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Simulating the impact of acidifying farming systems on Australian soils
E(OLOGI(gL mODELLInG ELSEVIER
Ecological Modelling 86 (1996) 207-211
Simulating the impact of acidifying farming systems on Australian soils J. Brett Robinson *, Keith R. Helyar N S W Agriculture, Agricultural Research Institute, PMB, Wagga Wagga, N.S. W. 265, Australia
Abstract The Hunter Valley is a productive farming and pastoral area in eastern Australia. Land use ranges from viticulture and dairying to extensive grazing and forestry. Soil acidity may become a problem if the soil pH is reduced to critical levels by acid accumulated when nutrient cycles are open (especially N, C and S). This study uses a model to forecast pH changes due to acids derived from land use. Soil samples from a survey in the Hunter Valley were analysed for pH (1 : 5 soil : 0.01 M CaCI2), effective cation exchange capacity ( c m o l ( + ) / k g ) and pH buffer capacity (pHBC, k m o l ( + ) ha -a 0.1 m a pH a). Data from three contrasting soil profiles were used in SPAM (Soil Profile Acidification Model) to forecast soil pH after 25 and 50 years. Results indicate the pH of a soil under native forest is stable. Only small amounts of acid are generated by this land use and the pH is buffered by both cation exchange reactions and aluminium dissolution. However, the acid generated from an intensive pastoral system (2.5 k m o l ( + ) ha 1 year 1) is sufficient to strongly acidify a well-buffered soil (pHBC 0-10 cm = 38 k m o i ( + ) ha -1 0.1 m -1 pH -1) to 50 cm depth. A less intensive pastoral system (0.33 k m o l ( + ) ha -1 year -a) was estimated to acidify a soil with below average buffer capacity (pHBC 0-10 cm = 9 k m o l ( + ) ha -1 0.1 m -1 pH -1) to 50 cm depth in 50 years. Verification of these results and the usefulness of SPAM are discussed. Keywords: Acidification; Agricultural ecosystems
1. Introduction A c i d soils a r e c o m m o n in A u s t r a l i a . T h e r e a r e m a n y highly w e a t h e r e d soils in a n c i e n t l a n d scapes, a n d have i n t r o d u c e d acidifying f a r m i n g systems to m a n y soils t h a t w e r e slightly acidic. A b o u t 29 m i l l i o n h e c t a r e s a r e d e f i n e d as acid, with p H less t h a n 4.8. N e w S o u t h W a l e s is s e v e r e l y
* Corresponding author.
affected, with 9.5 m i l l i o n h e c t a r e s ( B u r e a u of R e s o u r c e Sciences, 1991). A c i d soils a r e infertile t h r o u g h a n u m b e r o f m e c h a n i s m s which restrict p l a n t growth. B e l o w p H 4.8, toxic e l e m e n t s such as a l u m i n i u m a n d m a n g a n e s e i n c r e a s e in c o n c e n t r a t i o n in t h e soil solution, s o m e e s s e n t i a l nutrients (Ca, Mg, K) a r e d i s p l a c e d a n d l e a c h f r o m t h e r o o t zone, a n d o t h e r n u t r i e n t s (P a n d M o ) a r e m a d e less available to p l a n t roots. A l u m i n i u m r e l e a s e d at low p H t h a t finds its way into surface w a t e r s is highly toxic to fish a n d o t h e r a q u a t i c species.
