Predicting the soil-available nitrogen (N) to grain corn over a growing season in humid temperate regions is the key for improving fertilizer N recommendations. The objective of this study was to evaluate a suite of soil-N tests to predict soil N availability to grain corn over two growing seasons at 13 individual sites with long-term history of synthetic N fertilization in Ontario, Canada (13 site-years). At each site, fertilizer N was applied at various rates (0–224 kg N·ha−1) to determine the crop response to N fertilizer, relative yield (RY), and the most economic rate of N (MERN). Across the entire dataset, water-extractable mineral N (WEMN) was the only soil test that strongly correlated to both RY (r = 0.74**) and MERN (r = −0.56*) indicating that in grain corn fields with long-term history of N fertilization, mineral forms of N in soil solution can be used for fertilizer N recommendations in southern and eastern Ontario. We also provide evidence that grouping soils based on clay content could further refine fertilizer-N recommendations for grain corn in Ontario. A multi-year validation of the WEMN test with more field sites and development of a fertilizer recommendation table for this soil test are recommended.
Introduction
Accurately predicting plant-available nitrogen (N) during the growing season is essential for enhancing the sustainability of grain corn (Zea mays L.) production (Ransom et al. 2020). However, predicting corn-available N is complicated due to interactions among drivers of the soil N cycle such as precipitation, soil moisture and temperature, soil properties, crop management history, and current management practices (Morris et al. 2018). Currently, N fertilizer recommendations for corn in Ontario are mostly based on the pre-plant nitrate test (PPNT), or the corn N calculator (GOCorn.net 2010) requiring the yield goal, soil texture, previous crop, and market prices for corn and N fertilizer data. The quantity of labile N fractions and their relative importance (Wu et al. 2008; Zebarth et al. 2009; Luce et al. 2011; Osterholz et al. 2016) in suppling N to grain corn in Ontario soils with long-term history of N fertilization has not been studied. A more reliable soil N test for grain corn can result in more accurate N recommendations and less risk of adverse environmental impacts (Sharifi et al. 2007b; Luce et al. 2011; Morris et al. 2018).
The PPNT is the pre-plant soil nitrate-N (NO3-N) concentration in 0–15 cm soil depth, which can be a useful soil-N test for adjusting fertilizer recommendations based on carryover of NO3-N from previous growing seasons and early season N mineralization (Sharifi et al. 2007b; Ransom et al. 2020). However, the reliability of the PPNT in humid temperate regions has been questioned due to the high mobility of NO3-N in soil (Sharifi et al. 2007b; Luce et al. 2011). Another limitation of the PPNT is its inability to predict the amount of mineralizable soil organic-N, which represents a portion of potential soil N supply (SNS) for corn in humid temperate climates (Wu et al. 2008; Zebarth et al. 2009; Whalen et al. 2013). One alternative to the PPNT is the pre-side-dress NO3-N test (PSNT). This soil test has gained popularity in northeastern USA and eastern Canada because it accurately determines NO3-N levels when the corn plant is at the V6 stage just prior to the highest rate of N uptake, enabling timely N fertilizer adjustments (Fox et al. 1989). The PSNT has been shown to predict fertilizer N needs for corn over a wide geographic range (Magdoff et al. 1984; Blackmer et al. 1989; Magdoff 1991; Ransom et al. 2020). Although the PSNT has shown greater accuracy in predicting crop N requirements compared to PPNT, the need for producers to have access to side-dressing equipment, soil sampling during the growing season in a manner to capture soil NO3-N spatial variability and potential changes in soil NO3-N concentrations over a short period of time has hindered its widespread use (Beauchamp et al. 2004; Ma et al. 2007). The limitations of the PSNT justifies examination of pre-plant soil-N indicators that account for both pre-plant-available N and organic N that mineralizes to become plant available during the growing season. A more in depth review of N rate recommendation methods for corn was presented by Morris et al. (2018).
