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Environmental impact of soil erosion under different cover and management systems
SOIL TECHNOLOGY
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Cremlingen 1993 ]
E n v i r o n m e n t a l I m p a c t of Soil Erosion U n d e r Different Cover and M a n a g e m e n t S y s t e m s G. C h i s c i & V. M a r t i n e z
S-mmary Runoff, soil loss and physical and chemical composition of surface soil, runoff water and eroded sediment were measured for three erosive rainstorms of the 1988 Autumn-Winter season, on clay soil slopes (Typic Chromoxerert) under different cover and management systems in a locality of Sicily. Four years reconsolidated natural grass-sod (Arena fatua and Lolium tumuleatum), natural grass-sod with implanted forage shrubs (Atriplez halimus) and natural grass-sod afforested with Pine trees (Piaus halepensis), reduced significantly runoff and soil loss in comparison with tilled fallow following four years durum wheat cultivation. While differences in runoff and soil loss between reconsolidated systems were not significant, the higher biomass yield (Stringiet al. 1991) and the better soil cover (Chisci et al. 1991) of the Atriplez system, increased O.M. content of the soil and prevented soil erosion under very intense rainstorms of the semiarid Mediterranean area. The comparison of textural and aggregate grain-size composition of surface soil and sediment confirmed that physico-mechanical composition of sediISSN 0933-3630 (~)1993 by CATENA VERLAG, 38162 Cremlingen-Destedt,Germany 0933-3630/93/5011851/US$ 2.00 + 0.25 SOIL
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ment detached and transported by overland flow on clayey soils is better estimated by pseudo-textural grain-size composition of surface soil (Chisci et al. 1989) than from textural composition. Soil loss amounts, and O.M., N and P enrichment ratios, combined in a specifically devised Environmental Impact Index (EI), demonstrated the excellent environmental protection value of reconsolidation of arable soils. However, Pz'aus system was somewhat less efficient than Atriplez and good natural grasssod systems.
1
Introduction
In the semi-arid Mediterranean area clayey hill soils have been used for a long time for wheat cropping in rotation with fallow and/or annual forage crops. This cropping system of marginal hill land, imposed by the prevalent large estate land-tenure system (~Latifondo") and by the agricultural population pressure of the past, was responsible for onsite soil degradation and off-site environmental impacts (Chisci 1980). In the last decades, changes in land tenure and a decrease in population pressure on agricultural land in the E C Mediterranean area have resulted in abandonment of some marginal arable land. Moreover, in the last years, reconsolidation of arable land has also
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been aided by the EC set-aside policy, which basically confers financial support to farmers to put arable land to sodbuster or reforestation programmes. While the main purpose of set-aside policy is to reduce EC agricultural surplus production, reconsolidation of arable land is also intended as a means for soil and environmental recovery in the most degraded marginal areas of the Community. As a m a t t e r of fact, a number of studies and observations have pointed out that clayey soils of the semi-arid climate, when continuously tilled for wheat cropping, are subject to soil O.M. depletion, soil structure deterioration and accelerated soil erosion. On the other hand, reconsolidation of arable soils, with different forms of cover and management system, is generally expected to induce a better O.M. turnover an a better control of runoff generation and soil erosion. Different types of reconsolidated cover and management systems may exert an i m p o r t a n t role not only on runoff generation and soil loss control on hillslopes, but also on reducing enrichment of finer soil particles, O.M. and nutrients in runoff and sediment exported from the soil surface, an aspect of soil degradation and environmental impact that is relatively less known (Barrows & Kilmer 1963, Chisci & Spallacci 1983). To our knowledge, scarce experimental evidence is available for the evaluation of recovery potential of reconsolidation measures on clayey semi-arid Mediterranean area. For this reason, the experiment commented on in this paper was carried out on clayey slopes in the interior of Sicily. SOIL
2 2.1
Material
and
methods
Site conditions
The experimental site was located in Pietranera-S. Stefano di Quisquina (Agrigento, Italy), an area with typically semi-arid Mediterranean climate with a long-period average annum rainfall of 560 m m and an annum E T P of 904 m m (Oliveri et al. 1986). Intense rainstorms occur in the fall and winter seasons producing remarkable runoff and soil erosion. The site of the experiment represents the hilly environment and the soil is uniformly classified as a Typic Chromoxerert (USDA 1975) derived from the Miocene clay Gypsum-Solfureus series.
2.2
Experimental layout
On a S W facing slope of 27-31% plots under different cover and management systems, represented by plantations of Atriplez halimus and Pinus halepensis in rows 4 m apart aligned up-and-down slope, were set up in 1984 on arable land on which durum wheat had been continuously cultivated for a long time. P a r t of the area, with slope around 19%, was left to continuous durum wheat cultivation for a comparison. In 1988, special devices were installed for monitoring and sampling runoff and sediment, providing six replications for each one of the following treatments: AF = Four years reconsolidated natural grass-sod (Arena fatua and Lolium tumulentum) partly covered by Atriplez halimtts canopy. AI = Four years reconsolidated natural grass-sod in the interrows between A triplex rows. PF = Four years reconsolidated natural grass-sod partly covered by Pinus T~CHNOLOGY--A
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halepensi8 canopy. PI = Four years reconsolidated natural grass-sod in the interrows between the Pinus rows. D = Tilled fallow after four years of continuous durum wheat cultivation. A more detailed description of the different plant cover architectures for the reconsolidated cover and management systems is reported by Chisci et al. (1991). 2.3
S a m p l i n g a n d analytical determinations
Two to five samples of the Ap horizon were collected in each experimental unit, in the area upslope of each collector for initial analysis. Rainfall d a t a were monitored by a local raingage station, while runoff was sampled using six randomly replicated Gerlach collectors for each cover and management system (Zachar 1982, De Ploey & Gabriels 1984). Runoff samples collected for the erosive events were analysed in the laboratory, separating water and sediment. For runoff water, O.M. was analysed using Biancucci & Ribaldone method (1980). For soil and sediment analysis S.I.S.S. methods (1985) were adopted. Grain-size distribution analysis of primary soil particles and water-stabh aggregates was performed with the methods illustrated by Chisci et al. (1989).
3 8.1
Results
and discussion
Rainfall and soil p a r a m e t e r s
In 1988, three erosive events occurred at Pietranera, on the 21st of September, 13th of November and the 1st of December, with rainfall totalling 167.6 mm. SOIL
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It was especially the second rainstorm of 60.8 mm that provided the most significant data because of its high peak intensity (I30 = 89.2 m m / h ) . The kinetic energy of this storm was 1279 MJ m m / h a / h , while the energy of the other two storms was 227 M3 m m / h a / h . Such values correspond to events with returnperiod probability of ten years (D'Asaro & Santoro 1983). Soil analysis of samples collected from different cover and management units (tab. 1), whih indicating a remarkable uniformity of semi-permanent soil physical and chemical characteristics, show some significant differences between the treatments after four years of reconsolidation. O.M., N and P were significantly higher in the Atriplez areas. Considering the higher biomass growth (Stringi et al. 1991) and the better soil cover (Chisci et al. 1991), it may be sustained that the improved O.M. turnover and soil protection against erosion are accounted for by the observed improvement in the analysed parameters of soil fertility.
