bY M. G. Wagger

Department of Soil Science North Carolina State University

October 7 996

UNC-WRRI-96-303

REDUCTION OF NITRATE LEACHING IN AGRICULTURAL SOILS V I A COVER CROPS

M. G. Wagger

Department of S o i l Science

and

College of Agriculture and Life Sciences North Carol i na State University Raleigh, North Carol i na 27695

The research on which th i s report i s based was financed in part by the United States Department of the Interior, Geological Survey, th rough the N.C. Water Resources Research Insti tute.

Contents of t h i s publication do n o t necessarily re f lec t the views and policies of the United States Department o f Interior nor does mention of trade names or commercial products constitute the i r endorsement by the United States Government.

WRRI Project No. 70127 Agreement No. 14-08-0001-G2037

USGS Project No. 1 7 (FY '93) September 1996

ACKNOWLEDGMENTS

This work was performed w i t h i n the Department of Soil Science of the College of Agriculture and Life Sciences a t North Carolina State University.

The author appreci ates the excel 1 ent cooperati on provided by Mr , Sandy Barnes, Superintendent , Lower Coastal P l a i n Tobacco Research Stat ion and other s ta t ion personnel. A special note of appreciation is expressed t o Mr. Fred Averette and Mr. Noah Ranells for technical assistance i n handl ing the field work. F ina l ly , I would like t o t h a n k Ms. Gail Regan for her assistance i n preparing this report.

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ABSTRACT

The inherent inefficiency of fert i l izer N u t i l i za t ion by corn can lead to a relatively large pool of residual soil N subject t o leaching and possible contami n a t i on of groundwater suppl i es , parti cul arly on sandy soi 1 s i n the North Carolina Coastal P l a i n . The objectives of this research were t o : ( I ) determine the extent of NO3 leaching w i t h respect t o fertilization scheme and ( i i ) evaluate the potential of several cover crops (crimson clover, rye, spring o a t , wheat, and native weeds) t o recover residual fert i l izer N from a corn producti on system. Soi 1 inorganic N concentrations fol 1 owing corn harvest i n September were two-to threefold greater a t most depths when the previous corn N rate was 300 vs 150 kg ha? Moreover, the largest differences i n residual soil N levels between N rates occurred i n the 45- t o 75-cm depth range. Rye was the most effective cover crop i n reducing profile soil inorganic N concentrations, followed by oat> wheat= crimson clover= native weeds. The greatest soil inorganic N concentrations just prior t o corn p l a n t i n g the following spring occurred i n the 75- t o 90-cm soil layer, ranging from 1 mg kg-1 under rye t o 9 mg kg-1 under native weeds. Estimates of N re1 eased from decomposi ng cover crops duri ng the corn growing season were 59 kg ha-' for crimson clover, 32 kg ha" for rye, 25 kg ha-' for spring oa t , and 17 kg ha-' for wheat. In another experiment using 15N methodology, cover crop recovery of fa l l applied 15N fert i l izer immediately prior t o corn p l a n t i n g was 10% for native weeds, 5% for crimson clover, and 35% for rye. The corresponding residue 15N released by corn maturity was i n the order of crimson clover (23%) > native weeds (14%) > rye ( 7 % ) .

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TABLE OF CONTENTS

Page

Acknowledgments

Abstract

L i s t o f Figures

L i s t o f Tables

Summary and Concl usi ons

Recommendations

In t roduct ion

Materi a1 s and Methods

Objective 1 - Leaching o f f e r t i l i z e r N Objective 2 - U t i l i z a t i o n o f residual N

Results and Discussion

Unl abel ed N Experiment Residual s o i l N and cover crop performance Cover crop N release rates

I5N- Labeled Experiment Cover crop recovery o f 15N Corn recovery o f residue I5N

L i t e r a t u r e Ci ted

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1

3 4

7

7 7

13

16 16 19

21

23 G1 ossary

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LIST OF FIGURES

Page

1. Distribution of soil inorganic N i n September ( a ) and December ( b ) 1992 as affected by previous corn N fertilization rate.

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2. Dry matter accumulation patterns by fal low ( F ) , oat ( 0 ) . rye ( R ) , 9 wheat (W),and crimson clover (C) cover crops following the previous corn crop fertilized w i t h 150 or 300 kg N ha-'.

3. Nitrogen accumulation patterns by fallow (F) . oat ( 0 ) . rye ( R ) . 11 wheat ( W ) , and crimson clover (C) cover crops following the previous corn crop fertilized w i t h 150 or 300 kg N ha?

4 . Distribution of soil inorganic N i n March ( a ) and April ( b > 12 1993 as affected by cover crops following the previous corn crop fertilized w i t h 300 kg N ha''.

5 . Percentage of i n i t i a l cover crop N remaining during the 1993 corn growing season.

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V

LIST OF TABLES

1. Precipitation during the 1992-1993 cover crop growing season and departures from 30-yr averages ( i n parentheses).

2 . Cover crop recovery of ferti 1 i zer 15N and profi l e soi 1 inorganic N content a t various times during the 1992-1993 cover crop season.

3. Distribution of I5N recovered from cover crop residues a t various times during the 1993 corn growing season.

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17

18

20

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SUMMARY AND CONCLUSIONS

Field experiments on a Coastal P l a i n soil (Norfolk loamy sand) served as a basi s for characteri zing NO, 1 eachi ng potenti a1 and the subsequent potenti a1 of winter annua l cover crops (crimson clover, rye, spring o a t , wheat, and native weeds) t o recover and recycle residual fert i l izer N . Two approaches were used t o evaluate these N dynamics. The f i r s t method established two levels of residual soil N v i a fert i l izer N applied t o the previous corn crop, while the second method employed I5N- enriched potassium nitrate applied t o microplots immediately prior t o p l a n t i n g rye and crimson clover cover crops i n early f a l l .

