VARIATIONS IN SOIL AGGREGATE STABILITY AND ENZYME ACTIVITIES IN A TEMPERATE AGROFORESTRY PRACTICE

Authors: UDAWATTA, RANJITH - UNIVERSITY OF MISSOURI; Kremer, Robert; ADAMSON, BRANDON - UNIVERSITY OF MISSOURI; ANDERSON, STEPHEN - UNIVERSITY OF MISSOURI

Submitted to: Applied Soil Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/1/2008
Publication Date: 6/1/2008
Citation: Udawatta, R.P., Kremer, R.J., Adamson, B.W., Anderson, S.H. 2008. Variations in soil aggregate stability and enzyme activities in a temperate agroforestry practice. Applied Soil Ecology. 39:153-160.

Interpretive Summary: Soil conservation practices, implemented on a large scale in the U.S. during the 1930’s in response to devastating soil loss and degradation brought on by the Dust Bowl, are designed to reduce soil erosion and improve soil productivity. Current practices widely used in soil conservation include agroforestry and grass buffers. Grass buffers are composed of forage or native grasses and legumes planted in narrow strips in fields following the landscape contour (i.e., perpendicular to slope). Established vegetation in these strips slow the rate of runoff and filter suspended soil contained in runoff after periods of rainfall. Agroforestry is a multiple-cropping land use system that involves simultaneous production of trees and agricultural crops. Both grass buffers and trees can diversify production systems through harvests of forage and tree crops that potentially increase farm profits. Despite the known benefits in reducing soil loss, little information is available about the effects on biological properties that influence plant productivity and soil quality. This information would be valuable in assessing effectiveness of land management practices associated with agroforestry/grass buffer systems. Our objective was to examine changes in soil carbon and nitrogen, soil microbiological activity, and soil structure in an agroforestry system established on a silt loam within a northeast Missouri watershed. A 10-year stand of pin oak trees in rows is established along the landscape contour on one-half of the watershed; narrow strips (10 to 15 ft wide) of cool-season grasses along the contour planted in the other half in 1997. Permanent grass waterways are located at the outflow of the watershed. The crop production area of the watershed has been continuously cultivated under a corn-soybean rotation since 1991; soils were sampled in 2006 during the corn rotation cycle. Soil samples at 4-in depths were removed from all treatment sites within the watershed in June 2006. Soil enzyme activities, soil quality parameters that indicate levels of nutrient cycling processes important in plant growth, were greater in grass and agroforestry soils compared with soil planted to corn. Soil carbon and nitrogen contents were lowest for the corn soil suggesting that soils under continuous vegetation (grass and agroforestry) are more efficient in improving soil quality by increasing soil organic matter, which stores carbon and nitrogen. Higher soil enzyme activities and carbon and nitrogen contents under grass and agroforestry were directly related to improved soil structure detected in these areas, indicated by higher stable soil aggregates, which provided better aeration, water infiltration, and organic matter stabilization than soils in the cropped area. Results demonstrated that soil conservation practices established with grass and agroforestry buffers significantly improve biological properties of soil quality. The biological parameters measured in this study differentiated management of soil under permanent conservation practices from that where intensive row cropping was practiced. Thus, results are important to farmers, conservationists, extension personnel, and other scientists because they illustrate the value of simple conservation practices in improving soil quality and that the measurements reported can be easily applied to other areas for assessing soil conservation effects.

Technical Abstract: Agroforestry and grass buffers have been shown to improve soil properties and overall environmental quality. The objective of this study was to examine management and landscape effects on water stable soil aggregates (WSA), soil carbon, soil nitrogen, enzyme activity, and microbial community DNA content. Treatments were row crop (RC), grass buffer (GB), agroforestry buffer (AG), and grass waterways (GWW). A corn (Zea mays L.)-soybean (Glycine max L.) rotation under no-till management was established in a watershed in northeast Missouri in 1991; grass buffers were implemented in 1997. Grass buffers, 4.5 m wide and 36.5 m apart, consisted of a mixture of redtop (Agrostis gigantea Roth), brome grass (Bromus spp.), and birdsfoot trefoil (Lotus corniculatus L.) on contour within the watershed. Agroforestry buffers have pin oak (Quercus palustris Muenchh.) trees distributed down the center of the grass buffers on one half of the watershed. Soils were collected from two transects extending from the summit to lower landscape positions within the grass and agroforestry portions of the watershed in June 2006. Soil enzymes studied included: fluorescein diacetate hydrolase, ß-glucosidase, glucosaminidase, and dehydrogenase. Soil DNA content was determined as an alternative to microbial biomass. WSA was significantly different among treatments and landscape positions. WSA decreased from GWW>AG>GB>RC management treatments and also decreased in order from lower>middle>summit landscape positions. Soil carbon and nitrogen were highest for the GWW treatment and lowest for RC. The landscape position effect was significant for RC and AG treatments. Fluorescein diacetate, ß-glucosidase and glucosaminidase enzyme activities were significantly higher in buffers and GWW areas than RC areas. Dehydrogenase activity was different between grass (GB and GWW) and crop areas. The landscape effect was insignificant for enzyme activity. Although soil DNA may be a good indicator of microbial biomass, it did not appear to differentiate among management systems as selectively as other microbial parameters. Results of the study show that establishment of AG, GB, and GWW increased WSA, soil carbon, soil nitrogen, and enzyme activity.