3.2
R u n o f f and soil loss
As expected, runoff and soil loss were significantly reduced by all the tested reconsolidated cover and management systems in comparison to the tilled fallow (tab. 2). It is remarkable that the Atriplex system exerts a complete control of runoff and soil loss, even when a high magnitude erosive rainstorm is experienced. In relation to sediment composition (tab. 3), soil loss from the tilled fallow has significantly higher amount of fine sand + silt particles when compared to that from all the reconsolidated systems. This indicates that erosion is more selec-
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Cover and management system: AI PF PI D
AF Fine sand+silt (0.1-0.002 mm)% Clay (< 0.002 mm)% Organic matter % E.C. mmhos/cm N%
p%
K + meq/100 g Ca ++ meq/100 g Mg ++ meq/100 g Na + meq/100 g
35.91 63.34 2.04 0.47 0.056 0.020 0.86
35.76 63.62 1.97 0.53 0.052 0.020 0.86 36.97 3.87 1.29
37.05
3.88 1.29
36.19 63.20 0.85 0.39 0.040 0.017 0.83 35.56 3.72 1.24
35.98 63.51 0.78 0.21 0.039 0.009 0.83 35.80 3.75 1.24
35.49 64.00 1.21 0.68 0.044 0.024 0.86 36.92 3.87 1.29
L.S.D. 0.05 n.s. n.s.
0.31 0.35 0.007 0.008 n.8. n.8. n.8. n.8.
T a b . 1: Physical and chemical composition of surface soil (Typic Chromozerert) in
the different cover and management systems. Cover system
Runoff l/m
AF
Soil loss g/m
--
AI PF PI D L.S.D. 0.05
67.52 136.49 326.55 1423.90 521.60
--
508.07 1006.82 2434.27 10618.59 3892.44
T a b . 2: Runoff and soil loss of different cover and management systems, total of
three erosive rainstorms of 1988.
AF Fine sand+silt (0.1-0.002 mm)% Clay (<0.002 mm)% Organic matter % N % P %
Cover and management system: AI PF PI D
---
33.99 65.24 2.23 0.083 0.018
-
---
32.55 66.90 1.16 0.089 0.029
33.11 66.34 1.28 0.078 0.028
L.S.D. 0.05
35.31 63.45 1.26 0.080 0.031
1.88 1.96 0.41 n.s. 0.01
T a b . 3: Physical and chemical composition of sediment produced by water erosion
in different cover and management systems. t i v e in t h e c o n s o l i d a t e d p l o t s in c o m p a r ison to fallow plots where the runoff rate is m o r e i n t e n s e . O . M . c o n t e n t in t h e s e d i m e n t is sign i f i c a n t l y h i g h e r for t h e g r a s s - s o d of t h e Atriplez area, w h e r e a h i g h e r c o n t e n t of O . M . w a s f o u n d in t h e soil s a m p l e s .
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O n t h e o t h e r h a n d , P c o n t e n t in t h e s e d i m e n t is s i g n i f i c a n t l y l o w e r in t h e a b o v e s y s t e m t h a n in t h e o t h e r s y s t e m s . Since t h e P c o n t e n t of t h e s u r f a c e soil is t h e s a m e for t h e d i f f e r e n t t r e a t m e n t s , t h e r e is n o t an e a s y e x p l a n a t i o n t o t h i s finding.
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100 90r-
--~ 6)
80-
>, .(3
70
~> ,
60-
/ //"
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MWD=O0086_~0.0006
/
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¢,i~, ,)
MWD=O.442,tO.010
/
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40-
----
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20
(O 10 0
~K .e~.....1~
/
0,d02 0,b2 0,b5
MWD=O.706,,tO.021 (line 3)
011 012 015 Aggregate or grain size (mm)
i
Primary particles grain-size distributions of both soil and sediment (line 1); grain-size undispersed aggregate distribution of sediment and pseudo-teztural aggregate grain-size distribution of surface soil (Chisci et al. 1989) (line ~); standard grain-size aggregate distribution of surface soil (Kemper & Chepil 1965) (line 8).
F i g . 1:
From the above data on the soil and Utilizing some grain-size distribution sediment physical composition, some analysis carried out for soil and sediment comments can be made on the mechan- during the present experiment, it can ics of soil particle detachment and trans- be observed that water-stable surface port in clayey soils. soil aggregates represent the main core The peculiarity of such soils in re- of sediment transported by the overlation to prediction of their erodibility land flow (fig. 1). In fact, the arith(Wischmeier et al. 1971) using surface metic Mean Weight Diameter ( M W D ) soil characteristics has been pointed out of the grain-size distribution of undisby m a n y authors (Toni & Sfalanga 1980, persed sediment is 0.442 m m , signifiof the 1982, De Ploey et al. 1984, Govers 1985, cantly higher than the M W D primary particle grain-size distributions, Torri 1987, Torri et al. 1987, Chisci et al. 1989, Chisci 1989). From the lit- which are equal for both soilsamples and erature, it seems that the single most sediment samples to 0.0086 ram. important soil parameter for evaluating Water-sieving analysis on surface soil soil erodibility is the amount of stable samples using intense shaking, as resoil aggregates of the fine sand and silt ported by Chisciet al. (1989) for deterclasses present at the soil surface at the mining pseudo-textural aggregate grainmoment of the erosive rainfall event. size distribution, shows that the grainSOIL
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size aggregate distribution of sediment is equal to the surface soil pseudo-textural aggregate particle distribution (MWD -0.442 mm), while the standard watersieving aggregate analysis, as reported by Kemper & Chepil (1965) yields a MWD equal to 0.706 mm. It must be noticed that, within the repeatibility of the measurements, soil and sediment textural distributions of primary particles coincide. The same happens for the aggregates of eroded sediment and of the soil, when the analysis of the latter is made following the Chisci et al. (1989) methodology (fig. 1). From the data, it may be inferred that the mass of soil material detached and transported during erosion in clayey soils is mainly represented by aggregated clay particles. The disruption and selection of aggregates during the erosion process, selects very stable clay aggregates of the silt and fine sand classes. 8.8
Enrichment ratios and environmental impact
O.M. and nutrients losses due to water erosion on slopes, have been often used to assess on-site soil degradation and off-site environmental impact potential (Chisci & Spallacci 1984). Also the present data on losses of O.M. and nutrients under different cover and management systems show that all the consolidated systems reduce signifiantly the release of soil chemical compounds to water bodies (tab. 4). However, using O.M. and nutrient losses from upslope, relative total exports depend mainly on the amount of runoff and soil loss from each system. In evaluating such data m a n y authors (Thompson 1957, Barrows & Kilmer 1963, Chisci & Spallacci 1984) have SOIL
given less emphasis to the relative quality of physical and chemical composition of runoff water and sediment than to the amount of runoff and soil loss, in assessing potential soil degradation and environmental impact. O n the other hand, the relative protection value of different cover and management systems would be better assessed taking into account the quality of the soil material exported by soil erosion processes from upslope. Such quality can be determined as enrichment ratios (ER), obtained by expressing physical and chemical characteristics of the runoff water and sediment as a function of the soil composition (tab. 5). Our data show significant differences in enrichment ratios between different cover and management systems for O.M., P and EC. O.M. and P enrichment ratios are significantly higher for Pinus consolidated soil, notwithstanding that O.M. and P content of the surface soil are lower. Moreover, while for the studied clayey soil, containing soluble salt, the E C enrichment ratio is generally high for all the different cover and management systems however, for the tilled fallow, it is significantly higher than for the consolidated systems. In fact, the EC of water extract of the tilled soil is higher than for the consolidated soil as a consequence of bringing deep soil layers to the surface by ploughing. It may be added that the good relationship between soluble O.M. and EC in runoff water (r = 0.902), supports the fact that, for soils with a high EC of the water extract, the export of O.M. can be high, even when the O.M. content of the soil is low. From the above considerations, the TECHNOLOGY--A
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Cover and management system1): AF AI PF PI D Organic mat. g/m 1)2)
--
N g / m 1) P g / m 1)
---
14.03 0.37 0.08
1 6 . 8 1 33.17 0.92 1.59 0.27 0.50
L.S.D. 0.05
204.59 8.61 3.30
95.50 4.51 1.30
1) Mean exports were calculated for each replication for statistical analysis. For this reason average values are different from that obtained by the mean data of tab. 2 and 3 2) O.M. export includes also soluble O.M. of runoff water. T a b . 4: Average losses of O.M. and nutrients from different cover and management
systems. Enrichment ratio 1)
Cover and management system: AF AI PF PI D
sand+silt (0.1-0.002 mm)~ Clay (< 0.002 mm)% Organic matter N% P% E.C. mmhos/cm Environm. Impact (EI)
Fine
L.S.D. 0.05
0.97 1.02
0.91 1.06
0.93 1.05
0.99 1.08
1.18
1.39
1.68
0.36
1.59 1.01 1.45 0.014
2.19 2.09 1.18 0.149
2.01 3.21 1.57 0.433
1.08 1.86
1.42 5.64 1.000
1.02 3.69 0.39
n.s. n.s.