Considerably higher concentrations of residual soi 1 i norgani c N occurred Because the greatest following corn fertilized w i t h 300 vs 150 kg N ha-'.

concentrations (33 mg kg-l) a t the h i g h fert i l izer N rate were found i n the 45- t o 75-cm depth interval, i t i s likely t h a t further downward movement during winter months would remove this inorganic N from the effective crop rooting zone. In this regard, a winter annua l cover crop of rye was quite effective i n accumulating residual fert i l izer N and thereby minimizing further N losses from the plant-soi l system. Spring o a t showed some moderate potential for accumulating soi l N and reducing NO, leaching while wheat, crimson clover, and native weeds were relatively ineffective i n this role.

Estimates of the subsequent cover crop N pool potentially available t o corn by 16 weeks was i n the order of crimson clover > rye > spring o a t > wheat. Based on a recommended fert i l izer N rate of 150 kg ha-' for corn grown i n the North Carolina Coastal P l a i n , the percentage of this N requirement met by cover crop N release ranged from 12% w i t h wheat t o 40% w i t h crimson clover.

In general, results from the I5N experiment confirmed f indings from the unlabeled N experiment. Rye recovery of f a l l -applied I5N-enriched fer t i l izer was 35% by the following April compared t o 10% by native weeds and only 5% by crimson clover. As a percentage of the to t a l residue I5N, nearly 23% of the crimson clover N was released by corn maturity compared t o 14% of the native weed N and 7% of the rye N . Low to t a l N accumulation by rye and crimson clover, however, severely limited actual N contributions t o corn growth and yield.

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RECOMMENDATIONS

Specific recommendations based on one site-year of d a t a i n the Coastal P l a i n are difficult t o make. Given the dynamic role climate plays i n field research, mu1 t iple year studies are requi red t o properly assess treatment effects. Nevertheless, there are some clear patterns t h a t emerged i n these experiments. Results of studies using two approaches t o assess NO3 leaching potential and the subsequent cover crop recovery of residual fert i l izer N i n soil indicated the a b i l i t y of rye over t h a t of spring o a t , wheat, crimson clover, and native weeds t o f i l l this niche. In situations (environmental stress or pest-related pressures) where low yields of a high-N requirement crop such as corn result i n relatively high levels of residual soil inorganic N , rye should be the cover crop of choice for remedial action. With respect t o cover crop release of N , crimson clover would serve better i n this capacity compared t o the grasses evaluated. Therefore, i n order t o optimize the inherent capabi 1 i t i es of grasses and 1 egumes , a grass-1 egume bi cul ture may be more appropriate i n cover-crop based production systems. investigation i s warranted on the role o f bicultures i n soil water and nutrient dynami cs .

Further

Final ly , i f cover crops are t o be widely used as a soil management t o o l , the potential scope of application should be expanded. This expansion m i g h t include the evaluation of rye cultivars w i t h respect t o nutrient recovery and the use of cover crops t o recycle nutrients contained i n animal wastes.

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INTRODUCTION

Groundwater contami n a t i on by agricultural chemical s i s one of the major problems facing agriculture i n the 1990's. because relatively low concentrations (10 ppm NO,-N) impair water for human consumpti on and because many shall ow groundwater suppl i es now exceed recommended NO,-N drinking water standards (Hal 1 berg, 1986; Keeney , 1986). Increases i n groundwater NO, 1 eve1 s have been associated w i t h major increases i n N fert i l ization, primarily w i t h respect t o corn (Zea mays L . ) (Hallberg, 1987).

Long-term experiments i n the Corn Belt have shown t h a t fert i l izer N removal by corn grain rarely exceeds 40% a t economically optimum corn yields (Blackmer, 1986; Oberle and Keeney, 1990 a , b ) . In cases where residual N from previous crop fertilization remains i n the soi l , the potential for NO, leaching also exists. Analysis of long-term climatic d a t a for North Carolina shows t h a t rainfall exceeds evapotranspiration during the winter and early spring months (van Bavel and Verlinden, 1956). With an annual crop such as corn, the l a n d i s often bare during this period when the greatest potential for si gni f i cant 1 eachi ng exi sts .

Nitrate is of particular concern

Winter annual cover crops have been recognized as an integral component of southern agricultural systems for many years because of their role i n soil erosion control and enhancement of soil productivity. With the refinement of conservation t i 1 lage technology, new strategies have evolved w i t h regard t o cover crop management. The role of nonleguminous cover crops i n efficient use o f water and N was recently reviewed by Wagger and Mengel (1988). Indirect evidence for cover crops u t i l i z i n g residual N can be found i n studies reported 2by Langdale e t a l . (1979). Pelchat (19861, and Utomo (1986). In these experiments, N content of winter wheat (Triticum aestivum L . ) or rye (Secale cereale L.) cover crops ranged from 12 t o 66 kg N ha-'. Pelchat (1986) found t h a t increasing N applications t o the previous corn crop from 0 t o 180 kg N ha'' resulted i n an increase of 16 kg ha-' i n N uptake by the subsequent cover crop. Information is limited, however, on the role of cover crops as sinks for residual N . Moreover, a better understanding of the management of cover crops i n this role is needed t o ensure t h a t the residual N trapped is effectively recycled t o subsequent summer crops.

Winter cover crop effects on NO, leaching were evaluated more directly i n drainage lysimeters by Karraker e t a l . (1950). leaching losses under bluegrass (h gratensis L . Large losses of NO, occurred under a lespedeza (Lespedeza slzg. sod. but not when a winter rye cover crop was grown. They concluded t h a t most of the NO, came from decomposing , senescent lespedeza residues. In Sweden, Berti 1 sson (1988) concluded t h a t a rape (Brassica napus L.) cover crop could greatly

They found only small sod and rye cover crops.

reduce NO, losses, even when farm yard manure was applied i n the autumn. These studies po in t t o a potential benefit of cover crops w i t h respect t o reduction i n NO, leaching . However, definitive da ta on the d i fferences among cover crops for their residual NO:, use-efficiency are not available.

