Understanding how the quality of organic soil amendments affects the synchrony of nitrogen (N) mineralization and plant N uptake is critical for optimal agronomic N management and environmental protection. Composting solid livestock manures prior to soil application has been promoted to increase N synchrony; however, few field tests of this concept have been documented. Two years of replicated field trials were conducted near Boone, Iowa to determine the effect of composted versus fresh solid swine manure (a mixture of crop residue and swine urine and feces produced in hoop structures) on Zea mays (maize) N uptake, in situ soil net N mineralization, and soil inorganic N dynamics. Soil applications of composted manure increased maize N accumulation by 25 % in 2000 and 21 % in 2001 compared with fresh manure applications (application rate of 340 kg N ha(-1)). Despite significant differences in net N mineralization between years, within year seasonal total in situ net N mineralization was similar for composted and fresh manure applications. Partial N budgets indicated that changes in soil N pools (net N mineralization and soil inorganic N) in the surface 20 cm accounted for 67 % of the total plant N accumulation in 2000 but only 16 % in 2001. Inter-annual variation in maize N accumulation could not be attributed to soil N availability. Overall, our results suggest that composting manures prior to soil application has no clear benefit for N synchrony in maize crops. Further work is required to determine the biotic and abiotic factors underlying this result.
