Soil organic matter gain by reduced tillage intensity: Storage, pools, and chemical composition

Abstract

Soil organic matter (SOM) loss due to intensive cultivation is the focus of studies on climate change and food security. Dropping tillage intensity has been widely reported as a potential tool for SOM increase; however, the chemical composition, storage mechanisms, and vertical distribution of SOM gain are not fully understood, especially in calcareous soils. This study aimed to analyze the increased SOM conditions among pools and depths in an eighteen-year-long field experiment comparing SOM under conventional moldboard plowing (PT), deep cultivation (DC), and no tillage (NT) systems on a base-saturated Endocalcic Chernozem (Loamic). Soil samples were collected from the 0–10 cm and 30–40 cm soil layers and divided into mineral phase-associated organic matter (<63 µm, stable pool; MPAOM) and aggregate related organic matter (>63 µm, labile pool). The aggregate related organic matter was further divided into particulate organic matter (POM, <1.0 g cm-3) and aggregates-associated organic matter (AAOM, >1.0 g cm-3). Results indicated an overall increase in soil organic carbon (SOC) concentration in the topsoil in the order of PT<DC<NT. The surplus did not manifest as POM but as either MPAOM or AAOM fractions. This increase likely resulted from the surplus above-ground plant residue input, as the SOC content in the 30–40 cm layer did not change. The additional SOM, stabilized in the soil, did not affect SOM composition between depths and fractions, suggesting preferential SOM binding by the fine fraction and aggregates. This suggests preferential binding of the more aromatic and less complex SOM to the fine fraction, even for the surplus SOM in recent years. This fractionation maintains or even increases the difference between organic carbon pools (soil fractions) in terms of SOM composition. In addition, vertical differentiation as a result of tillage intensity mitigation was established by an increase in the stratification of aromaticity. These results suggest the key role of dissolved SOM movement in the profile as a potential driving force for the differentiation of aromaticity with depth. The results also emphasize the role of local conditions on OM composition changes, establishing the complexity of the process and difficulties of holistic model construction.