Abstract

Repeated application of animal manure to agricultural fields as a source of plant nutrients has led to questions concerning the impact of this practice on soil organic carbon (C) and biochemical properties, specifically the activity of soil enzymes. There are also some environmental concerns of using livestock manure. The objectives of this study were to determine the effects of repeated applications of liquid swine manure (LSM) on total organic C (TOC), light fraction organic C (LFOC) and on the activity of the soil enzymes (arylsulfatase, alkaline phosphatase and urease) in two contrasting soil-climatic zones and cropping systems in Saskatchewan. Liquid hog manure was applied annually at 37,000 L ha^-1 and 74,000 L ha^-1rates for three years at Melfort (Mollic Cryoboralf) and for four years at Plenty (Typic Boroll) in Saskatchewan, Canada. Soil samples were collected to a depth of 0-15 cm in the spring of 2003 and analyzed for TOC, LFOC and enzyme activities. Annual application (37,000 L ha^-1) and larger application made every two years (74,000 L ha^-1) of LSM at Melfort increased LFOC, which was attributed to stimulation of plant growth and thus residue inputs, from the nutrients contained within the manure. Applications of LSM at 37,000 L ha^-1 and 74,000 L ha^-1 at the Plenty site increased both TOC and LFOC concentration when compared to the control. Soil at the Plenty site is a Typic Boroll of heavy clay texture, which aids in protecting soil organic matter (SOM) from decomposition by soil microorganisms. Melfort was the only site that responded to LSM applications in terms of increased enzyme activity, which may be a result of a shorter application history. The Melfort site also had significantly higher LFOC in the manure treatments compared to the other site and LFOC has been linked to enzyme activity. The results of this study indicate that it may take a long period of time for addition of LSM to produce measurable changes in TOC and LFOC, as the effect from LSM is mainly from the stimulation of plant growth rather than from direct additions of C, and the nutrients contained in the LSM may potentially enhance microbial decomposition. In soils that receive repeated applications of LSM, nutrient loading may contribute to reduced enzyme activity after a period of time.

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