COS 87-4 - Consideration of soil aggregate habitat on extracellular enzyme activity in prairie and conventional agriculture

Thursday, August 11, 2011: 9:00 AM
Ballroom F, Austin Convention Center
Elizabeth M. Bach, Ecology, Evolution, and Organismal Biology, Iowa State University, Ames, IA and Kirsten S. Hofmockel, Pacific Northwest National Laboratory, Richland, WA

In a rapidly changing world, human demand for food and energy continues to tax Earth’s ecosystems.  High production of commodity crops used for processed foods and biofuels depends on the rich soil legacy left by the tallgrass prairie ecosystem, now nearly extinct throughout central North America.  In an effort to better understand the processes that produced the most fertile soils in the world, we investigated the effects of soil structure (aggregate distribution) on soil extracellular enzyme activity in replanted prairie in contrast to traditional agricultural methods.  We hypothesized that enzymatic activity would be greatest in large aggregates due to more available organic matter and pore space heterogeneity facilitating aerobic and anaerobic microsites.  Furthermore, we expected prairie soils would have lower rates of enzymatic activity indicative of slower turnover of soil C and N pools.  We tested these hypotheses on soil collected at the Iowa State University Comparison of Biofuel Systems (COBS) site in spring 2010.  Soil was separated into 6 aggregate fractions through a wet sieving technique and sub-samples were frozen immediately and later analyzed for soil enzyme activity.  Six enzymes were analyzed:  β-1,4-glucosidase, cellobiohydrolase, β-xylosidase, acid phosphatase, N-acetyl-glucosaminidase, and leucine aminopeptidase.


Consistent with our hypotheses, all enzymes had greatest activity in the 1-2 mm aggregate fraction and lowest in 0.25-0.5 mm aggregate fraction and whole soil (P­ <  0.0004).  Whole soil measurements of enzyme activity were consistently lower than values found in isolate aggregates, underscoring the importance of considering soil structure in microbial community function.  Within aggregate fractions, activities of β-1,4-glucosidase, acid phosphatase, cellobiohydrolase, and leucine aminopeptidase were also affected by cropping system.  When differences were present, enzyme activity was always lower in prairie systems compared to corn, supporting our hypotheses.  Cellobiohydrolase and leucine aminopeptidase only differed between prairie and corn in the largest aggregate fraction, 4-10 mm (P < 0.02).  β-1,4-glucosidase activity was lower in prairie compared to corn systems within the 4-10 mm and 2-4 mm aggregates and whole soil (P < 0.05).  Acid phosphatase activity showed the opposite trend with prairie systems supporting lower activity than corn systems in the 0.25-0.5 mm and < 0.25 mm fractions and whole soil (P < 0.03).  These data imply that soil structure and associated habitat differences may carry a stronger signal of soil microbial community function than whole soil measurements in contrasting planting practices.

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