Relationships between carbon dioxide emission and soil properties in salt-affected landscapes
Research highlights
► Soil respiration was measured in 120 soils from two salt-affected landscapes. ► Salinity (EC) was the main factor influencing the soil respiration. ► Soil respiration was negatively correlated with EC and clay content. ► Soil respiration was positively correlated with particulate organic carbon content.
Introduction
Soil salinity and sodicity are major forms of land degradation in arid and semi-arid regions where precipitation is too low to maintain a regular percolation of rain water through the entire crop root zone of the soil. Under such conditions, soluble salts accumulate in the soil and negatively affect soil properties (Farifteh et al., 2006) and crop productivity (Rengasamy, 2010). The global extent of primary salt-affected soils is about 955 million hectare (Mha), whereas secondary salinization affects 77 Mha (Metternicht and Zinck, 2003).
Saline soils are characterized by high concentrations of salts in the soil solution (electrical conductivity of saturated extract (ECe) >4.0 dS m−1), whereas sodic soils have a high percentage of sodium (Na) on the exchange sites of the soil particles (determined from sodium adsorption ratio (SAR) >13). Saline-sodic soils are characterized by both high EC and high SAR (ECe > 4.0 dS m−1 and SAR > 13) (Sumner et al., 1998). Salinity reduces plant growth and the activity of microorganisms as a result of low water availability (due to low osmotic potential of the soil solution) and ion toxicity (primarily Na, Cl) (Marschner, 1995). High SAR leads to dispersion of clay particles, slaking and poor soil structure as well as surface crusting and hard setting (Sumner et al., 1998, Qadir and Schubert, 2002). This can affect water and air movement, root penetration and organic matter decomposition.
In general, soils are very often first saline and then become sodic as Na replaces Ca and Mg from the cation exchange sites which causes dispersion of clay particles and thereby affects oxygen availability in the soils. Sodic soils that are also saline contain high concentrations of both sodium and sodium chloride. Such soils will usually not exhibit symptoms of sodicity (poor aggregation, dispersion and hard setting) because the sodium and chloride ions formed by the dissolved sodium chloride in the soil solution prevent clay particles from dispersing. Rengasamy et al. (1984) found that Red Brown Earths of South Australia with EC1:5 (EC of 1:5 soil:water extract) >0.6 dS m−1 are flocculated. However, such high concentration of ions (6.0 me l−1 or more) in soil solution decrease osmotic potential, thus inducing osmotic stress for microorganisms and then reducing their metabolic activity.
Soil respiration is the total efflux of CO2 produced from soil metabolic processes (Lundegårdh, 1927), mainly microbial decomposition of soil organic matter and root respiration (Singh and Gupta, 1977). Soil microbial respiration from decomposition of organic matter constitutes approximately 50% of soil CO2 emission (Rastogi et al., 2002). The measurement of CO2 emission from the soils is a sensitive indicator of availability of soil C to decomposition (Al-Kaisi and Yin, 2005). It is important to quantify the effect of physical and chemical soil properties to accurately predict CO2 release and understand SOM turnover in a landscape. The impact of temperature, water and clay on CO2 emissions are well-studied in non-salt-affected soils (Zak et al., 1993, La Scala et al., 2000, Aciego Pietri and Brookes, 2008) but less is known about the relationship between soil properties and CO2 emission in salt-affected landscapes.
High EC causes osmotic stress and has been shown to reduce soil microbial activity (Pathak and Rao, 1998), but the impact of sodicity or saline sodicity on microbial activity is unclear with some studies showing no effect (Pathak and Rao, 1998) and others showing increased microbial activity at high SAR (Nelson et al., 1996). Many of these studies have been carried out in salt amended soils that may not be representative of the ionic composition of salt-affected soils in the field (McClung and Frankenberger, 1987). Moreover, salt addition results in a sudden osmotic stress; hence unlike field conditions, where salinity develops more gradually, the microbial community may not have been able to adapt to salinity.
The largest pool of actively cycling carbon (C) in terrestrial ecosystems is the soil (Janzen, 2004). On a global scale, C stored in soils (1580 Pg C) accounts for more than that contained in vegetation (610 Pg C) and the atmosphere (750 Pg C) combined (Kimble and Stewart, 1995). Thus, changes in soil organic matter (SOM) content have a major effect on atmospheric CO2 concentration (Lal et al., 2007). Decomposition of SOM results in the release of CO2, but soils can also act as a sink for atmospheric CO2 if added plant material is transformed into stable SOM forms (Raich and Potter, 1995). Soil degradation by salinity and sodicity will have an impact on SOM turnover by affecting the amount of plant material entering the soil as well as the rate of SOM decomposition, which in turn, may influence the role of soil as a sink for atmospheric CO2.
