Water and Salt Extractable Organic Matter As Affected by Soil Depth and Tillage System.

Soluble organic matter (OM) has been suggested to reflect shifts in soil management. We characterized the pool size and properties of soluble OM along soil profile to 125 cm in a maize-based agricultural system that was managed under conventional tillage (CT) and no-tillage (NT) systems for 14 years. Water extractable organic matter (WEOM) and salt extractable organic matter (SEOM) were obtained in a sequential extraction procedure using dilute (CaCl₂) and concentrated salt (K₂SO₄) solutions, respectively. WEOM and SEOM were characterized using XAD resin fractionation, specific UV absorption (SUVA₂₅₄) and humification index (HI). The pool size (C and N) of SOM, WEOM and SEOM declined with increasing soil depth. The pool size (C and N) of WEOM were lower in CT than NT soils along soil profile. While the proportion of hydrophilic-C increased, the proportion of more humified and aromatic OM of WEOM and SEOM decreased with soil depth. The proportion of SOM that was recoverable as WEOM and SEOM increased with soil depth and was positively related to CaCO₃ content of the soil. WEOM has been of less interest in, specifically long-term, management studies due to its high spacial/temporal variability. From the other hand,  the observed influence of tillage management on the properties of SEOM, in addition to the strong relationship between C and N concentrations and C/N ratio of SEOM with those of SOM suggest that the properties of SEOM can be monitored to reflect effects of tillage management on soil, with evidence suggesting further decomposition of SEOM in soils under conventional tillage