0304-3800/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0304-3800(95)00053-4
208
J.B. Robinson, K.R. Helyar/ Ecological Modelling 86 (1996) 207-211
The causes of acidification are complex, but agriculturally the most important factors are the export of produce, leaching of nitrate and sulphate ions, and accumulation of soil organic matter (Helyar and Porter, 1989). All of these processes create residual H ÷ ions, which decrease soil pH. Also, the release of sulphur and nitrogen oxides (SO 2 and NO x) from the combustion of fossil fuels and the roasting of mineral ores leads to the deposition of dilute sulphuric and nitric acids via rainfall. Dry deposition of SO 2 and NO x on soils and vegetation is also strongly acidifying. The high economic and ecological costs of acidification have stimulated much research into causes, rates and amelioration technologies. Several criteria for estimating soil sensitivity to acid loads have been developed. These rely on the soil's ability to either resist pH changes (e.g. McFee, 1980) or the total alkalinity of the soil (e.g. Reuss and Johnson, 1986). The usefulness of these measures have seldom been tested in the field. Also, recent improvements in our understanding soil acidification have resulted in the development of simulation models of soil profile behaviour under acid loads. These range from simple one-layer models which treat acidification as a titration through to complex multi-layer models with numerous chemical equilibria and reaction kinetics represented in each layer (Levine and Ciolkosz, 1986; Reuss and Johnson, 1986; Turchenak et al., 1994). The objective of this study is to test the application of simulation modelling for identifying soils in the Hunter Valley that are sensitive to acid loads. The Hunter Valley is one of the larger and more agriculturally important coastal valleys in eastern Australia. It is centred north of Sydney (Figs. 1 and 2). The land use is predominantly agricultural with about 2000 holdings occupying 1.4 million hectares. It is an important area for dairying, beef production and lucerne hay making. Wine is produced from about 60 vineyards. The soil types in the Hunter Valley form a complex mosaic. Infertile soils derived from the Hawkesbury sandstones occur to the south and west of the river. However, the fertile alluvial floodplain is fed by deposits from basaltic plateaux in the upper reaches which are covered with deep
Q
I
jf
Fig. 1. The location of the Hunter Valley within eastern Australia. prairie and krasnozem soils. Non-alluvial soils in the middle Hunter are often highly weathered podsolics, soloths or solodics derived from shales or sandstone.
2. M e t h o d s Soil samples were collected at 51 sites and the p H (1:5 soil:0.01 M CaC12), effective cation
209
J.B. Robinson, K.R. Helyar/ Ecological Modelling 86 (1996) 207-211
Table 1 Soil chemical data for three Hunter Valley soils differing in their pH and pH buffer capacities Site Depth pH pHBC ECEC (cm) (0.01 M (kmol ha- 1 (cmol( + CaCI 2) 0.1 m -1) kg -I ) 352
Newcaste ~
N
Ei
t o o kl.
0-10 10-20 20-40 50-70 80-100 0-10 10-20 20-40 50--70 80--100 0--10 10-20 20-40 50-70 80-100
y 361
I
377
Y
Fig. 2. The Hunter Valley in relation to the Sydney-Hawkcs-
5.01 6.20 6.80 7.62 7.81 4.85 5.20 5.55 6.40 6.42 4.12 4.62 4.28 4.12 4.80
38.0 22.0 25.0 49.0 49.0 9.0 a 5.5 a 8.0 a 10.0 a 12.0 a 5.8 a 3.7 a 9.4 a 9.7 a 10.0 a
13 16 25 35 32 2 2 2 5 9 2 2 10 10 13
a Estimated from ECEC, pH and depth of layer. See Fig. 3 and text for details.
bury region.
