*Article* **Simulated Cropping Season Effects on N Mineralization from Accumulated No-Till Crop Residues**

**Rashad S. Alghamdi 1,\*, Larry Cihacek <sup>1</sup> and Qian Wen <sup>2</sup>**


**\*** Correspondence: rashad.alghamdi@ndsu.edu

**Abstract:** The adoption of no-till management practices has increased in the United States over the last decade. In the state of North Dakota, approximately 5.7 million hectares of cropland is managed under no-till or conservation tillage management practices. Even though conservation tillage is known for building soil health, increasing soil organic matter, capturing soil moisture, and reducing wind and water erosion, it also presents a unique best management practice since an increased mass of crop residue remains on the soil surface. Producers are concerned about whether plant needs are being met by nitrogen fertilizer that is currently being applied based on current North Dakota recommendations for long-term no-till systems. A Forman clay loam soil (fine-loamy, mixed, superactive, frigid Calcic Argiudolls) was used in this study, as it represented glacial till soils of the region. We examined whether N mineralization from surface-applied crop residue would result in similar or different results when compared to crop residue mixed into the soil. Soil freeze-thaw contribution to soil N mineralization was also evaluated. Six residue treatments with different C/N ratios including corn (*Zea mays* L.), soybean (*Glycine max* L.), forage radish (*Raphanus sativus* L.), winter pea (*Pisum sativum* L.), spring wheat (*Triticum aestivum* L.), and winter wheat (*Triticum aestivum* L.) were used. Five 10–14-week cycles with a three-week freeze period between each cycle at 0 ºC were evaluated for NO3-N production. Crop residues with a narrow C/N ratio contributed to greater instances of N mineralization during each incubation cycle, and the accumulation of crop residues with a wide C/N ratio over each incubation cycle following the first incubation did not offset the immobilization trends observed in the first incubation. A change in N mineralized in the untreated control soil during the last two incubation cycles may have been caused by freeze-thaw effects or a shift in microbial population due to a lack of fresh C inputs.

**Keywords:** N mineralization; C/N ratio; crop residue; N availability

**1. Introduction**

In a no-till agricultural system, crop residue remains on the soil surface, whereby in a drier environment with lower soil temperatures may slow and/or reduce nitrogen mineralization, with the potential to affect crop yields. In areas where there is a short growing season, no-till practices have been perceived by producers to delay planting and slow crop emergence as a result of delayed soil warming and drying [1–4]. In a frigid environment, such as that in the Northern Great Plains, these concerns are more pronounced, as crop residue tends to accumulate when high residue crops (corn, small grains) are left on the soil surface as a result of no-till practices. Alghamdi et al. [5] examined soil warming and drying in a frigid environment for corn-soybean systems and provided evidence to suggest that these perceived delays are not related to moisture and temperature of the soil. Daigh et al. [6] have also reported research on full-production scale farms in the Red River Valley of Minnesota and North Dakota (i.e., frigid environment), finding that producer perceptions of delayed warming and drying of the soil do not translate into yield losses. This begs the question of where these perceived losses are coming from.

**Citation:** Alghamdi, R.S.; Cihacek, L.; Wen, Q. Simulated Cropping Season Effects on N Mineralization from Accumulated No-Till Crop Residues. *Nitrogen* **2022**, *3*, 149–160. https:// doi.org/10.3390/nitrogen3020011

