The site is located at the UAES Greenville Research Farm (41°45’56.6”N 111°48’52.2”W) in North Logan, Utah, USA. The soil is a highly calcareous Millville silt loam (coarse-silty, carbonatic, mesic Typic Haploxeroll) with a pH of 8.2 (1:2 soil: water). The plots were established in 2011 to investigate N cycling and different N transformations under contrasting N management, as outlined in previous studies (Ouyang, 2016; Kakkar, 2017). Prior to 2011 the field was utilized for conventional cultivation of small grains, involving an annual application of 70 kg N ha^-1 in the form of urea. The experimental design in this study was a randomized complete block design (RCBD) with four N fertility source treatments and four replications, totaling 16 plots. Each plot measured 9.1 m in length and 3.8 m in width. Treatments were assigned to the same plot each year. The treatments include a no N control (Control), low ammonium sulfate at 112 kg N ha^-1 year^-1 (AS100), high ammonium sulfate at 224 kg N ha^-1 year^-1 (AS200), and steer manure compost at 224 kg total N ha^-1 year^-1 (Compost). Compost was obtained commercially and consisted of composted steer manure, slaughter by-products and woodchips (Miller companies LLC, Hyrum, Utah). Compost N and dry matter content were determined yearly, and these parameters were used to apply the desired total N rate of 224 kg total N ha^-1 year^-1 equivalent to approximately 14 ± 1.8 metric ton of dry weight compost ha^-1 year^-1. Average compost analysis was 27.1%C, 1.7%N and 14.5 C/N ( Supplementary Table S1 ). Silage corn was planted every May from 2012 until 2021 except for 2017 when a cover crop of vetch was grown.

During early spring of each year pre-plant soil samples were collected from each plot using a Giddings probe with two cores per plot at depths of 0–15 cm, 15–30 cm and 30–60 cm. Soil was weighed, sieved (2 mm) and air-dried before analysis for available P and K. To meet the crop requirement of P and K, fertilization for P and K in each plot was carried out according to the recommendations outlined in the Utah Fertilizer Guide for silage corn (James and Topper, 1993). The fertilizer applications and compost amendments took place in early May of each year. N, P, K fertilizers were applied to the field using an Edge Guard mini push broadcast spreader (The Scotts Company LLC. USA). For compost treatment, the amendment was applied manually and subsequently, bow rakes were utilized to evenly distribute the fertilizers and compost amendments within individual plots. Following this, the amendments were incorporated into the soil through tillage within one day of application.

After the amendments were added and incorporated, the seedbed was prepared, and seeds (DEKALB^® Corn Hybrids (glyphosate tolerant) were planted with a row spacing of 76 cm. Within each block, approximately 4 rows of silage corn were planted at a density of 50,000 plants per hectare using a John Deere planter. Throughout the growing season, an overhead sprinkler irrigation system was used to apply water on a weekly basis as required and as available. To control weed growth, glyphosate herbicide (Killzall 41% glyphosate) was applied at a rate of 1.12 kg ha^-1. This application was done once via broadcast before the corn reached a height of 30 inches.

To analyze STN, topsoil samples were manually collected every year from 2012–2021 in August from the 0–15 cm layer (four cores per plot) using slide hammer soil probe. The soil samples were sieved through a 2 mm mesh, air-dried, and then a subsample was finely ground to pass through a 0.25 mm sieve (60-mesh) for TN analysis using dry combustion with a PrimacsSN (Skalar, Inc. GA, USA). Soil macro and micronutrients were analyzed using the ammonium bicarbonate -DTPA method (Soltanpour, 1985) followed by inductively coupled plasma spectrophotometric analysis (USU Analytical Laboratory, Logan UT USA).

For leaf tissue N analysis, samples of the corn ear leaf were collected approximately 80 days after planting each year. Four corn leaves from each row, located in the middles of the plots, were harvested. In total, eight leaves were sampled per plot. Leaves were dried at 60°C to constant weight, followed by grinding using a Wiley Mill. Subsequently, the subsample was further ground to achieve a particle size equivalent to 0.25 mm (60 mesh).