e x c h a n g e c a p a c i t y ( E C E C , c m o l ( + ) k g - 1 ) , ext r a c t a b l e a l u m i n i u m (/zg A1 m1-1 1 : 5 soil :0.01 M C a C I 2) a n d p H b u f f e r c a p a c i t y ( p H B C , k m o l h a - 1 0.1 m - 1 ) w e r e d e t e r m i n e d . T a b l e 1 shows t h e results for t h r e e o f t h e soils. T h e s e a r e r e p r e s e n tative o f significant l a n d - u s e units in t h e valley t h a t a r e highly b u f f e r e d (site 352) a n d p o o r l y b u f f e r e d (sites 361 a n d 377). T h e s e d a t a w e r e u s e d in S P A M (Soil P r o f i l e A c i d i f i c a t i o n M o d e l ) , an a n n u a l t i m e s t e p C S M P m o d e l o f p H a n d ionic c h a r g e b a l a n c e d o w n a soil p r o f i l e ( R o b i n s o n et al,, 1994). T h e m o s t i m p o r t a n t c h e m i c a l a n d physical p r o c e s s e s o f a c i d i f i c a t i o n a r e r e p r e s e n t e d in t h e m o d e l . Soil b u f f e r p H c a p a c i t i e s d u e to c a t i o n exchange reactions were estimated from the relationship between pHBC and ECEC/pH. This was n e c e s s a r y for soils 361 a n d 377 w h e r e s o l u b l e a l u m i n i u m m a s k e d p H b u f f e r i n g by e x c h a n g e reactions. Fig. 3 shows t h e lines t h a t fit s u r f a c e soils a n d subsoil layers. A c i d l o a d s f r o m l a n d use w e r e s e l e c t e d using t h e L i m e - i t m o d e l o f a g r i c u l t u r a l a c i d i f i c a t i o n ( F e n t o n a n d H e l y a r , 1994). P r o d u c t removal, n i t r o g e n fertility a n d t h e s e a s o n a l i t y o f
n i t r o g e n s u p p l y a n d d e m a n d a r e a c c o u n t e d for in this m o d e l . T h e r a t e for site 352 is high (2.5 k m o l ( + ) ha -1 y e a r - 1 ) b e c a u s e t h e system has a m o d e r a t e n i t r o g e n status s u p p o r t i n g g r a z e d p a s ture. Site 361 (0.33 k m o l ( + ) h a -1 y e a r - ] ) is less fertile, a n d n i t r a t e l e a c h i n g a n d p r o d u c t e x p o r t w o u l d b e low, while site 377 (0.16 k m o l ( + ) ha -1 y e a r - 1 ) is a d r y sclerophyll w o o d l a n d which has very l i m i t e d n i t r o g e n t u r n o v e r , a d e e p - r o o t e d p e r e n n i a l t r e e c r o p a n d very low p r o d u c t e x p o r t rates. A r e f e r e n c e " n i l p o l l u t i o n " rainfall p H o f 5.0 was u s e d in s i m u l a t i o n s to allow for b a c k -
0
+ *10
~..e
•
lO- lOOcm depth
~-20 -25
• i 1
i 2
\
O-lOcm depth i
3 ECEC/pH (cmol(+)lkglpH)
4
= 5
Fig. 3. Relationships between pH buffer capacity and pH-corrected effective cation exchange capacity (ECEC) for soils with negligible extractable aluminium.
J.B. Robinson, K.R. Helyar/ Ecological Modelling 86 (1996) 207-211
210
ground deposition of SO 2 and NO~ from non-anthropogenic and diffuse anthropogenic sources. This value is adjusted from the 330 ppm CO 2 equilibrium p H of 5.7.
8
~
~
.
,,,.
,,
"
1993
~:L 8
÷--
5
,
4
2018
- -- i- -- 2043
.'"
3. Results
The p H profiles predicted for 25 and 50 years from 1993 are shown in Figs. 4 to 6 below. Fig. 4 shows that over 50 years a considerable depth of soil is acidified. Fig. 5 shows an even greater degree of acidification at 30-50 cm depth, even though the agricultural acid load is very much less than site 352. Fig. 6 shows that little change occurs on weakly buffered soils that are already acidified. Site 377 is virtually in a steady state with respect to pH.
t~'--:- -A'"
3 20
40
80
80
100
Depth (cm)
Fig. 4. Current and predicted pH changes at site 352 where the acidification rate is approximately 2.5 k m o l ( + ) ha -1 year t.