With the aforementioned factors i n mind, the objectives of this research were t o : (1) determine the extent of NO, leaching w i t h respect t o fertilization scheme and ( 2 ) evaluate the potential of several cover crops t o capture residual fer t i l izer N from a corn production system.

2

MATERIALS AND METHODS

Two methods were employed t o accompl i sh each objective, each provi di ng unique information. The research was conducted on a Norfolk loamy sand (fine-loamy, si 1 i ceous , thermic Typic Pal eudul t 1 a t the Lower Coastal P1 a i n Research S t a t i o n i n Kinston. T h i s and similar Coastal P l a i n soils are quite suscepti b l e t o leaching 1 osses of ferti 1 i zer N i n crop producti on systems. Selected soil physical and chemical characteristics of the surface 0.15 m prior t o the i n i t i a t i o n of the experiment were as follows: 86% s a n d , 8% s i l t , 6% clay, 2.3 cmol, kg-' CEC, and pH 5.8.

0b.iective 1: Leaching of Fertilizer N

Unl abel ed N Experiment

With the f i r s t method, a gradient of residual fert i l izer N was established by applying 150 or 300 kg N ha'', as ammonium nitrate, t o corn during the 1992 growing season. These rates represent 100 and ZOO%, respectively, of the recommended amount for corn grown i n the Coastal P l a i n . All plots received a broadcast application of 50 kg N ha-' as ammonium nitrate a t corn planting and either 100 or 250 kg N ha-' surface banded 10 cm t o the side of each row approximately 6 weeks after p lan t ing . Following corn harvest i n September 1992, 5-cm diameter soil cores (3 per p lo t ) were taken t o a depth of 90 cm i n 15-cm increments. Soil samples were air dried, extracted w i t h 2M KC1 (10 g soil i n 100 mL) for 1 hr, and analyzed for NH4 and NO:, on a Lachat au to- analyzer.

Labeled N Experiment

A second approach employed 15N methodology under field conditions. Prior t o p l a n t i n g rye and crimson clover cover crops i n early 0 area was chisel plowed and disked. The experiment was hed i n an area t h a t had a previous crop of corn fertilized w i t h 150 kg N ha? A fallow (EO cover crop) treatment and the two monocultures comprised the 3 treatments i n a randomized complete block design experiment w i t h three replications.

Field mi crop1 ots consisting of gal vani zed steel flashing were used t o prevent the lateral movement of water and fert i l izer 15N. The microplots measured 2 by 3 m , w i t h f l a s h i n g installed approximately 10 mm i n to the soil and extending 10 mm above the soil surface. Microplots were divided in to 4 quadrants t o faci 1 i ta te fert i 1 i zer appl i cation. w i t h 10 atom % 15N was uniformly added as a solution t o the soil surface a t 50

Potassi um nitrate 1 abel ed

3

kg N ha- l (ca. 30 g 15N per microplot) on 13 October 1992. simulate a profile distribution of residual fert i l izer N t h a t . m i g h t occur after a corn growing season, the fert i l izer solution was moved i n t o the soil over a 10-day period by approximately 8.0 cm of natural rainfall and applied water. Four soil cores (1.59-cm diameter) were obtained per microplot (1 core per quadrant) and composited i n 15-cm increments t o a depth of 90 cm following the last application of water i n mid-October. Holes were filled w i t h soil from the adjacent untreated area t o minimize any alteration i n soil water dynami cs .

In order t o

Soil samples were air-dried, ground w i t h a mortar and pestle, and analyzed for to ta l i norgani c N as previ ously described. Once the inorganic N concentrati on was determined, a n extract volume containing 40 t o 100 pg N was incubated i n 120-mL specimen cups w i t h 0 .4 g MgO and 0 . 2 g Devarda's a l loy for 6 days (Sorensen and Jensen, 1991). Glass fiber disks (ca. 7 mm diameter) were acidified w i t h 20 pL KHS04 and sealed i n a Teflon packet t o trap gaseous ammonia. Following the 6-day incubat ion period, glass fiber disks were removed from the Teflon packet and desiccated over concentrated H2S04 for 48 h . All 15N analyses were conducted on a Europa Scientific Tracer Mass Stab1 e Isotope Detector.

0b.iective 2: Uti l izat ion of Residual N

For each approach outlined under Objective 1. winter annual cover crops were evaluated for their a b i l i t y t o recover residual fert i l izer N t h a t might otherwise be lost t o leaching during the winter months.

Unl abel ed N Experiment

The experimental area was chisel plowed and disked prior t o p l a n t i n g cover crops i n early October. Crimson clover (Trifolium incarnation L . ) , rye, wheat, and spring o a t (Avena sativa L . by 15.2 m . Seeding rates were 28 kg ha'' for crimson clover and 56 kg ha'' for the grasses. A fallow (no cover crop) and 4 cover crops comprised the 5 main p lo t treatments and two levels of residual soil N represented the s p l i t - p l o t factor i n a randomi zed complete block design w i t h 4 rep1 i cati ons . Wi nter annual weeds i n the fallow treatment were allowed t o grow during the cover crop phase of the study and consisted primarily of henbit (Lamium amptexicaule L . ) and chickweed (Stellaria media L . 1.

were drilled i n p lo ts measuring 5.8

Aboveground cover crop DM was determined i n December, early March, and mid April by harvesting a 0.5-m2 quadrat from each plo t on a l l sampling dates. After the f i r s t sampling date, care was taken t o provide adequate distance

4

between newly. and previously harvested areas. P l a n t samples were dried a t 65OC, weighed, ground, and analyzed for t o t a l N and C on a Perkin Elmer 2400 CHN Elemental Analyzer. Soil sampling was conducted t o a depth of 90 cm i n 15-cm increments a t each p l a n t sampling date. Soil samples were analyzed for to ta l inorganic N i n the same manner as previously described.

