Salinity and sodicity affect soil respiration and dissolved organic matter dynamics differentially in soils varying in texture
Highlights
► We studied the interactive effect of sodicity and salinity on soil respiration and DOM. ► Low EC combined with high SAR increases the risk of DOC and DON leaching in the soil. ► Soil texture and water content play an important role in determining the response of microbes to salt.
Introduction
Land degradation by salts is a major threat to sustainable crop production in many arid and semi-arid regions of the world (Bossio et al., 2007). Low rainfall and high potential evapotranspiration in these regions promote the upward movement of salts in the soil solution which adversely affects soils physical, chemical and biological properties (Rengasamy, 2006). Worldwide more than 831 million hectares of land is salt-affected (Martinez-Beltran and Manzur, 2005) and this area is likely to increase in the future because of secondary salinisation due to irrigation and clearing of native vegetation (Pannell and Ewing, 2006). Therefore, it is important to understand the processes in salt-affected soils particularly those involved in nutrient cycling.
Salt-affected soils are classified as saline, sodic and saline-sodic on the basis of EC (electrical conductivity), SAR (sodium absorption ratio) and pH (Brady and Weil, 2002). Soils with ECe (saturated extract) > 4 dS m−1 and SAR > 13 are classified as saline-sodic (US Salinity Laboratory Staff, 1954). Saline soils have an ECe of the saturation extract > 4 dS m−1 (SAR < 13) and contain Na+, Mg2+, and Ca2+ as dominating cations and Cl− and SO42− are the dominant anions. The effects of excessive salts in the soil solution include reduced water uptake due to low osmotic potential (Harris, 1980), high pH, and ion competition limiting nutrient uptake (Keren, 2000) which not only reduce plant growth but also have a negative influence on the size and activity of soil microbial biomass and biochemical processes essential for maintenance of soil organic matter (Rietz and Haynes, 2003, Tripathi et al., 2006, Yuan et al., 2007). Some microbes respond to low osmotic potential by accumulating osmolytes to retain water whereas sensitive microbes die (Hagemann, 2011). In sodic soils (ECe < 4 and SAR > 13 according to USDA classification, but SAR > 3 in the Australian classification system (Isbell, 2002)), Na is the dominant cation on the exchange sites of the soil particles. Increasing Na saturation on the exchange sites results in dispersion of organic matter and clay particles, thus destroying aggregates and soil structure.
Oades (1988) showed that high electrolyte concentration plays a major role in linking organic matter to clays. Therefore, high clay content and increasing electrolyte concentration will make organic matter less accessible to microbes for decomposition in salt-affected soils (Nelson et al., 1997). Hence, clay dispersion is negatively correlated with EC and positively correlated with SAR and pH (Nelson et al., 1998).
Dissolved organic matter (DOM which includes C, N and other organically bound nutrients) is the most mobile and dynamic non-living organic matter fraction. It comprises only a small part of soil organic matter (< 1%); nevertheless, it is a primary source of mineralizable C, N and P and affects many processes in soil such as nutrient translocation and leaching, microbial activity, mineral weathering and nutrient availability (Evans et al., 2005, Kalbitz et al., 2000, Zsolnay, 2003). The concentration of dissolved organic carbon (DOC) can decrease as a result of sorption, precipitation or mineralisation by soil microorganisms. The ease with which DOC can be degraded by microbes is related to its content of aromatic C compounds (Marschner and Kalbitz, 2003). High solubility of organic matter due to sodicity can cause loss of DOM by leaching (Peinemann et al., 2005). Conversely, increasing salinity causes soils to flocculate, offsetting the effects caused by sodicity (Shainberg and Letey, 1984).
Contradictory results have been reported on the effect of salinity and sodicity on soil respiration and microbial biomass (Laura, 1976, Nelson et al., 1996, Pathak and Rao, 1998, Rietz and Haynes, 2003, Sarig et al., 1993, Wong et al., 2008). These contradictory observations may be due to differences in soil properties, especially the levels of salinity and soil pH (Muhammad et al., 2008), but there are few studies that have assessed the interaction of salinity and sodicity on soil microbial activity and organic matter decomposition in soils of different texture.
Based on the findings reported in the literature outlined above, we hypothesised that (1) salinity would decrease microbial activity but increase DOC concentration because of decreased organic matter decomposition, and (2) sodicity would increase microbial activity and DOC concentration because of increased soil organic matter solubility.
Section snippets
Soils
Two non-saline and non-sodic soils, of sand and sandy clay loam texture were collected from A horizon (0–30 cm) of a soil near Monarto, South Australia (35°05′ S and 139°06′ E) (Table 1). The area has a dry Mediterranean climate, and the average temperature is 30.1 °C in summer and 15.9 °C in winter with mean annual rainfall of 352 mm. Samples were thoroughly mixed, air dried, passed through a 2 mm sieve and stored air-dry at room temperature. Textures were assigned according to the Australian
Results
In the sand, cumulative respiration decreased significantly with increasing salinity after 10 and 42 days (Fig. 1). Cumulative CO2–C was lowest at EC 4.0 + SAR 20, being 60% lower than at EC 0.5 + SAR 20. Cumulative respiration was more strongly affected by EC than by SAR. It decreased by 8, 24 and 42% at EC 1.3, EC 2.5 and EC 4.0 respectively. SAR 20 increased cumulative respiration after 42 days of incubation by 9% at EC 0.5 and by 2.3% at EC 1.3. However, cumulative respiration was not
Discussion
Salinity affected soil respiration and DOC irrespective of SAR, whereas sodicity only had an effect at low EC levels. The study also highlighted that soil texture and water content play an important role in determining the response of microbes to salt stress.
Conclusions
The results show that increasing salinity adversely affects microbial activity and therefore increases DOC and DON concentration, whereas an increased DOC and DON concentration in response to sodicity was observed only at low EC. Therefore, high SAR in combination with low EC increases the risk of DOC and DON leaching and further soil degradation. In addition, the results of the experiment also indicate that soil texture and water content play an important role in determining the response of
Acknowledgements
The authors thank Harsimranjeet Mavi for assistance in the laboratory and with data, Colin Rivers for assistance in the field and laboratory and John Gouzos for analysis of the samples. This project is being funded by an Endeavour scholarship from The University of Adelaide to the senior author and by Future Farm Industries, CRC, Australia.
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