Nitrogen Cycling as Affected by Interactions of ...

67(1),1986,pp.80-87 Ecology, ^ 1986bytheEcological Societyof America

NITROGEN

CYCLING

AS

AFFECTED

IN A GEORGIA

COMPONENTS

BY

PIEDMONT

INTERACTIONS

OF

AGROECOSYSTEM1

P. M. Groffman,2 G. J. House,3 P. F. Hendrix, D. E. Scott, and D. A. Crossley, University of Georgia, Institute of Ecology, Athens, Georgia 30602 USA

Jr.

Abstract. Patterns of nitrogen cycling were studied in N-fertilized (N: 95 kg/ha, P: 45 kg/ha, K: 135 kg/ha) and "unfertilized" (no N, but the same levels of P and K) agroecosystems with heavy weed infestations. To calculate monthly standing crop nitrogen levels, nutrient concentrations were multiplied by biomass values for each month. Soils were also sampled monthly at three depths; precip? itation was sampled weekly and analyzed for N inputs. Nitrification and denitrification enzyme ac? tivities were measured, as well as rates of denitrification from unaltered soil cores. Nitrate leaching was measured with porous cup lysimeters. Weeds competed more vigorously with the crop in fertilized treatments than in unfertilized treatments, resulting in greater crop growth in unfertilized treatments. Nitrification, denitrification, and leaching losses of N were greater in fertilized treatments than in unfertilized treatments, especially after residue input following harvest of the summer crop. Soil organic nitrogen tended to be lower in unfertilized treatments than in fertilized treatments after summer cropping, whereas soil organic carbon showed an opposite trend. As a result the carbon : nitrogen ratio of soil organic matter was significantly higher in unfertilized treatments than in fertilized treatments. Fertilized treatments showed a positive N balance (47 kg/ha) while unfertilized treatments showed a net loss of N (57 kg/ha). Nitrogen use efficiency by plants was higher in unfertilized treatments than in fertilized treatments, but was low relative to many unmanaged ecosystems. Key words: denitrification; Georgia piedmont; leaching; nitrification; nitrogen budgets; nitrogen use efficiency. Introduction Nutrient cycling studies in both natural and agri? cultural ecosystems have frequently emphasized nitro? gen due to the importance of this nutrient to plant growth and its susceptibility to leaching and gaseous losses (Vitousek et al. 1982). Nitrogen is the most heavily applied fertilizer nutrient in agroecosystems, but crop recovery of applied N is often low, and leach? ing and gaseous losses of N are often high (Keeney 1982, Rosswall and Paustian 1984). Analysis of N cy? cling patterns in simplified ecosystems, such as agro? ecosystems, can help to clarify the major processes that affect N-use efficiency and losses in natural ecosystems (Coleman et al. 1984). Studies of N cycling in agro? ecosystems aid in understanding ecosystem structure and function, and provide information for agroecosystem management (Crossley et al. 1984). Nitrogen fertilizers are known to increase the growth and yield of crops (Olson and Kurtz 1982), but they also affect weed composition and growth (Hoveland et al. 1976) and soil physical and chemical properties (Hauck 1981). These factors affect N cycling in agro? ecosystems. Construction of nutrient budgets is the most common method for analysis of whole system nutrient dynamics (Frissel 1977, Legg and Meisinger 1982, Lowrance et al. 1985). Agroecosystems are ideal for 1 Manuscript received 30 August 1984; revised 25 February 1985; accepted 1 March 1985. 2 Present address: Department of Crop and Soil Science, Michigan State University, East Lansing, Michigan 48824 USA. 3 Present address: Department of Entomology, North Car? olina State University, Raleigh, North Carolina 27596 USA.

nutrient budget studies because they have simple vege? tation structure, well-defined boundaries, and easily manipulated processes of input and output (Odum 1984). Research on nutrient cycling in agroecosystems has been carried out at the University of Georgia since 1978. Previous studies have analyzed the effects of tillage on nutrient cycling and mechanisms that pre? vent nutrient losses from agroecosystems (House et al. 1984, Stinner et al. 1984). In this study, N cycling was studied in N-fertilized and unfertilized agroecosys? tems. Nitrogen budgets were constructed, and patterns and mechanisms of nutrient loss were observed. The goal of this research was to analyze the effects of N fertilizer on different agroecosystem components and processes and their interactions. Methods This research was conducted at the Horseshoe Bend Experimental Area, near Athens, on the Georgia pied? mont. The soil is a Hiwassee series (typic Rhodudult), a well-drained soil with a sandy clay loam Ap horizon (66% sand, 13% silt, 21% clay) found on 0-2% slopes. Physical and chemical properties of this soil have been described elsewhere (Groffman 1984) and are summarized in Table 1. The area has been under conventional and no-tillage treatments since 1978. Conventional tillage consisted of moldboard plowing to a depth of 10 cm, followed by disking before planting of both summer and winter crops. There were four 28 x 28 m plots per tillage treatment. Prior to 1978 the area was in old-field vegetation. Grain sorghum {Sorghum bicolor L. Moench) was grown