n.s.
x) Enrichment ratios were calculated for each replication for statistical analysis. For this reason average values are different from that obtained by the mean data of tab. 1 and 3. T a b . 5: Enrichment ratios (ER) and Environmental Impact Index (El) of runoff
water and eroded sediment coming from different cover and management systems. following g e n e r a l s t a t e m e n t s can be m a d e , w h i c h can be v a l u a b l e for p r o p o s ing a n i n d e x e v a l u a t i n g t h e e n v i r o n m e n tal hazard: 1. G i v e n a c e r t a i n soil type, in-situ soil c h a r a c t e r i s t i c s c a n be s t r o n g l y influe n c e d b y different soil cover a n d m a n agement systems: (a) directly, b y t h e i r influence on soil f e r t i l i t y p a r a m e t e r s (e.g., o r g a n i c m a t t e r t u r n o v e r , soil s t r u c t u r e a m e lioration); and (b)
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indirectly, b y its i n t e r a c t i o n w i t h h y d r o l o g i c a l p h e n o m e n a (e.g. leaching, runoff, soil erosion).
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2. Off-site e n v i r o n m e n t a l h a z a r d is linked to t h e release of s e d i m e n t , soil O.M. a n d n u t r i e n t s to t h e g e n e r a l env i r o n m e n t along w i t h runoff w a t e r a n d e r o d e d m a t e r i a l . Such losses are n o t tot a l l y e x p l a i n e d b y t o t a l runoff a m o u n t a n d soil loss p r o d u c e d in a given cover a n d m a n a g e m e n t s y s t e m . In fact, relative p h y s i c a l a n d c h e m i c a l c o m p o s i t i o n of runoff w a t e r a n d s e d i m e n t is also influenced b y t h e different d y n a m i c s of t h e erosion process a n d c h e m i c a l c o m p o s i t i o n of t h e soil in t h e different syst e m s (e.g. rainfall kinetic e n e r g y for soil loss, electrical c o n d u c t i v i t y for O . M . loss). It follows t h a t for e v a l u a t i n g t h e
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global impact on the environment of A second partial environmental Imdifferent management techniques, nutri- pact Index can be calculated for off-site ents as well as soil and O.M. losses must effects: be taken into account; impacts cannot Ei. OMi • Pi • Ni be expressed solely by runoff and soil E I i " = E d . O M d . P d . N d (3) loss. where soil loss (El, d} has been introA general index of environmental imduced considering that release of sedipact of different cover and management ment in the environment is in itself a systems should take into account two factor of environmental impact. main components: one expressing the A global EI index based on the two soil amelioration in-situ and the other partial indices reported above for on-site expressing the potential off-site impact. and off-site impact can then be proposed In order to keep the index as simple as follows: as possible, it may be expressed with a EIi= EIi"/EIi' (4) formula of the following type:
EI=fl*f2*.../i*...*/n
(1)
where f i is a generalized characteristic value for the i-th treatment. Let us now define fi. Primarily such a term should express the relative variation in conditions with respect to a reference situation that, in our case, could be identified with the worst possible condition. Generally speaking, such a situation can be approximately identified with tilled fallow (with zero conservation measures). Hence, f i can be represented by the ratio between the value of the ith characteristic in the present situation and the reference one. To make an example, the phosphorous f-value for the P F treatment is given by:
fP(Pf)
= P(PF)/P(D)
where: P ( P F ) and P ( D ) are P contents of soil or sediment respectively in the PF and D treatments. Following the above criteria, a partim Environmental Impact Index can be calculated concerning on-site effects for each consolidated system:
EIi'=
OMi •Pi. Ni OMd. Pd. Nd
(2)
The ratio between the two previously defined partial indices has been chosen so that when the global index increases the impact on environment increases. When E I ~ increases, the on-site conditions are improving, while when E I i " increases, the off-site impacts worsen (more sediment or more N or more P or more O.M. are released inducing pollition and eutrophication problems). In order to have a global index in agreement with the two aspects, i.e., expressing improvement when both E I ~ and E I " indicate improvement, the ratio in the above formulation is the only simple solution. Taking into account the ER definition that was given above, by substituting eqs. 2 and 3 in eq. 4 and rearranging, a global "Environmental Impac Index" can be calculated as follows: El. ER(OM)i. ER(N)i. ER(P)i EI = ~ - ~ - ~ :ER(N)d. ER(P)i where E I = Environmental Impact Index: 0 > E l > 1; Ei and Ed = Soil loss respectively on the i-th reconsolidated system and tilled fallow;
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Environmental impact of soil erosion ER(OM)i and ER(OM)d = O.M. enrichment ratios respectively in the i-th reconsolidated system and tilled fallow; ER(N)i and ER(N)d = N enrichment ratios respectively on the i-th reconsolidated system and tilled falow; ER(P)i and ER(P)d -- P enrichment ratios respectively on the i-th reconsolidated system and tilled fallow.
