Chemical composition of dissolved organic matter in agroecosystems: Correlations with soil enzyme activity and carbon and nitrogen mineralization

Abstract

Soil enzyme-catalyzed depolymerization of organic matter results in the production of low molecular weight and dissolved organic compounds. This fraction of soil organic matter is the immediate energy, carbon and other nutrient substrates for microbial catabolic pathways and thus likely plays an important role in soil processes. The purpose of this study was to elucidate interrelationships among dissolved organic matter, soil enzyme activity, and soil C and N mineralization from diverse agroecosystems. These systems included a conventional cropping, organic cropping, integrated crop–livestock, plantation forestry, and succession from an abandoned agricultural field. We collected surface soil samples from 0 to 10 cm depth in early spring 2009 and examined the concentrations of soil-derived dissolved organic C and N, soluble phenolics, reducing sugars, and amino acids, the activities of β-glucosidase, exoglucanase, phenol oxidase, peroxidase, and β-glucosaminidase, and the rates of soil C and N mineralization. The integrated crop–livestock system showed the highest concentrations of dissolved soil organic C (78 μg C g −1 soil) as well as phenolic compounds (1.5 μg C g −1 soil), reducing sugars (23 μg C g −1 soil), and amino acids (0.76 μg N g −1 soil), and these components were up to 3-fold greater than soils under the other systems. However, soil β-glucosidase activity in the integrated crop–livestock system was significantly lower than the other systems and appeared to reflect the inhibitory role of soluble phenolics on this enzyme; this enzymatic disparity was also revealed in our preliminary study conducted in 2008. Among the five enzyme activities examined, only peroxidase activity was correlated significantly with the chemical composition of dissolved organic matter as well as soil C and N mineralization. Soil peroxidase activity was negatively related to the relative abundance of reducing sugars (i.e., reducing sugar C as a fraction of dissolved organic C, r = −0.92, P < 0.05) and positively with soil C and N mineralization ( r = 0.86, P < 0.1 for C mineralization; r = 0.85, P < 0.1 for N mineralization). Furthermore, relative abundance of reducing sugars was negatively associated with soil C mineralization ( r = −0.80, P < 0.1) and so was relative abundance of amino acids with soil N mineralization ( r = −0.97, P < 0.01). Our results suggested that diverse agroecosystems differed in the chemical composition of dissolved organic matter and the differences could be correlated with soil peroxidase activity and soil C and N mineralization.