Fall Fertilizer Research Articles

Distinct driving mechanisms of non-growing season N2O emissions call for spatial-specific mitigation strategies in the US Midwest

Agricultural N2O emission is a growing concern for climate change. Recent field evidence suggests that non-growing seasons (NGS) may contribute one-third to half of the annual N2O emissions, but implications on management adaptations remain unclear. Here we used an advanced process-based model, ecosys, to investigate the magnitude and drivers of NGS N2O emissions from the US Midwest. Results showed that simulated NGS N2O emissions accounted for 6–60% of the annual fluxes under continuous corn systems, peaking in counties with NGS precipitation (PNGS) around 300 mm. Divergent patterns of spatial-temporal correlations between NGS N2O emissions and environmental variables were shown in the southeast (PNGS > 300 mm) and the northwest (PNGS < 300 mm) of the study area by simulations. Causal analysis indicates that more intensive freezing caused by decreased air temperature (Ta) is the dominant driver that leads to NGS N2O emissions increasing within the southeast of the study area, while increased PNGS and increased Ta cooperatively result in soil moisture decreasing at soil thaws that enhances NGS N2O production within the northwest of the study area. Scenario simulations suggest that annual N2O emissions in the US Midwest are likely to reduce under climate change primarily due to the reduction of NGS N2O emissions. Our estimates on monetized social benefits inform the necessity to implement spatial-specific mitigation strategies, i.e. determining fertilizer timing and use of nitrification inhibitors (NI). Spring fertilizer application is more beneficial than fall fertilizer application for most counties, however, the latter can bring extra benefits to some counties in the west of the study area. Introducing NI with either spring or fall applications can greatly increase social benefits by reducing N2O emissions and N leaching. This study addresses possibly effective adaptations by providing seasonal- and spatial-explicit mitigation potentials.

Timing of potassium chloride application effect on soil and potato uptake of chloride

AbstractPotassium chloride (KCl) is often used to supply K for potato (Solanum tuberosum L.) production; it is the least costly K fertilizer. However, crop uptake of Cl can reduce diagnostic petiole NO3–N and reduce tuber specific gravity. This trial evaluated the effect of K fertilizer source (KCl vs. sulfate of potash [SOP] and sulfate of potash magnesia [KMag]) and fertilizer application timing on Cl, S, N, and K in soil, petioles, and whole plant biomass. Fertilizers were applied at 224 kg K2O ha–1 at three application timings: 210 d before planting (fall preplant), 14 d before planting (spring preplant), or 35 d after planting (Layby) to a sandy loam soil in Hermiston, OR. Nitrogen fertilizer was applied by overhead sprinklers during the growing season. Potassium source and timing had minimal effect on crop S, N, and K uptake. Crop Cl response reflected a fertilizer source × timing interaction. Fall‐applied KCl did not increase plant Cl above that observed for KMag and SOP treatments. Spring or summer‐applied KCl increased plant Cl concentrations. The most likely explanation for this response pattern was leaching of Cl below rooting depth with fall fertilizer application. Findings of importance for potato production practices in our region included: (a) fall‐applied Cl was not taken up by the crop, and so did not represent a risk to potato tuber quality; (b) elevated concentrations of petiole Cl from preplant or in‐season KCl application did not affect petiole NO3–N.