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ter abandonment, and arable soil conditions. Moreover, although the prominent role of the vegetation cover in the control of surface erosional processes is, in principle, well known, the data of the present experiment emphasize how the different kinds of vegetation cover influence the physical and chemical quality of runoff water and eroded material, and thereby As expected, the EI index (tab. 5) have important consequences for both confirms substantially a remarkable sig- the dynamics of soil degradation and the nificant reduction of environmental im- potential environmental impact. pact for all the reconsolidated systems. In the given conditions, a reconsolHowever, while there is not any ques- idated system with Atriplex halimus tion about the very high soil and en- plantation and undergrowth of natural vironment protection offered by the grasses (Arena fatua and Lolium tuAtriplex system (EI practically 0), and mulentum), provMing canopy protection the fact that tilled fallow gives the worst and mulch material to the soil, is capable soil protection by definition (EI = 1), of preventing soil erosion almost comthe index offers a more detailed eval- pletely, even in the case of very intense uation of the other reconsolidated sys- rainstorms such as those that afflict, tems. For instance, a good four year nat- however rarely, the semi-arid Mediterural grass-sod protects the soil very well ranean environment. against the erosional power of the inMoreover, the stratified canopy of tense rainstorms of the area. Contrarily, Atriplex and annual grasses growing a bad grass-sod, as that present in the naturally in the area provides higher Piaus area, reduces the EI index only to biomass yield for grazing and extends one half in comparison to tilled fallow. the grazing season in late Spring and Where Piaus canopy is present, the EI early Summer by the prolonged growth index results are better. of the Atriplez shrub (Stringi et al. 1991). A good control of soil erosion is at4 Conclusions tained also by a good reconsolidated Reconsolidated cover and management natural grass-sod, without implanted systems exert a remarkable potential for shrubs, providing live green cover in soil protection and environmental recov- Winter and Spring and dry mulch in ely. Summer and Autumn. Indeed, the exHowever, while this statement seems periment confirms that, in relation to conceptually obvious, quantitative data surface soil erosion, it is the protection were not available to compare the rel- at ground level that exerts the best conative value of different steered recon- trol. solidation systems, obtained with shrub It was also demonstrated that, if for and tree plantation, in comparison with some reason the formation of a good both natural grass-sod, as expected af- grass sward is prevented and a bad SOIL
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sward develops, the protection value of such cover is badly affected. It follows that simple abandonment of arable land is an uncertain practice for soil and environmental protection, depending on the natural grass sward formation, the characteristicsof which cannot be easily predicted. T h e reconsolidated Pinus halepensis system m a y p r o t e c t the soil much b e t t e r t h a n the arable condition, b u t it is less efficient t h a n the other reconsolidated systems in controlling erosion. F r o m one side, the P i n e p l a n t reduces the u n d e r g r o w t h of a n n u a l grasses u n d e r its canopy. F r o m the other, the clayey soil of our e x p e r i m e n t is n o t suitable for three growth. In fact, after four years, the s h r u b s were more developed t h a n Pinus p l a n t s (Chisci et al. 1991). A group of e n v i r o n m e n t a l indices has b e e n proposed o n the basis of the experi m e n t here described. T h e indices try to q u a n t i f y the i m p a c t a given m a n a g e m e n t practice can have on soil fertility in situ. Off-site i m p a c t was also examined, considering the release of eroded m a t e rial a n d n u t r i e n t s to the general environm e n t . T h e two indeces were combined in a t h i r d one, i n t e n d e d for a global evalu a t i o n of the effect of a change of landuse a n d m a n a g e m e n t on general environmental amelioration. O u r indices are still far from being definitive b u t they provide a useful quick e s t i m a t e of the global e n v i r o n m e n t a l impact a change of land-use a n d m a n a g e m e n t m a y produce.
R.P.C. Morgan, Silsoe College, Silsoe (UK) for their help in the revision and editing of the paper. References B A R R O W S , H. & K I L M E R , V.J. (1988): Plant nutrient losses from soil by water erosion. In: Advances in Agronomy 15, 303-316. B I A N C U C C I , G. & R I B A L D O N E , E. (1980)" L'analisi chimica delle acque naturali e inquinate. Hoepli (ed.), Milano, p. 319. CHISCI, G. (1980): Physical soil degradation due to hydrological phenomena in relation to change of agricultural system in Italy. Ann. Ist. Sper. Studio e Difesa del Suolo, vol. XI, Firenze, 271-283. CHISCI, G. & S P A L L A C C I , P. (1984): Nutrient losses by leaching and runoff and possibilities of their control. In: Nutrient Balance and Fertilizers Needs in Temperate Agriculture. 18th Colloquium of the International Potash Institute, Gardone Riviera
(Italy). CHISCI, G. B A Z Z O F F I , P. & M B A G W U , J.S.C. (1989): Comparison of aggregatestabilityindicesfor soilclassification and assessment of soil management practices.SOIL T E C H N O L G O Y 2, 113-133. CHISCI, G. (1989): Measures for runoff and erosion control on clayey soils: a review of trials carried out in the Apennines hilly area. In: U. Schwertmann, R.J. riekson & K. Auerswald (Eds.), SOIL TECHNOLGOY SERIES 1, 53-71. CHISCI, G., S T R I N G I , L., M A R T I NEZ, V., A M A T O , G. & G R I S T I NA, L. (1991)" Ruolo di arbusti foraggeri nell'ambiente semi-arido siciliano. 2. Funalone protettiva contro l'erosione idrometeorica. Rivista di Agronomia 2,332-340.
Acknowledgement
D ' A S A R O , F. & S A N T O R O , M. (1983): Aggressivith della pioggia hello studio dell'erosione idrica del territorio siciliano. Arti Graflche Siciliane, Palermo, p. 28.
The Authors are grateful to Dr. D. Torri, of the ~CNR-Centro per la Genesi, Classificazione e Cartografia del Suolo, Firenze (Italy)" and to Prof.
DE PLOEY, J. & G A B R I E L S , D. (1980): Measuring soil loss and experimental studies. In: M.J. Kirkby & R.P.C. Morgan (Eds.): Soil Erosion. Wiley & Son, Chichester, 63-108.
SOIL
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D E P L O E Y , J . & P O E S E N , J . (1984): Aggregate stability, runoff generation and interriU erosion. In: K.S. Richards, R.R. Arn e t t & S. Ellis (Eds.), Geomorphology and Soil. Allen & Unwin, London. GOVERS, G . (1985): Selectivity and transport capacity of thin flows in relation to rill erosion. CATENA 12, 35-49. KEMPER,
W.D.
&
CHEPIL,
W.S.
(1965): Size distribution of aggregates. In: C.A. Black (Ed.), Amer. Madison, Wis., 499-510.
Soc. Agronomy,
OLIVERI, G., D A Z Z I , C. & R A I MONDI, S. (1986): Studi sui suoli dell'aziende di Pietranera (AG). Quaderni di Agronomia 2, 103-162. S T R I N G I , L., A M A T O , G., G R I S T I o N A , L., C H I S C I , G . & A L I C A T A , M . L . (1991): Ruolo di arbusti foraggeri nell'ambiente semi-arido siciliano. 1. Funzione produttiva e valore nutritivo. Rivista di Agronomia 2,325-331. T H O M P S O N , L. (1967): Soil and Soil FertiliW. McGraw-Hill, N.Y., 2nd. Ed. T O R R I , D. & S F A L A N G A , M . (1980): Stima dell'erodibilit~ dei suoli mediante simulazione di pioggia in laboratorio. Nota 2. Preparazione del campioni di suolo. Ann. Ist. Sper. Studio e Difesa del Suolo, vol. XI, Firenze, 141-157. T O R R I , D. & S F A L A N G A , M. (1982): Some aspects of soil erosion mechanics. Laboratory experiments. Ann. Ist. Sper. Studio e Difesa del Suolo, vol. XII, Firenze, 75-89. T O R R I , D. (1987): A theoretical study of soil detachability. CATENA SUPPLEM E N T 10, 15-20. T O R R I , D., S F A L A N G A , M . & D E L S E T T E , M . (1987): Splash detachment: runoff depth and soil cohesion. CATENA 14, 149-155. WISCHMEIER, W.H., JOHNSON, C . B . & C R O S S , B . V . (1971): A soil erodibility nomograph for farmland and construction sites. Jour. of Soil and Water Conservation 26, 189-193. Z A C H A R , D. (1982): Soil Erosion. Development in Soil Science 10, Elsevier, Amsterdam, p. 548.
