Phosphorus and nitrogen losses associated with runoff and erosion on an Aridisol in northern Iraq

Transcription

1 Hydrological Sciences-Journal-des Sciences Hydrologiques, 44(5) October Phosphorus and nitrogen losses associated with runoff and erosion on an Aridisol in northern Iraq MOHAMMAD H. HUSSEIN Department of Civil Engineering, College of Engineering, PO Box 61160, Hoon, Libya ABBAS J. HUSSEIN Department of Soil and Water, College of Agriculture, Mosul University, Mosul, Iraq MUHSEN M. AWAD Department of Geography, College of Education, PO Box 187, Jeffrin, Libya Abstract Runoff and soil erosion are known to cause a degradation in soil and water quality. Six natural runoff plots (three 10 m and three 30 m ) were established on 6% uniform slope area for the study of P and N losses associated with runoff and soil erosion in northern Iraq. The soil at the site bes to the Calciorthid suborder which dominates in the low rainfall zone of northern Iraq. Runoff, erosion, and associated P and N losses, were recorded from these plots for three rainfall seasons. Results illustrated that eroded sediment is always rich in available P and inorganic N compared to the original soil. Concentrations of soluble P and soluble N in runoff illustrated significant variability both between storms and between seasons. Both sediment-bound P and soluble P were significantly correlated with the ratio of runoff to rainfall. Pertes en phosphore et en azote dues au ruissellement et à l'érosion dans le nord de l'irak Résumé On sait le rôle du ruissellement et de l'érosion des sols dans la dégradation des sols et de la qualité des eaux. Six parcelles d'étude du ruissellement naturel (trois de 10 m de et trois de 30 m de ) de pente égale à 6% ont été mises en place en vue d'étudier les pertes en P et N dues au ruissellement et à l'érosion, sur un soussol calcaire très courant dans la région peu arrosée du Nord de l'irak. Le ruissellement, l'érosion et les pertes en P et N ont été mesurés durant trois saisons des pluies. Les résultats ont montré que les sédiments provenant de l'érosion sont beaucoup plus riches en P et en N que le sol original. La concentration du phosphore mobilisable et de l'azote minéral dans les eaux de ruissellement présente une importante variabilité selon les averses et saisons étudiées. Aussi bien le P dissous que le P particulaire sont significativement corrélés au rapport du ruissellement aux précipitations. INTRODUCTION The region of northern Iraq has a semiarid Mediterranean type climate. It is often divided into high-, medium- and low-rainfall zones according to the mean seasonal rainfall. The high-rainfall zone receives more than 600 mm of mean seasonal rainfall. The medium- and low-rainfall zones receive, respectively, between 400 and 600 mm, and below 400 mm of mean seasonal rainfall. Aridisol (mainly Calciorthids) is the main soil order in the low-rainfall zone of northern Iraq. The area is mainly used for wheat and barley production in a biennial crop-fallow rotation. Conventional tillage of moldboard ploughing and/or disking Open for discussion until 1 April 2000