Conceptually and methodologically, soil organic matter (SOM) can be divided into different pools (Baldock and Skjemstad, 1999). The bulk of plant residues are initially part of the particulate organic matter (POM) pool which consists of fragments of organic matter less than 2 mm that exhibit a recognizable cellular structure. During decomposition, a portion of the substrate C will be used by microorganisms and retained within the microbial biomass and some will be respired. The remaining portion is transformed into other SOM pools – dissolved organic matter (DOM), and humus (Baldock, 2002). Humus consists of altered and unaltered biomolecules with no recognizable cellular structure, and is less decomposable than DOM or POM. Many soils also contain OM that has a very low decomposability – the inert SOM pool. In many Australian soils, inert SOM is primarily charcoal (Skjemstad et al., 1998). The presence of salts may affect turnover of SOM pools by influencing SOM availability and microbial activity. The soil organic C pools have been determined in non-salt-affected soils (Marriott and Wander, 2006), but very little is known about the size and turnover of SOC pools and their relationship with CO2 emission in salt-affected soils.
The aim of the present study was to assess the effect of various physical and chemical soil properties on CO2 emission from salt-affected soils. To achieve this aim, soils from two salt-affected landscapes in India and Australia were collected covering a wide range of salinity and sodicity levels. Soil respiration of unamended and amended (with 2% w/w mature wheat straw) soils was measured over 120 days under controlled conditions. Regression analysis was used to assess the effect of chemical and physical properties on cumulative respiration to test the following hypotheses: (i) in unamended soils, CO2 release will be C-limited, therefore respiration will be positively correlated with soil organic carbon (SOC) pools and negatively correlated with clay because binding to clay decreases accessibility of SOC as reported by Hassink, 1997, Baldock and Skjemstad, 2000 and Krull et al. (2003) (ii) in amended soils, C availability will be greater, therefore SOC pools will be less important and CO2 release will be more strongly affected by clay and other soil properties, and (iii) in unamended or amended salt-affected soils, the main factors influencing CO2 release will be EC and SAR with other soil properties being of less importance.
Section snippets
Soils
Two regions were used for collection of soil samples: Muktsar, Punjab, India (area 2630 km2, between latitudes 29° 54′ and 30° 40′ N and longitudes 74° 14′ and 74° 49′ E) where irrigation induced salinity is dominant; and Kadina, South Australia (area 159 km2, 33° 57′ S, 137° 42′ E) where dry saline land (salinity not associated with the ground water table) and dry land salinity (salinity associated with the ground water table) are wide spread. The climate at the Indian site is sub-tropical and
India
The Indian soils were characterized by pH values >7 and low to high salinity (EC1:5 from 0.09 to 6.9 dS m−1) and ranged from being non-sodic to saline-sodic (SAR from 0.19 to 20.3) (Table 1). The clay content varied from 46 to 348 g kg−1 with a median value of 167 g kg−1. The median value of TOC was 10.3 g kg−1, ranging from 2.5 to 23.3 g kg−1. The particulate organic carbon content varied from 0.08 to 17.3 g kg−1, char-C from 0.0001 to 1.4 g kg−1 and humus-C from 1.1 to 3.5 g kg−1. The
Discussion
The differences in soil properties between Indian and Australian soils (Table 1) can be explained by differences in soil age, bedrock, climate and cropping history. Indian soils are only 1–2 m deep and no more than 500 years old, whereas Australian soils are very deep and millions of years old (CSIRO, 1983). The lower organic C in the Indian soils can be explained by the higher temperature and soil water content (due to irrigation) at the Indian site compared to the Australian site as these
Conclusions
Overall, CO2 release in salt-affected landscapes is affected by C availability (size of C pools), clay content and EC. A negative effect of bulk density on CO2 release suggests that CO2 emission is not only affected by biological and chemical properties of the soil but also by physical properties. Electrical conductivity has a negative impact on CO2 release in soils of India and Australia, which shows the universal effect of EC on CO2 release irrespective of climate and origin of salinity.
Acknowledgments
The authors would like to acknowledge Dr P.K. Sharma, Director, Punjab Remote Sensing Centre, India for providing necessary facilities for collection of soil samples from Indian site, Mr Sean Forrester for MIR analyses and predictions, Ms Athina Massis for technical support, North and Yorke Natural Resources Management Board, Department of Climate Change and The Future Farm Industries CRC for funding, and Dr Atul Nayyar for comments on the manuscript. The first author also acknowledges the
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