SOIL
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A d d r e s s e s of a u t h o r s : Prof. Dr. Gianearlo Chisci Univ. degli Studi di Firenze Dipt. di Agronomia e Produzioni Erbacee P. le delle Cascine 18 1-50144 Firenze Italy Dr. Victoria Martines University of Las Palmas Gran Canaria Spain
of C:ATBNA
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Cremlingen 1993 ]
E n v i r o n m e n t a l I m p a c t of Soil Erosion U n d e r Different Cover and M a n a g e m e n t S y s t e m s G. C h i s c i & V. M a r t i n e z
S-mmary Runoff, soil loss and physical and chemical composition of surface soil, runoff water and eroded sediment were measured for three erosive rainstorms of the 1988 Autumn-Winter season, on clay soil slopes (Typic Chromoxerert) under different cover and management systems in a locality of Sicily. Four years reconsolidated natural grass-sod (Arena fatua and Lolium tumuleatum), natural grass-sod with implanted forage shrubs (Atriplez halimus) and natural grass-sod afforested with Pine trees (Piaus halepensis), reduced significantly runoff and soil loss in comparison with tilled fallow following four years durum wheat cultivation. While differences in runoff and soil loss between reconsolidated systems were not significant, the higher biomass yield (Stringiet al. 1991) and the better soil cover (Chisci et al. 1991) of the Atriplez system, increased O.M. content of the soil and prevented soil erosion under very intense rainstorms of the semiarid Mediterranean area. The comparison of textural and aggregate grain-size composition of surface soil and sediment confirmed that physico-mechanical composition of sediISSN 0933-3630 (~)1993 by CATENA VERLAG, 38162 Cremlingen-Destedt,Germany 0933-3630/93/5011851/US$ 2.00 + 0.25 SOIL
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ment detached and transported by overland flow on clayey soils is better estimated by pseudo-textural grain-size composition of surface soil (Chisci et al. 1989) than from textural composition. Soil loss amounts, and O.M., N and P enrichment ratios, combined in a specifically devised Environmental Impact Index (EI), demonstrated the excellent environmental protection value of reconsolidation of arable soils. However, Pz'aus system was somewhat less efficient than Atriplez and good natural grasssod systems.
1
Introduction
In the semi-arid Mediterranean area clayey hill soils have been used for a long time for wheat cropping in rotation with fallow and/or annual forage crops. This cropping system of marginal hill land, imposed by the prevalent large estate land-tenure system (~Latifondo") and by the agricultural population pressure of the past, was responsible for onsite soil degradation and off-site environmental impacts (Chisci 1980). In the last decades, changes in land tenure and a decrease in population pressure on agricultural land in the E C Mediterranean area have resulted in abandonment of some marginal arable land. Moreover, in the last years, reconsolidation of arable land has also
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been aided by the EC set-aside policy, which basically confers financial support to farmers to put arable land to sodbuster or reforestation programmes. While the main purpose of set-aside policy is to reduce EC agricultural surplus production, reconsolidation of arable land is also intended as a means for soil and environmental recovery in the most degraded marginal areas of the Community. As a m a t t e r of fact, a number of studies and observations have pointed out that clayey soils of the semi-arid climate, when continuously tilled for wheat cropping, are subject to soil O.M. depletion, soil structure deterioration and accelerated soil erosion. On the other hand, reconsolidation of arable soils, with different forms of cover and management system, is generally expected to induce a better O.M. turnover an a better control of runoff generation and soil erosion. Different types of reconsolidated cover and management systems may exert an i m p o r t a n t role not only on runoff generation and soil loss control on hillslopes, but also on reducing enrichment of finer soil particles, O.M. and nutrients in runoff and sediment exported from the soil surface, an aspect of soil degradation and environmental impact that is relatively less known (Barrows & Kilmer 1963, Chisci & Spallacci 1983). To our knowledge, scarce experimental evidence is available for the evaluation of recovery potential of reconsolidation measures on clayey semi-arid Mediterranean area. For this reason, the experiment commented on in this paper was carried out on clayey slopes in the interior of Sicily. SOIL
2 2.1
Material
and
methods
Site conditions
The experimental site was located in Pietranera-S. Stefano di Quisquina (Agrigento, Italy), an area with typically semi-arid Mediterranean climate with a long-period average annum rainfall of 560 m m and an annum E T P of 904 m m (Oliveri et al. 1986). Intense rainstorms occur in the fall and winter seasons producing remarkable runoff and soil erosion. The site of the experiment represents the hilly environment and the soil is uniformly classified as a Typic Chromoxerert (USDA 1975) derived from the Miocene clay Gypsum-Solfureus series.
2.2
Experimental layout
On a S W facing slope of 27-31% plots under different cover and management systems, represented by plantations of Atriplez halimus and Pinus halepensis in rows 4 m apart aligned up-and-down slope, were set up in 1984 on arable land on which durum wheat had been continuously cultivated for a long time. P a r t of the area, with slope around 19%, was left to continuous durum wheat cultivation for a comparison. In 1988, special devices were installed for monitoring and sampling runoff and sediment, providing six replications for each one of the following treatments: AF = Four years reconsolidated natural grass-sod (Arena fatua and Lolium tumulentum) partly covered by Atriplez halimtts canopy. AI = Four years reconsolidated natural grass-sod in the interrows between A triplex rows. PF = Four years reconsolidated natural grass-sod partly covered by Pinus T~CHNOLOGY--A
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halepensi8 canopy. PI = Four years reconsolidated natural grass-sod in the interrows between the Pinus rows. D = Tilled fallow after four years of continuous durum wheat cultivation. A more detailed description of the different plant cover architectures for the reconsolidated cover and management systems is reported by Chisci et al. (1991). 2.3
S a m p l i n g a n d analytical determinations
Two to five samples of the Ap horizon were collected in each experimental unit, in the area upslope of each collector for initial analysis. Rainfall d a t a were monitored by a local raingage station, while runoff was sampled using six randomly replicated Gerlach collectors for each cover and management system (Zachar 1982, De Ploey & Gabriels 1984). Runoff samples collected for the erosive events were analysed in the laboratory, separating water and sediment. For runoff water, O.M. was analysed using Biancucci & Ribaldone method (1980). For soil and sediment analysis S.I.S.S. methods (1985) were adopted. Grain-size distribution analysis of primary soil particles and water-stabh aggregates was performed with the methods illustrated by Chisci et al. (1989).
3 8.1
Results
and discussion
Rainfall and soil p a r a m e t e r s
In 1988, three erosive events occurred at Pietranera, on the 21st of September, 13th of November and the 1st of December, with rainfall totalling 167.6 mm. SOIL
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It was especially the second rainstorm of 60.8 mm that provided the most significant data because of its high peak intensity (I30 = 89.2 m m / h ) . The kinetic energy of this storm was 1279 MJ m m / h a / h , while the energy of the other two storms was 227 M3 m m / h a / h . Such values correspond to events with returnperiod probability of ten years (D'Asaro & Santoro 1983). Soil analysis of samples collected from different cover and management units (tab. 1), whih indicating a remarkable uniformity of semi-permanent soil physical and chemical characteristics, show some significant differences between the treatments after four years of reconsolidation. O.M., N and P were significantly higher in the Atriplez areas. Considering the higher biomass growth (Stringi et al. 1991) and the better soil cover (Chisci et al. 1991), it may be sustained that the improved O.M. turnover and soil protection against erosion are accounted for by the observed improvement in the analysed parameters of soil fertility.
