۱٫ Introduction

۲٫ Materials and methods

۳٫ Results and discussion

۴٫ Conclusions

Acknowledgements

Appendix A. Supplementary data

References

Abstract

Soil organic matter (SOM) or humus is essential for the agricultural and environmental functionality of soils. Humus comprises small-size heterogeneous organic molecules arranged in complex meta-stable suprastructures, whose composition can be greatly affected by land management. Here, we report the molecular dynamics of the fractions extracted from an agricultural soil cropped with wheat after one and three years of tillage. The total molecular components, named Humeome, of soil under wheat found in both hydrosoluble and organosoluble fractions isolated by the Humeomic procedure, as determined by GC–MS and high-resolution Orbitrap LC–MS, were compared to the Humeome characterized in the same soil when cropped with maize. While the three-years tillage did not vary the total soil organic carbon under both wheat and maize, the carbon recovered for the sum of Humeomic fractions isolated from soil was significantly larger for maize than for wheat, thus suggesting a general destabilization of SOM under wheat cropping. Moreover, the soil Humeome under wheat resulted more hydrophilic than under maize. While fatty acids and carbohydrates were periodically replenished by crop residues, nitrogen-containing molecules, such as amides and heterocyclic nitrogen, and iron-bound molecular systems were the SOM components mostly reduced under wheat. The losses of these compound classes from the soil Humeome was possibly attributed to the exudation differences between wheat and maize cropping. These results reveal that the molecular dynamics and stability of SOM molecular components can be controlled by crops even in a short term.

Introduction

World soils contain the greatest reservoir of organic carbon (OC) in the biosphere (Batjes, 2014), and innovative measures to reduce OC losses from soils are required to limit global changes (Minasny et al., 2017; Piccolo et al., 2018). Nevertheless, agriculture intensification to produce food for an increased population will inevitably put soil at risk of SOM degradation and fertility losses (Lal, 2009), thus menacing human sustainability on the planet. Due to the still large uncertainty in strategies to sequester OC in soil (Baveye et al., 2018), there is an urgent need to enlarge knowledge on SOM dynamics, in order to control soil OC and prevent degradation of soil fertility.

Humus represents the metabolic substrate for soil microbial activity that, in turn, continues to transform the bioavailable humic pools until a new equilibrium is established between microbial communities, humic matter and plant species (Basler et al., 2015). While the concept of ecological succession was introduced as a theory (Putnam, 1994), few reports were so far published on the molecular changes of humus due to plant species, even though plant rhizodeposition is reckoned to play an important role in OC turnover in soil (Hütsch et al., 2002; Wang et al., 2006). Larger microbial and CO2-C exudates were reported in wheat than in maize (Marx et al., 2007), while wheat residues were found to rapidly and persistently stimulate a microbial degradation of fresh organic matter (Bernard et al., 2007), thereby enhancing mineralization of rhizospheric SOM (Schenck zu Schweinsberg-Mickan et al., 2012).