Green manure as part of organic management cycle: Effects on changes in organic matter characteristics across the soil profile

Green manure (GM) cultivation and incorporation (i.e., GM management) may change soil organic matter (SOM) composition and the agroecosystem functioning. However, the understanding of GM effects on SOM composition, specifically in deeper soil layers, is limited. The objectives of this study were to examine the effects of GM management (as part of an organic agriculture practice), following two years of various doses of compost application, on the changes in SOM and water-extractable organic matter (WEOM) compositional characteristics up to a soil depth of 60 cm. Soil samples from a two years compost amended field that was subsequently subjected to GM management were taken in four intervals to a depth of 60 cm (0-5, 5-15, 15-30 and 30-60 cm) and characterized for organic C content and SOM composition, by FT-IR transmission. Characterization of WEOM included excitation-emission matrices (EEMs) of fluorescence, UV absorbance and dissolved organic C (DOC) measurements. The response of SOM and WEOM to GM management resulted in some opposite trends. The SOM became rather aromatic, with aliphatic CH-containing structures contributing to a greater extent to SOM composition at deeper layers following GM management. By contrast, GM management resulted in (i) a substantial increase in dissolved organic C concentration, (ii) WEOM becoming enriched by hydrophilic aliphatic organic compounds, and (iii) aromatic and fluorescent components increasingly being found at deeper soil layers and hydrophilic aliphatic components at the surface soil. Fluorescent portion of WEOM became enriched by relatively less biodegradable and weaker-soil adsorbing humic-like components; this enrichment increased in deeper soil layers. The effects of past compost application rates on the changes in SOM and WEOM characteristics were either non-significant or negative. In the latter case, an increase in application rate decreased the changes in SOM content of hydrophilic groups and in the fractions of fluorescent components in WEOM. Growing and incorporating plant biomass may mask, at least on a short time scale, the effects of earlier compost applications on changes in SOM and WEOM compositional characteristics.