In the humid temperate region that characterizes eastern Canada, various laboratory and field-based methods have been tested for assessing the contribution of N mineralization to crop N uptake (Sharifi et al. 2007b; Nyiraneza et al. 2009; Sharifi et al. 2009; Nyiraneza et al. 2012; Luce et al. 2014; Thomas et al. 2016b). Recent developments for predicting the contribution of mineralizable soil N to crop N uptake have focused on measuring readily mineralizable N, including a biologically active fraction of soil organic-N (pool I) (Sharifi et al. 2007a), water-extractable C and N (Luce et al. 2014; Thomas et al. 2016a, 2016b), and particulate organic matter C (POMC) and N (POMN) (Sharifi et al. 2008; Luce et al. 2014; Thomas et al. 2016b). Pool I is the initial flush of N produced in the laboratory by drying and rewetting soil in the first 2 wk of incubation at 25 °C (Sharifi et al. 2007a). The water-extractable organic C and N (WEOC and WEON) were considered by Haynes (2000) to be the most dynamic and bioavailable fractions of soil organic matter and have been used as indicators of plant-available N (Zsolnay 2003; Haney et al. 2012; Thomas et al. 2016b). The POMC and POMN are composed of partially decomposed plant residues and organic amendments, representing a transient pool of physically uncomplexed organic matter that undergoes decomposition and mineralization processes to supply plant-available N (Gregorich et al. 2006; Thomas et al. 2016b).
Although some scientists have found PPNT to be a good predictor of potential corn yield and N uptake (Nyiraneza et al. 2009), others attempted to predict SNS and N fertilizer recommendations for grain corn with soil-N tests that extract biologically active organic-N fractions (Sharifi et al. 2007a; Nyiraneza et al. 2012). Nyiraneza et al. (2012) reported that UV absorbance of a 0.01 mol·L−1 NaHCO3 extract at 205 nm, and pool I plus PPNT were the most promising N availability indicators for grain corn (0.28 ≤ r ≤ 0.62) across 25 sites in cold humid temperate regions of Canada. They managed to improve the predictions by grouping the soils based on soil texture. Pool I alone or combined with PPNT were recommended as reliable predictors of available soil N due to their strong correlation with plant N uptake (R2 ≥ 0.42) in some studies (Sharifi et al. 2009). Particulate organic matter C and POMN were also reported to be reliable indicators of soil mineralizable N accounting for 30%–70% of the variation in plant N uptake (Luce et al. 2011; Luce et al. 2014; Thomas et al. 2016b). These indicators have been also used to predict soil-available N in soils that receive high organic matter inputs (Sharifi et al. 2008). Water-extractable C and N have recently received attention for their ability to predict SNS to various crops (Thomas et al. 2016a; Curtin et al. 2017). Yet, the above promising N availability indicators have not been evaluated for grain corn in Ontario.
In this study, grain corn N response trials were conducted in southern and eastern Ontario over two growing seasons, for a total of 13 site-years. The objectives were to evaluate the relationship between the soil-N tests, and relative yield (RY) and the maximum economic rate of nitrogen (MERN) to select the most appropriate soil-N test for grain corn in the major corn growing regions of Ontario. We hypothesized that soil-N tests that include a measure of readily mineralizable organic-N would more accurately predict N fertilizer recommendations for grain corn compared with PPNT or the corn N calculator.
Materials and Methods
Field sites
The study was conducted in 2013 and 2014 on 13 farmers’ fields and (or) university research farms in southern and eastern Ontario, Canada. Site descriptions and cropping history are presented in Table 1. All long-term sites were managed using conventional tillage with synthetic N fertilizers except for the trial at the Trent University Sustainable Agriculture Experimental Farm, which was under organic management for 5 yr prior to establishing the trial. The average growing season air temperature ranged from 15 to 17 °C in 2013 and 14 to 17 °C in 2014, whereas the growing season rainfall ranged from 492 to 748 mm in 2013 and 381 to 585 mm in 2014 (Table 1). The 30 yr mean growing season (May–October) temperature and precipitation for southwestern Ontario are 16 °C and 530 mm, respectively. Soil physical and chemical characteristics are summarized in Table 2.
Table 1.
Summary of the selected experimental sites’ description and management history in Ontario.
Table 2.