Determining Nitrogen Fertilizer Cost Using Turfgrass Response

Numerous nitrogen (N) sources are used in turfgrass management and vary from soluble to slow-release. Determining the least expensive N source can be confusing for consumers. Price per ton and price per pound N are common price comparison methods. An improved approach could use longevity of the N source to balance the price. The objective of this study was to determine the longevity of turfgrass response to N sources and to determine the cost to achieve such responses. This study was conducted in Ft. Lauderdale and Jay, FL, from 1 Jan. to 31 Dec. 2018 on ‘Riley’s Super Sport’ (Celebration®) bermudagrass (Cynodon dactylon). Treatments included nontreated turfgrass, urea, ammonium sulfate, stabilized urea, methylene urea, ureaformaldehyde, two natural organics, sulfur-coated urea, and two polymer-coated urea fertilizers. Treatments were arranged in a split-plot design with N sources as whole plots and N rate (N applied at 49 and 98 kg·ha−1 every 4 months) as subplots. Turf quality was recorded on a scale of 1 to 9, where 1 = dead/brown turf and quality, 6 = minimal acceptable, and 9 = optimal healthy/green turf. Turf quality ratings were recorded weekly and used to determine response longevity (days quality ≥6.0) and area under the turfgrass response curve (AUTRC). Urea resulted in response longevity greater than or equal to other N sources during each season except when applied at 98 kg·ha−1 of N during the fall fertilizer cycle in Jay. Natural organics were ≈6-fold more expensive than urea in Jay and Ft. Lauderdale using turfgrass response longevity and AUTRC. Urea and sulfur-coated urea were the least expensive soluble and slow-release N source, respectively, using dollars per pound N, dollars per acre per day, and dollars per acre per quality-day during each fertilizer cycle and annual average in Jay and Ft. Lauderdale. No evidence was found supporting the use of turfgrass response as a more effective method of determining fertilizer cost than dollars per pound N.

Nutrient Leaching in Soil Affected by Fertilizer Application and Frozen Ground

Core Ideas Preferential flow is prevalent in clay soil under both frozen and thawed conditions. Preferential flow dominates the infiltration regime under frozen soil conditions in silt loam. Subsurface placement of fertilizer can limit subsurface nutrient leaching. Subsurface placement is particularly effective in soil with abundant preferential flow. Subsurface placement is recommended for fall fertilizer application. Agricultural runoff containing P and N from drainage tiles contributes to nutrient loading in waterways, leading to downstream eutrophication. Recent studies suggest that nutrient losses through tile drains can be reduced if nutrients are applied in the subsurface. This study explored interactions between nutrient supply and infiltrating water during a simulated nongrowing season using a laboratory experiment to understand how water and nutrients move through partially frozen and unfrozen soil and if fertilizer placement influences NO3− and dissolved reactive P (DRP) leaching. Intact silt loam and clay soil monoliths (28 by 30 by 30 cm) were fertilized with P and N via subsurface placement or surface broadcast and subjected to simulated rainfall under unfrozen (10°C) and partially frozen (∼0°C) conditions. Conservative tracers (Br−, Cl−, and D2O) applied to characterize subsurface flow paths throughout a subset of events indicated that matrix flow dominated in unfrozen silt loam soil. However, preferential flow paths dominated in unfrozen clay and in both soil types under partially frozen conditions, transporting applied nutrients while minimizing contact with the soil matrix. The subsurface placement of inorganic fertilizer relative to surface broadcast reduced both NO3− (by 26.85 kg ha−1 [23%] in silt loam and 65.73 kg ha−1 [61%] in clay) and DRP losses (by 2.33 kg ha−1 [60%] in silt loam and 4.25 kg ha−1 [64%] in clay). This study demonstrates the advantage of subsurface placement of fertilizer in the reduction of nutrient leaching by limiting the interaction of the nutrient supply with preferential flow pathways.

Impacts of nitrogen application timing and cover crop inclusion on subsurface drainage water quality

Significant reductions in nitrogen loading from sub-surface drainage fields of the Upper Mississippi River Basin to the Gulf of Mexico will most likely be achieved from the mass adoption of nutrient loss reduction strategies at a watershed scale. Few studies have quantified the efficacy of cover crops to reduce NO3-N loading in nitrogen fertilizer management systems, where the dominant portion of the N rate is applied in the spring or fall, both of which are common practices in the Upper Mississippi River Basin. In this experiment we quantified the impact of N application timing and cover crop inclusion on NO3-N loss (leaching) from agricultural sub-surface drainage within five nitrogen management scenarios: a zero control, applying the dominant portion of the N rate in the spring, applying the dominant portion of the N rate in the fall, augmenting the a spring and Fall N application system with cover crop. Each of the five nitrogen management scenarios was replicated three times on individually monitored sub-surface drainage plots established in Lexington, IL. During the experiment, a cereal rye (Secale cereal L.) and radish (Raphanus sativus L.) blend was interseeded within both corn (Zea mays L.) and soybean (Glycine max L.). Fertilizer N application timing did not affect cover crop growth or N uptake. The inclusion of cover crop resulted in more consistent and greater NO3-N loss reductions relative to adjusting fertilizer N application timing from fall to spring. Cover crop reduced the flow-weighted NO3-N concentrations by 39% and 38% and the N load by 40% and 47% when added to spring and fall fertilizer N management systems, respectively. Cover crop proved to be effective in reducing NO3-N loss through sub-surface drainage across the spectrum of N fertilizer management systems common to the Upper Mississippi River Basin.