3.2
R u n o f f and soil loss
As expected, runoff and soil loss were significantly reduced by all the tested reconsolidated cover and management systems in comparison to the tilled fallow (tab. 2). It is remarkable that the Atriplex system exerts a complete control of runoff and soil loss, even when a high magnitude erosive rainstorm is experienced. In relation to sediment composition (tab. 3), soil loss from the tilled fallow has significantly higher amount of fine sand + silt particles when compared to that from all the reconsolidated systems. This indicates that erosion is more selec-
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Cover and management system: AI PF PI D
AF Fine sand+silt (0.1-0.002 mm)% Clay (< 0.002 mm)% Organic matter % E.C. mmhos/cm N%
p%
K + meq/100 g Ca ++ meq/100 g Mg ++ meq/100 g Na + meq/100 g
35.91 63.34 2.04 0.47 0.056 0.020 0.86
35.76 63.62 1.97 0.53 0.052 0.020 0.86 36.97 3.87 1.29
37.05
3.88 1.29
36.19 63.20 0.85 0.39 0.040 0.017 0.83 35.56 3.72 1.24
35.98 63.51 0.78 0.21 0.039 0.009 0.83 35.80 3.75 1.24
35.49 64.00 1.21 0.68 0.044 0.024 0.86 36.92 3.87 1.29
L.S.D. 0.05 n.s. n.s.
0.31 0.35 0.007 0.008 n.8. n.8. n.8. n.8.
T a b . 1: Physical and chemical composition of surface soil (Typic Chromozerert) in
the different cover and management systems. Cover system
Runoff l/m
AF
Soil loss g/m
--
AI PF PI D L.S.D. 0.05
67.52 136.49 326.55 1423.90 521.60
--
508.07 1006.82 2434.27 10618.59 3892.44
T a b . 2: Runoff and soil loss of different cover and management systems, total of
three erosive rainstorms of 1988.
AF Fine sand+silt (0.1-0.002 mm)% Clay (<0.002 mm)% Organic matter % N % P %
Cover and management system: AI PF PI D
---
33.99 65.24 2.23 0.083 0.018
-
---
32.55 66.90 1.16 0.089 0.029
33.11 66.34 1.28 0.078 0.028
L.S.D. 0.05
35.31 63.45 1.26 0.080 0.031
1.88 1.96 0.41 n.s. 0.01
T a b . 3: Physical and chemical composition of sediment produced by water erosion
in different cover and management systems. t i v e in t h e c o n s o l i d a t e d p l o t s in c o m p a r ison to fallow plots where the runoff rate is m o r e i n t e n s e . O . M . c o n t e n t in t h e s e d i m e n t is sign i f i c a n t l y h i g h e r for t h e g r a s s - s o d of t h e Atriplez area, w h e r e a h i g h e r c o n t e n t of O . M . w a s f o u n d in t h e soil s a m p l e s .
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O n t h e o t h e r h a n d , P c o n t e n t in t h e s e d i m e n t is s i g n i f i c a n t l y l o w e r in t h e a b o v e s y s t e m t h a n in t h e o t h e r s y s t e m s . Since t h e P c o n t e n t of t h e s u r f a c e soil is t h e s a m e for t h e d i f f e r e n t t r e a t m e n t s , t h e r e is n o t an e a s y e x p l a n a t i o n t o t h i s finding.
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100 90r-
--~ 6)
80-
>, .(3
70
~> ,
60-
/ //"
/
MWD=O0086_~0.0006
/
/ /
¢,i~, ,)
MWD=O.442,tO.010
/
/
/
0i -
¢~ "-'1
50
0"
40-
----
30
E
20
(O 10 0
~K .e~.....1~
/
0,d02 0,b2 0,b5
MWD=O.706,,tO.021 (line 3)
011 012 015 Aggregate or grain size (mm)
i
Primary particles grain-size distributions of both soil and sediment (line 1); grain-size undispersed aggregate distribution of sediment and pseudo-teztural aggregate grain-size distribution of surface soil (Chisci et al. 1989) (line ~); standard grain-size aggregate distribution of surface soil (Kemper & Chepil 1965) (line 8).
F i g . 1:
From the above data on the soil and Utilizing some grain-size distribution sediment physical composition, some analysis carried out for soil and sediment comments can be made on the mechan- during the present experiment, it can ics of soil particle detachment and trans- be observed that water-stable surface port in clayey soils. soil aggregates represent the main core The peculiarity of such soils in re- of sediment transported by the overlation to prediction of their erodibility land flow (fig. 1). In fact, the arith(Wischmeier et al. 1971) using surface metic Mean Weight Diameter ( M W D ) soil characteristics has been pointed out of the grain-size distribution of undisby m a n y authors (Toni & Sfalanga 1980, persed sediment is 0.442 m m , signifiof the 1982, De Ploey et al. 1984, Govers 1985, cantly higher than the M W D primary particle grain-size distributions, Torri 1987, Torri et al. 1987, Chisci et al. 1989, Chisci 1989). From the lit- which are equal for both soilsamples and erature, it seems that the single most sediment samples to 0.0086 ram. important soil parameter for evaluating Water-sieving analysis on surface soil soil erodibility is the amount of stable samples using intense shaking, as resoil aggregates of the fine sand and silt ported by Chisciet al. (1989) for deterclasses present at the soil surface at the mining pseudo-textural aggregate grainmoment of the erosive rainfall event. size distribution, shows that the grainSOIL
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Chisci& Martinez
size aggregate distribution of sediment is equal to the surface soil pseudo-textural aggregate particle distribution (MWD -0.442 mm), while the standard watersieving aggregate analysis, as reported by Kemper & Chepil (1965) yields a MWD equal to 0.706 mm. It must be noticed that, within the repeatibility of the measurements, soil and sediment textural distributions of primary particles coincide. The same happens for the aggregates of eroded sediment and of the soil, when the analysis of the latter is made following the Chisci et al. (1989) methodology (fig. 1). From the data, it may be inferred that the mass of soil material detached and transported during erosion in clayey soils is mainly represented by aggregated clay particles. The disruption and selection of aggregates during the erosion process, selects very stable clay aggregates of the silt and fine sand classes. 8.8
Enrichment ratios and environmental impact
O.M. and nutrients losses due to water erosion on slopes, have been often used to assess on-site soil degradation and off-site environmental impact potential (Chisci & Spallacci 1984). Also the present data on losses of O.M. and nutrients under different cover and management systems show that all the consolidated systems reduce signifiantly the release of soil chemical compounds to water bodies (tab. 4). However, using O.M. and nutrient losses from upslope, relative total exports depend mainly on the amount of runoff and soil loss from each system. In evaluating such data m a n y authors (Thompson 1957, Barrows & Kilmer 1963, Chisci & Spallacci 1984) have SOIL
given less emphasis to the relative quality of physical and chemical composition of runoff water and sediment than to the amount of runoff and soil loss, in assessing potential soil degradation and environmental impact. O n the other hand, the relative protection value of different cover and management systems would be better assessed taking into account the quality of the soil material exported by soil erosion processes from upslope. Such quality can be determined as enrichment ratios (ER), obtained by expressing physical and chemical characteristics of the runoff water and sediment as a function of the soil composition (tab. 5). Our data show significant differences in enrichment ratios between different cover and management systems for O.M., P and EC. O.M. and P enrichment ratios are significantly higher for Pinus consolidated soil, notwithstanding that O.M. and P content of the surface soil are lower. Moreover, while for the studied clayey soil, containing soluble salt, the E C enrichment ratio is generally high for all the different cover and management systems however, for the tilled fallow, it is significantly higher than for the consolidated systems. In fact, the EC of water extract of the tilled soil is higher than for the consolidated soil as a consequence of bringing deep soil layers to the surface by ploughing. It may be added that the good relationship between soluble O.M. and EC in runoff water (r = 0.902), supports the fact that, for soils with a high EC of the water extract, the export of O.M. can be high, even when the O.M. content of the soil is low. From the above considerations, the TECHNOLOGY--A
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Cover and management system1): AF AI PF PI D Organic mat. g/m 1)2)
--
N g / m 1) P g / m 1)
---
14.03 0.37 0.08
1 6 . 8 1 33.17 0.92 1.59 0.27 0.50
L.S.D. 0.05
204.59 8.61 3.30
95.50 4.51 1.30
1) Mean exports were calculated for each replication for statistical analysis. For this reason average values are different from that obtained by the mean data of tab. 2 and 3 2) O.M. export includes also soluble O.M. of runoff water. T a b . 4: Average losses of O.M. and nutrients from different cover and management
systems. Enrichment ratio 1)
Cover and management system: AF AI PF PI D
sand+silt (0.1-0.002 mm)~ Clay (< 0.002 mm)% Organic matter N% P% E.C. mmhos/cm Environm. Impact (EI)
Fine
L.S.D. 0.05
0.97 1.02
0.91 1.06
0.93 1.05
0.99 1.08
1.18
1.39
1.68
0.36
1.59 1.01 1.45 0.014
2.19 2.09 1.18 0.149
2.01 3.21 1.57 0.433
1.08 1.86
1.42 5.64 1.000
1.02 3.69 0.39
n.s. n.s.