Soil physical and chemical characteristics for 13 selected experimental sites in Ontario, Canada (n = 4).
Experimental design
At each field site, four to five N fertilizer rates were applied to plots arranged in a randomized complete block design with four replicates. Plot size was typically 32 m × 16 m. Rates of N fertilizer ranged from 0 to 222 kg N·ha−1, and the N source was urea–ammonium-nitrate or urea (Table 2). The majority of N fertilizers were applied pre-plant, but some sites received “Starter N” through the planter (Table 2). Sites were planted to grain corn (recommended variety and seeding rate for each site according to guidelines provided by the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) or a regional crop advisor) in May to early June of each year with 76.2 cm inter-row spacing. Phosphorous and potassium fertilizers were applied at planting according to soil test and OMAFRA recommendations.
Soil sampling and analyses
Eight soil cores (2.5 cm diameter, 30 cm depth) were collected from each replicated zero-N plot with a soil probe 5 to 10 d prior to planting and fertilizer application. Soil samples were thoroughly mixed and then divided into two subsamples. One subsample was kept moist and stored at 4 °C, whereas the other was air-dried and sieved (<2 mm) until analysis. Soil moisture content was determined by drying a field moist subsample at 105 °C for 24 h. Soil pH was determined in a 1:2 soil to deionized water suspension. Particle size distribution was determined following organic matter removal by the pipette method (Gee and Bauder 1986).
Eight laboratory biological and chemical soil test indicators of the SNS were evaluated (Table 3). Soil mineral N was extracted from field moist soil with 2 mol·L−1 KCl (1:5 soil to extractant ratio), and the NO3-N and ammonium-N (NH4-N) concentrations were determined by colorimetry using the modified indophenol blue technique (Sims et al. 1995) with an Epoch microplate spectrophotometer (BioTek Instruments Inc., Winooski, VT, USA). The soil mineral N at planting is referred to as SMNp. The KCl-extractable NO3-N in spring soil samples is referred to as PPNT. Water extractions were conducted by shaking 4 g air-dried soil in 20 mL of room temperature deionized water for 60 min (Curtin et al. 2006; Chantigny et al. 2009). Water extracts were then centrifuged at 4500g for 20 min, and the supernatant was decanted and analyzed for WEOC using a Schimadzu TOC-VCPH (Schimadzu, Kyoto, Japan) and analyzed for water-extractable mineral N (WEMN) using the modified indophenol blue method as described above. Total water-extractable N (WEN) was determined using the persulfate oxidation method (Cabrera and Beare 1993). The WEON was calculated by subtracting the WEMN from the WEN. For POMC and POMN, 25 g of air-dried soil was dispersed in 100 mL of a 5 g·L−1 sodium hexametaphosphate solution in a 250 mL nalgene bottle by shaking for 16 h, and then dispersed soil was passed through a 53 μm sieve (Gregorich et al. 2003). The retained sand and macro-organic matter were air-dried overnight and then oven-dried at 50 °C for 24 h. The concentration of POMC, POMN, organic C, and total N of each soil was determined following carbonate removal using a CNS analyzer (VarioMAX cube, Elementar Analysensysteme GmbH, Hanau, Germany). Pool I, the flush of readily mineralizable N at the second leaching event 2 wk after the initial time 0 leaching, was measured as described by Thomas et al. (2015).
Table 3.
Mean values (n = 4) for soil nitrogen (N) availability indicators for 13 selected experimental sites in Ontario, Canada.
Corn yield and N uptake
Grain yield was measured in each plot either by hand harvesting an area of at least 6 m2 (two corn rows, 8 m in length) or machine harvesting using a plot or commercial scale combine. Grain yields were reported at an adjusted moisture content of 155 g·kg−1. Corn N uptake was measured by harvesting 10 random plants per plot at the same time as harvest. Harvested corn plants were partitioned into kernels, cobs, and stover, and then oven-dried at 60 °C until constant dry mass was achieved. Tissue samples were finely ground to pass a 1 mm sieve, and the N concentrations were determined by dry combustion with a CNS analyzer (VarioMAX Cube, Elementar Analysensysteme GmbH, Hanau, Germany).