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NITROGEN CYCLING IN AGROECOSYSTEMS

February 1986

81

Table 1. Properties of conventional (CT) and no-tillage (NT) soils at the Horseshoe Bend Experimental Area, Athens, Georgia.

Depth (cm)

f Cation ? Values * Tillage XTillage

Concentration (% soil dry mass) Organic C Organic N CT

NT

CT

NT

Bulk density (g/cm3) NT CT

CEC (mmol/kg)t NT

CT

pH? CT

NT

exchange capacity, expressed as the concentration of exchangeable charges. from February 1983; all other data are annual means. treatments significantly different at a = .05. treatments significantly different at a = .10.

in 1978 and 1979, soybeans {Glycine max L.) were grown in 1980 and 1981, and sorghum was again grown in 1982. From 1978 through 1981, winter rye {Secale cereale L.) was grown as a winter cover crop on all plots. Dolomitic limestone (2300 kg/ha) was applied to all plots in May 1980. In the fall of 1981, the plots were split and crimson clover {Trifolium incarnatum L.) was seeded as a win? ter cover crop on half of each plot, while rye was grown on the remainder. Establishment of crimson clover was very poor due to late seeding and to severe cold (? 18?C). The resulting vegetation was primarily an assortment of nonleguminous winter weeds. This study was initiated in spring 1982. Glyphosate (4.5 L/ha) was applied to all winter vegetation (rye or on 31 May 1982. All plots received P clover/weeds) (45 kg/ha) and K (135 kg/ha) on 10 June 1982. Plots that had rye as a winter cover received (95 kg/ha) as NH4NO3 on 10 June 1982 (fertilized treatment). Plots that had clover/weeds as a winter cover received no N fertilizer (unfertilized treatment). Sorghum (Funks 522) seeds were drilled in 1-m rows (15 kg/ha) on 11 June 1982. Rye (70 kg/ha) and clover 15 kg/ha) were seeded as winter cover crops in the fertilized and unfertilized plots, respectively, on 26 October 1982. Establishment of both winter cover crops was successful in 1982. Plant analysis Plant material (crop plants and grain, weeds and surface litter) was sampled at 1-mo intervals using two 0.75-m2 quadrats per plot for summer crops and one 0.25-m2 quadrat per plot for winter crops. Plant ma? terial was dried at 60?C to constant mass, then ground in a Wiley mill to pass through a 1.0-mm screen. Plant and litter tissue were digested using the micro-Kjeldahl procedures (Alien et al. 1974) and analyzed for NH4+-N using a Technicon Autoanalyzer II. Standing crop of N in plants and litter was calculated by multiplying monthly nutrient concentrations by monthly biomass values. Soil analysis Soils were sampled monthly at three depths (0-5 cm, 5-13 cm, 13-21 cm). Percent moisture was determined

by mass loss upon drying at 105? for 24 h. For organic C and N analysis, soils were air-dried and passed through a 2.0-mm sieve. Organic N was analyzed fol? lowing the procedure described for plant tissue. Or? ganic C was determined by the Walkley-Black method (Nelson and Sommers 1982). Soil NH4+-N and N03~-N were extracted with 2 mol/L KC1 within 3 d of sampling and analyzed with a Technicon Autoanalyzer II. Soils were stored at 4? between the time of sampling and all analyses. Denitrification

