Chemistry and Biodegradability of Dissolved Soil Organic Matter in Diverse Farming Systems

View the project final report

Commodities

• Animals: poultry

Practices

• Animal Production: pasture fertility
• Crop Production: nutrient cycling
• Education and Training: decision support system
• Production Systems: organic agriculture
• Soil Management: nutrient mineralization, organic matter, soil analysis, soil chemistry, soil microbiology, soil quality/health

We assessed the impacts of agricultural management on the quantity and chemical properties of dissolved organic matter (DOM) and examined the relationships of DOM with other soil chemical and biological properties in a well-designed farming-systems unit that includes five systems: conventional farming, organic farming, integrated crop and livestock, plantation forestry, and agricultural field succession. Integrated crop-livestock system contained the highest concentrations of dissolved soil organic carbon as well as phenolic compounds, reducing sugars, and amino acids; These components were up to three-fold greater than the respective ones in the other systems. However, soil beta-glucosidase activity in the integrated crop-livestock system was about three times lower than the other systems and appeared to reflect the inhibitory role of soluble phenolics on enzymes. Among the five enzymes examined, peroxidase was the only one correlated significantly with the chemical composition of dissolved organic matter. Reducing sugars as a fraction of dissolved organic carbon were negatively related to peroxidase activity. Furthermore, relative abundance of reducing sugars was inversely associated with soil carbon mineralization and so was relative abundance of amino acids with soil nitrogen mineralization. Our results indicated that soil peroxidase played a preponderant role in regulating the chemical composition of dissolved organic matter in agroecosystems, which in turn dictated soil carbon and nitrogen mineralization.

Introduction

Soil quality has become a focal point for sustaining agricultural productivity in intensive and extensive agricultural practices. While microbial biomass and mineralization of soil organic carbon and nitrogen are often-used biological parameters to evaluate soil quality, they are unlikely comprehensive to diagnose soil organic matter and microbially-mediated soil functions. Mounting evidence supports that dissolved soil organic matter (DOM) contributes to numerous soil chemical, physical, and biological processes and, thus, may present a holistic picture on soil quality. To date, there has been little information on the agricultural and ecological significance of DOM in diverse farming systems.

DOM is produced by soil enzyme-catalyzed depolymerization of organic matter and is comprised of low molecular weight chemicals that are often water soluble and thus more assessable to microbial assimilation as energy, carbon, and nutrient sources. Microbial utilization of these soluble compounds leads to microbial biosynthesis as well as soil carbon and nitrogen mineralization. DOM comprises chemicals varying in biodegradability, i.e., labile and recalcitrant pools. Laboratory incubation studies have shown that not all dissolved chemicals could be degraded. While DOM is a small fraction of soil organic matter, it is a direct source of available carbon and nutrients for microbes, and thus the chemistry and biology of the dissolved organic pool may dictate overall soil carbon and nitrogen cycling.

We hypothesized that the chemical composition and biodegradability of dissolved organic matter were tightly associated with soil enzyme activities and could be used to predict soil carbon and nitrogen mineralization. Specifically, we addressed two questions: (1) Could chemical composition of dissolved organic matter be explained by the activities of soil enzymes involved in carbon and nitrogen mineralization? and (2) Were the chemical composition and biodegradability of dissolved organic matter correlated with soil carbon and nitrogen mineralization? The investigation was conducted in diverse, long-term managed farming systems.

1. Quantify and characterize dissolved soil organic matter (DOM) in various farming systems.

2. Examine the relationships of DOM with key soil properties that are often used for evaluating soil quality, including soil organic matter, microbial biomass, soil respiration, and soil enzyme activity.