Calculations
The corn N uptake in the zero-N treatment (PNU0N) was calculated as the product of tissue N concentration and dry matter yield for each replicated plot minus any fertilizer N applied at planting as starter N.
Relative yield for each replicate was calculated as follows (Sharifi et al. 2007b):
where GY0N is the grain yield from the plot receiving no N fertilizer, and GYoptimal-N is the grain yield from the highest yielding N fertilizer rate. To estimate the SNS, at corn harvest, soil samples were collected from the zero-N rate treatments (0–30 cm depth) as described above and stored at 4 °C until analysis. The composite soil samples collected at harvest were analyzed for mineral N (SMNh) as described above. The SNS was calculated as sum of the PNU0N and SMNh with an assumed bulk density of 1.1 Mg·m−3.
The corn N response (Dahnke and Olson 1990) for each site was calculated using a quadratic regression equation (McGonigle et al. 1996; Rashid et al. 2004):
where Y is the corn grain yield (kg·ha−1); N is the fertilizer N applied (kg N·ha−1).
The derivative of the quadratic equation was used to determine MERN:
where dY/dN is the price ratio of 1 kg fertilizer to 1 kg of grain corn defined as R below to solve for the MERN:R was determined using the 2014 grain corn and N fertilizer prices: corn price = $0.18·kg−1, N fertilizer price =$1.38·kg−1. The maximum economic yield (MEY) was then calculated using the MERN for each site:
Statistical analyses
All statistical analyses were performed using SAS version 9.2 (SAS Institute Inc. 2011). Data were first tested for normality using the Kolmogorov–Smirnov test and then for outliers. Sites were grouped based on clay content into two groups of ≤240 and >240 g clay·kg−1. The field-based indices of N supply, soil characteristics, and the soil-N test mean values were correlated with RY and MERN using PROC CORR. Correlations were assessed by Pearson’s correlation for the parameters with normal distribution or Spearman’s rank correlation where parameters did not have a normal distribution. Regression analysis was used to determine the associations between the laboratory-based measures of N availability and RY to select the best predictive soil-N test. Stepwise regression (probability of F to enter the model ≤0.05, probability of F to remove from the model ≤0.10) were used in an attempt to improve the relationship between the selected laboratory-based measures of N availability and RY by including soil characteristics.
Results
Soil-N tests
Soil organic C and total N ranged from 14.0 to 35.0 g C·kg−1 and 1.30 to 2.80 g N·kg−1, respectively (Table 2). The SMNp and PPNT values represented 2.8%–18.8% and 5.5%–19% of the total SNS across both years, respectively (Table 3). The WEON concentrations varied among sites, ranging from 26 to 48 mg N·kg−1, representing an average of 1.6% the total soil N. The WEN ranged from 29 to 65 mg N·kg−1 with an average of 76% in the organic-N form. The WEOC values averaged about 1% of the soil organic C, ranging from 156 to 403 mg C·kg−1. The WEOC/WEON ratio was between 5.6:1 and 12.0:1, which is on average 70% of the soil C/N ratio (8.6:1–13.0:1). The POMC ranged from 684 to 6758 mg C·kg−1, and the POMN ranged from 49.5 to 266 mg N·kg−1. On average, POMC and POMN concentrations were 12% of the soil organic C and 6.0% of the soil total N, respectively. The POMC/POMN ratios varied (7.3–33.2), averaging 1.8 times the soil C/N ratio. Pool I ranged from 24 to 60 mg N·kg−1 and represented 1.3%–3% of the total soil N (Table 3). The pool I values represented 56%–197%, and 42%–497% of the SNS in 2013 and 2014, respectively. Soil mineral N at harvest (SMNh) values varied and ranged from 7.2 to 75 kg N·ha−1 (average 24 kg N·ha−1) over the 13 site-years (Table 4).
Table 4.
Mean values (n = 4) for field-based indicators of soil nitrogen (N) supply for 13 selected experimental sites in Ontario in 2013 and 2014.
