alkyl C were the predominant C functional groups in Spodosols.

Soil organic carbon (C) is a key component regulating grazing land ecosystem production and sustainability. In this study, we investigated the impacts of wet vs. dry physical separation of soil C fractions in representative sandy Coastal Plain Spodosols subjected to different levels of management intensification. X-ray diffraction and solid-state 13C nuclear magnetic resonance (NMR) spectroscopy were used to characterize the structural composition of the fine (< 53 μm) particle size. Dry sieving resulted in ~ 10-15% greater total C recovery as compared to wet sieving. Sieving method also affected the proportion of soil C associated with the various size fractions. In general, dry sieving resulted in greater proportion of soil C (48 to 73% of the total C in the 0 to 10 and 10 to 20 cm, respectively) associated with the 150-53 and < 53 μm fractions. Conversely, the proportion of C associated with large aggregates (2000-250 and 250-150 μm fractions) increased 1.5 to 2-fold when wet sieving was performed. X-ray diffraction demonstrated that mineral composition of the fine (< 53 μm) fraction was dominated by quartz and, thus, offers limited protection against mineralization. Results also demonstrated that alkyl C and total O-alkyl C dominated soil C chemical composition of both whole soils and < 53 μm fractions. In whole soils, abundance of alkyl C and total O-alkyl C decreased while methoxyl, carboxylic, and total aromatic-C increased with management intensification (native rangeland < silvopasture < sown pasture). However, in the < 53 μm fraction, total O-alkyl C abundance was greater in native rangeland than that in the silvopasture and sown pasture ecosystems. Both sieving methods were sensitive tools to evaluate the long-term impacts of grazing land intensification on soil C distribution among aggregate size fractions. However, because dry sieving is easier to perform and preserves more C than wet sieving, dry sieving can be a reliable method of soil C separation for determining the impacts of land use management on soil C dynamics in sandy soils.