Academic Editor: Jacynthe Dessureault-Rompré

Received: 1 March 2022 Accepted: 28 March 2022 Published: 31 March 2022

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Alghamdi et al. [7] concluded that any differences observed in yield were attributed to fertilizer application methods and rates. Many earlier studies concluded that an addition of nitrogen fertilizer may be necessary under a no-till system to increase nitrogen (N) in the deeper soil profile [8–10], yet modern recommendations often overlook previous decades of research [11]. The rate of soil N mineralization is dependent upon crop residue factors that include, but are not limited to, the type and quantity of residue and other soil factors, such as temperature and moisture in the residue environment and organic matter (OM) in the soil [12]. As residue continues to accumulate on the soil surface in a no-till system, it is necessary to determine if enough N is being provided to subsequent crops to offset the potential immobilization in the system. Schoenau and Campbell [13] reported that conservation tillage such as reduced and no-till systems results in greater surface accumulation of crop residue on top of the soil surface, which slows residue decomposition of wide C/N ratio crops. No-till systems provide added benefits of increased soil moisture and organic matter, although reduced availability of oxygen from aeration needed by microbes may slow the mineralization of nitrogen from the crop residue. In addition to increasing soil moisture, no-till systems can be susceptible to water saturation as a result of increased crop residue on the soil surface. Thus, the activity of microbes may be reduced along with soil shading and slower soil drying due to cooler temperatures [14]. A laboratory study on North Dakotan soils concluded that the type of crop residue and organic matter present may increase N immobilization when crop residue has a wide C/N ratio (>25:1) as compared to crop residue with a narrow C/N ratio (<25:1) [7]. Green and Blackmer [15] examined the rate of N fertilization and its effect on residue decomposition on corn–corn and corn–soybean fields in Iowa. They determined that N immobilization tended to decrease with increased N fertilization as a result of the fertilizer contributing N to the residue biomass. Increasing rates of N fertilization decreased the time required for N mineralization to occur. This process was expedited when soybean residue was the proceeding crop due to the nature of the soybean residue. Li et al. [16] found that N mineralization occurred more quickly with crop residue placed on the soil surface versus crop residues incorporated into the soil. Satchell [17] explained the biological process of decomposition, where plant tissues are broken down by microorganisms. These microorganisms can break down residue more quickly when plant residue is incorporated with the soil [18] as compared to remaining on the soil surface [19]. Coppens et al. [20] found that incorporated crop residues decomposed faster than leaving residue on the soil surface and that higher N fluxes were more pronounced with crop residue incorporation. For the residue that did remain on the soil surface, the absence of moisture was a greater limitation to decomposition than N itself. Often, to aid in the breakdown of high lignin content crop residue, such as corn, many producers apply liquid N to their fields after harvest. Al-Kaisi et al. [21] examined corn residue decomposition and the application rate of liquid urea ammonium nitrate (UAN) following harvest and concluded that increased rates of decomposition were not attributed directly to the application rates of N, but to increased air and soil temperature. They recommended the limiting of liquid N application under less-than-ideal air temperature conditions and/or limited availability of soil moisture during critical periods of residue decomposition. Additional studies have determined similar finding where decomposition effects were attributed to temperature and moisture availability [22–24]. In cooler climates of the Northern Great Plains, such as in North Dakota, highly variable precipitation and temperature conditions during crucial periods of crop N needs can lead to the rate of residue decomposition being less predictable, resulting in less certain N mineralization for crop needs. Vigil and Kissel [25] examined the impact of temperature on nitrogen mineralization and residue decomposition in Kansas soils. Their findings indicated that higher incubation temperatures resulted in increased mineralization. At temperatures lower than 35 ◦C, microbial activity decreased. Nitrogen from sorghum and soybean residues mineralized faster at 35 ◦C than 25 ◦C, 15 ◦C, and 5 ◦C, respectively. Aher et al. [26] examined C/N ratios and the mass of residue on the soil surface near Forman, ND, and indicated potential N deficits to succeeding crops

ranging from 56 to 105 kg N ha−<sup>1</sup> following winter weathering of the crop residue. North Dakota has a frigid climate where residue decomposition and nutrient mineralization from crop residues can be limited by the short frost-free period of 100–135 days [27,28]. In the Canadian prairie, an incubation study was conducted to examine the factors influencing the stability of microbes in the soil [29]. Results indicated that freezing and thawing periods had a strong effect on releasing N and other nutrients back into the soil when compared with wetting and drying cycles. The reason for this was that the freeze and thaw periods enabled microorganisms to feed on decomposed residue. During freezing and thawing, soil aggregates are disturbed, exposing sheltered residue to microbes that help release nutrients back to the soil [30,31] or may break up residue particles to provide a greater residue surface area for greater microbial access. During the winter, native soil organic matter mineralizes to release N [32] due to microorganism abate and releases amino acids and simple sugars back into the soil [33]. During thaw periods, surviving microorganisms are active and feed on the nutrients released from dead microorganisms [34]. As a result, many studies have noted the increase in microbial respiration following freezing and thawing periods [33–36]. Current recommendations for N management in North Dakota do not regard temporal residue decomposition and N mineralization and immobilization effects that vary with different types of crop residue in their recommendations. Instead, standardized recommendations are cited for long-term no-till systems (i.e., managed for six or more years) where producers are to take a 34 kg N ha−<sup>1</sup> credit [37]. Over the last ten years, recommendations for North Dakota have varied. Standardized N recommendations are useful in warmer and moister climates, which allow for optimum N mineralization from crop residue. Management recommendations for nitrogen fertilization of soil should aim to address concerns of the producer and advance best management practices. The objectives of this study are: (1) evaluate if the type and quantity of crop residue are offsetting the N immobilization, (2) evaluate the influence of freezing and thawing on crop residue decomposition, and (3) evaluate changes in N mineralization due to repeated residue addition over several simulated growing seasons.

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