Corn yield and total N uptake
Corn grain yields ranged from 3.5 to 9.0 Mg·ha−1 in the zero-N plots and 5.6 to 12.8 Mg·ha−1 in the non-limiting N rate plots, with each site exhibiting a strong quadratic response to fertilizer N application (R2 = 0.93–0.99, Table 5). Over both growing seasons, RY ranged from 42% ± 4.9% to 101% ± 7.9%. The MERN values ranged from 103 to 257 kg N·ha−1. The PNU0N ranged from 67 to 126 kg N·ha−1 in 2013 and from 50 to 194 kg N·ha−1 in 2014 (Table 4).
Table 5.
Recommended rate of nitrogen (N) fertilizer for selected experimental sites in 2013 and 2014 using quadratic equations based on corn yield response to fertilizer N rates, and the recommended rate based on the corn N calculator.
Soil test correlation
A strong relationship between field-based indices of N availability (i.e., MERN, grain corn yield in zero-N fertilized plots, PNU, and SNS) and RY was observed (Table 6; r = 0.60–0.78). Although the relationships were improved when sites with clay >240 g·kg−1 were excluded (Table 6), the correlation result after grouping based on clay content was not reliable for sites with clay >240 g·kg−1 due to the low number of sites (n = 5). The soil N availability indicators were correlated to RY and MERN (Table 6). Only the indicators that had a significant correlation with RY were considered as reliable soil N tests, and their relationship with MERN was then assessed for fertilizer recommendations. The significant correlation between an indicator and RY confirms that variation in corn RY as a result of N availability in soil can be predicted by the indicator. Relative yield was positively correlated to only WEMN (r = 0.74) among tested indicators (Table 6 and Fig. 1). Among the evaluated laboratory-based indices of N availability only WEMN was significantly correlated with both RY and MERN (r = −0.56). A wider range of WEMN, RY, and MERN were observed in sites with clay ≤240 g·kg−1 than sites with greater clay content. Inclusion of soil properties in the relationship between WEMN and RY or MERN using stepwise regression did not result in any improvement (data not shown).
Table 6.
Correlation coefficients (r) of soil nitrogen (N) availability indicators with relative yield (RY) and maximum economic rate of N (MERN) at 13 selected experimental sites in Ontario in 2013 and 2014.
Discussion
Soil mineral N at planting and harvest is highly variable across Ontario soils
The wide range of potentially mineralizable N observed in the soils from the selected field sites was probably due to interactions among the broad range of soil properties, cropping history and differences in rainfall and temperature across southern and eastern Ontario. Using an assumed bulk density for all sites (1.1 Mg·m−3), the SMNp in this study is estimated to have contributed similar amounts to the SNS as observed in different studies for grain corn in eastern Ontario and Quebec that showed the mineral N at planting represented 16% to 27% of the SNS (Ma et al. 2007; Wu et al. 2008; Nyiraneza et al. 2009). This suggests that more than 70% of corn N uptake may derive from soil organic-N mineralization during the growing season in non-fertilized soils.
A wide range in SMNh was also observed, but there was no consistent pattern across sites. The variation in SMNh has been related to differences in precipitation or irrigation during the growing season (Jokela and Randall 1989), soil properties, and management practices (Rasouli et al. 2014). Similar to SMNp, the majority of SMNh was NO3-N. In Ontario, between 1981 and 2006, an average of 57 kg NO3-N·ha−1 remained in agricultural soils at harvest (De Jong et al. 2009). High residual NO3-N at harvest indicates asynchrony between SNS and crop N demand, which may result in N losses over the fall, winter, and spring months (Power et al. 1998; Dinnes et al. 2002; Whalen et al. 2019). Ultimately, limiting the amount of residual soil NO3-N at harvest minimizes potential N losses to the surrounding environment during the non-growing season (Rasouli et al. 2014).
