*3.2. C Mineralization*

The effects of endogenous and exogenous N on microbial decomposition during the 60 d incubation period are documented by temporal (Figure 2) and cumulative (Figure 3) data for CO2-C production. As expected, the amount of CO2-C collected was always greater for incubations with rather than without residue (Figure 2), the cumulative effect being at least a four-fold difference (Figure 3) with O2 concentrations that always exceeded 0.20 kPa O2 kPa−<sup>1</sup> to ensure aerobic conditions. Examination of the inset panel in Figure 2A reveals that CO2 production without residue was significantly (*p* < 0.01) reduced by the presence of KNO3 (PN) or (NH4)2SO4 (AS), although neither effect was significant when evaluated using the entire dataset (Figure 3). Such findings can be explained by 'microbial N mining', whereby N limitation stimulates microbial attack on indigenous organic matter [60–62], the result being greater C mineralization than would occur when microbial N demand is alleviated by the input of mineral N. Moreover, the high salt index of KNO3 [63] would have limited microbial activity and biomass due to plasmolysis caused by osmotic stress [64–66], whereas the acidifying effect of (NH4)2SO4 [67], which is documented by Figure 4, would have been more important than salinization for inhibiting heterotrophic C oxidation [68–70]. The findings in Figures 2 and 3 are consistent with previous reports that, in the absence of residue inputs, N fertilization usually decreases soil respiration [22,71,72].

Given that the two residue mixtures studied differed considerably in their N contents, an increase in CO2 production was expected in comparing the HNR with the LNR treatment. This was indeed observed, as collection of CO2 was significantly greater for HNR than LNR during the first 10 d of incubation but not thereafter (Figure 2B,C). Due to the initial enhancement, a significant increase also occurred in the cumulative emission of CO2 (Figure 3), which in terms of the C applied was equivalent to 66% for HNR and 50% for LNR. The latter finding is in line with previous studies showing that a higher N content promotes microbial decomposition when crop residues incubate following their incorporation in soil [73–76], but also reflects the fact that HNR was substantially greater in the water-soluble fraction (Table 2). According to Shi and Marschner [55], this fraction serves as a key source of energy to support active growth by the heterotrophic microflora, which promotes residue decomposition during the early stage of incubation.

**Figure 2.** Total quantity of CO2-C produced by soil in 10 d intervals during a 60 d aerobic incubation involving the following nine treatments: (**A**) unamended control, potassium nitrate (PN), ammonium sulfate (AS); (**B**) high N residue (HNR) with or without PN (HNR + PN) or AS (HNR + AS); and (**C**) low N residue (LNR) with or without PN (LNR + PN) or AS (LNR + AS). Data shown as a mean from triplicate incubations with standard error bars and a table for mean comparisons. Within a given incubation interval, treatments followed by the same letter do not differ significantly at *p* < 0.05. When compared at a smaller scale (**A**), CO2-C was significantly greater (*p* < 0.01) for the control than for the PN or AS treatment.

Regardless of which residue mixture was incorporated prior to incubation, cumulative CO2 production, ranging from 82 to 88% of the residue C applied, was significantly increased by the presence of exogenous NH4 <sup>+</sup> or NO3 − relative to the HNR and LNR treatments. This is evident from Figure 3, which also shows that the two N sources did not

differ in their effects on cumulative CO2 production, presumably because NO3 − utilization is promoted by the presence of carbonaceous residues. The finding that addition of mineral N promoted liberation of CO2 during decomposition of corn residue is consistent with results previously obtained in many relevant incubation studies [2,61,73,76–79] and can presumably be attributed to microbial N utilization for cellular synthesis and metabolism. In some cases, exogenous N has had no significant effect on soil respiration in the presence of corn residue [9,21,80], which may reflect variations in incubation procedure, the type of soil studied, and/or the relative rates of residue and N addition. Reports that decomposition is unaffected or even inhibited by the addition of mineral N are more common from studies with more ligneous plant materials such as wheat (*Triticum aestivum* L.) or rice (*Oryza sativa* L.) straw, tree bark, or sawdust [4,81–83].

**Figure 3.** Cumulative CO2-C produced by soil during half or all of 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 (LNR + AS). Data shown as a mean from triplicate incubations with standard error bars obtained for the total amount of CO2 collected. Treatments do not differ significantly (*p* < 0.05) for the entire 60 d incubation period when bars are accompanied by the same letter.

**Figure 4.** Soil pH 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 (LNR + AS). Data shown as a mean from duplicate incubations for 7 and 60 d with standard error bars. Treatments do not differ significantly (*p* < 0.05) when bars are accompanied by the same letter.

Besides increasing the cumulative production of CO2 from residue-treated soil, exogenous N shifted the temporal pattern of decomposition, such that 76 to 82% of the CO2 collected was liberated in the first month of incubation, as compared to 51% for the LNR and 57% for the HNR treatment (Figure 3). A substantial decline subsequently occurred for the fertilized but not the unfertilized treatments with residue, and the difference was usually significant (Figure 2). This shift has previously been observed in numerous incubation studies [2,4,76,80] and can be explained by microbial utilization and subsequent depletion of labile constituents released by residue decomposition.
