**1. Introduction**

Modern cereal production relies on intensive N fertilization to increase grain yield and thereby enhances the input of crop residues. The latter effect is important for agricultural sustainability, as residues protect the surface soil against erosion loss, serve as a source of nutrients for plants and soil microbes, and sustain the soil microflora by supplying C as an energy source. Moreover, crop residues are essential to the formation of soil organic matter, although the efficiency of this conversion is necessarily reduced by liberation of CO2 during microbial decomposition, which depends not only on environmental factors (temperature and moisture) but also N availability and residue quality (i.e., chemical composition and C:N ratio) [1].

The effect of exogenous N addition on residue decomposition has been the subject of numerous investigations, but contrasting results have been reported. A positive effect has been found in some studies [2,3], whereas in others, N addition has reduced C mineralization [4–6] or has had no net effect [7–9]. The disparities can be attributed in part to the type of residue, which affects the proportions of cellulose, lignin, and other constituents that differ in their ease of decomposition [10], and further interactions arise in relation to the size and placement of residues. Moreover, discrepancies can occur because of variation in the fertilizer N rate relative to the soil's N supplying capacity and can also reflect differences in the form of N applied, as microbial N utilization is greater with NH4 <sup>+</sup> than NO3 − [11,12] but can be inhibited by acidifying N sources such as (NH4)2SO4 or NH4Cl. Another source of inconsistencies in the effect of exogenous N on residue decomposition is the method of

**Citation:** Jesmin, T.; Mitchell, D.T.; Mulvaney, R.L. Short-Term Effect of Nitrogen Fertilization on Carbon Mineralization during Corn Residue Decomposition in Soil. *Nitrogen* **2021**, *2*, 444–460. https://doi.org/10.3390/ nitrogen2040030

Academic Editor: Jacynthe Dessureault-Rompré

Received: 17 September 2021 Accepted: 25 October 2021 Published: 27 October 2021

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incubation, which can be compromised if aerobic conditions are not maintained throughout a study period with continuous collection of CO2 [13].

The effect of N availability on C mineralization during residue decomposition can be clarified by investigating the impact on microbial biomass production and/or extracellular enzyme activities involved in microbial C and N cycling. Previous studies have shown that incorporation of N with residue or other carbonaceous substrates increases activities for cellulase and protease, two of the major enzymes responsible for C and N mineralization [14–17]; however, a negative effect is also possible when cellulase activity is limited by a low substrate concentration in ligneous materials [18] or when protease is repressed by a substantial concentration of NH4 <sup>+</sup> as the end product [16]. Microbial biomass content tends to follow changes in enzymatic activities and CO2 production during residue decomposition [19] and can either be increased [20] or decreased [21] by the addition of N. These changes would necessarily affect the dynamics of soil and residue N through mineralization and immobilization [21–23].

Despite a massive input of residues when corn (*Zea mays* L.) is repeatedly grown with synthetic N fertilization in long-term cropping experiments, the usual trend over time is a decline in profile storage of soil organic C (SOC) [24–26]. Such findings motivated the laboratory incubation study reported herein, which utilized continuous CO2 monitoring and relevant microbial indicators to test the null hypotheses that (1) exogenous or endogenous N increases C and N mineralization during corn residue decomposition, (2) the increased mineralization is due to stimulation of microbial biomass production and enzyme activities, and (3) the effectiveness of N for enhancing mineralization will be reduced by declining substrate availability.

#### **2. Materials and Methods**

#### *2.1. Soil Studied*

For use in comparing the decomposition of different residues, a bulk sample of surface (0–20 cm) soil was collected in early May 2019 from a Mollisol mapped as the Ipava series [Fine, smectitic, mesic Aquic Argiudolls (Chernozem)] near Farmer City (40◦15 12.6 N 88◦34 59.4 W) in central Illinois, USA. The sampling site had been cropped to a corn−soybean (*Glycine max* L. Merr) rotation for more than 40 years, during which the fertilizer N rate for corn was 180 kg ha<sup>−</sup>1. The soil sample, collected in a 38 L polyethylene tote box, was sieved (2 mm screen) in the field-moist condition with removal of macro residues from the 2018 corn crop, thoroughly homogenized, and then returned to the tote box for no more than 2 weeks of storage in a refrigerator at 4 ◦C. A subsample was air-dried for triplicate analyses to determine the properties reported in Table 1.


**Table 1.** Physicochemical properties of the soil studied.