With the relatively low price of N fertilizer in 2014, it was economical to apply higher rates of N for small gains in grain yield. The MERN values were comparable to the range reported by OMAFRA (107–237 kg N·ha−1) for corn response trials in southwestern Ontario in 2013 (GOCorn.net 2010). Other factors that impact N fertilizer recommendations include cropping history (Luce et al. 2011), soil properties (Dharmakeerthi et al. 2005; Subbarao et al. 2006), and soil moisture and temperature (Dessureault-Rompré et al. 2011). For example, in this study, Pinkerton and Teeswater sites had high soil organic C and total N, which probably contributed to a greater SNS (202–269 kg N·ha−1) compared with other field sites. Soils with greater soil organic C concentration have been shown to generally have a greater soil water-holding capacity (Manns et al. 2016), which may have also increased C and N mineralization rates and reduced the likelihood that water stress would limit crop performance.
Labile organic carbon and nitrogen fractions are used for interpretation of variations in nitrogen availability to corn
The WEOC concentrations were comparable to those reported for soils cropped to corn and corn–soybean (Glycine max L.) rotations in Ontario, Canada (Gregorich et al. 2003), and to soil under a corn–soybean–wheat (Triticum aestivum L.) rotation near Quebec City, QC, Canada (Thomas et al. 2016a). The mean proportion of WEON to total N (1.6%) was smaller than range (2.6%–8.7 %) reported for 30 New Zealand soils (Curtin et al. 2006); however, the mean WEOC to WEON ratio (7.7) was numerically lower than some previous studies (Gregorich et al. 2003; Curtin et al. 2006; Haney et al. 2012), or similar to sandy loam soils (6.7 ± 1.0) fertilized with calcium–ammonium-nitrate fertilizer in Quebec (Thomas et al. 2016b). The high proportion of total N as WEON indicates that these soils have a high supply of soluble organic-N compounds, containing biologically available forms of organic-N (Herbert and Bertsch 1995) and therefore may be an important N source for soil organisms and field crops at the selected field sites when they are not fertilized or are under-fertilized. Furthermore, the lower WEOC/WEON ratio may be the result of long-term inorganic-N fertilization without supplemental organic amendment application leading to mining of soil C and production of N rich water-soluble microbial byproducts.
The wide range of POMC and POMN values may be attributed to the differences in cropping history (Griffin and Porter 2004; Haynes 2005), or soil properties such as soil texture, given that previous work showed a silty clay soil had 84% greater POMC concentrations than a sandy loam soils in Quebec, Canada (Thomas et al. 2016a). The POMN values were lowest where fields were under continuous corn (e.g., AAFC-Ottawa, Woodslee and Elora Research Station). The proportion of total N as POMN was within the range reported in the literature (Gregorich et al. 2006; Sharifi et al. 2007a), and the high C/N ratio of this fraction is a characteristic of soils receiving plant residues as the sole source of organic residue (Luce et al. 2011; Sequeira and Alley 2011). The wide range in pool I to SNS ratio, expressed as a percentage, indicates that the sites used in this study had contrasting amounts of readily mineralizable N.
Soil clay content decreases nitrogen availability to corn
Soils with clay content >240 g·kg−1 showed weaker relationships between the soil-N tests and RY than soils with clay content ≤240 g·kg−1. These results are consistent with other work that has found clay content explained a substantial proportion of the variation in soil N mineralization (Dessureault-Rompré et al. 2011; Nyiraneza et al. 2012; Villar et al. 2014). Soil mineral N parameters have been related to potentially mineralizable N (N0) in coarse-textured soils (>300 g sand·kg−1; match with ≤240 g clay·kg−1 in this study except for one site) but not fine-textured soils (<300 g sand·kg−1; match with >240 g clay·kg−1 in this study except for two sites), whereas total N was related to N0 in fine-textured soils (Nyiraneza et al. 2012). The higher C content in soils with greater clay content can be attributed to the clay particles physically protecting organic matter from microbial decomposition through physio-chemical interactions and formation of aggregates (Jenkinson 1988; Angers et al. 1997; Six et al. 1999; Kölbl et al. 2006; Yoo and Wander 2006; Chivenge et al. 2011; Nyiraneza et al. 2012). The clay particles also may limit NH4-N availability for oxidation and nitrification reactions by binding NH4-N at negatively charged exchanges sites (Drury et al. 1989; Nieder et al. 2011).
