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Nitrogen Management

Soil Nitrate Concentrations Used to Predict Nitrogen Sufficiency in Relation to Yield in Perennial Grasslands

Dep. of Plant Sci., Univ. of Connecticut, 1376 Storrs Rd., Storrs, CT 06269-4067

^* Corresponding author (rmaugusta{at}juno.com)

Received for publication March 3, 2003.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
A method of evaluating soil N present in a perennial grassland system could help determine the need for additional N and enable more accurate N fertilization recommendations. The purpose of this study was to establish a relationship between a critical level (CL) of soil nitrate N under perennial grassland and optimum yield in a three-harvest system. A range of fertilizer treatments from 0 to 613 kg ha^–1 was applied across two growing seasons to mixed stands of perennial forage grasses at two sites in Connecticut. Soil samples were taken on a weekly basis, and soil ammonium N and nitrate N were measured. Nitrate N was correlated with relative yield for three harvests within each growing season, and CLs of nitrate were generated using Cate–Nelson, linear response plateau, and quadratic response plateau models. Ranges of critical nitrate levels during the first 2 wk of each cycle were established: 2.0 to 4.5 mg kg^–1 for the first harvest, 4.0 to 9.8 mg kg^–1 for the second harvest, and 2.0 to 11.0 mg kg^–1 for the third harvest. These values could be used to evaluate the necessity of adding N to a perennial grassland system in a three-harvest rotation and aid in a more correct fertilization of perennial grasses by offering soil-specific N evaluations.

Abbreviations: CL, critical level • CN, Cate–Nelson (model) • LRP, linear response plateau (model) • PSNT, presidedress nitrogen test • QRP, quadratic response plateau (model) • RCBD, randomized complete block design • RCG, reed canarygrass • TF, tall fescue

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
INORGANIC SOIL N under perennial grassland can be measured, but such data are meaningless as a predictive tool unless correlated with plant response. Any measurement of soil N under actively growing grass indicates the amount present at that particular time, not the N that will become available with mineralization. Soils under long-term grassland have large reserves of organic matter accumulated from the constant recycling of dead and decaying material (Whitehead, 1995). Organic N in the soil is mineralized to ammonium that is quickly nitrified to nitrite and then to nitrate (Jansson and Persson, 1982; Schmidt, 1982). Previous speculation that grasslands may inhibit nitrification (Theron, 1951; Soulides and Clark, 1958; Munro, 1966) has been disputed, and a recent review of this topic has concluded that grasses do not release compounds inhibiting nitrification (Bremner and McCarty, 1993). Grasses take up N in both the NH₄⁺ and NO₃^– forms (Barraclough and Smith, 1987), but temperature, moisture, and pH conditions under perennial grassland typically favor nitrification, and NO₃^– is taken up as soon as it becomes available (Dorsey and Brown, 1935). As a result, NH₄⁺ concentrations are typically higher than NO₃^– under perennial grasslands (Dorsey and Brown, 1935; Christensen, 1983; Davidson et al., 1990; Jarvis et al., 1990; Mallarino and Wedin, 1990).

Cool-season perennial grasses respond positively to N applications (Wedin, 1974; Allinson et al., 1992; Guillard et al., 1995; Whitehead, 1995). Quantities of N applied are largely determined by the species' yield potential but may be in excess of 200 kg N ha^–1 (Vetsch et al., 1999), and in temperate climates, the N applied is typically between 200 and 400 kg N ha^–1 (Whitehead, 1995). Although a typical recommendation for perennial grassland with less than 20% legume under high-level management may be 200 kg N ha^–1 (Jokela et al., 2004), yield responses have been seen in orchardgrass (Dactylis glomerata L.) with applications up to 400 kg N ha^–1 (Guillard et al., 1995) and in reed canarygrass (Phalaris arundinacae L.) with rates between 280 and 336 kg N ha^–1 (Collins and Allinson, 1995; Vetsch et al., 1999). Knowledge of N cycling and availability within the soil could assist in avoiding excessive N application.

Various laboratory methods exist for estimating potentially mineralizable N (Waring and Bremner, 1964; Stanford and Smith, 1972; Bundy and Meisinger, 1994; Cabrera et al., 1994). Laboratory incubations are often slow, but sample turnover time must be rapid if the producer is to effectively use the information. Field conditions are variable, and rates of N mineralization determined in the laboratory do not necessarily translate to the field (Wedin and Pastor, 1993). In situ field incubations have been tried using buried soil bags (Eno, 1960; Westermann and Crothers, 1980) as well as exclusionary pipes combined with resin bags (Hart and Binkley, 1985), which have been criticized as causing soil disturbance (Subler et al., 1995). The problem of soil disturbance affecting mineralization has been addressed by using two-dimensional anion exchange membranes when quantifying NO₃^––N under perennial grassland (Collins and Allinson, 1999; Ziadi et al., 1999; Kopp and Guillard, 2002).