7
6
~
5
•
1993 "~
+
2018
" - - *-- " 2043
4 3 20
40
60
80
100
Depth (cm)
Fig. 5. Current and predicted pH changes at site 361 where the acidification rate is approximately 0.33 k m o l ( + ) ha -1 year- 1
O
20
40
•
1993
-÷
2018
--'*"-
2043
60
80
100
Depth(cm)
Fig. 6. Current and predicted p H changes at site 377 where the acidification rate is approximately 0.16 k m o l ( + ) ha -1 year - 1
4. Discussion
Site 352, the high pH and high buffer capacity soil, was significantly affected by the agricultural acid load. For similar soils, which are widespread in the Hunter, lime use in intensive agriculture will be very important due to the large acid loads and the short time periods needed to significantly reduce the pH. However, the results from site 361 (Fig. 5) also indicate that with a low agricultural acid load the pH of poorly buffered soils declines to a low level in a moderately short time (25 years). The model predictions for site 377 were essentially unchanged from the existing pH profile. This gives some confidence that the low "endpoint" pH values that the model is predicting are valid, as the soil at site 377 has acidified over thousands of years at the "natural" rate, and should be very close to steady-state. The modest similarities of pH changes between 352 and 361 and the differences between 361 and 377 show that simple soil chemical parameters such as pH and E C E C are fallible indicators of soil changes with acidification. The use of SPAM has indicated that the results reflect more than one or two simple parameters, and acidification rates, aluminium-based pH buffering and other effects must be included in evaluating soil or site sensitivity. This is where simulation has an important role in assessing sensitivity. Simulation integrates pH, pHBC and other physical and biological processes and effects, and quantifies outcomes. This avoids the pitfalls of
J.B. Robinson, K.R. Helyar/ Ecological Modelling 86 (1996) 207-211 soil classification or c a t e g o r i s a t i o n into simple sensitivity classes. S P A M is also to b e used to assess the impact of acidic d e p o s i t i o n in the H u n t e r Valley. A small b u t significant p r o p o r t i o n (1 to 2 % ) of the l a n d area in the m i d d l e a n d u p p e r H u n t e r receives wet plus dry d e p o s i t i o n equal to a p p r o x i m a t e l y 0.5 k m o l ( + ) ha -1 year -~. This load is c o m p a r a b l e with large areas of N o r d i c E u r o p e that receive 0 . 5 - 1 . 0 k m o l ( + ) ha -1 year -a (Lovblad et al., 1992), b u t m u c h less t h a n in c e n t r a l a n d n o r t h e r n E u r o p e , w h e r e loads exceed 5 k m o l ( + ) ha -1 y e a r - 1 in some areas ( B r e d e m e i r et al., 1990).
Acknowledgements This work has b e e n financially s u p p o r t e d by Pacific Power. T h a n k s are d u e to G r e g Ayers a n d H u g h Malfroy for providing d e p o s i t i o n data, to Colin C h a r t r e s for collecting a n d classifying the soils, a n d K a t h y Shaw, P e t e r H a w k i n s a n d P e t e r O l s e n for soil chemical analyses.
References Bredemeir, M., Matzner, M. and Ulrich, B., 1990. Internal and external proton loads to forest soils in northern Germany. J. Environ. Qual., 19: 469-477. Bureau of Resource Sciences, 1991. Acid Soils in Australia: A summary of issues for government. BRS, Canberra.
211
Fenton, I.G. and Helyar, K.R., 1995. Applying information from research on soil acidity. In: R.A. Date, N.J. Grundon, G.E. Rayment and M.E. Probert (Editors), Plant-Soil Interactions at Low pH: Principles and Management. Kluwer, Dordrecht. Helyar, K.R. and Porter, W.M., 1989. Soil acidification, its measurement and the processes involved. In: A.D. Robson (Editor), Soil Acidity and Plant Growth. Academic Press, Sydney, pp. 61-101. Levine, E.R. and Ciolkosz, E.J., 1986. A computer simulation model for soil genesis applications. Soil Sci. Soc. Am. J., 50: 661-667. Lovblad, G., Amann, M., Andersen, B., Hovmand, M., Joffre, S. and Pedersen, U., 1992. Deposition of sulfur and nitrogen in the Nordic countries: present and future. Ambio, 21: 339-347. McFee, W.W., 1980. Sensitivity of soil regions to long-term acid precipitation. In: D.S. Shriner, C.R. Richmond and S.E. Lindberg (Editors), Atmospheric Sulfur Deposition: Environmental Impact and Health Effects. Ann Arbor Science, Ann Arbor, MI, pp. 495-506. Reuss, J.O. and Johnson, D.W., 1986. Acid Deposition and the Acidification of Soils and Waters. Springer-Verlag, New York. Robinson J.B., Helyar, K.R. and Hochman, Z., 1995. A model for understanding the importance of various chemical, physical and biological processes in the development of soil profile acidity. In: R.A. Date, N.J. Grundon, G.E. Rayment and M.E. Probert (Editors), Plant-Soil Interactions at Low pH: Principles and Management. Kluwer, Dordrecht. Turchenak, L.W., Abboud, S.A. and Lutwick, G.W., 1995. Modelling and monitoring acid deposition impacts on soils in Alberta, Canada. In: R.A. Date, N.J. Grundon, G.E. Rayment and M.E. Probert (Editors), Plant-Soil Interactions At Low pH: Principles and Management. Kluwer, Dordrecht.
