*2.8. Gross N Mineralization and Immobilization*

For comparing treatment effects on N mineralization and immobilization, pooldilution measurements were performed using quadruplicate 10 g (dry weight) samples of soil from each composite mixture. Two of these samples were treated with 100 mL of 2 M KCl followed by 0.64 mL of deionized water containing 40 μg of N as (15NH4)2SO4 (30 atom % 15N), and the remaining samples were incubated at 25 ◦C for 3 d after the same addition of (15NH4)2SO4. In both cases, mineral N was extracted by shaking soil samples with 2 M KCl for 1 h and filtering the resulting soil suspension through Whatman no. 42 filter paper (GE Healthcare, Maidstone, UK) in a Büchner funnel under vacuum. The extracts were analyzed for exchangeable NH4 +-N by accelerated diffusion methods of Khan et al. [44], followed by 15N analysis using an automated Rittenberg system [45]. Gross rates of mineralization and immobilization were calculated using the zero-order equations derived by Kirkham and Bartholomew [46].

#### *2.9. Statistical Analyses*

Replicate data were characterized by computing means and standard deviations. PAST version 3.22 [47] was used to test for homogeneity (Levene's test) and normality (Shapiro−Wilk's test), to evaluate the significance of treatment effects by a one-way analysis of variance (ANOVA), and to carry out mean comparisons by Tukey's procedure (*p* < 0.05 to 0.001).

#### **3. Results and Discussion**

#### *3.1. Residue Quality*

Besides increasing grain yields and the quantity of above- and below-ground residue inputs, N fertilization has an intrinsic effect on residue quality by increasing biomass content of total N or crude protein [48–50], thereby lowering the C:N ratio [51,52]. These changes are apparent from Table 2, which shows that total N was significantly greater for the high N residue (HNR) collected after using fertilizer N, which caused a substantial decline in C:N ratio. Table 2 also shows that fertilization led to a significant increase in the water-soluble fraction obtained by proximate analysis. This fraction, consisting largely of monomeric sugars (predominately glucose and fructose) along with related alditols, aliphatic acids, and inorganic ions [53], enriches the soil in labile organic C when leached from residue and is readily utilized during microbial decomposition [54–56]. The proximate analyses reported by Table 1 are consistent with previous reports that N fertilization is more effective for increasing soluble than structural carbohydrates in corn residues [57–59].