The potentially mineralizable N under a perennial grassland after plowing has been estimated cumulatively to reach 4000 kg N ha^–1 over 20 yr (Whitmore et al., 1992) or as much as 400 kg^–1 N ha^–1 yr^–1 in the first year after plowing (Clement and Back, 1969). Obviously, only a fraction of the potential N would become available through mineralization when the grassland was not plowed. Nitrogen mineralization under unplowed perennial grassland may vary according to grass species (Wedin and Tilman, 1990), which has an effect on biomass allocation, root development, and rates of litter decomposition (Wedin and Pastor, 1993). A system for measuring and using the actual N available should reduce the application of excess N as well as assure the application of sufficient N for optimum plant growth.

The problem of estimating potentially available soil N exists in many crops, especially in a humid climate where NO₃^––N could be rapidly lost from the system through leaching. A widely adopted method used to evaluate N need with field corn (Zea mays L.) is the presidedress N test (PSNT) introduced in Vermont in 1984 (Magdoff et al., 1984). When a composite soil sample is collected to a depth of 30 cm and plants are between 15 and 30 cm tall, concentrations of soil NO₃^––N between 21 and 25 mg kg^–1 consistently indicate sufficient N for corn to attain optimum yield (Blackmer et al., 1989; Fox et al., 1989; Cerrato and Blackmer, 1991; Binford et al., 1992; Fox et al., 1992; Klausner et al., 1993). This test has proven most reliable as an indicator of the CL of N beyond which added N will not increase yield. The PSNT has been less successful in determining what rates of N fertilizer to apply when soil NO₃^––N levels are less than the CL (Fox et al., 1989; Klausner et al., 1993). This test has also been used successfully with sweet corn (Heckman et al., 1995) and fall-grown cabbage (Brassica oleracea L.) (Heckman et al., 2002) and was adapted for use with other vegetables such as lettuce (Lactuca sativa L.) and celery (Apium graveolens L.) (Hartz et al., 2000). In all cases, it worked best in situations where N had been applied as manure in substantial quantities over time and substantial mineralization could be expected.

One crop for which the PSNT has not been adapted is perennial grassland grown for hay or pasture. The obstacles are obvious. As a perennial crop, the well-developed root system takes up NO₃^––N as soon as it becomes available. There is no readily accumulated reservoir of NO₃^––N to measure as there would be for an annual crop such as corn. Therefore, it was doubtful that a measure of soil NO₃^––N would have any practical connection with the potential yield of the crop. In addition, a cool-season grass hayfield is typically harvested three times during the growing season. Thus, the period for taking a soil sample and applying N if necessary must be early in the growing season and immediately after each harvest. The purpose of our research was to evaluate the potential for adapting the PSNT to cool-season perennial grassland harvested three times during the growing season. Our goal was to determine if CLs of soil nitrate could be determined early enough in each growth cycle to allow for the corrective application of fertilizer N. Soil samples collected on a weekly basis, over two growing seasons, in this system, were evaluated for their potential to establish CLs of N for each growth cycle.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Field Procedures
Research was performed at two sites at the University of Connecticut Plant and Soil Science Research Farm, located in Storrs, CT (41°48' N, 72°15' W). The first site had originally been seeded to reed canarygrass (RCG) and the second to tall fescue (Festuca arundinaceum Schreb.). At the time of the experiment, however, other species, including orchardgrass, sweet vernalgrass (Anthoxanthum odoratum L.), Kentucky bluegrass (Poa pratensis L.), and smooth bromegrass (Bromus inermis Leyss.) were present. Both sites had been in continuous sod for more than 16 yr and were treated before experimental use with Crossbow (2,4-D [(2,4-dichlorophenoxy)acetic acid] and triclopyr {[(3,5,6-trichloro-2-pyrdinyl)oxy]acetic acid}), a broadleaf herbicide that was used to remove any stray legumes present in the plots, either as red clover (Trifolium pratense L.) or white clover (Trifolium repens L.). The amount of legume initially present never exceeded 1 to 3%, so any N possibly entering the system directly as a result of the legume kill would have been negligible. The herbicide was applied, however, to avoid any proliferation of red or white clover during the course of the research at the 0 or low-N treatments as had been observed in an earlier project when low levels of N fertilizer had been applied (Collins and Allinson, 1995). The soil was a Paxton fine sandy loam (coarse-loamy, mixed, active, mesic Oxyaquic Dystrudept).

