Pushing the limits of fertilizer N recovery in a semiarid climate with improved N management

Introduction and Justification
A common goal of N fertility programs should be to provide for the highest recovery of applied fertilizer N in the targeted crop species consistent with maximum yield and/or economic returns. Fertilizer N is subject to a number of biochemical processes in the N cycle that limit its recovery in crops, including losses due to denitrification (DEN), leaching, and volatilization. In Montana, volatilization of fertilizer N represents the most important pathway of N loss. There are several considerations here, including the fact that urea represents the most common N fertilizer source, and often it is surface applied to soils where it is susceptibility to volatility as ammonia. In contrast fertilizer N loss as a result of DEN and leaching is less problematic. Montana’s climate is semiarid and as such soils are rarely wet and conducive for DEN for prolonged periods. Also, precipitation does not exceed ET so fertilizer N typically does not leach below the crop rooting zone. Immobilization of inorganic N by soil microbes represents another biochemical pathway that impacts fertilizer N recovery by crops. Previous reports on the effect of inorganic N form (NH4 vs. NO3) have frequently found that NH4 is preferentially immobilized compared with NO3 (Recous et al., 1990; Christie and Wasson, 2001; Romero et al., 2015). The preferential absorption of NH4 over NO3 has been attributed to the reduced energy requirements for NH4 assimilation into microbial cells (Murphy et al., 2003), and also to NH4 which can inhibit NO3 assimilation (Rice and Tiedje, 1989).

To improve fertilizer N recovery in semiarid climates such as Montana consideration should be given to use of urease inhibitors and nitrification inhibitors, as well as the inclusion of NO3 sources. Addition of urease inhibitors like Agrotain® to urea have been shown to reduce N volatilization by 66% over untreated urea (Engel et al., 2011). Nitrate production or addition in soils is sometimes viewed as an impediment to achieving high N use efficiency, or fertilizer N recovery by crops. However, utilization of NO3 fertilizer sources may represent a benefit over NH4 containing fertilizers (including urea) in semi-arid climates. Not only are NO3 fertilizer sources not subject to volatility but they are less susceptible to immobilization. Recent dryland field trials in Montana (2011-2015) have consistently shown that fertilizer N recovery by winter wheat is greater from NaNO3 compared to urea (Figure 1). Given this response, opportunities exist to push the limits of fertilizer N recovery by inclusion of NO3 sources, e.g. in soil fertility programs. The overall goal of this research project will be to evaluate the impact of different fertilizer management strategies on fertilizer N recovery in winter wheat, along with yield and grain protein. This past season, we observed FNR up to 60% for winter wheat fertilized with NaNO3 (105 kg N ha-1). We desire to push the limits of fertilizer N recovery even further by inclusion of early spring (spring green-up) and midseason (e.g. stem elongation and/or heading) application timings that are synchronized with crop N uptake. As in previous trials, fertilizer N recovery will be quantified using 15N labelled (5% enrichment) materials. A large field trial is planned at the Montana State University - Central Agricultural Research Center near Moccasin. One or two smaller field trials are planned on private-farms in central Montana.


Figure 1. Nitrogen fertilizer source and timing trials revealed that 15N recovery in winter wheat was greater for NaNO3 compared to urea for both fall subsurface band and spring surface broadcast applications at field site near Moccasin, Montana in 2014-2015. Similar responses were observed in three earlier growing seasons. N rate = 105 kg N ha-1.

Objectives:

1. To compare fertilizer 15N recovery, nitrogen use efficiency, yield and grain protein of winter wheat of winter wheat from several different N sources including urea + inhibitors (urease and nitrification), urea, ammonium nitrate (or Ca-ammonium nitrate, CAN-27), Ca(NO3)2 or NaNO3 applications, and utilizing different application timing strategies.
2. To push the limits of fertilizer N recovery by utilization or consideration of different fertilizer N strategies, including use of nitrate fertilizer sources which are less susceptible to immobilization than ammonium or urea sources, and urease and nitrification inhibitors which are less susceptible to volatilization and leaching.