and nitrification

methods

To obtain estimates of denitrification rates, 2 cm diameter soil cores (three per plot per depth) were placed in glass vials and capped with rubber serum stoppers in the field (Hendrickson 1981). These cores were im? mediately brought back to the laboratory and aired for 15-20 s. The vials were recapped and acetylene (C2H2) was added to a concentration of at least 10% of the headspace of the vials. Samples were then incubated for 6 h, outside in the shade, and air samples were removed and stored in evacuated glass tubes. These air samples were later analyzed for nitrous oxide (N20) using a Tracor MT-220 gas chromatograph equipped with a 63Ni electron capture detector. Headspace of the incubation vials was measured by displacing the air in the vials with water, assuming that the density of water equals 1 g/cm3. Nitrous oxide values were corrected for ambient N20 concentrations and for differences in pressure between incubation vials and gas sample tubes. Nitrous oxide dissolved in soil water was also esti? mated, using the method of Moraghan and Buresh (1977). Potential denitrification enzyme activity (phase I assay; Smith and Tiedje 1979) was assayed by measuring N20 production from 25-g samples of field-moist soils (soil with no water added or removed after collection) incubated in 25 mL of medium containing dextrose (mass per unit soil mass: 40 Mg/g), N03~-N (200 Mg/g), and chloramphenicol (10 Mg/g)- These soils were in? cubated anaerobically, in the presence of 10% C2H2, for 2 h at 22?. Soils were made anerobic by purging incubation flasks with 5% H, 95% Ar. Nitrous oxide was sampled and analyzed as described above.


Ecology, Vol. 67, No. 1

P. M. GROFFMAN ET AL.

82

jTOTALplant BIOMASS

WEED BIOMASS -g Fertilized ?+?Unfertilized

JUN JUL AUG SEP MONTH

* o



Fig. 1. Plant biomass in fertilized and unfertilized treatments, June through September 1982. For comparison of treatments on a given date, * indicates significance at a = .05. Potential nitrification enzyme activity was assayed by measuring nitrite (N02~-N) production from fieldmoist soils (15 g) amended with NH4+-N (45 Mg/g) in the presence of NaC103 at 10 mmol/L in 50 mL of pH 7.2 phosphate buffer (chlorate inhibition method; Belser and Mays 1980). These soils were incubated on a rotary shaker for 24 h at room temperature. Nitrite in filtered extracts was analyzed with a Technicon Autoanalyzer II. For both the nitrification and denitrifi? cation enzyme assays, soils from replicate plots were composited and three replicates per treatment per depth were used. Solution chemistry Inputs of N in precipitation (wetfall and dry fall) were and N03~-N con? quantified by measuring NH/-N centrations in precipitation sampled weekly in Nalgene

collectors (Likens et al. 1967) at three locations within the experimental area. Leaching losses of N were quantified by measuring NH/-N and N03~-N concentra? tions in Soil Moisture Company ceramic porous cup lysimeters (one lysimeter per plot) placed 60 cm below the soil surface. Samples were collected monthly by evacuating lysimeters to ?30 kPa pressure for 24 h. Organic N in precipitation and leachate was not ana? lyzed. Water balance was determined using the formula of Thornthwaite and Mather (1957), with the exception that changes in soil water status were assessed from soil moisture data collected in the field. Statistical

analysis

Data were analyzed using analysis of variance for a split-plot design with a plot term incorporated for a repeated measures effect (Winer 1971), where possible.

Table 2. Summary of N cycle processes for fertilized and unfertilized treatments for the summer season (June-September 1982) and for the entire study period June 1982 to February 1983.

* Treatments significantly different at a = .05. t Negative value indicates net immobilization. i Treatments significantly different at a = . 10. Standing stock of N in litter was significantly greater in the fertilized treatment when all months were included in an analysis.


83

NITROGEN CYCLING IN AGROECOSYSTEMS

February 1986

Tillage treatments were considered main plots, with the fertilizer treatments as subplots. There were a total of eight 14 x 28 m replicate plots per fertilizer treat? ment. In this paper, only differences between N-fertilized and nonfertilized plots are discussed. Tillage ef? fects are discussed elsewhere (Groffman 1984, 1985). Results presented are the means of fertilized and non? fertilized treatments over conventional and no-tillage treatments. Soil N03~-N, organic C and N, and denitrification to hovalues were subjected to log transformations mogenize variances before analysis. Results of soil analyses were expressed in kilograms per hectare, using bulk density values; this permitted comparison of soils of different density and aggregation of data from dif? ferent depths.