Characterizing Cereal Rye Biomass and Allometric Relationships across a Range of Fall Available Nitrogen Rates in the Eastern United States

Core Ideas Cereal rye has the capacity for substantial biomass and N accumulation. Cereal rye shoots accumulated roughly 50% of the fertilizer N applied in our study. Cereal rye required 72.4 kg added N ha−1 to reach maximum biomass production. The average maximum biomass was 2853 kg ha−1 at GS25 and 9739 kg ha−1 at GS60. Cereal rye (Secale cereale L.) is widely grown due to its winter hardiness and adaptability to varied soil and environmental conditions. Fall and spring climate and available soil N drive biomass production. However, there is limited empirical information on the effects of these factors on cover crop performance. Farmers need early spring indicators of cereal rye performance to guide management. A 3‐yr experiment was initiated to test and model the effects of climate and soil N fertility on cereal rye growth and allometric relationships in Pennsylvania, Maryland, and North Carolina under five‐six fall fertilizer rates. We hypothesized that allometric relationships between early spring growth indicators can guide management decisions. Measurements included tillering, biomass, tissue N, and normalized difference vegetation index (NDVI) at Zadoks growth stages (GS) 25, 30, and 60. Nitrogen application increased biomass: maximum average biomass was 2853, 4844, and 9739 kg ha−1, respectively, at GS25, GS30, and GS60. At GS25, biomass accounted for the greatest amount of model variation and better predicted GS60 biomass than shoot density and NDVI. Variance attributed solely to GS25 and GS30 biomass constituted 38.5 to 65.2% of total model variance. Models accurately predicted biomass and N accumulation 34 to 60% of the time. This study illustrates the difficulty in predicting late season biomass and N content based on early measurements. One extension of this research would be the development of a simple protocol to accurately sample cereal rye biomass at GS25 to estimate potential N accumulation and biomass at GS60.

Commentary on “A possible trade-off between clean air and clean water” by Smith et al. (2017)

A uthors of the recent feature article by Smith et al., which was published in the A Section of the July/August 2017 issue of the Journal of Soil and Water Conservation (A Section articles are not peer-reviewed while articles in the Research Section of the journal are peer-reviewed), have conducted water quality research in the Western Lake Erie Basin (WLEB) and demonstrated multiple potential causes of increased soluble phosphorous (SP) loading, including agricultural practice changes such as increased no-till, tile drainage, surface application of fall fertilizer, and weather. In their article, “A possible trade-off between clean air and clean water,” these authors propose an additional cause: the connection between the success of the Clean Air Act in improving air quality in the United States and increases in SP loading that have contributed to harmful algal blooms in the WLEB. Although we agree that scientists must always be vigilant for pernicious consequences of well-meaning actions, we believe that there are flaws in the study design and data interpretation that undermine the conclusion of article. Specifically, what we see as flaws in the interpretation of the data presented in table 1 and figures 2 and 3 are described below. Furthermore, the authors do…

Variation in riverine nitrate flux and fall nitrogen fertilizer application in East-central illinois.