Once the silage reached maturity in late September, aboveground plant material from the inner two rows of each plot, covering a distance of 3 meters, was harvested using machetes. Plant counts and fresh wet weight were recorded for each row per plot. The harvested corn was subsequently dried at 60°C for approximately one week, and its dry weight was determined. The dried stalks were then coarsely chopped, and a subsample was finely ground using a cutting mill (Wiley Mill). The subsamples were then finely ground with a rolling ball mill to 0.25 mm sieve before total N analysis by combustion (PrimacsSN Skalar, Inc., GA, USA).

Uptake efficiency (UE), agronomic efficiency (AE), partial factor productivity (PFP), partial nutrient balance (PNB) are important metrics for interpreting NUE. The equations for NUE are adapted from previous studies (Sindelar et al., 2015; Augarten et al., 2019). The metrics and their equations are shown Table 1 .

The parameters in this study included 80-day leaf nitrogen content, dry matter yield, N uptake at harvest, NUE indicators (including uptake efficiency (UE), agronomic efficiency (AE), partial factor productivity (PFP), and partial nutrient balance (PNB), and STN content collected annually from the years 2012 to 2021. There was no data collected during 2017 due to the planting and management of a cover crop of hairy vetch.

For each year within the study duration, we performed an analysis of variance (ANOVA) to assess the impact of different fertilizer sources on the above-mentioned parameters. The PROC MIXED procedure available in SAS^® OnDemand was utilized. Our examination focused on the significant differences among the treatment groups at each year. Mean differences were considered significant at p ≤ 0.05.

To gain a comprehensive understanding of the overall treatment effects across the study years, we employed repeated measures analysis of variance (ANOVA) using the PROC MIXED procedure. In this analysis, year was considered a fixed and repeated effect. Blocks and interactions with treatment were considered as random effects. Several covariance structures were evaluated, and the compound symmetry (CS) covariance structure was used. The mean separations were conducted at p ≤ 0.05 using Tukey’s test. To ensure the validity of our statistical tests, we assessed the normality of residuals using the UNIVARIATE procedure in SAS. Additionally, we generated scatterplots of residuals against predicted values to ascertain the presence of common variance. These steps were undertaken to verify the assumptions or to indicate that transformations were needed. This approach enables the detection of treatment differences in datasets collected over multiple years in agronomic field trials (Pagliari et al., 2022).

Contrasting N sources showed inconsistent effects on corn silage yield from 2012 to 2021 ( Figure 1 ). In 2012, Compost displayed the lowest yield, whereas yield for AS200 and AS100 were not significantly different. In 2013, 2014, 2018, and 2021, the yield of Compost treatment was higher than Control. From 2012 to 2021, yields for AS200 were not different from AS100, except for the year 2020. In some years, the yield in the AS100 and AS200 treatments was comparable to Compost treatment (2013, 2016, and 2018), while in other years, AS100 and 200 treatments yielded more than Compost (2012, 2014, and 2020) ( Figure 1 ). Similarly, the pattern of plant uptake of N at harvest was variable year to year ( Figure 2 ). The N uptake for Compost treatment was significantly higher than Control only in the year 2014 ( Figure 2 ). Nitrogen uptake under Compost tended to be lower than AS treatments; however, this was only significant in 2013 and 2020.

The inconsistent year-to-year response in yield and plant N at harvest complicated the determination of treatment effects in the individual years. However, the impact of N source treatments on yields was significant based estimates from repeated measures analysis for the complete record of 2012-2021. The response of corn silage yield to N source was: AS200 and AS100 yielded the highest, followed by Compost, and then Control ( Figure 3 ). The estimated yields from repeated measure for control, compost, AS100, and AS200 were 7.9, 11.1, 14.9, and 17.2 Mg ha^-1, respectively ( Figure 3 ). While Compost significantly increased yield by 3.21 Mg ha^-1 (40.5%) compared to Control, this treatment still yielded 3.74 Mg ha^-1 (25.5%) and 6.12 Mg ha^-1 (35.51%) less than the AS100 and AS200 treatments, respectively.