In order t o better understand the recycling of N trapped by cover crops, and thereby potentially reduce the N required by the subsequent summer crop, cover crop decomposition was monitored w i t h nylon mesh bags contai n i ng p l a n t residue from the respective cover crops following corn fertilized w i t h 150 kg N ha- l only. prior t o chemical desiccation i n mid April, air-dried on greenhouse benches, and 18.0 g placed i n 1-mm mesh nylon bags (15.2 by 30.5 cm) . This add i t ion corresponded t o a residue loading rate of 3.7 Mg dry matter ha-'. Growth stage a t harvest was heading for rye, boot for o a t and wheat, and mid-bloom for crimson clover. The samples were careful l y hand1 ed t o mi nimi ze detachment or breakage of various p l a n t parts. Prior t o mesh bag placement i n the field, corn was planted v i a no-tillage i n chemically desiccated cover crops. were placed on the soil surface i n the corn interrow on 4 May 1993 and retrieved a t 1,2,4,6,8, and 16 weeks after field placement. Bag contents were dried a t 65"C, weighed, ground, and analyzed for t o t a l C and N. Soil contamination was accounted for by ash ing a 1 -g subsample a t 550°C and a l l p l a n t constituents are reported on a n ash-free basis.

Aboveground who1 e pl a n t materi a1 was col 1 ected immedi ately

Bags

Nonlinear regression equations for percent original N remaining a t each mesh bag retrieval date were determined by f i t t i n g the da ta t o one and two pool models, previously described by Ranells and Wagger (19921, using the Marquardt option of the NLIN procedure developed by SAS (SAS, 1985). A single pool model assumes t h a t all of the p l a n t N will mineralize a t the same rate. The two pool model attempts t o segregate p l a n t N i n t o two groups of mineralizable N; one t h a t quickly mineralizes and a second, more passive pool t h a t releases N more slowly. The general form o f the equations were as follows:

E q . [l] PNR = P + (lOO-P)e-kt Eq. [ Z ] PNR = 100Pe-'lt + 100(1-P)e-k2t where: PNR = Percent nitrogen remaining a t time ' t '

P = N pool(s) K , k l , k 2

An appropriate model was chosen for successful regression and root mean

= rate constant of N release

each cover crop treatment based on square error values.

Label ed N Experiment

Cover crop u t i l i z a t i o n of residual fert i l izer N i n the 15N experiment followed an approach similar t o t h a t previously described: however, a more detailed N balance should be obtained w i t h this method. After profile distribution of "N-enriched fert i l izer occurred, the cover crops and native weeds i n the fallow microplots were allowed t o grow u n t i l corn p l a n t i n g the following April. season i n December, early March, and mid April. In order t o estimate cover crop dry matter and N accumulation for the December and March sampling dates, a 534-cm2 area was harvested i n each microplot quadrant and then composited i n t o one sample. Care was taken t o provide adequate distance between the December and March harvested areas. A l l p l a n t material was dried a t 65% weighed, ground, and analyzed for to t a l C , N , and 1 5 N . For the April sampling date, aboveground biomass was completely harvested i n each mi crop1 o t and allowed t o a i r dry on greenhouse benches before i t was weighed and subsampled for moisture, t o t a l N , and 15N determinations. Soil sampling was conducted t o a depth of 90 cm i n 15-cm increments a t each p l a n t sampling date, w i t h samples processed and analyzed as previously described.

P l a n t and soil samples were taken during the cover crop

Air-dried cover crop material from the April harvest was placed i n newly established microplots where the same unlabeled cover crops were harvested and removed. Corn ('Dekalb-Pfizer 689') was hand planted on 27 April 1993. All microplots received a broadcast application of 33 kg N ha-' as ammonium nitrate a t p l a n t i n g and 67 kg N ha'' as ammonium nitrate surface banded 1 0 cm t o the side of each row approximately 6 weeks after p l a n t i n g . Th i s was done so t h a t corn recovery of cover crop 15N would not be constrained by low available soil N . Previous research i n North Carolina has shown t h a t fert i l izer N a t 90 kg ha-I was sufficient t o optimize corn yields when preceded by crimson clover or hairy vetch cover crops (Wagger, 19896). P , and K were applied broadcast according t o soil tes t recommendations for corn. Residual weed control was provided w i t h 2.24 kg a. i . ha-' alachlor [2-chloro-N-(2', 6 ' -diethyl phenyl -N-(methoxymethyl )-acetamide] and 2.24 kg a . i . ha-' atrazine [6-chloro-N-ethyl -"-methylethyl )-1,3,5-triazine-2, 4-di ami ne]. application of 1.08 kg a . i . ha-' linuron [N'-(3,4-dichlorophenyl)-N-methoxy-N- methylurea] 6 weeks after p l a n t i n g . A t maturity, corn was hand harvested from each microplot, weighed, and subsampled for moisture, t o t a l N , and 15N determinations . Soi 1 sampl i ng was conducted after corn harvest and analyzed for inorganic N and 15N as previously described. Data was analyzed by treatment us ing SAS GLM procedures (SAS, 1985).

