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.
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.
Our study was conducted in a farming systems-unit, located in the Center for Environmental Farming Systems (CEFS), Goldsboro, NC. This unit included five farming systems: conventional cropping system subjected to best management practices (BMP), organic cropping system (ORG), integrated crop-livestock system (CA), plantation forestry system (TREE), and successional system from abandoned agricultural field (SUCC). Details on farming systems are available at www.cefs.ncsu.edu.
Soil samples were collected in March 2009. Twenty soil cores (2.5 cm x 10 cm) were collected randomly from each field plot and pooled to form a composite soil sample. Soil samples were placed in plastic bags, transported in a cooler back to the laboratory, sieved and stored at 4 ºC until analysis of soil chemical and biological properties.
Soil organic carbon and nitrogen, inorganic nitrogen, soil pH, and microbial biomass carbon and nitrogen were determined using standard methods. Mineralization of soil organic carbon and nitrogen was determined via a three-week incubation. In this study, DOM was referred to 0.45-?m filtered water extractable organic matter and extracted following soil equilibration with distilled water at a soil-to-extractant ratio of 1:4. To gain multifaceted and insightful information on DOM, we used several analytical methods to determine chemical composition of DOM, including dissolved organic carbon, dissolved organic nitrogen, aromaticity of DOM by UV absorbance at 280 nm, soluble phenolic compounds, amino acid nitrogen, and reducing sugar carbon. Furthermore, mineralization of dissolved organic carbon and nitrogen was determined via an incubation experiment.
A 96 well microplate method was used to measure activities of soil enzymes involved in depolymerization of soil organic carbon and nitrogen, including oxidative enzymes of peroxidase (EC 18.104.22.168) and phenol oxidase (EC 22.214.171.124), and hydrolytic enzymes of exoglucanase (EC 126.96.36.199), ?-glucosidase (EC 188.8.131.52), and ?-glucosaminidase (EC 3.21.30).
Analysis of variance (ANOVA) for a completely randomized block design was performed to examine significant differences of soil chemical and microbiological properties among the five farming systems. The mean values were compared via Waller’s T test. Linear regressions were used to test relationships among soil carbon and nitrogen mineralization, soil enzyme activities, and chemical composition of DOM.
Long-term diverse farming practices had generated divergences in soil chemical and microbial properties. As often-used biological indicators for assessing management practices, soil organic carbon and microbial biomass carbon varied moderately among the five farming systems with less than 25% of coefficient of variation. By contrast, the five farming systems differed greatly in soil enzyme activities with up to 79% of coefficient of variation. Our results supported the notion that soil enzyme activities were very sensitive to management practices
Soil enzyme activities appeared to reflect the characteristics of individual farming systems. TREE and SUCC were woody plant dominant ecosystems where soil organic matter might contain more lignin and/or lignin derivatives, the substrates of phenol oxidase, than the cropping systems. Consequently, the higher activity of soil phenol oxidase was found in TREE and SUCC. Despite that CA had a higher concentration of soil organic and similar microbial biomass as compared to the TREE, it produced lower activity of soil glucosidase. On the contrary, soil exoglucanase activity in CA was higher than the TREE. It is very interesting that the activities of the two carbon-mineralization enzymes behaved differently in the two systems, but soil carbon and nitrogen mineralization were similar.
Of the five enzymes examined, soil peroxidase was the only one associated significantly with soil carbon and nitrogen mineralization. Variation in some components of DOM was well explained by the variation of soil peroxidase among the five farming systems. Tight relations exited between soil peroxidase activity and the relative abundance of reducing sugars and amino acids. Furthermore, relative abundance of amino acids interpreted reliably the variation in soil nitrogen mineralization among the five farming systems. Relative abundance of reducing sugars also explained the variation in soil carbon mineralization but with less certainty than for amino acids and soil nitrogen mineralization.
This study is the first report regarding the interrelationships among soil enzyme activity, chemical composition and biodegradability of DOM, and soil carbon and nitrogen mineralization. It helps to gain new insights into enzymatic controls on soil carbon and nitrogen dynamics.
This study has extended current knowledge on relationships between DOM and soil enzymes. Relative abundance of reducing sugars and amino acids could reliably interpret soil carbon and nitrogen mineralization and these components of DOM were well correlated with soil peroxidase activity. Soil peroxidase activity appeared to have preponderant effects on soil carbon and nitrogen mineralization and the chemistry of DOM.
Areas needing additional study
Our results represented a snapshot in time in our diverse farming systems. It is unclear, since controlling factors for soil peroxisase activity are unknown, whether the strong correlations between soil peroxidase and chemical composition of DOM would remain through seasons and years. Probably, a temporal study examining relations among soil enzymes, chemical composition of DOM, and soil carbon and nitrogen mineralization in agroecosystems of other geographic locations and subjected to other management practices could help in solidifying the predictive ability of soil peroxidase in identifying the extent of soil processing, specifically carbon and nitrogen mineralization.