The results obtained from repeated measures analysis (log -transformed) revealed that the average estimates of N uptake by corn silage were 42.0, 70.5, 105.3, and 163.1 kg N ha^-1 for the Control, Compost, AS100, and AS200 treatments, respectively ( Figure 3 ). Compared to Control, N uptake under Compost, AS100, and AS200 were 68%, 105% and 288% increased over control uptake, respectively.

From 2012 to 2021, corn ear leaves at 80 days showed N concentrations of 1.37% in the control, 1.53% with compost, 1.81% with AS100, and 2.36% with AS200 ( Supplementary Figure S1 ). AS200 had the highest N concentration, followed by AS100, while the control and compost treatments had similar N levels ( Supplementary Figure S1 ). The observed N concentrations in the corn ear leaves of this study were found to be below the sufficiency range when compared to the recommendations from the (University of Wisconsin, 2016).

The results from this study showed that the STN response to fertilizer treatments varied by year ( Supplementary Figure S2 ). Compost treatment had the highest STN content in 2013, 2014, and 2021. Control treatment exhibited the lowest STN content in 2013 and 2014, and AS100 had the lowest STN in 2021. In the remaining years, N fertilization treatments did not significantly affect STN, although the STN levels under compost treatment were numerically higher than the others. Based on the repeated measures analysis from 2011-2021, the STN content of 1.28 g  kg − 1 was highest under Compost, which was significantly greater than Control (1.05g kg^-1 ) , AS100 (1.01 g kg^-1 ) , and AS200 (1.06 g kg^-1) treatments ( Figure 5 ). Overall, the STN under Control was not significantly different from that under AS100 and AS200 treatments. The compost treatment resulted in significant increases in STN levels compared to the Control, AS100, and AS200 treatments, with percentage increases of approximately 21.90%, 26.73%, and 20.75%, respectively. Specifically, compost treatment elevated STN levels by about 0.24 g  kg − 1 (23.1%) compared to the other treatments, demonstrating its effectiveness in enhancing levels ( Figure 5 ).

Micronutrients such as Cu, Fe, Mn, Ni, and Zn were in the high range, while S was in the medium range and P was in the very high range. Fertilizer treatments did not significantly impact soil nutrients except for P. The P concentrations in control, compost, AS100, and AS200 were 13.1, 19.3, 9.38, and 8.5 mg/kg, respectively. P in compost was significantly increased ( Supplementary Table S3 ). This finding agreed with previous studies (Eghball and Power, 1999; Reeve et al., 2012).

In this study, there was a considerable yield variation ranging from 2-20, 5-24, 8-24, and 8–29 Mg ha^-1 for the control, compost, AS100, and AS200 treatments, respectively from 2012 to 2021 ( Figure 1 ) demonstrating that yields of corn silage was influenced by the growing season (Biswas and Ma, 2016). In 2012, the compost treatment had the lowest yield possibly attributed to N immobilization, where soil microbes compete with the growing crop for available nitrogen, potentially limiting crop growth and yield (Geisseler et al., 2021).

Overall results showed that the yield of corn silage was improved by application of compost and ammonium sulfate fertilizer ( Figure 3 ). However, there was no significant difference in corn yield between the AS100 treatment, which received 112 kg of N ha^-1, and the AS200 treatment, which received 224 kg of N ha^-1 ( Figure 3 ). For sustainable maize production on volcanic soil in Bea Cameroon, an N fertilization rate between 50 and 100 kg of N ha^-1is considered optimal (Ngosong et al., 2019). However, in the midwestern United States, optimizing N rates for maximum ecosystem value requires an N rate of about 156 kg of N ha^-1 (Ewing and Runck, 2015). Meanwhile, there are several studies have suggested that applying fertilizer rates ranging from 0 to 101 kg of N ha^-1 can increase corn yield, but this increase levels off at 101 kg of N ha^-1 (McSwiney and Robertson, 2005; Hejazi and Soleymani, 2014; Biswas and Ma, 2016).