In east-central Illinois, fertilizer sales during the past 20 yr suggest that approximately half of the fertilizer nitrogen (N) applied to corn ( L.) occurs in the fall; however, fall fertilizer N sales were greatly reduced in 2009 as wet soil conditions restricted fall fieldwork, including fertilizer N applications. In 2010, we observed unusually low flow-weighted nitrate concentrations (approximately 40% below the long-term average) in two east-central Illinois rivers (5.7 mg N L in the Embarras River and 5.6 mg N L in the Lake Fork of the Kaskaskia River). Using long-term river nitrate data sets (1993-2012 for the Embarras and 1997-2012 for the Kaskaskia), we examined nitrate concentrations and developed regression models to estimate the association between fall fertilizer N application on riverine nitrate yields in these tile-drained watersheds. During these periods of record, annual riverine nitrate yields ranged from 8 to 57 kg N ha yr (30 kg N ha yr average) for the Embarras River and 2.6 to 59 kg N ha yr (32 kg N ha yr average) for the Kaskaskia. Multivariate linear regression relationships with the current and previous year's annual water yields, previous year's corn yield, and nine-county fall fertilizer sales accounted for 96% of the annual variation in nitrate yield in both watersheds. Running the regression models with fall fertilizer sales set to the 2009 amount suggests that the average reduction in nitrate yield (for the period of record) would be 17 and 20% for the Embarras and Kaskaskia Rivers, respectively. These data suggest that shifting fertilizer N application to the spring can be detected in watersheds as large as 481 km.

Combined effects of pre-hardening and fall fertilization on nitrogen translocation and storage in Quercus variabilis seedlings

Maintaining proper seedling nitrogen status is important for outplanting success. Fall fertilization of evergreen conifer seedlings is a well-known technique for averting nitrogen (N) dilution caused by continued seedling growth during hardening. For deciduous seedlings, this technique is much less understood, and regardless of foliage type, the interaction of N status prior to fall fertilization and the rate of fall fertilization have yet to be fully explored. Therefore, we fertilized Quercus variabilis container seedlings with either 25, 100, or 150 mg total N seedling−1, applied exponentially, during a 23-week pre-hardening regime, followed by either 0, 12, or 24 mg total N seedling−1 applied during hardening (i.e., fall fertilization) in equal aliquots for 4 weeks. For seedlings without supplemental N during hardening, N concentration in stems and roots increased significantly despite substantial growth. The absence of N dilution was attributed to N translocation from foliage to these tissues, which was independent of pre-hardening N status. Overall, 32 % of foliar N was translocated and accounted for 75 % of the total N increase in stems and roots. Final stem N status was a function of pre-hardening fertilization, whereas root N concentration was affected by the interaction of pre-hardening and fall fertilization. Roots appear to be the main site of N storage, and root N content was significantly affected by pre-hardening and fall fertilization, but not their interaction. A combination of pre-hardening and fall fertilizer at a rate of 100 and 24 mg total N seedling−1, respectively, yielded seedlings with the largest root systems.

Fertilizer Application Has No Effect on Large (Digitaria sanguinalis) or Smooth (Digitaria ischaemum) Crabgrass Germination and Emergence in Residential Turfgrass in a Northern Climate

Given the importance of emergence level and timing to the competitiveness and success of annual crabgrass species in turfgrass, particularly in the context of increasing synthetic pesticide bans and the common cultural practice of fertilization, a study was conducted in a northern region of North America (Ontario, Canada) to determine the effect of fertilizer application on large and smooth crabgrass emergence in residential lawns. In petri dish experiments, we reconfirmed that KNO3has a significant positive effect on large and smooth crabgrass seed germination but we showed that there is only an effect on fresh seed and no effect on aged seed, suggesting that the treatment affects dormancy level and not germination per se. In two other experiments using turf cores and commercial lawn fertilizer in growth room conditions and in field trials at three sites, we confirmed this result showing that neither fall nor spring fertilizer application had any effect on the emergence level of either smooth or large crabgrass. These results have practical relevance to homeowners and turf managers in this region because they are dealing with crabgrass emerging in the spring from seed shed the previous fall. The results also show that fertilizer can be used to aid turf quality and competitiveness without impacting true infestation level (density) of crabgrass in the spring.