2 658 Mohammad H. Hussein et al. (ploughing with a disk plough) is normally practised. Rainfall insufficiency and limited rooting depth keep the normal crop yield at a level below 1 t ha" 1. Soil erosion by rainfall and runoff is a major problem in northern Iraq (Hussein, 1998). Nutrient loss associated with rainfall-runoff events is due to surface flow, erosion and leaching. Soluble nutrients are transported by surface flow. Insoluble forms of nutrients and nutrients adsorbed to sediment particles are transported by erosion. Nutrient leaching by percolating water to groundwater is of minor importance in the low-rainfall zone due to the low seasonal rainfall and the relatively deep groundwater. Data on nutrient loss with runoff and sediment in the region are essential for accurately evaluating the impact of erosion on soil productivity, and for assessing the probable environmental impact including deterioration in surface water quality. Phosphorus and nitrogen are the two plant nutrients most frequently associated with water quality and soil fertility problems. STUDY SITE AND METHODS Six natural runoff plots were established in March 1988 on a 6% uniform slope area at Hammam Al-Alil (36 10'N, 43 20'E) in the northwestern part of Iraq. Site elevation is about 250 m a.m.s.l. Three plots were 30 m followed by three 10 m. Plot width was 3 m and the distance between adjacent plots was 1 m. Plot design and installation were done according to the procedure outlined by Mutchler (1963). Each autumn, and after rain showers had moistened the soil, the plots were tilled by spading, then smoothed and left in the fallow condition throughout the rainfall season. Gramaxon was used to control weeds. The experimental site was used primarily as grazing land, and the soil is classified as fine, mixed, thermic, calcareous, Xerrollic Calciorthid. General soil characteristics are given in Table 1. Mica and chlorite are the dominant clay minerals, followed by kaolinite and montmorillonite. The rainfall season in the region usually extends from October to May. The mean seasonal rainfall at the site is about 340 mm. The site was equipped with a nonrecording raingauge. During the rainfall season, a recording raingauge was used. Differences in mean temperature between winter and summer months are usually more than 20 C. During the winter months, the minimum daily temperature occasionally drops below freezing point. During the summer months, the maximum daily temperature is usually above 40 C. Table 1 Soil characteristics at the experimental site at the beginning of experiment. Location on soil profile Topsoil (0-30 cm) Subsoil (30-70 cm) Particle size distribution (%): Clay 36 Silt 44 Sand Organic matter (%) Av. water ph cap.* (cm) CaC0 3 (%) CEC* Inorg. N (C mol kg') (ugg 1 ) * Average water capacity = difference between water contents at 3"' and 15 bar suctions. + CEC: cation exchange capacity. Av. P (m g')

3 Phosphorus and nitrogen losses associated with runoff and erosion 659 After each runoff-producing rainstorm, runoff in the collecting tank at each plot outlet was mixed thoroughly, then sampled for P and N measurements using one litre plastic bottles. In addition, runoff volume in the collecting tank was measured and a sample was taken for determining sediment concentration. This last sample was weighed and evaporated on a hot plate then weighed again to determine sediment concentration in the runoff water. For normal rainfall events, sediment concentration in runoff was generally low. For this reason, eroded sediment samples were collected only after heavy showers, where a substantial amount of sediment was lost by erosion. Phosphorus and nitrogen sampling was discontinued in May 1990 at the end of the third rainfall season. Runoff and sediment samples were returned immediately to the laboratory for analysis. Inorganic N (NH* and NOj) in eroded sediment was determined by the method of Bremner & Keeney (1965). Available phosphorus in eroded sediment (HP 4 2 "and H 2 P04) was extracted by using the Olson method and determined by the method of Murphy & Riley (1962). In addition, runoff samples in the plastic bottles were filtered and soluble P and N in the filtrate were determined by using the methods mentioned above. RESULTS Within the replicated plots, plot-to-plot variability for both sediment-bound and soluble P and N losses was not significant at the 95% probability level. Hence, average values from the replicated plots will be considered in the subsequent analysis. Sediment-bound loss Values of rainfall, runoff, sediment yield and enrichment ratios for single storm events are shown in Table 2. The enrichment ratios for available P and inorganic N were generally similar for and plots. Phosphorus and nitrogen concentrations in sediment reported above were regressed against total rainfall, total runoff, runoff ratio and sediment yield (Table 3). Generally, Table 2 Rainfall, runoff, sediment yield and sediment enrichment ratio for single storms. Date of storm 17/12/88 19/12/88 13/02/89 30/03/89 30/11/89 14/03/90 06/04/90 Rain (mm) Runoff: Depth ( mm) Ratio Sediment yield: (gnf 2 ) Enrichment ratic,*. P N *Enrichment ratio is the ratio of concentration of a nutrient in sediment to concentration in the original soil