Ecological Modelling 86 (1996) 207-211
Simulating the impact of acidifying farming systems on Australian soils J. Brett Robinson *, Keith R. Helyar N S W Agriculture, Agricultural Research Institute, PMB, Wagga Wagga, N.S. W. 265, Australia
Abstract The Hunter Valley is a productive farming and pastoral area in eastern Australia. Land use ranges from viticulture and dairying to extensive grazing and forestry. Soil acidity may become a problem if the soil pH is reduced to critical levels by acid accumulated when nutrient cycles are open (especially N, C and S). This study uses a model to forecast pH changes due to acids derived from land use. Soil samples from a survey in the Hunter Valley were analysed for pH (1 : 5 soil : 0.01 M CaCI2), effective cation exchange capacity ( c m o l ( + ) / k g ) and pH buffer capacity (pHBC, k m o l ( + ) ha -a 0.1 m a pH a). Data from three contrasting soil profiles were used in SPAM (Soil Profile Acidification Model) to forecast soil pH after 25 and 50 years. Results indicate the pH of a soil under native forest is stable. Only small amounts of acid are generated by this land use and the pH is buffered by both cation exchange reactions and aluminium dissolution. However, the acid generated from an intensive pastoral system (2.5 k m o l ( + ) ha 1 year 1) is sufficient to strongly acidify a well-buffered soil (pHBC 0-10 cm = 38 k m o i ( + ) ha -1 0.1 m -1 pH -1) to 50 cm depth. A less intensive pastoral system (0.33 k m o l ( + ) ha -1 year -a) was estimated to acidify a soil with below average buffer capacity (pHBC 0-10 cm = 9 k m o l ( + ) ha -1 0.1 m -1 pH -1) to 50 cm depth in 50 years. Verification of these results and the usefulness of SPAM are discussed. Keywords: Acidification; Agricultural ecosystems
1. Introduction A c i d soils a r e c o m m o n in A u s t r a l i a . T h e r e a r e m a n y highly w e a t h e r e d soils in a n c i e n t l a n d scapes, a n d have i n t r o d u c e d acidifying f a r m i n g systems to m a n y soils t h a t w e r e slightly acidic. A b o u t 29 m i l l i o n h e c t a r e s a r e d e f i n e d as acid, with p H less t h a n 4.8. N e w S o u t h W a l e s is s e v e r e l y
* Corresponding author.
affected, with 9.5 m i l l i o n h e c t a r e s ( B u r e a u of R e s o u r c e Sciences, 1991). A c i d soils a r e infertile t h r o u g h a n u m b e r o f m e c h a n i s m s which restrict p l a n t growth. B e l o w p H 4.8, toxic e l e m e n t s such as a l u m i n i u m a n d m a n g a n e s e i n c r e a s e in c o n c e n t r a t i o n in t h e soil solution, s o m e e s s e n t i a l nutrients (Ca, Mg, K) a r e d i s p l a c e d a n d l e a c h f r o m t h e r o o t zone, a n d o t h e r n u t r i e n t s (P a n d M o ) a r e m a d e less available to p l a n t roots. A l u m i n i u m r e l e a s e d at low p H t h a t finds its way into surface w a t e r s is highly toxic to fish a n d o t h e r a q u a t i c species.
