Plant and Soil 44, 193-200 (1976) Ms. 2689

I.

S O M E M E A S U R E S O F R E D U C I N G L E A C H I N G

LOSS O F N I T R A T E S B E Y O N D P O T E N T I A L

R O O T I N G Z O N E

PROPER CO-ORDINATION OF NITROGEN SPLITTING

WITH WATER MANAGEMENT

by BIJAY SINGH and G. S. SEKHON

Department of Soils, Punjab Agricultural University, Ludhiana (India)

SUMMARY

Distribution patterns of nitrate in field are studied in twelve treatments comprising of different N splits and irrigation schedules, after the harvest of wheat. Total amount of irrigation and nitrogen application were kept same for each treatment. The curves show tha t heavy irrigation at greater intervals can result in larger amount of unutilised NOa--N, which will eventually be lost beyond potential rooting zone. As irrigation becomes lighter and frequent, nitrates travel slowly and thus remain for more time within the reach of roots and are lost to a less extent. When whole of the nitrogen is applied in one lot, considerably more NOa- -N is lost under all the irrigation schedules. As the number of splits are increased, susceptibility of nitrate nitrogen for leaching decreases to a greater extent under lighter and more frequent irrigation schedule than the other. Besides N-splitting and irrigation criteria, efficiency and depth of rooting system of plants seems to play a major role in defining nitrate leaching patterns towards unsaturated zone.

INTRODUCTION

The interest in nitrate movement through soils was for a long time restricted to an agricultural concern about loss of available nitrate from the root zone of soils. This loss results in lower yields of crops. In the last few years, with a tremendous increase in the concern for NO~- in underground waters, the emphasis has changed to the study of nitrate leaching towards unsaturated zone because of the potential pollution of waters.

Although many workers 2 4 6 have related the movement of

194 BIJAY SINGH AND G. S. SEKHON

nitrates through soil to one or more factors that affect its movement, a majori ty of the studies have been conducted with soil columns in laboratories. Some field studies on nitrate movement have also been conducted 1 3 7 9 but there is little or no information concerning N O 3 - status of soil profiles and unsaturated zone in relation to split application of nitrogen and irrigation schedules.

In Punjab, with its short rainy season and a long dry season, intensive irrigation is required for successful crop production. With a particular irrigation schedule, proper management of fertilizer nitrogen is equally essential so as to get a minimum loss of nitrate from rooting zone and thus also to avoid the risk of ground water pollution. Accordingly present s tudy aims to investigate nitrate leaching patterns from root zone to unsaturated zone under various irrigation schedules and N-splitting treatments.

MATERIALS AND METHODS

Soil s amples were t a k e n w i t h screw t y p e auge r a t 15 cm i n t e r v a l s u p t o 225 cm d e p t h in t h e twe lve p lo t s of t he field e x p e r i m e n t c o n d u c t e d a t P A U F a r m , L u d h i a n a . Twelve p lo t s were compr i sed of 4 N- sp l i t t i ng t r e a t m e n t s a n d 3 i r r iga t ion schedules . Var ious soil cha rac t e r i s t i c s of t h e e x p e r i m e n t a l s i te are g iven in T a b l e 1.

TABLE 1

Characteristics of the soil of experimental site

Soil depth Sand Silt Clay pH CaCOa O.C. cm % % % % %

0-- 45 93.8 2.1 4.1 8.5 0.2 0.21 45- 90 89.7 3.9 6.4 8.4 0.1 0.19 90-135 87.2 4.8 8.0 8.5 0.3 0.23

135-180 85.1 4.7 10,2 8.6 0.3 0.14 180-225 83.7 6.9 9.4 8.6 0.4 O. 13

Af te r a p re sowing i r r iga t ion of 8 cm, f i r s t sp l i t of N, 26.2 kg P / h a a n d 49.8 kg K / h a were i n c o r p o r a t e d in t he soil a n d K-227 v a r i e t y of w h e a t (Tr i t i - c u m a e s t i v u m L.) was sown on N o v e m b e r 9, 1973. A t o t a l of 150 kg N / h a as u r ea was app l ied in 4 sp l i t t i ng t r e a t m e n t s as whole a t sowing, two splits, t h r e e spl i ts a n d / o u r spli ts . Seven i r r iga t ions of 5.5 cm each, 5 i r r iga t ions of 7.5 cm each a n d 4 i r r iga t ions of 9.5 cm each c o n s t i t u t e d 3 i r r iga t iona l t r e a t m e n t s . I n each case, n e x t i r r iga t ion was app l i ed w h e n w a t e r loss as i nd i ca t ed b y p a n e v a p o r a t i o n b e c a m e equa l to t h e a m o u n t of w a t e r to be app l ied a t t ime.