Soils were analyzed initially and at the end of each growing season. Phosphorus was applied on 6 May 1996 to supply 49 kg P ha^–1. Potassium was applied in split applications with 93 kg K ha^–1 being applied on 6 May 1996 and 74 kg ha^–1 being applied after the second harvest, on 21 July. The same rates of P and K were applied in 1997, with the first application on 22 April and the remainder of the K applied after the second harvest on 29 July. All soils were extracted with a Modified-Morgan NH₄(OAc) extractant, pH 4.8, 4 g 20 mL^–1 (McIntosh, 1969). Soil analysis data are presented in Table 1.

Weekly Soil Samples
Soil Sampling
Soil samples were taken on a weekly basis to a depth of 20 cm, from approximately 1 wk after each N application until harvest, both in 1996 and 1997. Three cores per plot were taken, bulked, and immediately set out to dry. The soil was sifted to pass a 1.65-mm mesh and stored in airtight containers. Throughout the course of the experiment, the soil from all treatments was analyzed for organic matter using the weight loss-on-ignition method (Ball, 1964). Soil samples were also analyzed for total N and total C using a LECO Nitrogen/Protein Analyzer (Model FP-2000, LECO, St. Joseph, MI).

Ammonium Nitrogen and Nitrate Nitrogen Determinations
Ammonium and NO₃^– were extracted using the procedure outlined by Keeney and Nelson (1982). Extractants were analyzed colorimetrically for NO₃^––N and NH₄⁺–N on a Scientific Instruments continuous flow analyzer (Westco, Danbury, CT), using a Cd column for reduction of NO₃^––N to NO₂^––N (USEPA, 1983) and an automated phenate method for NH₄⁺–N.

Statistical Analysis of Weekly Soil Sample Data
Both NO₃^– and NH₄⁺ data for each week were subjected to Bartlett's test for homogeneity of variance, and log₁₀ transformations were performed where necessary. In some cases the data remained non-homogeneous after transformation, and a Friedman's non-parametric test was performed in those instances. The data were analyzed as a RCBD in both years, since only the same N treatments for both years were used in this analysis.

Mean NO₃^– and NH₄⁺ data for each N treatment from each week were plotted additively, that is the mean amount found in the soil each week for each separate N treatment was added to the amount for the previous weeks (Fig. 1 and 2). Thus, the value for any particular week can be determined by subtracting the amount from the previous data point from that particular week. The total additive values at the end of each season at each site were analyzed as a RCBD in 1996 and as a split-plot design in 1997. In 1997, the soil samples taken during Week 2, on 7 May, are not included in Fig. 2 because only the plots receiving the original four N treatments were sampled on that day.

View larger version (34K): [in this window] [in a new window]   Fig. 1. Additive soil NO3––N and NH4+–N concentrations measured for successive weeks at four N rates at both sites during the 1996 season. Nitrogen applications are indicated with an inverted arrow. The ** and *** indicate significance at the 0.01 and 0.001 levels of probability, respectively. L, linear; Q, quadratic.

View larger version (39K): [in this window] [in a new window]   Fig. 2. Additive soil NO3––N and NH4+–N concentration measured for successive weeks at eight N rates at both sites during the 1997 season. Nitrogen applications are indicated with an inverted arrow. The *, ***, and NS indicate significance at the 0.05 and 0.001 levels of probability, and not significant, respectively.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Harvest Yield
The two mixed stands of grasses used in this study, originally seeded to RCG and TF, were both high yielding and generally produced yields that fell within the range of 8000 to 15000 kg dry matter ha^–1. These yields are typical for intensively managed grasslands in temperate regions (Whitehead, 1995). The mean dry matter total yield, over both years and all N rates for the RCG site, was 11118 kg ha^–1, with a maximum of 14303 kg ha^–1 at 525 kg N ha^–1 in 1996 and 14244 kg ha^–1 at 613 kg N ha^–1 in 1997. Tall fescue produced a mean dry matter yield of 9811 kg ha^–1 over both years, with a maximum of 12508 kg ha^–1 at 350 kg N ha^–1 in 1996 and 11149 kg ha^–1 at 263 kg N ha^–1 in 1997. These compared well to maximum yields of RCG reported by Vetsch et al. (1999) of 11760 and 11558 kg ha^–1 at N rates of 280 and 336 kg ha^–1 in two consecutive growing seasons and 13104 kg ha^–1 at 672 kg N ha^–1 reported by Smith (1981). It was greater than the maximum yield of TF reported by Collins and Allinson (1995) of 9117 kg ha^–1 at 280 kg N ha^–1 and Buckner and Cowan (1973), who reported 7000 to >9000 kg ha^–1 as typical of well-fertilized TF plots and indicated that a plateau yield had been reached at both sites from which a corresponding CL of soil NO₃^––N was deduced.