Lime,

Post-emergent weed control was provi ded by a broadcast, d i rected

6

RESULTS AND DISCUSSION

Unlabeled N Experiment

Residual Soil N and Cover Crop Performance

I t was anticipated t h a t the 150 and 300 kg N ha-l rates applied t o the previous corn crop would provide a soil inorganic N gradient from which t o evaluate the a b i l i t y of various cover crops t o capture residual fert i l izer N. The i n i t i a l soil sampling following corn harvest i n September 1992 revealed si gni f i cant (p=O. 10) di fferences i n the distribution of inorganic N between N rates applied t o the previous corn crop ( F i g . l a ) . increase w i t h depth i n soil inorganic N levels under corn fertilized w i t h 150 kg N ha-', ranging from a low of 5 t o 8 mg kg-' i n the upper 30 cm t o 18 mg kg-I between 60 t o 90 cm. In contrast, residual soil inorganic levels following corn fertilized w i t h 300 kg N ha'' ranged from 11 t o 33 mg kg-', being significantly higher by approximately two-to threefold a t most depths compared t o the low N rate treatment. rates occurred i n the 45- t o 75-cm depth range. Without some form of intervention, these elevated soi 1 inorganic N concentrations (ca. 33 mg kg-') this deep i n the soil profile would likely be lost from the p l a n t - s o i l system before p lan t ing of next year's row crop. In this study, precipitation from cover crop seeding i n October t o termination of cover crop growth and corn p lan t ing the following April was 50 cm.

There was a modest

Moreover, the largest differences between N

The hypothesis was t h a t the above-mentioned gradient i n residual soil inorganic N would subsequently be influenced by cover crop species due t o differences i n rooting characteristics, growth rates, and N requirements . Cover crops were seeded i n early October 1992 and by December only rye had shown a dry matter (DM) response (F ig . 2) due t o residual soil inorganic N levels. wheat > fallow = crimson clover. The corresponding cover crop N accumulation values reflected a similar pattern, w i t h the N content i n rye following the h igh N rate nearly double (39 vs. 21 kg N ha-l) t h a t of rye after corn fertilized w i t h 150 kg N ha-l (F ig . 3) . There was no difference i n N accumulation among the other cover crop treatments due t o prior N rate, w i t h mean values across N rates ranging from 4 kg N ha-' i n crimson clover t o 17 kg N ha-' i n spring oa t .

I n general, DM accumulation values were i n the order of rye > o a t >

Even though cover crop N accumulation differed between species by December, there was l i t t l e difference i n soil inorganic levels due t o cover crop. Consequently, soil inorganic N results for this date are presented as mean values across cover crops for the 150 and 300 kg ha-' N rates ( F i g . l b ) . From

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NO3 + NH4, mg kg"

20

0

Q 60

80

0 10 20 30 I I I

0 10 20 30 I I

mber

8

3

> Q

3

1 Q

L

) December A50 kg N ha-'

L 8.1 A

LSD cover LSD N rate

LSD cover LSD N rats

0.32

1 .o 0.65

F 0 R w C Cover crop

Fig. 2. Dry matter accumulation patterns by fallow (F), oat (0), rye (R), wheat 0, and crimson clover (C) cover crops following the previous com crop fertilized with 150 or 300 kg N ha-'. LSD values are significant at the 0.05 level of probability.

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September t o December there was a marked decline i n soil inorganic N concentrations under both N rates : however, differences were sti 11 evident between N rates. Soil inorganic levels i n the upper 45 cm were low (1 t o 3 mg kg-l) and similar between N rates. These same relatively low concentrations persisted through the remainder of the profile under the low N rate b u t increased below 45 cm under the high N rate. center of highest inorganic N concentration (ca. 17 mg kg-') had shifted from 60 t o 75 cm i n September t o 75 t o 90 cm i n December.

I t i s also of note t h a t the

Cover crop DM accumulation increased appreciably from December t o March as spring regrowth proceeded ( F i g . 2a and b ) . Averaged across N rates, rye accumulated the highest DM (2 .64 Mg ha'') and wheat accumulated the least (0.87 Mg ha-'). Also noteworthy is the result t h a t mean DM i n the fallow treatments, which was composed o f winter annual weeds, exceeded DM values for both wheat and crimson clover. With regard t o prior N rate, and similar t o December results, only rye showed a marked DM increase (3.27 vs 2 . 0 1 Mg ha-') when following the 300 vs 150 kg ha-' N rate. Cover crop N accumulation paralleled the DM results, w i t h mean rye N content (40 kg ha-') approximately two t o three fo ld greater t h a n the other cover crop treatments and rye N accumulation increasing 55% under h i g h residual soil N (Fig. 3 ) . Spring oat a t the h i g h residual soil N level accumulated the same amount of N as rye a t the low residual N level. Nitrogen accumulation by wheat, crimson clover, and fallow treatments were similar and unaffected by prior N rate.

There were no significant differences due t o cover crop i n the distribution o f soil inorganic N w i t h the prior fert i l izer N rate of 150 kg ha-' by March 1993, consequently, d a t a are presented for the prior 300 kg N ha-' rate only (Fig. 4 ) . A distinct pattern was evident i n soil inorganic levels, w i t h significant (p=O.lO) treatment differences confined t o the lower two soil depths and concentrations i n the order of rye < spring o a t < wheat, crimson clover, and fa1 low. Soi 1 inorganic N concentrations under rye were general l y < 1 mg kg-1 throughout the profile compared t o a mean of 13 mg kg-I under wheat, crimson clover, and fallow plots a t the 75- t o 90-cm depth. These results, and the concomitant N accumulation values, reflect the a b i l i t y of rye t o effectively utilize residual soil N .

Just prior t o cover crop desiccation and corn p l a n t i n g i n April 1993, DM accumulation by rye, spring o a t , and crimson clover was similar and greater t h a n wheat and fallow treatments (Fig . 2 ) . These DM values were i n the range commonly reported i n other studies for the region. Only rye and spring oat responded t o residual soil N level, increasing an average of 45% (1.37 Mg ha-') from low t o h i g h residual soil N plots. The corresponding N accumulation values, averaged over residual soil N level, were i n the order o f

10

(a) December

40

28

0

0

0

60

0

20

cov

LSD cover 6.3

LSD N rats 6.8

LSD COVBB 18.2

Fig. 3. Nitrogen accumulation patterns by fallow (F), oat (0)' rye (R), wheat (W), and crimson clover (C) cover crops following the previous corn crop fertilized with 150 or 300 kg N ha-'. LSD values are significant at the 0.05 level of probability.