Water-extractable mineral nitrogen is a strong nitrogen availability indicator for corn
Overall, laboratory-based soil-N tests that extracted readily available forms of soil N (e.g., WEMN, SMNp, and PPNT) outperformed the indices that were associated with organic forms of N in soil (e.g., total N, POMN, and WEON) or the C-based indicators (e.g., WEOC, WEOC/N, and POMC/N). The organic-based C and N indicators consist of a combination of readily available and recalcitrant C and N, which may explain their weaker performance.
The PPNT has already been calibrated for corn in Ontario (OMAFRA 2009). Although some corn producers are using precision agriculture practices; others shifted towards using expected yields or visual observations of N deficiency/sufficiency to help predict their fertilizer N rates (O’Halloran et al. 2004). This is due to the large in-field variability that requires a high number of soil samples to be collected per hectare. Furthermore, the PPNT has shown varying success as a predictor of N availability as it is highly dependent on early-season rainfall, which may result in substantial losses due to leaching between the time of sample collection and start of maximum crop N uptake (Sharifi et al. 2009).
The WEMN is the N form that is readily available in the soil solution with a consistent positive correlation with RY and negative correlation with MERN among soil textures. The high importance of WEMN in supplying N to grain corn can be attributed to long-term history of N fertilization in this region. The WEMN represented an average of 22% of the WEN, which is comparable to the 15% found for soils under unfertilized corn monoculture, and the 20% for soils under a corn–soybean rotation receiving mineral fertilizer (Gregorich et al. 2003). Literature on WEN is rare as most studies report only the organic portion (Curtin et al. 2006; Haney et al. 2012; Luce et al. 2014; Thomas et al. 2016a). The WEON is hypothesized to contain mobile forms of bioavailable organic-N that is the by-product of microbial decomposition of crop residues and organic amendments (Murphy et al. 2000; Gregorich et al. 2003). The composition and, therefore, biodegradability of the WEON pool is important as this pool can also be composed of recalcitrant compounds that are resistant to further microbial decomposition (Smolander et al. 1995; Gregorich et al. 2003; Wander 2004).
It is apparent that the WEMN is a reliable index of the SNS for corn in soils primarily fertilized with inorganic-N sources. However, similar to PPNT, WEMN is highly mobile in soil, may show great in-field variability and can be affected by early season rainfall. Our findings reject our hypothesis; therefore, labile organic-N fractions were not significant predictors of soil-available N to grain corn in Ontario. However, our findings suggest that WEMN outperformed PPNT and corn N calculator methods in predicting soil-available N (Tables 5 and 6). Future research may focus on readily available pools of N in the main soil texture classes to develop regional-based indicators for N availability under field conditions.
Conclusion
The mineral N in the soil at planting (SMNp) was estimated to represent about 30% of the SNS available to a corn crop during the growing season in Ontario. Across the entire dataset, WEMN was the only indicator that strongly correlated with both RY and MERN; indicating that in soils with a long-term history of N fertilization, mineral forms of N in soil solution may be used to make fertilizer N recommendations for corn in Ontario. Using WEMN instead of KCl-extractable mineral N, can reduce the cost of analysis while generating more reliable recommendations. However, the variability due to the soil properties and weather conditions will still exist and requires careful attention. Evidence from this research suggests that grouping soils based on soil texture improved predictions of corn-available N in the soils with clay <240 g·kg−1. A multi-year calibration of the WEMN soil test with more field sites and development of a fertilizer recommendation table for this test are recommended.
Acknowledgements
Funding for this project was provided by the Grain Farmers of Ontario (GFO). The majority of the sites selected in this experiment were part of the Ontario Ministry of Agriculture Food and Rural Affairs (OMAFRA) corn response trials. Technical support was provided by Scott Baker and Melissa Johnston. Dr. Mehdi Sharifi’s position at Trent University at the time of this research was funded by Canadian Research Chairs (CRC) program of Canada Foundation for Innovations (CFI) and by Agriculture and Agri-Food Canada during the preparation of this manuscript.