Nitrate–nitrogen and oxygen isotope ratios for identification of nitrate sources and dominant nitrogen cycle processes in a tile-drained dryland agricultural field

Agricultural systems are a leading source of reactive nitrogen to aquatic and atmospheric ecosystems. In this study environmental δ15Nnitrate and δ18Onitrate are used to identify the dominant nitrogen cycle processes and sources of NO3− leached from a tile-drained, dryland agricultural field. Tile-drain water discharge δ18Onitrate values suggest nitrification is the dominant soil nitrogen cycle process throughout the 5-year study period, because the expected δ18Onitrate from nitrification is indistinguishable from the measured value of −1.3 ± 1.5‰. Given this there is no evidence that denitrification was occurring at a large enough scale to influence [NO3−]. Values for δ15Nnitrate varied seasonally during the high-discharge season (January through May) and low-discharge season (June through December) with weighted means of 1.0 ± 1.0‰ and 4.7 ± 2.3‰ respectively. This suggests that during the high-discharge season NO3− originates from nitrification of NH4+ fertilizer, and during the low-discharge season NO3− originates from mineralized soil organic nitrogen. The estimated travel time through the soil for nitrified NH4+ fertilizer leached during the high-discharge season is less than 6 months, from fall fertilizer application to leaching through the tile-drain. This study suggests that understanding the hydrology of a region is necessary before dominant nitrogen cycling processes can be reliably determined.

Water Quality Modeling of Fertilizer Management Impacts on Nitrate Losses in Tile Drains at the Field Scale

Nitrate losses from subsurface tile drained row cropland in the Upper Midwest U.S. contribute to hypoxia in the Gulf of Mexico. Strategies are needed to reduce nitrate losses to the Mississippi River. This paper evaluates the effect of fertilizer rate and timing on nitrate losses in two (East and West) commercial row crop fields located in south-central Minnesota. The Agricultural Drainage and Pesticide Transport (ADAPT) model was calibrated and validated for monthly subsurface tile drain flow and nitrate losses for a period of 1999-2003. Good agreement was found between observed and predicted tile drain flow and nitrate losses during the calibration period, with Nash-Sutcliffe modeling efficiencies of 0.75 and 0.56, respectively. Better agreements were observed for the validation period. The calibrated model was then used to evaluate the effects of rate and timing of fertilizer application on nitrate losses with a 50-yr climatic record (1954-2003). Significant reductions in nitrate losses were predicted by reducing fertilizer application rates and changing timing. A 13% reduction in nitrate losses was predicted when fall fertilizer application rate was reduced from 180 to 123 kg/ha. A further 9% reduction in nitrate losses can be achieved when switching from fall to spring application. Larger reductions in nitrate losses would require changes in fertilizer rate and timing, as well as other practices such as changing tile drain spacings and/or depths, fall cover cropping, or conversion of crop land to pasture.

Fall Fertilization Timing Effects on Nitrate Leaching and Turfgrass Color and Growth

Fall season fertilization is a widely recommended practice for turfgrass. Fertilizer applied in the fall, however, may be subject to substantial leaching losses. A field study was conducted in Connecticut to determine the timing effects of fall fertilization on nitrate N (NO3-N) leaching, turf color, shoot density, and root mass of a 90% Kentucky bluegrass (Poa pratensis L.), 10% creeping red fescue (Festuca rubra L.) lawn. Treatments consisted of the date of fall fertilization: 15 September, 15 October, 15 November, 15 December, or control which received no fall fertilizer. Percolate water was collected weekly with soil monolith lysimeters. Mean log(10) NO3-N concentrations in percolate were higher for fall fertilized treatments than for the control. Mean NO3-N mass collected in percolate water was linearly related to the date of fertilizer application, with higher NO3-N loss for later application dates. Applying fall fertilizer improved turf color and density but there were no differences in color or density among applications made between 15 October and 15 December. These findings suggest that the current recommendation of applying N in mid- to late November in southern New England may not be compatible with water quality goals.