11

NO3 + NH4, mg kg"

20

40

60

80

0 3 6 9 1 2 1 5 INS (a) March

0 3 6 9 1 2 1 5 - - I I I I I

(b) April

Fig. 4. Distribution of soil inorganic N in March (a) and April (b) 1993 as affected by cover crops following the previous corn crop fertilized with 300 kg N ha-'. The symbols 4- and * indicate significant treatment differences at 0.1 0 and 0.05 probability levels. respectively, while NS = nonsignificant.

12

crimson clover = rye > oat > wheat> fallow (Fig . 3 ) . As w i t h earlier sampling dates. rye N accumulation increased sharply (108%) under h i g h compared t o low residual soil N levels. Spring oat also responded t o residual soil N level by April. accumulating 27 kg N ha"' a t the low residual soil N level compared t o 50 kg N ha-l a t the h i g h residual N level. Only the fallow treatment, comprised of winter annual weeds whose growth had terminated by the April sampling date, reflected a net decrease i n N accumulation. Associated w i t h these relatively low N accumulation values i n fallow plots were the greatest soil inorganic N concentrations, most notably i n the 60- t o 75- ( 7 mg kg-l) and 75- t o 90-cm (9 mg kg-') depth intervals ( F i g . 4). Wheat, crimson clover, and spri ng oat treatments had i norgani c N concentrati ons i ntermedi ate t o fallow and rye (1 mg kg-l) . While this range i n soil inorganic N concentrati ons was re1 a t i vely smal 1 just prior t o p l a n t i n g corn, NO3 movement below the 90-cm sampling depth appears likely given the differences i n cover crop N accumulation.

Cover Crop N Release Rates

Nitrogen release patterns, expressed as a percentage of the i n i t i a l residue N remaining, are illustrated i n F ig . 5. An exponential equation reflected the decline i n residue N over time for a l l curves, w i t h only marginal differences between cover crop residues. These results are somewhat surpri si ng , as previous work has shown distinct differences between cover crop N release rates, particularly between grasses and legumes (Wilson and Hargrove, 1986; Wagger. 1989a). Residue decomposition and N release proceeded a t a relatively rapid rate, such t h a t by 4 weeks i n the field the N remaining i n the respective p l a n t residues ranged from 42 t o 50%. By 8 weeks, which corresponded w i t h corn tasseling/silking and a period of high corn N demand, the percentage N remaining ranged from a low of 18% for wheat t o 27% for rye. The general order of cover crop N release by 16 weeks was crimson clover > wheat = rye > o a t , w i t h a mean residue N remaining value o f approximately 8%.

Inspection o f the i n i t i a l residue C:N ratios provides some explanation for the relatively small differences i n N release rates between the residues. The N concentration or C : N ratio of p l a n t residues has frequently been used as a tool for predicting the rate o f decomposition. O a t , rye, and wheat had C:N ratios i n a very narrow range of 35 t o 37:l while the C : N ratio of crimson clover was 19: l . Although the C:N ratio of crimson clover was below the theoretical value (25: l ) above which net N immobilization occurs (Allison, 19661, and the grasses were above this threshold value, these differences are not overly large. Another contributing factor may have been the cover crop developmental stage a t the cessation of growth. Ranells and Wagger (1992) reported t h a t a greater proportion of crimson clover N was released from clover collected a t the late vegetative stage compared t o harvest dates

13

ranging from early bloom t o late seed se t . Averaged over 2 years, C:N ratios increased from 1 4 : l t o 18:l between late vegetative and late bloom while cellulose and l i g n i n concentrations increased a n average of 48%. Of the structural carbohydrates, 1 i gni n i s the most resistant t o decomposition by microorganisms and i t s presence i n cell walls can retard the degradation of cell u l ose (Szegi , 1988; Waksman and Hutchi ngs , 1936) .

I t is important t o ob ta in some index of N a v a i l a b i l i t y based on the i n i t i a l residue N content. Estimates of N released from each cover crop were calculated from values generated by the decomposition curves and then multiplied by the cover crop N content a t the time of desiccation for cover crops following the previous low ferti l izer N rate (150 kg ha-'). Using this approach, the potentially available N by 16 weeks was 59 kg N ha-l for crimson clover, 32 kg N ha-' for rye, 25 kg N ha-' for o a t , and 17 kg N ha-' for wheat.

14

1

W

z a> 3 U u) Q)

.II

61:

00

80

60

40

20

sy.x= 5 Sy.x=ll sy.x= 4

-o.iax. - Rye y=4+96e -0.28~. - - Oat y=5+95e

- - Wheat y=6+94e I -0.28~. ...........I lover y=3+97e -0**2x; sy,x- -

Fig. 5. Percentage of initial cover crop N remaining during the 1993 corn growing season. Sy.x represents the root mean square error.

15

15N- Label ed Experiment

Following the application of 15N labeled KNO, t o microplots planted t o the respective cover crops, soi 1 sampl i ng was conducted t o determi ne the residual profile NO3 distribution. Averaged across microplots, approximately 55% of the labeled fer t i l izer was i n the surface 15 cm of soil and 34% i n the 15- t o 30-cm depth (da ta not presented). was relatively uniform, inc luding detectable enrichment i n the 75- t o 90-cm soil layer. The intent was t o facil i tate a somewhat deeper and more uniform distribution of 15N i n the soil profile b u t apparently 7 . 5 cm of irrigation water was not sufficient for this t a s k . t o characterize the precipitation environment from cover crop p l a n t i n g t o termination of growth the following spring. general l y bel ow normal precipitation prevai 1 ed (Tab1 e 1) , which would moderate further downward movement of NO3.