0304-3800/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0304-3800(95)00053-4
208
J.B. Robinson, K.R. Helyar/ Ecological Modelling 86 (1996) 207-211
The causes of acidification are complex, but agriculturally the most important factors are the export of produce, leaching of nitrate and sulphate ions, and accumulation of soil organic matter (Helyar and Porter, 1989). All of these processes create residual H ÷ ions, which decrease soil pH. Also, the release of sulphur and nitrogen oxides (SO 2 and NO x) from the combustion of fossil fuels and the roasting of mineral ores leads to the deposition of dilute sulphuric and nitric acids via rainfall. Dry deposition of SO 2 and NO x on soils and vegetation is also strongly acidifying. The high economic and ecological costs of acidification have stimulated much research into causes, rates and amelioration technologies. Several criteria for estimating soil sensitivity to acid loads have been developed. These rely on the soil's ability to either resist pH changes (e.g. McFee, 1980) or the total alkalinity of the soil (e.g. Reuss and Johnson, 1986). The usefulness of these measures have seldom been tested in the field. Also, recent improvements in our understanding soil acidification have resulted in the development of simulation models of soil profile behaviour under acid loads. These range from simple one-layer models which treat acidification as a titration through to complex multi-layer models with numerous chemical equilibria and reaction kinetics represented in each layer (Levine and Ciolkosz, 1986; Reuss and Johnson, 1986; Turchenak et al., 1994). The objective of this study is to test the application of simulation modelling for identifying soils in the Hunter Valley that are sensitive to acid loads. The Hunter Valley is one of the larger and more agriculturally important coastal valleys in eastern Australia. It is centred north of Sydney (Figs. 1 and 2). The land use is predominantly agricultural with about 2000 holdings occupying 1.4 million hectares. It is an important area for dairying, beef production and lucerne hay making. Wine is produced from about 60 vineyards. The soil types in the Hunter Valley form a complex mosaic. Infertile soils derived from the Hawkesbury sandstones occur to the south and west of the river. However, the fertile alluvial floodplain is fed by deposits from basaltic plateaux in the upper reaches which are covered with deep
Q
I
jf
Fig. 1. The location of the Hunter Valley within eastern Australia. prairie and krasnozem soils. Non-alluvial soils in the middle Hunter are often highly weathered podsolics, soloths or solodics derived from shales or sandstone.
2. M e t h o d s Soil samples were collected at 51 sites and the p H (1:5 soil:0.01 M CaC12), effective cation
209
J.B. Robinson, K.R. Helyar/ Ecological Modelling 86 (1996) 207-211
Table 1 Soil chemical data for three Hunter Valley soils differing in their pH and pH buffer capacities Site Depth pH pHBC ECEC (cm) (0.01 M (kmol ha- 1 (cmol( + CaCI 2) 0.1 m -1) kg -I ) 352
Newcaste ~
N
Ei
t o o kl.
0-10 10-20 20-40 50-70 80-100 0-10 10-20 20-40 50--70 80--100 0--10 10-20 20-40 50-70 80-100
y 361
I
377
Y
Fig. 2. The Hunter Valley in relation to the Sydney-Hawkcs-
5.01 6.20 6.80 7.62 7.81 4.85 5.20 5.55 6.40 6.42 4.12 4.62 4.28 4.12 4.80
38.0 22.0 25.0 49.0 49.0 9.0 a 5.5 a 8.0 a 10.0 a 12.0 a 5.8 a 3.7 a 9.4 a 9.7 a 10.0 a
13 16 25 35 32 2 2 2 5 9 2 2 10 10 13
a Estimated from ECEC, pH and depth of layer. See Fig. 3 and text for details.
bury region.