4 660 Mohammad H. Hussein et al. Table 3 Regression equations significant at the 95% probability level describe nutrient-storm interactions. Equation P srf = /?,. P s = R r P s = /j P s = O.OI/30 df r~ Season all all third third P irf = sediment-bound P (ng g" 1 ); P s = soluble P (mg l' 1 ); R r = runoff ratio; Is = maximum 5-min intensity and / 30 = maximum 30-min intensity (mm If 1 ). df = degrees of freedom. Plot runoff ratio was the variable which gave the highest coefficient of determination (r 2 ) when regressed against P and N concentrations in sediment. Regression analysis indicates that the linear regression model gives a fair fit to the data of runoff ratio and concentration of available P in eroded sediment. On the plots, the two variables were directly related and the relationship was significant at the 95% probability level (Table 3). On the plots, however, the relationship was not significant at the same probability level. Concentration of inorganic N in eroded sediment was not significantly correlated with runoff ratio at the 95% probability level for both types of plots. In spite of this, the above conclusions may change if more data points were involved in the regression analysis (Menzel, 1980). Soluble loss Soluble P and N values from and plots were not significantly different at the 95% probability level (Table 4). The small experimental site may be the principal reason for having small plot-to-plot variability for both soluble and sediment-bound P and N. Variation in surface soil properties within the site is small. However, for soluble P and N data, noticeable season-to-season differences on plots were observed especially for inorganic N. Soluble P levels recorded during the first and second rainfall seasons were considerably lower than the ones recorded during the third rainfall season (Table 4). The third season also recorded the highest sediment-bound available P loss (Table 2). Storm-to-storm variability in soluble P concentration in runoff was high except during the second season. In this season, coefficients of variation on the and plots were 35% and 15% respectively. However, for soluble N, the highest concentration in Table 4 Statistical summary of soluble P and N data from and plots*. Nut. P N Mean cone, (mgl" 1 ) for the season specified: First Second Third ( ) ( ) ( ) Standard deviation for the season First Second specified: Third * Recorded single storm events were four in the first season, six in the second season and seven in the third season.

5 Phosphorus and nitrogen losses associated with runoff and erosion 661 runoff, as well as the highest storm-to-storm variability occurred during the second dry season. Soluble P and N from individual storms were regressed against sediment yield and the rainfall runoff characteristics mentioned previously. In addition, since rainfall intensity records were available for the third season, maximum 5-min and 30-min rainfall intensities for storms were included in the regression analysis. Only soluble P showed significant correlation with runoff ratio and other rainfall-runoff characteristics (Table 3). Table 3 shows that soluble P increases with rainfall intensity on the plots. On the plots, the relationship was not significant at the 95% probability level. Total loss Totals of rainfall, runoff, sediment yield, and P and N losses for the second and third seasons are shown in Table 5. The first season was excluded, since measurements started in the middle of the season. Total sediment-bound loss for a season was calculated from mean enrichment ratio for the season and total sediment yield. Total soluble loss was calculated from mean concentration in runoff (see Table 4) and total runoff volume for the season. For available P, the sediment-bound loss was dominant in the second season. Most researchers reported dominant sediment-bound loss for phosphorus (Casanova et al, 1987; Johnson et al, 1979; Monke étal, 1981; Schuman étal, 1973). However, the relatively high runoff ratio in the third season caused a substantial increase in the soluble loss, compared to the sediment-bound loss for available P (Table 5). For inorganic N, the soluble loss was dominant. This is because inorganic N attached to sediment amounts only to small fraction of total N. DISCUSSION P and N loss Experimental data on available P and inorganic N losses associated with runoff and soil erosion have been presented for three rainfall seasons: the first season ( ) was regarded as wet, having a total seasonal rainfall of 568 mm; the second season ( ) was considered dry, with total seasonal rainfall of Table 5 Soil, water and P and N losses from and plots. Season Second Third Total ram (mm) Plot Total runoff (mm) Total sediment yield (kg ha" 1 ) Nutrients loss (kg ha 4 x Sediment-bound P N Rainfall added nitrogen (mostly nitrate) is estimated at 2.5 g ha' 1 mm "' rain. 10' 2 \*. Soluble P N

