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.