NITRATE LEACHING BEYOND POTENTIAL ROOTING ZONE. I ] 95

Thus for each of t he twelve t r e a t m e n t s to ta l a m o u n t of n i t rogen and w a t e r appl ied were equal. The schedule of i r r igat ion and n i t rogen appl ica t ion is shown in t ab le 2.

TABLE 2

Schedule of irrigation* and nitrogen application

Irrigation Dates of irrigation and nitrogen application** rate (cm)

5.5

7.5

9.5

Nov. 27, Dec. 12 N, Jan. 20 (N), Feb. 2 (N), Feb. 18, Mar. 3, Mar. 12, Mar. 21

Nov. 27, Dee. 12 N, Jan. 29 (N), Feb. 18 (N), Mar. 6, Mar. 21

Nov. 27, Dec. 18 N, Jan. 29 (N), Feb. 24 (N), Mar. 14

* Rainfall of 4.2 cm was received on Dec. 16, 1973. ** N in parenthesis denotes date of nitrogen application as well as of irrigation.

Soil samples collected on Apri l 16, 1974, af ter w h e a t ha rves t were imme- d ia te ly b r o u g h t to t he l abora to ry and weighed. These were dr ied a t 60°C and reweighed and mois tu re con t en t was de te rmined . N i t r a t e was e s t ima ted f rom dr ied samples by phenol d isulphonic acid m e t h o d as descr ibed by B r e m n e r 5 A l i t t le modi f ica t ion was made in t h a t N a O H - E D T A ins tead of NH4OH was used to p rov ide alkaline m e d i u m to 6-ni t rophenol-2, 4 d isulphonic acid, in which the la ter c o m p o u n d gives yellow colour to be measured a t a wave- l eng th of 410 n m on Spectronic-20.

Concen t r a t ion of NO~- in soil solut ion was c o m p u t e d f rom mois ture per- cen tage (weight basis) and n i t r a t e c o n t e n t of dr ied soil.

RESULTS AND DISCUSSION

Depthwise nitrate distribution under twelve treatments is shown in Figure 1. Distribution pattern has been strongly influenced both by N-splitting and irrigation criteria. As the rate of irrigation is increased while its frequency decreased, more NO3--N remains unutilized by crop plants for subsequent leaching to deeper soil layers. With lighter and frequent irrigation schedule, nitrates from only third and fourth split of nitrogen remain unutilized. Thus considerably less amount of NO3- is leached beyond potential rooting zone in the latter case.

Absorption of nitrates by roots, the very purpose for which nitrogenous fertilizers are applied, seems to be the major mechanism through which nitrate loss to deeper soil layers can be prevented. For

196

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BIJAY SINGH AND G. S. SEKHON

180

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,~ 210-

; ag i~:351g e r a r y s o i l _ . l o , ~ . , 3 o , ~ , 5,0

A

//

I 0 2 0 30 50 i . . . . . &l

i " Fig. 1. Nitrate distribution in soil profiles under various

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irrigation and nitrogen split treatments after wheat crop (Sampled on April 16, 1974).

A 9.5 cm irrigation o single dose B 7.5 cm irrigation • 2 splits C 5.5 cm irrigation x 3 splits

zx 4 splits

I I

C

roots to be efficient nitrate absorbers, they must be ensured in initial stages of the growth with a proper supply of nutrients (including N) and water. When a heavy and infrequent irrigation is applied in initial stages of the crop, most of the water is lost beyond the reach of the roots. Along With water-soluble anions like NO3- also move to deeper layers and roots are deprived of them. Thus under 9.5 cm treatments in the present study, resulting rooting system is not fully developed. With such inefficient rooting system, most of the applied NO3- will be either lost to deeper soil layers or will remain unutilized in the root zone depending upon the irrigation schedule. A peak in the treatment 1/4 N 9.5 at 75 cm is also due to the relatively unutilized NOa--N applied in third and fourth splits.

In comparison to 9.5 cm treatment, rooting system with 7.5 cm irrigation at a time, is more efficient. Along with more N-utilization, there is also higher water uptake in this case. Under such conditions. nitrate distribution with various N split treatments, will closely resemble the patterns obtained in Figure IB. A peak at about

NITRATE L E A C H I N G B E Y O N D POTENTIAL ROOTING ZONE. I 197

180 cm in treatment NF 7.5 is due to unused nitrate fraction as whole of 150 kg N (per hectare) was applied at sowing. Probably such peak in the treatment NF 9.5 is already below 225 cm depth.