A significant linear response to N occurred at both sites for each harvest each year (Table 3). In many cases, there was a significant quadratic response as well, especially at the TF site where yield peaked at 350 kg N ha^–1 in 1996 and at 263 kg N ha^–1 in 1997. At the RCG site, significant quadratic responses were also noted. With the exception of Harvest 3 in 1997, the highest yields came in response to the highest N level. The same trends were true for the total yield at both sites each year.

Concentrations of NO₃^––N and NH₄⁺–N extracted on a weekly basis were significantly affected by N rate (Table 5). This was particularly so for NO₃^––N concentrations. Generally, the response was linear, but occasional quadratic and cubic responses were noted as well. While weekly NH₄⁺–N concentrations were sometimes significantly different (Table 5), the differences were evident primarily during the first growth period, and the inconsistent response would make it less useful as a tool for determining CLs needed for N application.

View larger version (49K): [in this window] [in a new window]   Fig. 3. Pooled data for years and sites, showing Weeks 1 and 2, using three models to determine percentage error (Cate–Nelson only), R2 values, and critical nitrate levels (CL). The *** indicates significance at the 0.001 level of probability.

The capacity of each model to describe the data was similar, based on the R² values. However, the CN model (Cate and Nelson, 1971) that partitions the data into four quadrants also describes the error rate. It is based on the percentage of points in the upper left quadrant (I), which would overpredict the need for N and the lower right quadrant (II), which would underpredict the need for N (Fox et al., 1989). A relative yield of 0.88 was used as the horizontal CL, as this best partitioned the data into the upper right and lower left quadrants, which is the goal of the CN model. In general, the points in the upper left quadrant indicate that even though a plateau yield had been reached, N would have been recommended because the NO₃^––N CL had not been reached. Similarly, points in the lower right quadrant indicate that the CL was reached, but the plots did not reach plateau yield. Thus, no fertilizer N would have been recommended, and reduced yields would have been obtained. In the first case, excess nutrient loading may have entered into the environmental system, and in the second, lack of N application would have resulted in reduced yields. The CLs produced using the CN model showed combined error rates of 16, 22, 14, 14, and 12% and therefore 84, 78, 86, and 88% accuracy rates. These values compare well to the 31% error rate reported in relation to the PSNT for corn by Sims et al. (1995), the 18.1, 22.2, and 23.8% error rates reported by Fox et al. (1992) for the Mid-Atlantic states, and the 16% error rate reported by Klausner et al. (1993) in New York. For the most part, the CLs reported for the PSNT were determined in fields that had been consistently fertilized with manure. Sims et al. (1995) reported that the greater portion of their error (24%) occurred in the upper left quadrant, thus indicating a low CL that was not responsive to N. This could result if the available N was below the rooting zone of the crop or if for some other reason, the N present in the soil had only been released after the soil sample had been taken. Similarly for our data, the percentage error was greater in the upper left quadrant for the first two growth periods, but not for the third, and would err on the side of recommending that N be added when plateau yield would have already been achieved.

The range of CLs was usually greater for the second week than for the first. The QRP model generally had the highest CLs, whereas the CN model tended to recommend the lowest. These three models followed a similar trend to that reported by Hooker and Morris (1999), who determined a range of CLs for the late-season cornstalk nitrate test. That is, the CN reported the lowest values, the LRP a midrange value, and the QRP the highest CL values. A summary of the range of CLs described by the three models for the three growth periods encompassing both years is presented for Weeks 1 and 2 in Table 6. Rather than select one value based on one model, it seems more prudent to use the range suggested by the three models taken together.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Critical NO₃^––N levels were generated in relation to optimum yield for the first 2 wk of three harvest cycles in each of two seasons, 1996 and 1997. The range of CLs generated could be used to indicate the soil NO₃^– concentration necessary for a field to reach optimum yield. If that value were reached, no further N should be needed. However if it were not reached, there would be sufficient time for N application when necessary to reach optimum yield and eliminate a potential N deficiency. The CN model generally recommended the lowest CLs whereas the QRP model recommended the highest. Using this data to analyze soil samples for NO₃^––N at the critical time in relation to each harvest could offer more specific information in relation to soil mineralization potential than is currently available through cropping history, manure application, and yield potential.

	ACKNOWLEDGMENTS

 
The authors gratefully acknowledge the help of Dr. Tom Morris and Dr. Karl Guillard with the statistical analyses used in this research and for early review of the manuscript.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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