The effect of organic structures on the water stability of macro-aggregates.
A study was conducted to identify the chemical structures of soil organic matter and examine their effect on the water stability of macro-aggregates (>250 um) in a SiL Brown Chernozem under wheat-fallow (WF) and continuous wheat (CW) . The proportion of water stable macro-aggregates were determined by wet sieving. Chemical structures of soil organic matter were characterized by pyrolysis field ionization mass spectrometry (Py-FIMS), and grouped into seven classes: carbohydrates, phenolic and lignin monomers, lignin dimers, lipids + alkanes and alkenes, sterols, alkyl aromatic and N-compounds. In comparison with CW, there was a reduction in the proportion of macro-aggregates and an increase of micro-aggregates in the WF crop rotation. Sixty-five percent of all the soil organic matter (SOM) was identified by Py-FIMS. The average concentration of each class of compound in whole soil and macro-aggregate samples ranged between 0.1 and 15% of the total identified SOM. Carbohydrates, phenolic, and lignin monomers were the most abundant compounds (>10%). Linear regression models (r2 ~ 0.96, p=0.05) showed that the stability of macro-aggregates was highly correlated with the concentration of the least abundant (<3. 5%) structures of sterols, lipids and lignin dimers. These compounds are metabolic products of plants and soil organisms.
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