e x c h a n g e c a p a c i t y ( E C E C , c m o l ( + ) k g - 1 ) , ext r a c t a b l e a l u m i n i u m (/zg A1 m1-1 1 : 5 soil :0.01 M C a C I 2) a n d p H b u f f e r c a p a c i t y ( p H B C , k m o l h a - 1 0.1 m - 1 ) w e r e d e t e r m i n e d . T a b l e 1 shows t h e results for t h r e e o f t h e soils. T h e s e a r e r e p r e s e n tative o f significant l a n d - u s e units in t h e valley t h a t a r e highly b u f f e r e d (site 352) a n d p o o r l y b u f f e r e d (sites 361 a n d 377). T h e s e d a t a w e r e u s e d in S P A M (Soil P r o f i l e A c i d i f i c a t i o n M o d e l ) , an a n n u a l t i m e s t e p C S M P m o d e l o f p H a n d ionic c h a r g e b a l a n c e d o w n a soil p r o f i l e ( R o b i n s o n et al,, 1994). T h e m o s t i m p o r t a n t c h e m i c a l a n d physical p r o c e s s e s o f a c i d i f i c a t i o n a r e r e p r e s e n t e d in t h e m o d e l . Soil b u f f e r p H c a p a c i t i e s d u e to c a t i o n exchange reactions were estimated from the relationship between pHBC and ECEC/pH. This was n e c e s s a r y for soils 361 a n d 377 w h e r e s o l u b l e a l u m i n i u m m a s k e d p H b u f f e r i n g by e x c h a n g e reactions. Fig. 3 shows t h e lines t h a t fit s u r f a c e soils a n d subsoil layers. A c i d l o a d s f r o m l a n d use w e r e s e l e c t e d using t h e L i m e - i t m o d e l o f a g r i c u l t u r a l a c i d i f i c a t i o n ( F e n t o n a n d H e l y a r , 1994). P r o d u c t removal, n i t r o g e n fertility a n d t h e s e a s o n a l i t y o f
n i t r o g e n s u p p l y a n d d e m a n d a r e a c c o u n t e d for in this m o d e l . T h e r a t e for site 352 is high (2.5 k m o l ( + ) ha -1 y e a r - 1 ) b e c a u s e t h e system has a m o d e r a t e n i t r o g e n status s u p p o r t i n g g r a z e d p a s ture. Site 361 (0.33 k m o l ( + ) h a -1 y e a r - ] ) is less fertile, a n d n i t r a t e l e a c h i n g a n d p r o d u c t e x p o r t w o u l d b e low, while site 377 (0.16 k m o l ( + ) ha -1 y e a r - 1 ) is a d r y sclerophyll w o o d l a n d which has very l i m i t e d n i t r o g e n t u r n o v e r , a d e e p - r o o t e d p e r e n n i a l t r e e c r o p a n d very low p r o d u c t e x p o r t rates. A r e f e r e n c e " n i l p o l l u t i o n " rainfall p H o f 5.0 was u s e d in s i m u l a t i o n s to allow for b a c k -
0
+ *10
~..e
•
lO- lOOcm depth
~-20 -25
• i 1
i 2
\
O-lOcm depth i
3 ECEC/pH (cmol(+)lkglpH)
4
= 5
Fig. 3. Relationships between pH buffer capacity and pH-corrected effective cation exchange capacity (ECEC) for soils with negligible extractable aluminium.
J.B. Robinson, K.R. Helyar/ Ecological Modelling 86 (1996) 207-211
210
ground deposition of SO 2 and NO~ from non-anthropogenic and diffuse anthropogenic sources. This value is adjusted from the 330 ppm CO 2 equilibrium p H of 5.7.
8
~
~
.
,,,.
,,
"
1993
~:L 8
÷--
5
,
4
2018
- -- i- -- 2043
.'"
3. Results
The p H profiles predicted for 25 and 50 years from 1993 are shown in Figs. 4 to 6 below. Fig. 4 shows that over 50 years a considerable depth of soil is acidified. Fig. 5 shows an even greater degree of acidification at 30-50 cm depth, even though the agricultural acid load is very much less than site 352. Fig. 6 shows that little change occurs on weakly buffered soils that are already acidified. Site 377 is virtually in a steady state with respect to pH.
t~'--:- -A'"
3 20
40
80
80
100
Depth (cm)
Fig. 4. Current and predicted pH changes at site 352 where the acidification rate is approximately 2.5 k m o l ( + ) ha -1 year t.