Nitrate nitrogen does not move rapidly to deeper soil layers when rate of irrigation is less. Even with frequent irrigations, nitrates will remain within the reach of roots for a longer time and thus will be more susceptible to absorption. The curves in Figure 1C elucidate these points by showing only a little amount of NO3--N in the pro- files. As the number of splits is increased, more NOa--N is available in the profile for absorption by roots throughout the season. These observations point towards a greater leaching loss of nitrogen when applied as a single dose or two splits because whole of the nitrogen is not utilized by the plants within these intervals and also sub- sequent irrigations leach a major part to depths where roots are not active.

These points become clear when one considers area under each of the twelve curves in Figure 1. In Table 3 are given amounts of NOs- -N present in various treatment profiles upto 180 cm depth expressed as percentages of the total fertilizer applied. These data are tabulated from the area under various curves, assuming bulk density to be 1.50 throughout the profiles. Maximum amount of unutilised NOa--N is found in 9.5 cm treatments while minimum in 5.5 cm treatments, showing the efficiency of the rooting system developed under these moisture regimes. These data emphasize that when whole of the nitrogen was applied in one lot, most of it was lost to deeper layers before the sampling date. When nitrogen was splitted, only a portion of the first split was lost, while rest remained in the profile maintaining a supply to the roots. With intermediate

TABLE 3

Amount of NOn--N, expressed as percentage of total ap- plied nitrogen, present in 180 em soil profiles of various

t rea tments on April 16, 1974

N applicat ion I r r igat ion

rate (cm) Single dose 2 splits 3 splits 4 splits

5.5 14.5 17.9 34.1 47.4 7.5 29.5 21.3 32.1 32.0 9.5 39.4 44.3 53.5 88.8

198 B I J A Y SINGH AND G. S. SEKHON

irrigation rate (7.5 cm) residual nitrogen is intermediate of the two extremes and as expected there is only little difference for various N-splitting treatments.

Various mechanisms occurring in the soil in relation to leaching of nitrates under different treatments of the present s tudy are also exhibited by the data shown in Table 4. Data depict the concentra- tion of NO3- in soil solution in the 195-225 cm layer on April 16, 1974. This layer has been selected on the assumption that plant roots do no affect the volume and composition of soil solution beyond 2 m depth. These data are also a reflection of all what had been happening in 0-2 m soil layer. Besides amount and time of nitrogen and water application, these data are also affected by nitrate and water uptake behaviour of plants. At the time of sampling, depthwise moisture content (weight basis) was almost constant in 180-225 cm layer.

Data in Table 4 show that under lighter and more frequent irrigations, as the number of N-splits increases, soil solution con- centration of nitrate decreases at 195-225 cm depth. Under heavy and infrequent irrigations the reverse is true. Thus these data again support the fact that maximum N-utilization was in 5.5 cm treat- ment followed by 7.5 and 9.5 cm treatments.

TABLE 4

Nitrate ion concentration (mg NOs-/I) of soil solution at 195- 225 cm depth on April 16, 1974

Irrigation N application

rate (era) Single dose 2 splits 3 splits 4 splits

5.5 111.2 117.1 84.1 74.0 7.5 246.7 102.9 93.5 70.2 9.5 86.9 95.0 126.4 186.8

When nitrogen was applied in one lot, concentration of soil solution in 5.5 cm treatment was low because relatively smaller portion was left for leaching. In 9.5 cm treatment most of NOs- -N along with water was available for leaching beyond 225 cm depth. Value 86.9 mg NO3-/litre of soil solution is the result of 3 dilutions which occured because of later irrigations. Highest figure is for 7.5 cm treatment when N is applied in one lot because there was a

NITRATE LEACHING BEYOND POTENTIAL ROOTING ZONE. I 199

moderate N-utilization and moderate movement of irrigation water beyond 225 cm depth at the time of sampling.

