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DIVISION S-3—SOIL BIOLOGY & BIOCHEMISTRY

Salinity-induced Patterns of Natural Abundance Carbon-13 and Nitrogen-15 in Plant and Soil

^a Alterra, Dep. of Water and Environment, P.O. Box 47, 6700 AA Wageningen, the Netherlands
^b Dep. of Agronomy and Range Science, University of California-Davis, 1 Shield Avenue, Davis, CA 95616

* Corresponding author (J.W.vanGroenigen{at}Alterra.wag-ur.nl)

Although it is estimated that salinity stress occurs in 50% of irrigated agroecosystems around the world, not much is known about its impact on C and N dynamics. This study was conducted to characterize the impact of salinity stress on natural abundance ¹³C and ¹⁵N (¹³C and ¹⁵N) on a Lethent Clay Loam (fine, smectitic, thermic typic natrargid) in the San Joaquin Valley (California) for a nonhalophyte, C₃ plant and soil organic matter (SOM) fractions. A total of 101 plant (Littleseed Canarygrass, [Phalaris minor Retz.]) and soil samples were collected from a 10-ha area. Electrical conductivity in a 1:5 soil/water paste (EC_1:5) ranged from 2.7 to 8.9 dS m^-1. The ¹³C_plant values varied from -29.8 to -24.0, and ¹⁵N from 2.2 to 19.1. Average values for ¹³C increased from -26.9 in the plant, to -25.3 in the light fraction (LF) and -24.1 in the SOM. Salinity explained 57% of variance in ¹³C_plant, 16% of ¹³C_LF and 6% of ¹³C_SOM. For ¹⁵N, these numbers were 41, 56, and 0%, respectively. There was a clear spatial pattern match between salinity, ¹³C_plant, ¹⁵N_plant, and ¹⁵N_LF. The lack of any salinity-induced signature in total SOM probably indicates that the salinity was of recent origin. The high positive correlation between salinity and ¹⁵N in crop and LF might be because of higher NH₃ volatilization caused by high pH, combined with a relative increase of NH⁺₄-uptake by the plant under saline conditions. Under certain conditions, ¹³C and ¹⁵N signatures of recalcitrant SOM fractions may be used to reconstruct historic salinity patterns.

Abbreviations: C_LF, C from light fraction • C_plant, C from plants • C_SOM, C from soil organic matter • EC, electrical conductivity • EC_1:5, electrical conductivity of a 1:5 soil/water paste • LF, light fraction • N_LF, N from LF • N_plant, N from plants • N_SOM, N from soil organic matter • SOM, soil organic matter • ¹³C, natural abundance of C • ¹⁵N, natural abundance of N