Results

45 o; 15 10

and Discussion

Plant dynamics Final crop biomass was higher {P = .05) in unfertil? ized plots than in fertilized plots, whereas weed bio? mass showed an opposite pattern {P = .05) (Fig. 1). There was a marked difference in weed species between treatments, with Amaranthus retroflexus and A. powellii dominant in fertilized plots and Cassia obtusifolia dominant in unfertilized plots. Total plant biomass (sorghum plus weeds) was not affected by N fertilizer input. Grain yield of sorghum averaged 3877 kg/ha in unfertilized treatments and 1050 kg/ha in fertilized treatments. Average sorghum yield for the state of Georgia in 1982 was 2634 kg/ha (Georgia Crop Reporting Service 1983). Total plant uptake of N was significantly greater {P = .05) in fertilized plots than in unfertilized plots, but the majority of this uptake was by weeds (64% in fer? tilized plots during summer; Table 2). Consequently, crop uptake of N was greater (by 59%) in unfertilized plots than in fertilized plots. Percent N in crop, weed, and grain tissue was higher {P = .05) in fertilized plots than in unfertilized plots at all dates (data not shown). Cassia obtusifolia is a non-nodulating legume species (Alien and Alien 1976); thus, symbiotic N fixation was not a factor in summer N dynamics. The greater plant uptake of N in the fertilized treat? ments was probably driven by the high levels of soil NO3-N that were observed in the fertilized plots (Fig. 2A). Available N may also have affected the partition? ing of N uptake between crops and weeds. Amaranthus spp., which dominated in the fertilized treatments, are known to accumulate high levels of N03~-N (Weaver and McWilliams 1980). Given the high levels of avail? able N03~-N in the fertilized treatments, these weeds were able to achieve a much greater biomass than the Cassia sp. weeds that dominated in the unfertilized treatments. As a result, sorghum was able to compete more effectively with weeds in the unfertilized treat? ments, producing more biomass and grain yield.

150 100 I 50

OCT NOV MONTH Fig. 2. (A) Soil N03-N (0-21 cm depth) in fertilized and unfertilized treatments, July 1982 through February 1983. (B) Potential nitrification enzyme activity (0-21 cm depth) in fertilized and unfertilized treatments, July 1982 through Feb? ruary 1983. (C) Potential denitrification enzyme activity (021 cm depth) in fertilized and unfertilized treatments, August 1982 through February 1983. For comparisons of treatments on a given date, * indicates significance at a = .05, Xindicates significance at a = .10.

Weed species differences between treatments may also have been affected by differences in the winter cover vegetation that preceded the sorghum crop (rye or assorted weeds). Rye, the winter cover crop on all plots before 1981, is known to contain allelopathic chemicals, but these generally affect plant growth more than community composition (Barnes and Putnam 1983). Cassia has generally been the dominant weed on these study plots (House 1983), which suggests that this weed is not suppressed by rye residues. Allelopathy interactions may also have affected the competitive between the sorghum crop and weeds in the different treatments (Bhowmik and Doll 1984). Although differences in winter cover vegetation and allelopathy may have affected the patterns of N uptake and community composition observed, it is probable that the N fertilizer input was the dominant factor. All


84

P. M. GROFFMAN ET AL.

Ecology, Vol. 67, No. 1

study are discussed elsewhere (Groffman 1985), but in general, this method tends to underestimate N losses to denitrification. Leaching losses of N were greater {P = .05) in fer? tilized treatments than in unfertilized treatments in November, December, and January (Fig. 3). Leaching loss values presented here do not include organic N and are an underestimate. In February, N03~-N con? centrations in lysimeters had decreased significantly in both treatments but were significantly higher (P = .05) in unfertilized plots than in fertilized plots (Fig. 3). FEB This was JAN NOV DEC probably due to a greater availablility of N MONTH in the root zone ofthe N-fixing clover cover crop (un? Fig. 3. Lysimeter N03~-N (60 cm depth) in fertilized and fertilized plots) relative to the rye cover crop (fertilized unfertilized treatments, November 1982 through February treatment). Despite the low concentration of N in ly1983. Symbols as in Fig. 2. simeter water in February, leaching losses were still significant due to the large amount of rainfall recorded in that month (141 mm). Total leaching losses of N plots had the same management prior to establishing for the period June 1982 through February 1983 were the winter cover treatments. Fertilizer input created a ovalmost twice as high {P = .05) in fertilized treatments in difference soil N03~-N levels, apparently major as in unfertilized treatments (Table 2). erriding effects from the different winter cover crops. Nitrogen uptake by winter cover crops was a sink Nitrogen losses of summer for N produced from the decomposition almost 40 residues. took were measured up kg/ha between crop through Rye Nitrogen cycle processes November and February. This N would have been February 1983 to assess the effects of crop residues on N losses from the agroecosystems (Table 2). Nitrogen susceptible to leaching and denitrification losses. Clo? ver took up very little N (