In this study, we found that N uptake increased with higher rates of fertilizer which agreed with previous studies (Amado et al., 2013; Biswas and Ma, 2016; Davies et al., 2020). However, our study also supports the claim that higher N uptake does not necessarily lead to increased biomass production (Anas et al., 2020).

In our study, AS100 and AS200 treatments were considered to have high UE, while Compost was below the typical range according to the NUE benchmarking for corn silage (Augarten et al., 2019). For AE, AS100 had the highest value, followed by AS200, with Compost having the lowest value. This indicates that nitrogen applied under the AS100 treatment improved productivity more per unit than the other treatments (Augarten et al., 2019). According to the same study, we also found that PFP under AS100 was in the high-use efficiency range, while Compost and AS200 were in the low-use efficiency range (Augarten et al., 2019). The results from this study illustrate that the AS100 treatment outperformed the AS200 treatment in terms of NUE. This finding supports previous studies indicating that higher application rates of AS fertilizer led to a decrease in AE and PFP (Amado et al., 2013; Chen et al., 2018; Boulelouah et al., 2022). The lower AE and PFP under compost treatment likely reflects the limited availability of nitrogen in this organic material and the slow release of nitrogen from compost fertilizer.

The AS100 treatment showed the highest PNB value, slightly exceeding 1. This increase in PNB above 1, as observed in Augarten et al.’s research (2019), indicates potential soil organic matter mining, where more N is removed in the crop than applied. However, it is noteworthy that the PNB value for AS100 remains within the acceptable range of high low-to-mid use efficiency (0.92 < PNB < 1.08). In contrast, the PNB value for AS200 treatment (PNB=0.79) falls within the range of low use efficiency, indicating that more N is being applied than removed by the crop (Augarten et al., 2019). A PNB value less than 1 signifies N surplus and can lead to potential nitrogen losses such as volatilization and leaching (Fageria and Baligar, 2005; Andrews et al., 2018). Therefore, reductions in application N may be necessary. Compost treatment had an extremely low PNB value (PNB=0.38) indicating that a considerable amount of N was being retained in the soil but unavailable for plant uptake due to slow N mineralization or even immobilization (Fageria and Baligar, 2005; Andrews et al., 2018).

The yield under compost treatment demonstrated a significant increase relative to the control. This finding contrasts with that of Lin et al. (2022), who observed that the yield of corn under the compost treatment was not significantly different from control treatment. The duration of the experiment can affect the accuracy of the results, and in this regard, the study conducted by Lin et al. (2022) spanned only two growing seasons. In contrast, our study continued for nine years (2012-2021), providing more comprehensive data to evaluate the impact of different N source treatments on crop yield. The limited duration of that study experiment may have contributed to the absence of significant differences in yield between the organic fertilizer and control treatments reported in their study (Lin et al., 2022). It is well-known that the yield of corn can be influenced by the growing season (Biswas and Ma, 2016), and the response to nitrogen fertilizer treatments can also vary from year to year. These factors could explain why Lin et al. (2022) results differ from ours and highlights the importance of conducting long-term experiments to account for variability in crop growth and nutrient uptake over time.

This study also found that Compost yield remained lower than the average yield observed under AS100 and AS200 ( Figure 3 ), aligning with previous studies (Chivenge et al., 2011; Seufert et al., 2012; Wei et al., 2016). An integrated analysis of long-term experiments conducted by Wei et al. (2016) indicated that despite the application of organic amendments over a decade, organic amendment still produced lower yield compared to chemical fertilizer. The effectiveness of organic amendments in increasing yield is contingent upon several factors, including the quality of organic resources, soil fertility status, farming system, management practices, and site characteristics (Chivenge et al., 2011; Seufert et al., 2012; Wei et al., 2016).