6 662 Mohammad H. Hussein et al. 251 mm; and the third season ( ) was deemed normal, with a total seasonal rainfall of 301 mm (Tables 2, 4 and 5). Data of sediment-bound loss are reported on storm basis (Table 2). This is because only severe storms produce sufficient eroded sediment that can be sampled. Storm-tostorm variability in P and N enrichment ratios is generally high. Rainfall intensity variation was probably the principal factor which caused this high variability. Data of soluble P and N in runoff were reported on seasonal basis (Table 4). Inorganic N concentration in runoff was highest during the dry season. However, total inorganic N lost during this season was lower than during the third (normal) season (Table 5), suggesting that this measured high concentration in runoff was probably related to the low runoff ratio during this season. The runoff ratio was the variable which gave the highest correlation with both sediment-bound and soluble P; the relationships were significant on the plots only (Table 3). Common erosion/sedimentation analysis relates the runoff ratio to the sediment delivery ratio (ratio of sediment yield to gross erosion) (Hussein & Othman, 1988). Only the plots, however, showed noticeable sediment enrichment in fines compared to the original soil (Hussein, 1996). This fine enriched sediment had higher P and N enrichment ratios. This may indicate that the plots had a more complete erosion/sedimentation process (i.e. detachment-sediment transport-deposition) and deposition caused sediment enrichments in fines not observed on the plots. For soluble P, the coefficient of variation at the plots was lower than at the plots (Table 4). The relationships between P and N enrichment ratios were not significant at the 95% probability level. Similar results were obtained by Flanagan & Foster (1989). Impact on soil and water quality Phosphorus and nitrogen are the most important nutrients from both soil productivity and pollution standpoints. Table 2 indicates that the removal of these two plant nutrients from soil is selective. This will reduce soil fertility unless they are replaced through the application of fertilizers. The organic matter content in eroded sediment was determined for several samples during the third season by using the Walkley- Black method (Jackson, 1958). The enrichment ratios obtained were between 2.08 and Excessive loss of soil organic matter by soil erosion causes further deterioration in soil quality. A major source of pollution to the River Tigris is agricultural runoff, since most of the drainage area is agricultural land under dryland farming. The amounts of nutrient reaching the Tigris and its tributaries in the region are substantial in spite of the fact that the sediment delivery ratio in the region is generally low. Both P and N stimulate algal growth and adversely affect water quality and fish habitat. The nitrate-n level in drinking water should be less than 10 mg T 1 for humans (WHO, 1984). For phosphorus, a soluble concentration above 0.01 and a total concentration above 0.02 mg f 1 may accelerate eutrophication (depletion or disappearance of available oxygen) of lakes and impoundments (Smith et al, 1994). Table 4 shows that P loss for all seasons and N loss for the second season exceed the acceptable limits. Losses of

