Using salt-amended soils to calculate a rate modifier for salinity in soil carbon models

Abstract

In salt-affected soils, soil organic carbon levels are usually low as a result of poor plant growth; additionally, decomposition of soil organic matter may be decreased. Thus, the CO2 evolution from salt-affected soils is likely to be lower than that from non-saline soils. Carbon models such as Rothamsted Carbon (RothC) that are used to estimate global CO2 emission do not consider the effect of salinity on CO2 emission. Given the large extent of salt-affected soils (19 percent of 20.8 billion hectares of arable land on Earth), this may lead to overestimation of CO2 release. Two laboratory incubation experiments were conducted to assess the effect of soil texture on response of CO2 release to salinity and to calculate a rate modifier for salinity in soil carbon models and study soil carbon dynamics: a sandy loam (18.8% clay) and a sandy clay loam (22.5% clay) in one experiment and a loamy sand (6.3 % clay) and a clay loam (42 % clay) in another experiment. Sodium chloride (NaCl) was used to develop a range of salinities viz. EC1:5 1.0, 2.0, 3.0, 4.0 and 5.0 dS/m. The soils were amended with 2% wheat residues and CO2 emission was measured over 4 months. Cumulative CO2-C/g soil decreased with increased salinity. Cumulative CO2-C expressed as percent of the control soil (without salt addition) showed a lower impact of salinity on organic matter decomposition with increasing clay content. A decrease in particulate organic carbon (POC) associated with incubation was less in the higher saline soils whereas total organic carbon, humus-C and charcoal-C did not change over time and were not significantly affected by salinity. A significant exponential relationship was obtained between EC and the salt rate modifier, suggesting that a new salt rate modifier should be incorporated into RothC in order to accurately model CO2 emissions from salt-affected soils.