Soil organic matter and degradation Sarah Pariente and Hanoch Lavee Laboratory of Geomorphology, Bar-Ilan University, Ramat-Gan, Israel.
[email protected] 1 Introduction The importance of soil organic matter (SOM) as an indicator of the sustainability of ecogeomorphic systems was emphasized by Imeson (1995), Swift and Woomer (1993), Sparling (1991) and others. This function of SOM springs from its effects on soil structural stability (its action as a bonding agent between primary and secondary mineral particles leads to enhanced amount, size and stability of aggregates) and soil water retention (as a water adsorbing agent it enhances water acceptance and availability) and, hence, on infiltration and percolation. At the same time, SOM controls soil nutrients that affect biomass. Both bonding and adsorption processes explain why SOM has often been found to be positively correlated to soil structure but negatively correlated to soil erosion (Kemper and Koch, 1966; Tisdall and Oades, 1982; Chaney and Swift, 1984; Imeson and Verstraten, 1985; Bartoli et al., 1988; Haynes and Swift, 1990; Lavee et al., 1991; Dutartre et al., 1993 Imeson et al., 1994; Boix-Fayos et al., 1995; Lavee et al., 1996). Dutartre et al. (1993) emphasized that soil structural stability is influenced by the type of organic matter, as well as its amount. Therefore, in some cases high SOM content is not accompanied by high structural stability. Voroney et al. (1981) pointed out that some fungi exude oxalic acid, which enhances dispersion and breakdown of aggregates. The organic matter content in the soil expresses the relationships between the sources of organic materials and the decomposing factors (soil biota) (Greenland and Nye, 1959). The main source of SOM is litter (characterized by its amount and type). Both the sources and the decomposing factors depend, to a large extent, on climate and lithology – factors that control the texture, structure, moisture content and temperature of the soil. Land use and fires can obscure the effect of climate on SOM. The sources and the decomposing factors of SOM vary in space and time, and on different scales. Whereas on a regional scale, the macro conditions of climate control these variations, on a local scale, the spatial differences within each region reflect the micro-environmental conditions that depend on the natural conditions (microtopography and surface cover components) and on the type of land use (Haynes and Swift, 1984). Regarding the temporal variations, SOM content varies in the long term – decades and centuries – because of changes in climatic conditions, and in the short term – months or years – because of fluctuations in weather conditions between seasons and between years. Each climatic region has a typical range of SOM values that reflects its tempo-spatial variations under natural and seminatural conditions. This means that environmental change can be indicated by SOM values that fall outside that typical range. Values below the bottom of a range indicate increasing aridity and land degradation, whereas values above the top of the range indicate improvements in soil structural stability and the soil water regime. This presentation aims at analysing the changes in SOM that result from differences in climatic conditions and land use. 83
2 Research sites The research was carried out in several research sites, representing Mediterranean (sub-humid), semi-arid, mildly arid and arid climates along a climatic transect, running from the Judean mountains (mean annual rainfall 700 mm, and annual mean temperature 17°C) to the Dead Sea (mean annual rainfall under 100 mm, and annual mean temperature 23°C) (Figure 1). Five research stations were established on hillslopes having similar topographic (azimuth 135-150° and gradients of 11-14°) and lithological (hard calcareous bedrock) conditions. The climatic characteristics (Table 1) vary widely among the sites except for sites MAB and MAL, in which they are the same. However, these two sites differ from each other in their surface cover characteristics: MAB has fewer shrubs and annuals but more rock fragments than MAL.