7 Phosphorus and nitrogen losses associated with runoff and erosion 663 these two elements from fertilized fields may raise their concentration in surface water supplies to even higher levels, unless protective measures against excessive runoff and soil erosion are taken (Molenaar et ah, 1993). Up to the 1970s, fertilization was not common in dryland farming in the Tigris basin. This may be the reason why the data on P and N concentrations in the River Tigris at Mosul (in the vicinity of the experimental farm) given by Hanna & Talabani (1970) were comparable to the values given in Table 4. CONCLUSIONS Runoff and soil erosion cause a deterioration in soil and water quality in the low rainfall zone of northern Iraq. The eroded sediment was always richer in available P and inorganic N compared to the original soil. The runoff ratio was the variable which significantly interacted with sedimentbound and soluble P. The relationships obtained are useful in modelling the effect of runoff and soil erosion on soil and water quality. Acknowledgements The authors are grateful to M. A. Tabatabai for critical reviewing of the manuscript. REFERENCES Bremner, M. & Keeney, O. R. (1965) Steam distillation methods for determination of ammonium, nitrate and nitrite. Anal. Chim. Acta 32, Casanova, E., Paez, M. L. & Rodriguez, O. S. (1987) Plant nutrient losses in eroded sediment under different soil management in two areas of Venezuela. In: Soil Conservation and Productivity (ed. by I. P. Sentis). Sociedad Venezolana delà Ciencia del Suelo, Maracay, Venezuela. Flanagan, D. C. & Forster, G. R. (1989) Storm pattern effect on nitrogen and phosphorus losses in surface runoff. Trans. Am. Soc. Agric. Engrs 32, Hanna, A. & Talabani, K. (1970) Quality and classification of river water in Iraq. Proc. First Conf. of the Arab Soc. Agric. Engrs (Kurtoom, Sudan). Hussein, M. H. (1996) An analysis of rainfall, runoff and erosion in the low rainfall zone of northern Iraq. J. Hydrol. 181, Hussein, M. H. (1998) Water erosion assessment and control in northern Iraq. Soil & Tillage Res. 45, Hussein, M. H. & Othman, A. K. (1988) Soil and water losses in a low intensity rainfall region in Iraq. Hydrol. Sci. J. 33(3), Jackson, M. L. (1958) Soil Chemical Analysis. Prentice-Hall, Englewood Cliffs, New Jersey, USA. Johnson, H. P., Baker, J. L., Shrader, W. D. & Laflen, J. M. (1979) Tillage systems effect on sediment and nutrients in runoff from small watersheds. Trans. Am. Soc. Agric. Engrs 22, Menzel, R. G. (1980) Enrichment ratios for water quality modelling. In: CREAMS A Field Scale Model for Chemicals, Runoff and Erosion from Agricultural Management Systems. Cons. Res. Report no. 26, US Dept of Agriculture, Agricultural Research Service. Molenaar, A., Bleuten, W., Zeylmans, M. J. & Van Leeuwen, N. F. M. (1993) Application of GIS in determining sources and loads of pollutants transported by regional rivers into The Netherlands. In: Application of Geographic Information Systems in Hydrology and Water Resources Management (ed. by K. Kovar & H. P. Nachtnebei), IAHSPubl.no Monke, E. J., Nelson, D. W., Beasley, D. B. & Botcher, A. B. (1981) Sediment and nutrient movement from the Black Creek watershed Trans. Am. Soc. Agric. Engrs 24, Murphy, J. & Riley, J. P. (1962) A modified single solution method for the determination of phosphate in natural waters. Anal. Chim. Acta 27, Mutchler, C. K. (1963) Runoff plot design and installation for soil erosion studies. Report no. ARS-41-79, US Dept of Agriculture, Agricultural Research Service. Richards, L. A. (ed.) (1954) Diagnosis and improvement of saline and alkali soils. US Dept of Agriculture, HB no. 60.

8 664 Mohammad H. Hussein et al. Schuman, G. E., Spomer, R. G. & Piest, R. F. (1973) Phosphorus losses from four agricultural watersheds on Missouri valley loess. Soil Sci. Soc. Am. Proc. 37, 424^127. Smith, S. }., Sharpley, A. N. & Jones, O. R. (1994) Effects of crop residue management on water quality. In: Crop Residue Management to Reduce Erosion and improve Soil Quality (ed. by B. A. Steward & W. C. Moldenhauer). US Dept of Agriculture, Agricultural Research Service, Cons. Res. Report no. 37. WHO (World Health Organization) (1984) Guidelines for Drinking Water Quality, vol. 1, Recommendations. World Health Organization, Geneva, Switzerland. Received 12 January 1998; accepted 30 November 1998