When nitrogen was applied in two parts, by the time second half was applied, plants had developed considerably extensive rooting system and absorbed nutrients and water accordingly. Probably in the 5.5 cm treatment both nitrogen and water were absorbed in greater amounts, as these remained within the reach of roots for a longer time. Thus soil solution concentration is little affected as compared to NF treatment. After column studies W a r r i c k et al. s

have found that NO3- or C1- ion peaks are at shallower depth than the water front, when water is applied. Reason being that maximum solute concentration occurs at depth above which the total water in the profile is iust equal to the cumulative infiltration. Thus while in the 5.5 cm treatment both nitrogen and water were in the reach of roots, in the 7.5 cm treatment only nitrogen is within the reach of roots and a good amount of water escapes unabsorbed by roots. This phenomenon results in a lower concentration of NO3- in soil solution since even after the application of second split 4 irrigations were to be applied. An increase in the soil solution concentration in the 9.5 cm treatment can also be explained on this basis. In this case both N and water were unavailable to plant roots, but the slight increase in concentration is probably because the second split of N was diluted only twice as compared to 3 times when N is applied in one lot.

When N-splits are increased to 3 or 4, a further increase in soil solution concentration in 9.5 cm treatment at 195-225 cm depth can be explained in term of the hypothesis of W a r r i c k et al. s. Perhaps only a small fraction of the first dose of N was lost by leaching, while rest two or three doses were still in the profile in an unabsorbed form because of inefficient rooting system. As most of the water percolated down before more than 50 per cent of applied N leached beyond 180 cm, resulting solution becomes concentrated and it was ex- hibited by the figures 126.4 and 186.8 mg/1 respectively for 3 and 4 split treatments. In the treatments having 3 or 4 splits and irrigation rate 5.5 and 7.5 cm, only a small amount of unutilized N is leached beyond rooting zone. I t was because water and nitrogen supply was equally distributed throughout the cropping season. Thus the resulting concentration of the soil solution just below the potential rooting zone is low as is shown in table 4. Although movement of

5 0 0 NITRATE LEACHING BEYOND POTENTIAL ROOTING ZONE. I

NOs- ion was slow in these treatments, but one must note that even after the application of fourth split, 5 and 3 irrigations were applied to 5.5 and 7.5 cm treatments respectively.

The data obtained in the present s tudy thus suggest an important role for crop plants in altering the leaching loss of NOa- under various fertilizer and water management systems. Ensuring a deep and extensive rooting system in early stages can exercise consider- able control over the amount and rate of enrichment of unsaturated zone and underground waters. Adopting a proper irrigation schedule and N-splitting programme, besides providing an efficient rooting system also ensures higher crop yields.

ACKNOWLEDGEMENTS

T h e sen io r a u t h o r is g r a t e f u l to I n d i a n Counc i l of A g r i c u l t u r a l R e s e a r c h ,

N e w Delhi , for f inanc ia l he lp in t h e f o r m of a s en io r fe l lowship .

Received November 22, 1974

REFERENCES

1 A d r i a n o , D. C., P r a t t , P. F. and T a k a t o r i , F. H., Nitrate in unsaturated zone of an alluvial soil in relation to fertilizer nitrogen rate and irrigation level. J. Environmental Quality 1,418-422 (1972).

2 B a t e s , T. E. and T i s d a l e , S. L., The movement of nitrate nitrogen through columns of coarse textured soil materials. Soil Sei. Soe. Am. Proc. 21, 525-528 (1957).

3 Boswe l l , F. C. and A n d e r s o n , O. E., Nitrogen movement comparisons in cropped versus fallowed soils. Agron. J. 62, 499-503 (1970).

4 Boswel l , F. C. and A n d e r s o n , O. E., Nitrogen movement in undisturbed profiles of fallowed soils. Agron. J. 56, 278-281 (1964).

5 B r e m n e r , J. M., Inorganic forms of nitrogen, in Methods of Soil Analysis 2, ed. C. A. B l a c k et al., Agronomy 9, 1179-1237. Am. Soe. Agron., Inc. Madison, Wise. (1965).

6 B u r n s , G. R. and Dean , L. A., The movement of water and nitrate around bands of sodium nitrate in soils and glass beads. Soil Sei. Soe. Am. Proe. 2~8, 470-474 (1964).

7 P r a t t , P. F., J o n e s , W. W. and H u n s a k e r , V. E., Nitrate in deep soil profiles in relation to fertilizer rates and leaching volume. J. Environmental Quality 1, 97-102 (1972).

8 W a r r i c k , A. W., B i gga r , J. W. and N ie l s e n , D. R., Simultaneous solute and water transfer for an unsaturated soil. Water Resources Research 7, 1216-1225 (1971).

9 W e t s e l a a r , R., Nitrate distribution in tropical soils I. Possible causes of nitrate accumulation near the surface after a long dry period. II . Extent of capillary ac- cumulation of nitrate during a long dry period. Plant and Soil 15, 110-120, 121-133 (1961).