Impact of total water potential and varying contribution of matric and osmotic potential on carbon mineralization in saline soils

In saline soils, microbial activity may be reduced by low matric (low water content) and low osmotic potential (presence of salts) but little is known about the impact of the relative contribution of matric and osmotic potential to water potential (sum of matric and osmotic potential) on microbial activity and biomass. A laboratory incubation experiment was conducted using a non-saline sandy loam; different osmotic potentials (−0.30 to −3.24 MPa) were achieved by adding different amounts of NaCl. After pre-incubation for 14 days, subsamples of these treatments were dried to achieve different contributions of matric potential (8–73%) and osmotic potential (27–92%) to water potential which ranged between −0.57 and −4.57 MPa. All treatments were amended with 20 g kg−1 pea residues to increase nutrient supply; carbon dioxide (CO2) emission was measured over 14 days. Microbial biomass C and K2SO4-extractable C were measured at the end of the experiment. Cumulative CO2–C (mg g−1 soil) was significantly (p < 0.05) lower at water potential −4 MPa than at water potential −1.5 MPa. Above water potential −4 MPa, cumulative CO2–C significantly decreased with increasing percentage contribution of osmotic potential to water potential, particularly if the contribution of osmotic potential was >50%. In contrast, K2SO4-extractable C and microbial biomass C were little affected by water potential above −4 MPa. Only at water potential −4 MPa, cumulative CO2–C and microbial biomass C were affected by matric potential and its contribution to water potential; that is when the soils are very dry. Our results show that cumulative CO2–C was more sensitive to decreasing water potential or the contributions of osmotic and matric potential than microbial biomass C. This suggests that not only water potential but also the contribution of osmotic and matric potential should be taken into account to understand microbial activity and growth in saline soils.

► Water potential (WP) and the contribution of osmotic and matric potential were adjusted to different values.
► Low WP had a stronger negative impact on microbial activity than on microbial biomass.
► Above WP −4 MPa, respiration was a function of osmotic potential and its contribution to WP.
► At WP −4 MPa or lower, respiration was mainly influenced by matric potential.