Figure 1. Locations of the study sites (mean annual rainfall in mm, is indicated by isohyets). Table 1. The main ecogeomorphological characteristics of the research sites. Research site
Mean annual rainfall
Soil type
Vegetation cover in March 2000
Mean annual temperature (°C ) 17
(mm) 620
Brown Terra Rossa
(%) 85
Ma’ale Adumim (MAL)
19
330
Brown Rendzina
40
Ma’ale Adumim (MAB)
19
330
Brown Rendzina
30
Mishor Adumin (MIS)
20
260
Pale brown lithosol
30
KALIA (KAL)
23
120
Very pale gypsic desert lithosol
10
Giv’at Ye’arim (GIV)
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3. Method At four sites, GIV, MAL, MIS and KAL, soil samples were taken four times a year, in January, March, May and September, from 1992 through 1993 and 1994 and in April and August 2000. In the last two months soil samples were taken in site MAB too. At each of the sites, in each season, four to eight points were sampled in open areas between shrubs. At each point soil samples were taken from two soil depths: 0-2 cm, and 2-10 cm. The organic matter content was determined by the wet combustion method (Head, 1984). Data were statistically evaluated by analysis of variance with SPSS 10 for Windows (SPSS Inc. Chicago, USA). Tukeys test, at α=0.05 level of significance, was used. 4 Results and discussion 4.1 Effect of climate conditions Comparison between the sites along the climatic transect shows that, except for site MAB, SOM increased significantly in both 0-2 cm and 2-10 cm, from the arid site, KAL, through the mildly arid site, MIS, and the semi-arid site, MAL, to the Mediterranean site, GIV. This increase is accounted for by differences between the climatic zones, in the relationships between the sources of the soil organic matter – mainly vegetation – and the decomposing factors, i.e., micro organisms. The arid zone is characterized by a low vegetation cover (Table 1), which exists for a short time, so that the sources of organic matter are limited in both quantity and availability. In this zone the conditions that favour micro organism activity in the soil also prevail for a short time, and are limited to the winter and spring, when the soil moisture and temperature do not restrict such activity (Li and Sarah, 2003a,b). Also, the high salinity that characterizes this zone inhibits microbial biomass (Sarah, 2001). The Mediterranean zone is characterized by a high vegetation cover which exists both in the winter and in the summer, and by soil moisture content and temperatures suitable for biotic activities during a large proportion of the year. These characteristics result in a high organic matter content in the soil. Most of this SOM is in the form of polymers that facilitate the establishment of strong and flexible connections between the inorganic soil particles that form the aggregates, as a result of which the disintegration and dispersion of the aggregates during wetting and drying are relatively low (Emerson et al., 1986). Table 2. Soil organic matter at different depths at the study sites. Means in the row within one depth followed by different capital letters varied significantly at the 0.05 probability level. Means in the column within one site followed by different letters varied significantly at the 0.05 probability level. Depth/Site n 0-2 cm
a
KAL 76 1.46
2-10 cm
b
1.07 D
E
MIS 76 a 2.56 C
MAB 16 a 2.12 D
MAL 76 a 3.77 B
GIV 76 a 6.15 A
b
a 1.89
b 2.96
b
2.05 C
C
B
4.39 A
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4.2 Effect of land use Figure 2 and Table 2 show significant differences, in each of the soil depths, between the two semi-arid sites, MAL and MAB, in spite of the fact that they are under the same climatic conditions. The SOM content in site MAB was significantly lower than that in site MAL in the two depths. The SOM content in the 0-2 cm depth in site MAB was significantly lower even than that in site MIS, which is more arid. 10
1/92 3 5
8
OM (%)
9 1/93
6
3 5
4
9 1/94
2
3 5
0
9 0
100
200
300
400
500
600
700
Mean annual rainfall (mm)
0 - 2 cm
4/00 8
10
1/92 3 5
8
OM (%)
9 1/93
6
3 5
4
9 1/94
2
3 5 9
0 0
2 - 10 cm
100
200
300
400
Mean annual rainfall (mm)
500
600
700
4/00 8
Figure 2. Soil organic matter content variations along the climatic gradient.