Below 45 cm the distribution of 15N

In this regard, i t i s also important

From October 1992 t o April 1993

Estimates of fert i l izer I5N recovered by cover crops i n December and March were based on small area samples i n each microplot. By mid December 1992, < 1% of the applied 15N had been recovered i n each cover crop treatment (Table 2 ) . The results were unexpected since there were substantial differences i n cover crop DM estimates, ranging from approximately 0.75 Mg ha-' for fallow and crimson clover treatments t o 1.85 Mg ha-' for rye. However, profile soil inorganic N was considerably lower under rye (16 kg N ha-') compared t o crimson clover (53 kg N ha-l) and fallow (71 kg N ha- ' ) . By early March 1993, estimates of cover crop I5N recovery had increased considerably from December values. Winter annual weeds i n fa1 1 ow mi crop1 ots had accumul ated l o % , crimson clover 5%, and rye 20% of the fert i l izer 15N applied the previous fa l l (Table 2 ) . The associated soil inorganic 15N values i n March reflected the continued a b i l i t y of rye t o better utilize residual soil N t h a n crimson clover or winter annual weeds. Profile soil inorganic N i n March decreased sharply from December levels, yet there was nearly a fourfold difference between rye (6 kg N ha-') and crimson clover/fallow (22 kg N ha-I) treatments.

Just prior t o termination of cover crop growth i n April, recovery of residual I5N i n crimson clover and fallow treatments was unchanged from March estimates (Table 2 ) . In contrast, rye I5N recovery increased from 20 t o 35%. results are similar t o those from a study conducted on the Maryland Coastal P l a i n , where average percent recoveries of f a l l N were 8% for native weed cover, 8% for crimson clover, and 45% for rye (Shipley e t a l . , 1992). Based on the i n i t i a l fer t i l izer 15N app l i ca t ion of 50 kg ha- ' , N recovery was 2.5, 5, and 17.5 kg ha-' for crimson clover, fallow, and rye, respectively. As might be expected, profile soil inorganic N declined t o relatively low amounts from March t o April but levels were approximately threefold greater under fallow and crimson clover treatments compared t o rye.

These

16

Table 1. Precipitation during the 1992-1993 cover crop growing season and departures from 30-yr averages ( in parentheses).

Month Precipitation

cm October 6 .3 ( -1 .4) November 4 .0 ( -3 .2) December 6 .6 (-2.3)

February 4 .0 (-5.5) March 6.2 (-3.9) Apri 1 8.5 ( - 0 . 4 )

January 14.1 ( 3.3)

17

Table 2. Cover crop recovery o f f e r t i l i z e r 15N and p r o f i l e s o i l inorganic N content a t various times during the 1992-1993 cover crop season.

Cover crop 15N recoverv P r o f i l e s o i l inorganic N Cover Crop Dect Mart APr Dec Mar APr

- % of t o t a l I5N - kg ha-'

Fallow < 1 10 10 a' 7 1 a 22 a 15 a

Crimson clover < 1 5 5 a 53 a 22 a 15 a

Rye < 1 20 35 b 16 b 6 b 5 b

t I5N recovery values are estimates based on small area samples w i th in

4 Means w i t h i n each date followed by the same l e t t e r are not s i g n i f i c a n t l y m i c rop lots

d i f f e r e n t a t p > 0.05.

18

Corn Recoverv of Residue 15N

Total cover crop N a t desiccation i n April was 19, 21, and 61.kg ha-I for fa l low, crimson clover, and rye treatments, respectively (da ta not presented). These values represent the cover crop N pool potentially available t o the subsequent corn crop. limited i t s potential N contribution t o corn,

Poor stands i n crimson clover mi crop1 ots severely

Approximately 5 wk after returning I5N labeled cover crop residues t o new microplots, estimates of corn recovery of residue N were < 1% for a l l treatments (Table 3) . when p l a n t demand for N i s relatively small compared t o later stages of development . during cover crop decomposition, i ndi cates very di sti nct d i fferences i n available residue N . As a percentage of the total residue I5N, about 6% of the rye N , 15% of the native weed N , and 34% o f crimson clover N resided i n the soil inorganic N pool. grass vs. legume C:N ratios w i t h respect t o governing residue decomposition. Three wk later (25 June), corn had recovered nearly 11% o f the N i n crimson clover compared t o 6% i n fa l low and 2% i n rye treatments. The amounts of residue 15N i n soil inorganic N pools a t this date were low, such t h a t treatment differences were general l y of no agronomic consequence.

Corn was a t the V3 t o V4 developmental stage, a period

The inorganic soi 1 15N pool , which a1 so represents N mineral i zed

The results i l lustrate the relative differences i n

Corn recovery of residue 15N by physiological maturity i n early September followed a pattern similar t o the mid-season sampling date (Table 2). In summing up the t o t a l residue 15N available dur'ing the corn growing season, including a n estimate of 15N i n corn roots, approximately 23% of the crimson clover N was released compared t o 14% of the native weed N and 7% of the rye N . Based on the i n i t i a l N content of the respective cover crops, these percentages equate t o only 3 , 5, and 4 kg ha-l of available N from fallow, crimson clover, and rye, respectively. These N contributions would have no impact on corn yield potential,

Finally, these corn recovery values o f residue I5N were less t h a n h a l f of the estimated 80 t o 90% of i n i t i a l N t h a t was released from grass and legume residues during the corn growing season i n the mesh bag experiment a t the same location. approach only provide information w i t h respect t o a potentially available N pool , and therefore do not afford a direct measurement of residue-N contributions t o a cropping system. 15N-labeled residues i n this study suggest considerable overestimation of N contributions from cover crop residues based on a mesh bag technique.

I t should be noted t h a t N-ava i l ab i l i t y estimates using a mesh bag

Nevertheless, results obtained w i t h

19

Table 3. Distribution of I5N recovered from cover crop residues a t various times during the 1993 corn growing season.

