ORGANIC MATTER IN DOUBLE-CROPPED LOWLAND RICE SOILS: CHEMICAL AND SPECTROSCOPIC PROPERTIES

Publisher Website
Google Scholar
Abstract
Limited studies have been conducted to characterize soil organic matter (SOM) in intensively cropped lowland rice soils that support multiple annual rice crops. Previous nuclear magnetic resonance spectroscopy analysis suggested that increased frequency of soil submergence in long-term field experiments on the International Rice Research Institute farm is associated with less humified SOM. The objective of this study was to gather broader evidence for this finding by applying additional chemical and spectroscopic methods to representative SOM fractions extracted from soils of two fertilizer treatments in a long-term double-cropped lowland rice trial conducted at three sites in the Philippines. The labile mobile humic acid (MHA) and the more recalcitrant calcium humate (CaHA) fractions were analyzed for elemental composition and acidic functional groups and by Fourier Transform infrared, fluorescence, and electron spin resonance spectroscopies. Results suggested incomplete humification of either fraction. The MHA was less humified than the CaHA, having fewer acidic functional groups, narrower C:N, C:H, and C:N:S ratios, richer apparent concentrations of amides and carbohydrates, and lower apparent concentration of carboxyls, higher fluorescence intensity, shorter wavelength of fluorescence emission maximum, and lower concentration of organic free radicals. Most MHA properties appeared to vary among sites in association with the degree of soil drying during one of the fallow periods, the driest site having the youngest, least humified MHA. Some MHA properties suggested greater humification in the optimal N+P+K fertilizer treatment than in the unfertilized control. Free radical concentrations were highly correlated with other indices of humification reported here and elsewhere. Sulfur cycling under submerged conditions seemed only loosely linked to C and N fluxes.