Ezanno, A.F., R.R. Harwood, and E.A. Paul
Presented at the All Scientist Meeting (1999-07-20 to 1999-07-21 )
Management strategies that are sustainable must provide yields equivalent to conventional agronomic systems, decrease environmental pollutants such as NO3, and increase the amount of plant available N in synchrony with crop uptake. Management practices that included crop rotations and compost change the flow of N in the active and slow soil pools on both a seasonal and long-term basis.Nitrification potential, the maximum capacity of nitrifying bacteria to transform NH4 to NO3, depends upon community-population size and composition, as well as, abiotic controls. It is sensitive to short-term changes in management practices such as crop rotations, compost or fertilizer additions, and cover crops. High nitrification rates often result in NO3 loss to groundwater and increased N2O emissions. Substitution of compost for N fertilizer decreased nitrification rates by 25% without reducing yield. First year corn yields (averaged across cover crop treatment) were 68 Mg ha-1 on N fertilizer plots and 70.8 Mg ha-1 on compost plots at Hickory Corners, MI in 1998. The rotation consisted of the following crops: corn (Zea mays L.)/ corn,/soybean (Glycine max)/wheat (Triticum aestivum). Nitrification rates in 1st year corn (220 ng N g-1 h-1) were greater than those of continuous corn (167 ng N g-1 h-1). Cover crop treatments had higher seasonal nitrification potentials (190 vs. 176 ng N g-1 h-1). Nitrification potentials on agronomic treatments peaked in October. The successional grassland treatment peaked in April. Low nitrification potentials appear to be indicative of both agronomic and unmanaged ecosystems that cycle N efficiently and experience less N loss.Crop rotations and compost applications caused long-term changes in soil N pools. The initial size of the pool of mineralizable organic N (N0) from a 150 d incubation significantly increased from 1994 to 1998 (47 vs. 70 g N kg-1 soil N) on compost plots. The amount of N0 did not fluctuate after 4 yr of N fertilizer applications, 42 vs. 44 g N kg-1 soil N. The mean residence time (MRT) increased in both treatments. Average MRT in April of 1994 on compost treatments was 246 d and had increased to 768 d by April of 1998. The average MRT on N fertilizer plots was 273 d in 1994 and 447 d in 1998. Lower initial mean residence times in both treatments during 1994 may be due in part to the 6 yr of alfalfa production prior to implementation of the rotation in 1993. The N0 pool does not appear to have reached a steady state under compost management. The compost system has been shown to cycle N more efficiently than the N fertilizer system on a seasonal and long-term basis. The active pool of N under compost management is smaller but sufficient for plant needs and is in a less volatile form than in the fertilizer system. In the compost system, N equivalent to N0 is predicted to become available for crop uptake in June of 2000.Figure 1. Nitrification potentials integrated compost and the successional grassland 1998Figure 2. Nitrification potentials integrated fertilizer 1998(Std. error agronomic treatments = 23 ng N g-1 soil h-1)
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