Corn 15N recovery Profi l e soi 1 inorganic 15N

Cover Crop 6 Junt 25 Junt 9 Sep 6 Jun 25 J u n 9 Sep

% o f total residue 15N

Fa1 1 ow < 1 6 .2 9.7(10. 9Itas 14.7 a 4 . 2 a 2.8 a

Crimson cl over < 1 10.6 16.8(18.8) b 33.8 b < 1 b 4 . 0 a

Rye < 1 2 . 2 6 .1 (6.9) a 5 . 7 ~ < l b < l a

t 15N recovery values are estimates based on sampling 3 corn plants in each mi crop1 o t .

$. The value in parentheses represents a n estimate of corn N recovery when the root 15N component i s included and i s based on roots comprising 15% o f the total plant dry weight a t physi 01 ogi cal maturity .

§ Means within each date followed by the same letter are not significantly different a t p > 0.05.

20

LITERATURE CITED

Allison, F. E. 1966. The fa te of nitrogen applied t o so i l s . Adv. Agron. 18: 219-258.

Bertilsson, G . 1988. Lysimeter studies o f nitrogen leaching and nitrogen balances as affected by agricultural practices. Acta Agric. Scand. 3813-11.

Blackmer, A. M. 1986. Potential yield response of corn t o treatments t h a t f e r t i l i zer nitrogen i n so i l s . Agron. 3. 78:571-575. conserve

Hallberg, G . R quality.

Hallberg, G . R i mpl i ca t

1986. From hoes t o herbicides : Agriculture and groundwater J . Soi 1 Water Conserv . 41 : 357 -364.

1987. Agri cultural chemical i n groundwater : Extent and ons. Am. J . Altern. Agric. 2:3-15.

Karraker, P . E . , C . E . Bortner, and E . N . Fergus. 1950. Nitrogen balance in lysimeters as affected by growing Kentucky bluegrass and certain legumes separately together. Bull. No. 557. Kentucky Agric. Expt . S t a . , Univ. of Ky.. Lexington, KY.

Keeney. D. R . 1986. Sources of nitrate t o groundwater. CRC Crit . Rev. Envi ron. Control. 16257-304.

Langdale, G . W., A. P . Barnett, R . A . Leonard, and W. G . Fleming. Reduction of soil erosion by the no-ti l l system in the Sout Trans. Am. SOC. Agric. Eng. 22:82-86 and 92.

Oberle, S. L. and D . R. Keeney. 1990a. Factors influencing c requirements in the Northern U.S. Corn Belt. 3 . Prod. A

Oberle, S . L . and D. R , Keeney. 1990b. Soil type, precipitat f e r t i l i ze r N effects on corn yields. J . Prod. Agric. 3:522-527.

Pelchat, J . A. 1986. Effects of t i l l age and winter cover crops on nitrogen requirements of corn in Indiana. Lafayette, Ind. Diss. Abst. DA8/00940. 186 pp.

Ph.D. D i s s . Purdue Univ., West

Ranells. N. N., and M. G . Wagger. 1992. Nitrogen release from crimson clover in relation t o plant growth stage and composition. Agron. 3. 84:424-430

SAS Inst i tute , Inc. 1985. SAS user’s guide: Stat is t ics . 5 ed. SAS Inst i tute , Inc,

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Shipley, P. R . , J . J . Meisinger, and A . M. Decker. 1992. Conserving residual corn fer t i 1 i zer nitrogen with. winter cover crops. Agron. J . 84:869-876.

Sorensen, P . , and E. S . Jensen. 1991. Sequential diffusion of ammonium and ni t ra te from soil extracts t o a polytetrafluroethylene trap for I5N determination. Anal. Chim. Acta. 252:201-203.

Szegi , J . 1988. Cell ulose decomposi t i on and soi 1 fer t i 1 i t y . Akademi ai Ki ado, Budapest , Hungary .

Utomo, M . 1986. Role of legume cover crops in no-tillage and conventional t i 11 age corn production. Ph . D. Di ss . Uni v . of Kentucky, Lexington , K Y ,

van Bavel , C. H . M., and F . T . Verlinden. 1956. Agricultural drought i n North Carolina. North Carolina Agric. Exp. S t n . Tech. Bull. No. 122.

Wagger, M. G . and D. B . Mengel. 1988. The role of nonleguminous cover crops in the efficient use of water and nitrogen. pp. 115-128. &I. W . L . Hargrove (ed.) Cropping strategies for efficient use of water and nitrogen. ASA Spec. P u b l . 51. ASA, CSSA, and SSSA, Madison, WI.

Wagger, M . G . 1989a. Time of desiccation effects on plant composition and subsequent nitrogen re1 ease from several w i nter annual cover crops. Agron. J . 81 236-241.

Wagger, M . G . 1989b. Cover crop management and nitrogen rate in relation t o growth and yield of no-ti l l corn. Agron. J . 81:533-538.

Waksman, S. A. and I . J . Hutchings. 1936. Decomposition of lignin by mi croorgani sms . Soi 1 Sci . 41 : 119-130.

Wilson, D . O . , and W . L. Hargrove. 1986. Release of nitrogen from crimson clover residue under two t i l l age systems. Soil Sci . SOC. Am. J . 50: 1251- 1254.

22

GLOSSARY

Abbrevi a t i ons (units o f measurements 1

mg kg-I milligrams per kilogram

kg ha-' Kilograms per hectare

Mg ha-' megagrams per hectare

23

AcknowledgmentsAbstractList of FiguresList of TablesSummary and Concl usi onsRecommendationsIntroductionObjective 1 - Leaching of fertilizer N

Results and DiscussionUnl abel ed N ExperimentResidual soil N and cover crop performanceCover crop N release rates

Labeled ExperimentCover crop recovery of 15NCorn recovery of residue I5N

Literature CitedG1 ossary