*3.4. N Mineralization*

Because soil C and N cycling is closely coupled, 15N pool dilution was used to evaluate the treatments under investigation with respect to gross N mineralization and immobilization. The results are summarized by Figure 8A, which shows that immobilization always exceeded mineralization, even when there was no residue addition with the control, PN, and AS treatments. Both findings can be attributed to the use of a soil that contained carbonaceous residues when collected from a field previously cropped to corn [102]. Despite limited replication that minimized significant differences, net immobilization (Figure 8B) tended to be higher for treatments involving the addition of NH4 <sup>+</sup> than NO3 −, as would be expected from the microbial preference previously documented for N utilization [11,12,22]. Gross mineralization and immobilization were highly correlated (*r* = 0.91, *p* < 0.001), as would be expected given their concurrent occurrence as the central processes in soil N cycling [22].

**Figure 8.** Gross mineralization/immobilization (**A**) and net immobilization (**B**) measured at five intervals (7, 14, 30, 45, and 60 d) during a 60 d aerobic incubation involving an unamended control and the following eight treatments: potassium nitrate (PN), ammonium sulfate (AS), high N residue (HNR) with or without PN (HNR + PN) or AS (HNR + AS), and low N residue (LNR) with or without PN (LNR + PN) or AS (HNR + AS). Values reported as a mean with standard error bars representing duplicate data collected before and after a 3 d incubation with (15NH4)2SO4. Treatments do not differ significantly (*p* < 0.05) when bars are accompanied by the same lowercase (mineralization) or uppercase (immobilization) letter.