Available N is the major factor that affects crop yield (Berry et al., 2002). Numerous studies have substantiated those organic amendments, such as compost, animal manure, or cover crops, slowly release N that is available to plants, yet they do not provide an adequate N supply to meet the demands of crops during the peak of the growing season (Pang and Letey, 2000; Berry et al., 2002; Seufert et al., 2012). Therefore, while farming systems that exclusively relied on organic amendments have the potential to substantially increase yield, there must be substantial resources accessible. Otherwise, this system may fail to generate enough yield to satisfy food demand and may create nutrient imbalance (Pang and Letey, 2000; Berry et al., 2002; Seufert et al., 2012; Wei et al., 2016). N rate is not the only factor affecting corn yield. Other factors, such as rainfall, irrigation, soil texture and quality, farming management practices, planting date, and environmental conditions throughout the growing season, also significantly affect corn yield variability (Chivenge et al., 2011; Seufert et al., 2012; Wei et al., 2016; Hlisnikovský et al., 2020).

In the current study, Compost treatment did not result in an improvement in NUE compared to the ammonium sulfate treatment. This observation is consistent with the findings of Lin et al. (2022), who reported lower NUE of corn under organic fertilizers compared to chemical fertilizers. Despite using the same quantity of total N in the compost and AS200 treatments, not all of the total N in the compost was readily available for plant uptake, which explains the lack of improvement in NUE and lower yield. Compost is considered a slow-release fertilizer that gradually releases plant-available nutrients over time. Sullivan et al. (2018) reported that within the first year of application, plant-available N released from compost was less than 10% of its total N content (Sullivan et al., 2018). The timing of fertilizer N availability versus plant demand is an especially crucial determinant of maize yields and NUE (Zhu et al., 2025). Additional nutrients in compost may become available over years, although at a slower rate (Geisseler et al., 2021). However, insufficient N supply from compost can lead to a decrease in crop yield, N uptake, and NUE as we observed. The availability of N from the compost applied did not build significantly over the length of the experiment while preseason available P and K were higher in the compost treatment plots for most years (data not shown).

The results from this study showed that the STN response to fertilizer treatments varied by growing season. Although compost did not have a significant impact on STN in many years, it consistently produced numerically higher levels STN. The results from this study underscored the variable response of STN to fertilizer treatments, which is contingent upon the specific growing season under investigation and our ability to detect small absolute changes in STN. These fluctuations in STN levels, influenced by seasonal variations and their intricate interactions with the timing of soil amendments (Turner et al., 2015; Hurisso et al., 2018), posed challenges in discerning the impacts of fertilizers on STN. Future studies should take additional seasonal samples for multiple nitrogen pools and soil health indicators.

Repeated measurements in this experiment (2011-2021) revealed that compost treatment led to an approximately 23% increase in STN compared to other treatments. In contrast, ammonium sulfate treatments did not yield similar improvements, consistent with findings from other (Steiner et al., 2007; Gao et al., 2022). Earlier studies on the same plots investigating various aspects of the soil N cycle also showed that compost treatment enhanced the diversity of microbial communities and promoted N mineralization compared to AS fertilizer treatments (Ouyang, 2016; Ouyang and Norton, 2020).

Although the application of compost significantly increased soil N, the yield, N uptake, and NUE were lower compared to the use of ammonium sulfate fertilizer treatments. These observations suggest that composts with similarly low N availability need to be supplemented with additional available N to maintain yields. Based on the observed yields, the compost N supply was roughly equivalent to 40 kg N/ha (< 20% of total N available) and so yields would be responsive to an additional 60–80 kg N/ha in a readily available form. Preseason soil testing should be used to adjust macronutrient P and K fertilization after compost or manure applications. Similarly, research conducted by Gao et al., 2022, demonstrated that compost fertilizer enhanced STN levels while commercial fertilizer did not (Gao et al., 2022). Our findings are in line with Steiner et al. (2007), which found that organic fertilizers improve soil fertility but do not sustain crop productivity (Steiner et al., 2007). Numerous studies have shown that incorporating both organic and inorganic fertilizers increases yield, STN, and NUE (Li et al., 2012; Ding et al., 2018; Gao et al., 2022). Therefore, farmers may want to consider combining composts, manures and fertilizers for optimum silage corn production.