n.s.
x) Enrichment ratios were calculated for each replication for statistical analysis. For this reason average values are different from that obtained by the mean data of tab. 1 and 3. T a b . 5: Enrichment ratios (ER) and Environmental Impact Index (El) of runoff
water and eroded sediment coming from different cover and management systems. following g e n e r a l s t a t e m e n t s can be m a d e , w h i c h can be v a l u a b l e for p r o p o s ing a n i n d e x e v a l u a t i n g t h e e n v i r o n m e n tal hazard: 1. G i v e n a c e r t a i n soil type, in-situ soil c h a r a c t e r i s t i c s c a n be s t r o n g l y influe n c e d b y different soil cover a n d m a n agement systems: (a) directly, b y t h e i r influence on soil f e r t i l i t y p a r a m e t e r s (e.g., o r g a n i c m a t t e r t u r n o v e r , soil s t r u c t u r e a m e lioration); and (b)
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indirectly, b y its i n t e r a c t i o n w i t h h y d r o l o g i c a l p h e n o m e n a (e.g. leaching, runoff, soil erosion).
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2. Off-site e n v i r o n m e n t a l h a z a r d is linked to t h e release of s e d i m e n t , soil O.M. a n d n u t r i e n t s to t h e g e n e r a l env i r o n m e n t along w i t h runoff w a t e r a n d e r o d e d m a t e r i a l . Such losses are n o t tot a l l y e x p l a i n e d b y t o t a l runoff a m o u n t a n d soil loss p r o d u c e d in a given cover a n d m a n a g e m e n t s y s t e m . In fact, relative p h y s i c a l a n d c h e m i c a l c o m p o s i t i o n of runoff w a t e r a n d s e d i m e n t is also influenced b y t h e different d y n a m i c s of t h e erosion process a n d c h e m i c a l c o m p o s i t i o n of t h e soil in t h e different syst e m s (e.g. rainfall kinetic e n e r g y for soil loss, electrical c o n d u c t i v i t y for O . M . loss). It follows t h a t for e v a l u a t i n g t h e
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global impact on the environment of A second partial environmental Imdifferent management techniques, nutri- pact Index can be calculated for off-site ents as well as soil and O.M. losses must effects: be taken into account; impacts cannot Ei. OMi • Pi • Ni be expressed solely by runoff and soil E I i " = E d . O M d . P d . N d (3) loss. where soil loss (El, d} has been introA general index of environmental imduced considering that release of sedipact of different cover and management ment in the environment is in itself a systems should take into account two factor of environmental impact. main components: one expressing the A global EI index based on the two soil amelioration in-situ and the other partial indices reported above for on-site expressing the potential off-site impact. and off-site impact can then be proposed In order to keep the index as simple as follows: as possible, it may be expressed with a EIi= EIi"/EIi' (4) formula of the following type:
EI=fl*f2*.../i*...*/n
(1)
where f i is a generalized characteristic value for the i-th treatment. Let us now define fi. Primarily such a term should express the relative variation in conditions with respect to a reference situation that, in our case, could be identified with the worst possible condition. Generally speaking, such a situation can be approximately identified with tilled fallow (with zero conservation measures). Hence, f i can be represented by the ratio between the value of the ith characteristic in the present situation and the reference one. To make an example, the phosphorous f-value for the P F treatment is given by:
fP(Pf)
= P(PF)/P(D)
where: P ( P F ) and P ( D ) are P contents of soil or sediment respectively in the PF and D treatments. Following the above criteria, a partim Environmental Impact Index can be calculated concerning on-site effects for each consolidated system:
EIi'=
OMi •Pi. Ni OMd. Pd. Nd
(2)
The ratio between the two previously defined partial indices has been chosen so that when the global index increases the impact on environment increases. When E I ~ increases, the on-site conditions are improving, while when E I i " increases, the off-site impacts worsen (more sediment or more N or more P or more O.M. are released inducing pollition and eutrophication problems). In order to have a global index in agreement with the two aspects, i.e., expressing improvement when both E I ~ and E I " indicate improvement, the ratio in the above formulation is the only simple solution. Taking into account the ER definition that was given above, by substituting eqs. 2 and 3 in eq. 4 and rearranging, a global "Environmental Impac Index" can be calculated as follows: El. ER(OM)i. ER(N)i. ER(P)i EI = ~ - ~ - ~ :ER(N)d. ER(P)i where E I = Environmental Impact Index: 0 > E l > 1; Ei and Ed = Soil loss respectively on the i-th reconsolidated system and tilled fallow;
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Environmental impact of soil erosion ER(OM)i and ER(OM)d = O.M. enrichment ratios respectively in the i-th reconsolidated system and tilled fallow; ER(N)i and ER(N)d = N enrichment ratios respectively on the i-th reconsolidated system and tilled falow; ER(P)i and ER(P)d -- P enrichment ratios respectively on the i-th reconsolidated system and tilled fallow.