Effect of Fall Fertilization on Freeze Hardiness of Deciduous versus Evergreen Azaleas

This experiment compared the effect of fall fertilization on freeze hardiness of evergreen vs. deciduous azaleas (Rhododendron). Beginning in Spring 2003, a 2 × 3 factorial experiment was conducted in Athens, Ga., on container plants grown outdoors under nursery conditions involving two taxa (R. canescens and R. ×satsuki `Wakaebisu') and three fall fertigation regimes (Aug.–Sept., 75 mg·L-1 of N; Aug.–Nov., 75 mg·L-1 of N; and Aug.–Nov., 125 mg·L-1 of N). On 15 Nov. and 17 Dec. 2003 and 16 Jan., 18 Feb., and 19 Mar. 2004, plant stem tissue was harvested and exposed to 10 progressively lower temperature intervals between –3 °C and –30 °C under laboratory conditions in order to estimate azalea freeze hardiness. Freeze hardiness was affected by fertilizer and taxa treatments, but there were no significant interaction effects in this study. The timing of freeze hardening was not significantly different among the two species over time, and the fall fertilizer treatments did not affect the timing of hardening. Compared to the industry standard (75 mg·L-1 of N, Aug.–Sept.), R. canescens that received extended fertilization at the high rate (125 mg·L-1 of N, Aug.–Nov.) was less freeze hardy in November, December, and January, and R. ×satsuki was less freeze hardy in December. However, when compared to the industry standard, the low rate of extended fertilization (75 mg·L-1 of N, Aug.–Nov.) did not affect azalea freeze hardiness.

A SURVEY OF CROP RESIDUE BURNING PRACTICES IN MANITOBA

Crop residue burning has become a concern in the Prairies, Canada due to its adverse impact on human health,the environment, and soil quality. A telephone survey was conducted in 2001 to investigate crop residue burning situationson farms in four rural municipalities of Manitoba, Canada. The survey questionnaire included 45 questions developed toidentify the types and the percentage of producers who used burning as a crop residue management practice. Of the 84 eligiblerespondents, 47% practiced or possibly practiced crop residue burning. The motivating factors included the timeliness of fieldoperations, such as fall tillage, fall fertilizer application and spring seeding, lower cost for residue disposal, increased cropyield, and better control of weeds and crop diseases. The survey also selectively gathered information on producers and farmsbackground, crop, field equipment and farming practices, as these factors were expected to have impacts on the choice ofcrop residue management practices. Data show likelihood of reducing burning when producers practice longer crop rotations,use low disturbance disc-type seeding tools, and apply fertilizer in the spring. Education and awareness that leads producersaway from the traditional crop residue burning practices may be started for young producers and large farms.

Late Fall and Winter Nitrogen Fertilization of Turfgrass in Two Pacific Northwest Climates

Late fall N fertilization of cool-season turfgrass in northern climates is a common practice. Previous research has been focused in climates where freezing temperatures prevail. Research in more moderate northern climates where turf may not go through winter dormancy is scarce. Four fertilizer N sources and an untreated control were applied in four different months (November, December, January, or February) to perennial ryegrass (Lolium perenne L.) in Puyallup, Wash., and to kentucky bluegrass (Poa pratensis L.) In Pullman, Wash., to compare their effects in moderate (Puyallup) and freezing (Pullman) winter climates. In Pullman, only November applications of ammonium sulfate (AmS) or polymer coated sulfur coated urea (PCSCU) enhanced winter turfgrass quality. In Puyallup, November or December application of AmS, PCSCU, or polymer coated urea (PCU) resulted in enhanced winter quality. Polymer coated urea yielded a delayed initial response and a longer residual effect in the spring. Isobutylidenediurea (IBDU) did not improve winter turf quality in either Pullman or Puyallup. Although there was no quality response following January fertilizer application, there was suppression of red thread [Laetisaria fuciformis (McAlpine) Burds.] symptoms in Puyallup, indicating N uptake. Late fall fertilizer N in eastern Washington should be confined to November, using soluble or more quickly available slow-release nitrogen fertilizers. The application window can be extended to December in western Washington, and more slowly available coated ureas can be effectively used.

