INTRODUCTIONSoil is more than just plant food, it is a large and complex ecosystem made up of a wide variety of microorganisms which perform a variety of functions, from decomposition of organic matter to sustaining biogeochemical cycles. Soil microbes are the only organisms which can biologically fix atmospheric nitrogen (N) (Schlesinger 1997). Other microbes are known for their ability to convert phosphorus (P) to phosphates (Schlesinger 1997). These, and others, are essential steps in the biogeochemical cycles which allow life as we know it to exist on Earth.

The state of Iowa has an agricultural based economy, and row crops comprise a significant portion of this. These crops require certain amounts of fixed N and P to grow, and many farmers have to fertilizer their fields to ensure crop success. However, erosion and continuous farming depletes soil reservoirs of these compounds. Finding natural methods to replenish soil nutrients has the potential to reduce the need for fertilizing and maintain soil quality without negatively affecting production yields. Understanding soil microbial growth and activity could help in this regard.

The study of soil microbes can help create new, efficient and effective land management strategies. A predominant method for measuring soil microbe activity is by assaying for the presence of known extra cellular enzymes released by soil microbes to digest complex organic matter. This study looks at the presence of acid phosphatase (AP), β-N-acetylglucosaminidase (NAG), and leucine aminopeptidase (LAP) in soil samples collected from the Comparison of Biofuel Systems (COBS) test site in May and June, 2011 under continuous corn growth, fertilized planted prairie and planted prairie.• NAG hydrolyses N-acetyl-β-D-glucosamine molecules from the terminus of chitin (Ekenler & Tabatabai 2004). Chitin, found in the cell walls of fungi, is a polymer of

glucose molecules containing an N-actyl group attached in place of oxygen off the C2. Monomers of N-acetylglucosamine may be absorbed by microbes, which can cleave the N-actyl group off as a source of N for producing various other N containing organic molecules such as DNA or amino acids (Paul 2007) and use the remaining glucose as a source of energy production.

• LAP hydrolyses terminal leucine amino acids from proteins (Taylor et al. 1992). These amino acids may then be taken up whole by microbes or broken down further as another source of organic nitrogen.

• AP hydrolyses phosophomonoesters resulting in a source of phosphorus for soil microbes (Eivazi & Tabatabai 1997). Phosphates are building blocks for many larger organic molecules like DNA and ATP.

Temporal Effects on Soil Enzymatic ActivityR. Justin Hanna, Elizabeth Bach and Kirsten Hofmockel.

Department of Ecology, Evolution & Organismal Biology, Iowa State University.

METHODSStudy SiteSoil was collected from the COBS site located near Madrid, IA. The study site includes 6 cropping system treatments, but only 3 were sampled for this study: continuous corn (Zea mays), planted tallgrass prairie (30 species), and fertilized planted tallgrass prairie. All treatments are managed without tillage. The corn is fertilized with ~100 lbs/acre/yr according to crop needs, and the fertilized prairie received 75 lbs/acre/yr. Four replicate blocks contain a randomly chosen plot for each crop which is 27 m x 61 m. The site was created in 2008 and was previously farmed with row crops.

Soil Sampling & AnalysisThree soil cores were taken from each test plot within 5 days of the corn being planted in May. Cores were 10 cm deep and 10 cm in diameter and stored at 4°C. Soil was hand sieved, and the cores for each plot were combined. Samples were then dried to 10% moisture content. Cores were also collected one month later. For enzymatic analysis, soil was homogenized in buffer, dispensed, incubated in the dark, NaOH was added to halt enzymatic activity, and the fluorescence of the substrate was read using a Biotech Synergy HT plate reader.

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NAG Activity

F1 = 12.82P = 0.002

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F1 = 2.2P = 0.16

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AP Activity

F1 = 9.24P = 0.007

RESULTS• There was no significant statistical difference between enzymatic activity in corn, fertilized prairie and prairie soil.• NAG activity showed the most drastic increase between May and June (81.8% increase; F1 = 12.82; P = 0.002).• AP activity increased of 39.1% from May to June (F1 = 9.24; P = 0.007).• LAP activity was not statistically different between May and June (F = 2.2; P = 0.16).

DISCUSSIONThe dramatic increase in the activity of NAG in the June soil samples indicates soil microbial communities are far more active in June. An increase in NAG indicates that microbial organisms are utilizing more organic N as biomass rapidly increases during the start of the growing season (Kaiser & Heinemeyer 1993).

The increase in AP activity also supports the conclusion that the soil microbe community is more active in June. The phosphorus cycle starts from weathering of calcium phosphate minerals and in general P is a rare element to find on Earth (Schlesinger 1997). Yet it is essential for vital molecules in all forms of life.

The lack of difference in LAP activity may be the result of a lack of protein substrate in the soil or the microbial community could be relying upon NAG to fulfill their N needs.

The fact that there was no detectable difference between the corn, fertilized prairie and prairie implies the crops are utilizing the N available from the fertilizer. The corn and fertilized prairie experienced rapid growth between the sampling dates.

ACKNOWLEDGEMENTS & REFERENCESI would like to thank Professor Hofmockel for the opportunity to partake in this study, as well as the members of the Hofmockel lab for their help and for making this a fun, educational and enjoyable experience.I would like to thank Adah Leshem-Ackerman and the Plant Genomics Outreach program, funded by the National Science Foundation, which made this experience possible.

Schlesinger, William H. 1997. Biogeochemistry an Analysis of Global Change. Second edition. Academic Press.Ekenler, M. and Tabatabai. 2002. ß-Glucosaminidase activity of soils: effect of cropping systems and its relationship to nitrogen mineralization. Biology and Fertility of Soils 36:367-376.Horwath, W. R. 2007. Carbon Cycling and Formation of Soil Organic Matter. Pages 303-339 in E. A. Paul, editor. Soil Microbiology, Ecology, and Biochemistry. Academic Press, Oxford, U.K Eivazi, F. and M. A. Tabatabai. 1977. Phosphatases in soils. Soil Biology and Biochemistry 9:167-172.Taylor et al. 1992. Use of azidobestatin as a photoaffinity label to indentify the active-site peptide of leucine aminopeptidase. Biochemistry 31: 4141-4150.DeForest, J. L. 2009. The influence of time, storage temperature, and substrate age on potential soil enzyme activity in acidic forest soils using MUB-linked substrates and L-DOPA. Soil Biology & Biochemistry 41:1180-1186.Kaiser, E & Heinmeyer, O. 1993. Seasonal-variations of soil microbial biomass carbon with the plow layer. Soil Biology & Biochemistry 25:1649-1655.

HYPOTHESISWe tested the effects of two factors on extracellular enzyme activity in soils: time within the growing season and land use. We hypothesized the soil microbial communities will increase activity, resulting in greater activities of NAG, LAP, and AP in June compared to May.

We hypothesized the prairie soil samples will contain greater activity of extracellular enzymes than the corn and fertilized prairie samples.

Photo by Tom Shultz.

Photos by Elizabeth Bach.