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Measurements of SOM in other natural semi-arid sites with similar topographic and lithological conditions show similar values to those of site MAL. The conclusion is that while SOM values in MAL represent the semi-arid zone under natural/semi-natural conditions, the values in site MAB express a deviation that indicates land degradation, probably because of intensive human interference, such as overgrazing and/or the establishment of nomad settlement (tents). This conclusion is strengthened by the surface cover characteristics at this site, i.e., relatively low density of shrubs and high density of rock fragments. Furthermore, Bedouin encampments are still seen in the neighborhood. Comparison between the two soil depths shows that whereas in all sites SOM in the 0-2 cm was significantly higher than that in 2-10 cm, the difference in site MAB was small and not significant (Table 2). No intensive human interference was observed in site MAB in the last 12 years. This means that site MAB went through a severe degradation and did not recover yet. To sum up, significant deviations from the typical expected SOM values in both the regional scale and the soil profile scale can be used as indices of land degradation. This emphasizes the importance of field long term monitoring. References Bartoli, F., Philippy, R. and Burtin, G., 1988. Aggregation in soils with small amounts of swelling clays. Aggregate stability. J. Soil Science 39, 593-616. Boix-Fayos, C., Soriano, M.D., Tiemessen, I.R., Calvo-Cases, A. and Imeson, A.C., 1995. Properties and erosional response of soils in a degraded ecosystem in Crete (Greece). Environmental Monitoring Assessment 37, 79-92. Chaney, K. and Swift, R.S., 1984. The influence of organic matter on aggregate stability in some British soils. J. Soil Science 35, 223-230. Dutartre, Ph., Bartoli, F., Andreux, F., Portal, J.M. and Ange, A., 1993. Influence of content and nature of organic matter on the structure of some sandy soils from West Africa. Geoderma 56, 459-478. Emerson, W.W., Foster, R.C. and Oades, J.M., 1986. Organo-Mineral Complexes in Relation to Soil Aggregation and Structure. In: Huang, P. M. and Schnitzer, M. (Eds.), Interactions of Soil Minerals with Natural Organics and Microbes, Soil Science Society of America Spec. Pub. no. 17, 521-548. Greenland, D.J. and Nye, P.H., 1959. Increase in the carbon and Nitrogen contents of tropical soils under natural fallows. J. Soil Science 10, 284-299. Haynes, R.J. and Swift, R.S., 1990. Stability of aggregates in relation to organic constituents and soil water content. J. Soil Science 41, 73-83. Head, K.H., 1984. Manual of Soil Laboratory Testing, 1, Soil Classification and Compaction Tests. ELE International Ltd. Fentech Press, London. Imeson, A.C., 1995. The physical, chemical and biological degradation of the soil. In Fantechi, R., Peter, D., Balabanis, P. and Rubio, J.L. (Eds.): Desertification in a European Context: Physical and socioeconomic aspects. Proceedings of the European School 0f Climatology and Natural Hazards course, Alicante, 1993, 399-409. Imeson, A.C. and Verstraten J.M., 1985. The erodibility of highly calcareous soil material from southern Spain. Catena 12, 291-306. Imeson, A.C., Perez-Trejo, F., Lavee, H. and Calvo-Cases, A., 1994. Modelling and exploring the impact of climate change on ecosystem degradation, hydrology and land use along a transect across the Mediterranean. In Troen, I. (Ed.), Global Change: Climatic Change and Climatic Change Impacts. Proceedings Copenhagen Symposium, September 1993, European Commission, EUR 15921 EN, 173185. Kemper, W.D. and Koch, E.J., 1966. Aggregate stability of soils from Western United States and Canada. USDA Tech. Bull. 1355, 52 p. Lavee, H, Imeson, A.C., Sarah, P. and Benyamini, Y., 1991. The response of soils to simulated rainfall along a climatological gradient in an arid and semi-arid region. Catena 19, 19-37. Lavee, H., Sarah, P. and Imeson, A.C., 1996. Aggregate stability dynamics as affected by soil temperature and moisture regimes. Geografiska Annaler 78A, 73-82. Li, X. and Sarah, H. 2003. Enzyme activities along a climatic transect in the Judean Desert. Catena (in press). Li, X. and Sarah, H. 2003. Arylsulfatase activity of soil microbial biomass along a Mediterranean-arid transect. Soil Biology and Biochemistry (in press).
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Sarah, P., 2001. Soluble salts dynamics in the soil under different climatic regions. Catena, 43: 307-321. Sparling, G.D., 1991. Organic matter carbon and microbial biomass carbon as indicators of sustainable land use. In Elliot, C. R., Latham, M. and Dumanski, J. (Eds.), Evaluation for Sustainable and Management in the Developing World. Vol. 2: Technical Papers. IBSRAM Proceedings No. 12. Bangkok, Thailand: IBSRAM. Swift, M.J. and Woomer, P., 1993. Organic matter and sustainability of agricultural systems: Definition and measurement. In Mulongoy, K. and Merckx, R. (Eds.), Soil Organic Matter Dynamics and Sustainability of Tropical Agriculture. John Wiley and Sons, 3-18. Tisdall, J.M. and Oades, J.M., 1982. Organic matter and water sTable aggregates in soil. J. Soil Science 33, 141-163. Voroney, R.P., van Veen, J.A. and Paul, E.A., 1981. Organic carbon dynamics in grassland soils. II. Model validation and simulation of the long-term effects of cultivation and rainfall erosion. Can. J. Soil Science 61, 211-224.
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