Crop rotational diversity alters soil organic matter composition in soil physical fractions

Abstract:

Crop rotation is an important part of sustainable agricultural and soil management. This practice is widely used to increase crop yield and maintain soil fertility, but how this practice alters organic carbon (OC) sequestration in soils is unclear. Soil organic matter (SOM) is essential for providing plant nutrients and maintaining soil structure through associations with soil minerals. Long-term stabilization of SOM is believed to be associated with the physical protection of SOM in the form of organo-mineral interactions (mainly in silt/clay fractions). However, there is a lack of information about whether crop rotational diversity could alter the distribution or stabilization of SOM constituents in soil physical fractions. To address this, soil samples from a long-term (37 years) agricultural site at Elora Research Station (Ontario, Canada) with six different crop rotation treatments were analyzed. Elemental analysis, targeted compound analysis and nuclear magnetic resonance (NMR) spectroscopy methods were used to obtain the distribution and degradation of SOM components in sand, silt, clay and light fractions from the samples. The relative degradation state of SOM (alkyl/O-alkyl ratios) enhanced with decreasing particle size as measured by solid-state ¹³C NMR even with different crop diversity. Soil OC decreased from light, clay, sand to silt fractions for all rotation practices, and varied among different crop rotations in these fractions (especially in light fraction). Major groups of aliphatic lipids (i.e., n-alkanes, n-alkanols and suberin-and cutin-derived lipids) accumulated in the clay-sized fractions for all crop rotation treatments. Cutin-derived lipids in the clay-sized fraction did not exhibit any differences in degradation. Lignin-derived compounds were more oxidized in the silt and clay fractions, with higher concentrations observed in the silt fraction. Molecular-level analysis showed that long-term crop rotation treatments altered the sequestration of soil OC and revealed changes in SOM composition and degradation in these fractions. This suggests that crop rotation treatments may change soil carbon biogeochemical cycling in long-term agroecosystems.