Impact of conventional agriculture on the concentration and quality of water-extractable organic matter (WEOM) in the surface horizons of Retisols—A case study from the Carpathian Foothills in Poland

Abstract

In spite of many studies focused on the impact of land use changes on soil organic carbon (SOC) and the total nitrogen (TN) and soil organic matter (SOM) budget, less is known about the fate of water-extractable organic matter (WEOM) after the conversion of forestland to conventional farmland. The main aims of this study were: 1) to determine the concentration of water-extractable organic carbon (WEOC) and water-extractable total nitrogen (WETN) in topsoil horizons of forestland soils and cropland soils in the Carpathian Foothills in southern Poland and 2) to determine the quality of WEOM in topsoil horizons of forestland soils and cropland soils. Soil samples from topsoil horizons of forestland soils (Oi and Ah horizons) and cropland soils (Ap horizons) were collected and the concentration of WEOC and WETN was determined. In addition, the quality of WEOM was determined using ultraviolet-visible (UV–vis) spectroscopy and Fourier-transform infrared attenuated total reflectance (FTIR-ATR) spectroscopy. The results of this comparative study clearly indicate that conventional agriculture significantly affects the concentration and quality of WEOM in surface soil horizons of Retisols in the Carpathian Foothills in southern Poland. Surface horizons of arable soils (Ap horizons) are characterized by a significantly lower mean concentration of WEOC and WETN in comparison with surface horizons of forestland soils (Oi and Ah horizons), and reflect changes in the content of bulk SOC and TN due to soil tillage. Mean concentrations of WEOC and WETN in the studied Ap horizons of arable soils are 1.98 mg l−1 and 0.15 mg l−1, respectively while the mean concentrations of WEOC and WETN in the Ah horizons of forestland soils are 15.53 mg l−1 and 0.84 mg l−1, respectively. The studied Oi horizons of forestland soils are characterized by the highest mean concentration of WEOC (44.53 mg l−1) and WETN (6.22 mg l−1). Mean specific UV absorbance values at wavelength of 254 nm (SUVA254) was the highest for WEOM from Ap horizons indicating the highest transformation of WEOM in these horizons. Mean FTIR-ATR absorption bands of WEOM obtained from Oi and Ah horizons are much more intensive than the bands of WEOM from Ap horizons, which is related to significantly lower concentration of WEOM in the latter horizons in comparison with Oi and Ah horizons of forestland soils.