7
6
~
5
•
1993 "~
+
2018
" - - *-- " 2043
4 3 20
40
60
80
100
Depth (cm)
Fig. 5. Current and predicted pH changes at site 361 where the acidification rate is approximately 0.33 k m o l ( + ) ha -1 year- 1
O
20
40
•
1993
-÷
2018
--'*"-
2043
60
80
100
Depth(cm)
Fig. 6. Current and predicted p H changes at site 377 where the acidification rate is approximately 0.16 k m o l ( + ) ha -1 year - 1
4. Discussion
Site 352, the high pH and high buffer capacity soil, was significantly affected by the agricultural acid load. For similar soils, which are widespread in the Hunter, lime use in intensive agriculture will be very important due to the large acid loads and the short time periods needed to significantly reduce the pH. However, the results from site 361 (Fig. 5) also indicate that with a low agricultural acid load the pH of poorly buffered soils declines to a low level in a moderately short time (25 years). The model predictions for site 377 were essentially unchanged from the existing pH profile. This gives some confidence that the low "endpoint" pH values that the model is predicting are valid, as the soil at site 377 has acidified over thousands of years at the "natural" rate, and should be very close to steady-state. The modest similarities of pH changes between 352 and 361 and the differences between 361 and 377 show that simple soil chemical parameters such as pH and E C E C are fallible indicators of soil changes with acidification. The use of SPAM has indicated that the results reflect more than one or two simple parameters, and acidification rates, aluminium-based pH buffering and other effects must be included in evaluating soil or site sensitivity. This is where simulation has an important role in assessing sensitivity. Simulation integrates pH, pHBC and other physical and biological processes and effects, and quantifies outcomes. This avoids the pitfalls of
J.B. Robinson, K.R. Helyar/ Ecological Modelling 86 (1996) 207-211 soil classification or c a t e g o r i s a t i o n into simple sensitivity classes. S P A M is also to b e used to assess the impact of acidic d e p o s i t i o n in the H u n t e r Valley. A small b u t significant p r o p o r t i o n (1 to 2 % ) of the l a n d area in the m i d d l e a n d u p p e r H u n t e r receives wet plus dry d e p o s i t i o n equal to a p p r o x i m a t e l y 0.5 k m o l ( + ) ha -1 year -~. This load is c o m p a r a b l e with large areas of N o r d i c E u r o p e that receive 0 . 5 - 1 . 0 k m o l ( + ) ha -1 year -a (Lovblad et al., 1992), b u t m u c h less t h a n in c e n t r a l a n d n o r t h e r n E u r o p e , w h e r e loads exceed 5 k m o l ( + ) ha -1 y e a r - 1 in some areas ( B r e d e m e i r et al., 1990).
Acknowledgements This work has b e e n financially s u p p o r t e d by Pacific Power. T h a n k s are d u e to G r e g Ayers a n d H u g h Malfroy for providing d e p o s i t i o n data, to Colin C h a r t r e s for collecting a n d classifying the soils, a n d K a t h y Shaw, P e t e r H a w k i n s a n d P e t e r O l s e n for soil chemical analyses.
References Bredemeir, M., Matzner, M. and Ulrich, B., 1990. Internal and external proton loads to forest soils in northern Germany. J. Environ. Qual., 19: 469-477. Bureau of Resource Sciences, 1991. Acid Soils in Australia: A summary of issues for government. BRS, Canberra.
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Fenton, I.G. and Helyar, K.R., 1995. Applying information from research on soil acidity. In: R.A. Date, N.J. Grundon, G.E. Rayment and M.E. Probert (Editors), Plant-Soil Interactions at Low pH: Principles and Management. Kluwer, Dordrecht. Helyar, K.R. and Porter, W.M., 1989. Soil acidification, its measurement and the processes involved. In: A.D. Robson (Editor), Soil Acidity and Plant Growth. Academic Press, Sydney, pp. 61-101. Levine, E.R. and Ciolkosz, E.J., 1986. A computer simulation model for soil genesis applications. Soil Sci. Soc. Am. J., 50: 661-667. Lovblad, G., Amann, M., Andersen, B., Hovmand, M., Joffre, S. and Pedersen, U., 1992. Deposition of sulfur and nitrogen in the Nordic countries: present and future. Ambio, 21: 339-347. McFee, W.W., 1980. Sensitivity of soil regions to long-term acid precipitation. In: D.S. Shriner, C.R. Richmond and S.E. Lindberg (Editors), Atmospheric Sulfur Deposition: Environmental Impact and Health Effects. Ann Arbor Science, Ann Arbor, MI, pp. 495-506. Reuss, J.O. and Johnson, D.W., 1986. Acid Deposition and the Acidification of Soils and Waters. Springer-Verlag, New York. Robinson J.B., Helyar, K.R. and Hochman, Z., 1995. A model for understanding the importance of various chemical, physical and biological processes in the development of soil profile acidity. In: R.A. Date, N.J. Grundon, G.E. Rayment and M.E. Probert (Editors), Plant-Soil Interactions at Low pH: Principles and Management. Kluwer, Dordrecht. Turchenak, L.W., Abboud, S.A. and Lutwick, G.W., 1995. Modelling and monitoring acid deposition impacts on soils in Alberta, Canada. In: R.A. Date, N.J. Grundon, G.E. Rayment and M.E. Probert (Editors), Plant-Soil Interactions At Low pH: Principles and Management. Kluwer, Dordrecht.