Effects of Fall Fertilizer Applications on Mitotic Index and Bud Dormancy of Loblolly Pine Seedlings

Abstract A series of studies examined the effects of fall fertilization with diammonium phosphate (DAP) on mitotic index and bud dormancy [as measured by mean days to budbreak (DBB)] of two half-sib seed sources of loblolly pine. The first study tested different rates of DAP (0, 67, and 202 kg N/ha), the second study compared DAP with ammonium nitrate, and the third study examined the effect of different application dates (September 28, October 19, and November 9). An increase in mitotic index of unfertilized seedlings was observed during October and was due to developmental activity which follows initial budset. Differences in mitotic index were observed between families in all three studies. Overall, the Georgia family has a higher mitotic index, but in one study, the Virginia family had higher values in the spring. Both families tended to reach a minimum level of mitotic index at the same time (mid- to late December). However, the Virginia family reached maximum rest (as measured by days to budbreak) about 1 to 2 weeks prior to the Georgia family. Fertilization with DAP in the fall (after budset in September) did not delay the progression of the bud dormancy cycle as measured by days to budbreak in a greenhouse. The overall effect of fall fertilization on increasing the mitotic index was temporary and only lasted for about three weeks after fertilization. These findings indicated that a direct relationship may not exist between the bud dormancy cycle and mitotic index. For. Sci. 38(2):336-349.

Nitrogen Content of Helleri Holly as Influenced by Ambient Temperature and Nitrogen Fertilization Rate

Rooted cuttings of Ilex crenata Thunb. ‘Helleri’ were grown at 3 day/night temperature regimes 18°/14°, 14°/10° and 10°/6°C (64°/57°, 57°/50° and 50°/43°F) and 3 N rates, 10, 20, 80 ppm in a factorial arrangement. Total plant N increased over time at progressively higher rates as both temperature and N rate increased. Nitrogen rate had a greater influence on N accumulation than temperature and based upon the increase in percent N at the various N rates, a recommendation for timing fall fertilizer application is made.

Comparison of Fall and Spring Application of 15N‐Labeled Urea to Douglas‐Fir: I. Growth Response and Nitrogen Levels in Foliage and Soil

AbstractMovement, transformation, and recovery of fertilizer N by analysis of soil and foliage of seven‐ and nine‐year‐old Douglas‐fir were followed over a two‐year period at two relatively high quality sites in western Washington. Urea fertilizer labeled at approximately 8 atom percent 15N was applied at 224 kg N/ha at the end of November 1977 and in late April 1978. High rainfall followed the fall fertilizer treatment and low rainfall occurred after the spring treatment. Differences in variability in fertilizer recovery, fertilizer movement, and tree response to fertilization appeared to be related to differences in rainfall patterns shortly after fertilization. On the average, fall fertilization gave better volume growth response than spring fertilization, but substantial variation in response was observed between sites and season of application.Fertilizer N did not appear in substantial concentrations in foliage until after 6 weeks, with high levels of 15N still evident in foliage after 2 years. With the spring treatment, fertilizer N could have appeared earlier in the newly formed foliage, but this foliage was not sampled until August. The increase in total percent N in foliage lasted only about 1 year following spring treatment and 15 months following fall treatment. Peak concentrations of ammonium N in soil occurred in samples collected 9 d to 3 weeks after fertilization. Ammonium content in the soil declined rapidly and approached control levels at 24 weeks. However, at one site, nitrification was active and relatively high nitrate content was present in the soil at 24 weeks.Recovery of fertilizer N from analysis of soil was variable, averaging 59 ± 10% of the application at 3 d and about 100% between 9 d to 3 weeks. After two growing seasons, recovery from analysis of soil averaged only 38 ± 6%. Substantial increases in mineral N levels were observed in the soil beyond the quantity applied as fertilizer (the priming effect). Data from early samples of foliage showed increased concentration of N possibly resulting from this phenomenon. However, after the middle of the first growing season following treatment, no increased concentration of N in foliage from soil organic matter was evident.