Experiment location and design
Winter wheat field trials will be established on a dryland field site at the MSU-Central Agricultural Research Center (CARC) near Moccasin (47° 3'21.64"N; 109°57'43.50"W), and on private farms near Coffee Creek and/or Highwood, Montana. The field sites will be conducted under no-till management and at sites that are expected to be responsive to N based on soil NO3-N analyses.
The fertilizer treatments described in Table 1 will be applied to the trial at CARC. This field experiment is being conducted on barley stubble. Fertilizer treatments described in Table 2 will be applied to the trials conducted at private farms. The field sites selected at private farms are under a fallow-wheat rotation. All field sites are under no-till management

Table 1. Proposed treatments to be applied to winter wheat at CARC. 2015-2016. Five replications.
Treatment	Place	N source		Timing and rate (kg N ha-1)
				Fall	Spring	Stem elong	Heading
1	-	Control		-	-	-	-
2*	sub-surface band	Urea – fall band		50	-	-	-
3*				100	-	-	-
4				150	-	-	-
5*	broad-cast	Urea		-	50	-	-
6*				-	100	-	-
7				-	150	-	-
8*	broad-cast	NaNO3 or Ca(NO3)2		-	50	-	-
9*				-	100	-	-
10				-	150	-	-
11*	broad-cast	Urea + UNI†		-	50	-	-
12*				-	100	-	-
13				-	150	-	-
14*	broad-cast	NH4NO3 or CAN-27‡		-	50	-	-
15*				-	100	-	-
16				-	150	-	-
17*	broad-cast	NaNO3 or CAN-17		-	50	50
18*				-	50	25	25
† UNI = Agrotain® or NBPT + Instinct II ‡ Yara Bela® - CAN 27 or dolomite + ammonium nitrate
* Includes an 15N fertilizer micro-plot

Parameters to be measured
Yield and grain protein measurements will be collected at all field plots. Fertilizer N recovery, straw yield and total above ground biomass will be applied to treatments that contain 15N microplots. Fertilizer N recovery will be estimated from plant samples collected at mid-season at that maturity (straw and grain) from 15N enriched (5%) fertilizer microplots (1.5 x 1.5 m). Utilization of 15N enriched fertilizer provides the most reliable method for estimating the portion of applied fertilizer that is recovered in the crop biomass. The primary limitation to 15N fertilizer use is related to its high cost. Urea, ammonium nitrate, Ca(NO3)2 and NaNO3 (15N-5% enrichment) are currently priced in the range of $7.90 to $8.00 per gram of N. Approximately 1500 grams of urea, 800 g NH4NO3, and 3000 g of NaNO3 or Ca(NO3)2 are needed for the proposed study (CARC site + 1 farm site only). Post-harvest and mid-season soil samples will be collected from 15N enriched microplots in order to follow fertilizer N fate and cycling during the growing season. Soil analysis of 15N micro-plots will be conducted to include organic 15N and/or plant labile 15N.

Expected results
We expect to find different in fertilizer N recovery among the applied treatments, as well as yield and protein responses to N. Given our belief that preferential immobilization of NH4 over NO3 is important to fertilizer N cycling, the FNR recoveries are predicted to higher for the spring applied nitrate sources (in particular the split application) compared to urea. Also, inclusion of urease inhibitors with urea may result in higher fertilizer N recover relative to untreated urea for spring surface applications that are susceptible to volatility.

Table 2. Proposed spring N fertilizer (broadcast) treatments to be applied to winter wheat on private farms near Coffee Creek and Highwood, Montana. 2015-2016. Four replications.
Treatment		N source			Timing and rate (kg N ha-1)
					Spring	Stem Elong	Heading
1		Control			-	-	-
2		Urea+UNI			50	-	-
3*					100	-	-
4					150	-	-
4		Urea			50	-	-
5*					100	-	-
6					150	-	-
7		NaNO3 or CAN-17			50	-	-
8*					100	-	-
9					150	-	-
10		NH4NO3 or CAN-27			50	-	-
11*					100	-	-
12					150	-	-
13*		NaNO3 or CAN-17			50	50
14*					50	25	25

* Includes an 15N fertilizer micro-plot

References
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Romero, C.M., R.E. Engel, C. Chen, R. Wallander. 2015. Microbial immobilization of nitrogen-15 labelled ammonium and nitrate in an agricultural soil. Soil Sci. Soc. Am. J. 79:595–602. doi:10.2136/sssaj2014.08.0332.