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TECHNICAL REPORTS

Waste Management

Carbon and Nitrogen Mineralization Rates after Application of Organic Amendments to Soil

School of Earth and Geographical Science, Faculty of Natural and Agricultural Science, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia

^* Corresponding author (dmurphy{at}cyllene.uwa.edu.au)

Received for publication January 21, 2005. The objective of this study was to quantify C and N mineralization rates from a range of organic amendments that differed in their total C and N contents and C quality, to gain a better understanding of their influence on the soil N cycle. A pelletized poultry manure (PP), two green waste–based composts (GWCa, GWCb), a straw-based compost (SBC), and a vermi-cast (VER) were incubated in a coarse-textured soil at 15°C for 142 d. The C quality of each amendment was determined by chemical analysis and by ¹³C nuclear magnetic resonance (NMR). Carbon dioxide (CO₂–C) evolution was determined using alkali traps. Gross N mineralization rates were calculated by ¹⁵N isotopic pool dilution. The CO₂–C evolution rates and gross N mineralization rates were generally higher in amended soils than in the control soil. With the exception of GWCb all amendments released inorganic N at concentrations that would be high enough to warrant a reduction in inorganic N fertilizer application rates. The amount of N released from PP was high indicating that application rates should be reduced, or alternative amendments used, to minimize leaching losses in regions where ground water quality is of concern. There was a highly significant relationship between CO₂–C evolution and gross N mineralization (R² = 0.95). Some of the chemically determined C quality parameters had significant relationships (p < 0.05) with both the cumulative amounts of C and N evolved. However, we found no significant relationships between ¹³C NMR spectral groupings, or their ratios, and either the CO₂–C evolved or gross N mineralized from the amendments.

Abbreviations: GWCa and GWCb, 18- and 24-week green waste–based composts, respectively • MB-C, microbial biomass carbon • MB-N microbial biomass nitrogen • NMR, nuclear magnetic resonance • PP, pelletized poultry manure • SBC, straw-based compost • SOM, soil organic matter • VER, vermi-cast