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ter abandonment, and arable soil conditions. Moreover, although the prominent role of the vegetation cover in the control of surface erosional processes is, in principle, well known, the data of the present experiment emphasize how the different kinds of vegetation cover influence the physical and chemical quality of runoff water and eroded material, and thereby As expected, the EI index (tab. 5) have important consequences for both confirms substantially a remarkable sig- the dynamics of soil degradation and the nificant reduction of environmental im- potential environmental impact. pact for all the reconsolidated systems. In the given conditions, a reconsolHowever, while there is not any ques- idated system with Atriplex halimus tion about the very high soil and en- plantation and undergrowth of natural vironment protection offered by the grasses (Arena fatua and Lolium tuAtriplex system (EI practically 0), and mulentum), provMing canopy protection the fact that tilled fallow gives the worst and mulch material to the soil, is capable soil protection by definition (EI = 1), of preventing soil erosion almost comthe index offers a more detailed eval- pletely, even in the case of very intense uation of the other reconsolidated sys- rainstorms such as those that afflict, tems. For instance, a good four year nat- however rarely, the semi-arid Mediterural grass-sod protects the soil very well ranean environment. against the erosional power of the inMoreover, the stratified canopy of tense rainstorms of the area. Contrarily, Atriplex and annual grasses growing a bad grass-sod, as that present in the naturally in the area provides higher Piaus area, reduces the EI index only to biomass yield for grazing and extends one half in comparison to tilled fallow. the grazing season in late Spring and Where Piaus canopy is present, the EI early Summer by the prolonged growth index results are better. of the Atriplez shrub (Stringi et al. 1991). A good control of soil erosion is at4 Conclusions tained also by a good reconsolidated Reconsolidated cover and management natural grass-sod, without implanted systems exert a remarkable potential for shrubs, providing live green cover in soil protection and environmental recov- Winter and Spring and dry mulch in ely. Summer and Autumn. Indeed, the exHowever, while this statement seems periment confirms that, in relation to conceptually obvious, quantitative data surface soil erosion, it is the protection were not available to compare the rel- at ground level that exerts the best conative value of different steered recon- trol. solidation systems, obtained with shrub It was also demonstrated that, if for and tree plantation, in comparison with some reason the formation of a good both natural grass-sod, as expected af- grass sward is prevented and a bad SOIL
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sward develops, the protection value of such cover is badly affected. It follows that simple abandonment of arable land is an uncertain practice for soil and environmental protection, depending on the natural grass sward formation, the characteristicsof which cannot be easily predicted. T h e reconsolidated Pinus halepensis system m a y p r o t e c t the soil much b e t t e r t h a n the arable condition, b u t it is less efficient t h a n the other reconsolidated systems in controlling erosion. F r o m one side, the P i n e p l a n t reduces the u n d e r g r o w t h of a n n u a l grasses u n d e r its canopy. F r o m the other, the clayey soil of our e x p e r i m e n t is n o t suitable for three growth. In fact, after four years, the s h r u b s were more developed t h a n Pinus p l a n t s (Chisci et al. 1991). A group of e n v i r o n m e n t a l indices has b e e n proposed o n the basis of the experi m e n t here described. T h e indices try to q u a n t i f y the i m p a c t a given m a n a g e m e n t practice can have on soil fertility in situ. Off-site i m p a c t was also examined, considering the release of eroded m a t e rial a n d n u t r i e n t s to the general environm e n t . T h e two indeces were combined in a t h i r d one, i n t e n d e d for a global evalu a t i o n of the effect of a change of landuse a n d m a n a g e m e n t on general environmental amelioration. O u r indices are still far from being definitive b u t they provide a useful quick e s t i m a t e of the global e n v i r o n m e n t a l impact a change of land-use a n d m a n a g e m e n t m a y produce.
R.P.C. Morgan, Silsoe College, Silsoe (UK) for their help in the revision and editing of the paper. References B A R R O W S , H. & K I L M E R , V.J. (1988): Plant nutrient losses from soil by water erosion. In: Advances in Agronomy 15, 303-316. B I A N C U C C I , G. & R I B A L D O N E , E. (1980)" L'analisi chimica delle acque naturali e inquinate. Hoepli (ed.), Milano, p. 319. CHISCI, G. (1980): Physical soil degradation due to hydrological phenomena in relation to change of agricultural system in Italy. Ann. Ist. Sper. Studio e Difesa del Suolo, vol. XI, Firenze, 271-283. CHISCI, G. & S P A L L A C C I , P. (1984): Nutrient losses by leaching and runoff and possibilities of their control. In: Nutrient Balance and Fertilizers Needs in Temperate Agriculture. 18th Colloquium of the International Potash Institute, Gardone Riviera
(Italy). CHISCI, G. B A Z Z O F F I , P. & M B A G W U , J.S.C. (1989): Comparison of aggregatestabilityindicesfor soilclassification and assessment of soil management practices.SOIL T E C H N O L G O Y 2, 113-133. CHISCI, G. (1989): Measures for runoff and erosion control on clayey soils: a review of trials carried out in the Apennines hilly area. In: U. Schwertmann, R.J. riekson & K. Auerswald (Eds.), SOIL TECHNOLGOY SERIES 1, 53-71. CHISCI, G., S T R I N G I , L., M A R T I NEZ, V., A M A T O , G. & G R I S T I NA, L. (1991)" Ruolo di arbusti foraggeri nell'ambiente semi-arido siciliano. 2. Funalone protettiva contro l'erosione idrometeorica. Rivista di Agronomia 2,332-340.
Acknowledgement
D ' A S A R O , F. & S A N T O R O , M. (1983): Aggressivith della pioggia hello studio dell'erosione idrica del territorio siciliano. Arti Graflche Siciliane, Palermo, p. 28.
The Authors are grateful to Dr. D. Torri, of the ~CNR-Centro per la Genesi, Classificazione e Cartografia del Suolo, Firenze (Italy)" and to Prof.
DE PLOEY, J. & G A B R I E L S , D. (1980): Measuring soil loss and experimental studies. In: M.J. Kirkby & R.P.C. Morgan (Eds.): Soil Erosion. Wiley & Son, Chichester, 63-108.
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D E P L O E Y , J . & P O E S E N , J . (1984): Aggregate stability, runoff generation and interriU erosion. In: K.S. Richards, R.R. Arn e t t & S. Ellis (Eds.), Geomorphology and Soil. Allen & Unwin, London. GOVERS, G . (1985): Selectivity and transport capacity of thin flows in relation to rill erosion. CATENA 12, 35-49. KEMPER,
W.D.
&
CHEPIL,
W.S.
(1965): Size distribution of aggregates. In: C.A. Black (Ed.), Amer. Madison, Wis., 499-510.
Soc. Agronomy,
OLIVERI, G., D A Z Z I , C. & R A I MONDI, S. (1986): Studi sui suoli dell'aziende di Pietranera (AG). Quaderni di Agronomia 2, 103-162. S T R I N G I , L., A M A T O , G., G R I S T I o N A , L., C H I S C I , G . & A L I C A T A , M . L . (1991): Ruolo di arbusti foraggeri nell'ambiente semi-arido siciliano. 1. Funzione produttiva e valore nutritivo. Rivista di Agronomia 2,325-331. T H O M P S O N , L. (1967): Soil and Soil FertiliW. McGraw-Hill, N.Y., 2nd. Ed. T O R R I , D. & S F A L A N G A , M . (1980): Stima dell'erodibilit~ dei suoli mediante simulazione di pioggia in laboratorio. Nota 2. Preparazione del campioni di suolo. Ann. Ist. Sper. Studio e Difesa del Suolo, vol. XI, Firenze, 141-157. T O R R I , D. & S F A L A N G A , M. (1982): Some aspects of soil erosion mechanics. Laboratory experiments. Ann. Ist. Sper. Studio e Difesa del Suolo, vol. XII, Firenze, 75-89. T O R R I , D. (1987): A theoretical study of soil detachability. CATENA SUPPLEM E N T 10, 15-20. T O R R I , D., S F A L A N G A , M . & D E L S E T T E , M . (1987): Splash detachment: runoff depth and soil cohesion. CATENA 14, 149-155. WISCHMEIER, W.H., JOHNSON, C . B . & C R O S S , B . V . (1971): A soil erodibility nomograph for farmland and construction sites. Jour. of Soil and Water Conservation 26, 189-193. Z A C H A R , D. (1982): Soil Erosion. Development in Soil Science 10, Elsevier, Amsterdam, p. 548.
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A d d r e s s e s of a u t h o r s : Prof. Dr. Gianearlo Chisci Univ. degli Studi di Firenze Dipt. di Agronomia e Produzioni Erbacee P. le delle Cascine 18 1-50144 Firenze Italy Dr. Victoria Martines University of Las Palmas Gran Canaria Spain
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