Soil organic matter and metolachlor sorption characteristics as affected by soil management

Soil organic matter (SOM) plays an essential role in agriculture and in environments. It influences the productivity and physical well-being of soils. Therefore, it is important from both an economic and environmental standpoint to determine how agricultural practices will affect SOM and the biological cycling of carbon. Based on the changes of SOM characteristics, we further investigated the sorption dynamics and mechanisms of organic compounds in soil and SOM fractions under different soil managements. Several experiments were conducted and the results and conclusions were summarized as below. Long-term tillage experiment was initiated in 1979 at the Clemson University Pee Dee Research and Education Center (Darlington, SC). Cover crop experiment was conducted in the Connecticut River Valley at the Massachusetts Agricultural Experiment Station in South Deerfield, Massachusetts (since 1990). Cross-Polarization Magic-Angle-Spinning (CPMAS) and Total Sideband Suppression (TOSS) solid-state 13C Nuclear Magnetic Resonance (NMR) and Diffuse Reflectance Fourier Transform Infrared (DRIFT) spectroscopic techniques were used to identify the structural and compositional changes of SOM. Metolachlor sorption experiments (3 days) were conducted using a batch-equilibrium method. Three desorption steps were carried out for a total of 9 days in a sequential decant-refill steps following the sorption experiment. Our NMR results indicated that humic acid (HA) from rye alone cover was more aromatic and less aliphatic in character than other cover crop systems without nitrogen fertilizers. Based on the DRIFT peak height O/R (reactive/recalcitrant) ratios, the highest ratio was found in the HA from vetch/rye system with nitrogen fertilizer. Compared to conventional tillage (CT), conservation tillage (CnT) held a higher SOM and light fraction (LF) in the surface layer (0–5 cm). Aromaticity of HA increased with soil depth in both tillages. Conversely, the aliphatic-C content decreased with soil depth. However, the aromaticity of HA in CT was higher than that of CnT in the top soil (0–5 cm). Sorption nonlinearity (N) of metolachlor and hysteresis were dependent on the structure and composition of SOM. These results showed that agricultural practices not only changed the structure of SOM, but also changed the pesticide sorption behavior in the soil