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

vs. C4 ), age of SOM, depth in the profile, and the amount of fossil fuel derived CO2 in the atmosphere (Shearer Although it is estimated that salinity stress occurs in 50% of irriand Kohl, 1986). However, since plants derive all their gated agroecosystems around the world, not much is known about its impact on C and N dynamics. This study was conducted to characC from the air, modeling 13C concentrations in the plant terize the impact of salinity stress on natural abundance 13C and 15N is relatively easy. Nitrogen, which can be taken up from ( 13C and 15N) on a Lethent Clay Loam (fine, smectitic, thermic typic 4 different sources (NH 4 , NO 3 , organic N, and atmonatrargid) in the San Joaquin Valley (California) for a nonhalophyte, spheric N2 ) via the roots and foliarly is much more C3 plant and soil organic matter (SOM) fractions. A total of 101 plant difficult to model (Marriott et al., 1997; Farquhar et (Littleseed Canarygrass, [Phalaris minor Retz.]) and soil samples al., 1980). were collected from a 10-ha area. Electrical conductivity in a 1:5 soil/ A relationship between 13C of crop tissue and salinity water paste (EC1:5) ranged from 2.7 to 8.9 dS m 1. The Cplant values (measured as NaCl concentration in nutrient solution) varied from 29.8 to 24.0‰, and 15N from 2.2 to 19.1‰. Average was first reported by Guy et al. (1980) for the C3 halovalues for 13C increased from 26.9‰ in the plant, to 25.3‰ in phytes Salicornia europaea L. ssp. rubra (Nels.) Breitung the light fraction (LF) and 24.1‰ in the SOM. Salinity explained 57% of variance in Cplant, 16% of CLF and 6% of CSOM. For and Puccinellia nuttalliana (Schultes) Hitchc. In a con15N, these numbers were 41, 56, and 0%, respectively. There was a trolled experiment with different concentrations of clear spatial pattern match between salinity, Cplant, Nplant, and NaCl, they found positive correlation coefficients (r 2 ) NLF. The lack of any salinity-induced signature in total SOM probaof 0.88 and 0.96, respectively. These findings were later bly indicates that the salinity was of recent origin. The high positive confirmed under field conditions for the same species correlation between salinity and 15N in crop and LF might be because (Guy et al., 1986a and 1986b). Along a soil water potenof higher NH3 volatilization caused by high pH, combined with a tial gradient ranging from 1.3 to 6.0 MPa, 13C values relative increase of NH 4-uptake by the plant under saline conditions. ranging from 27 to 21‰ were found for Puccinellia Under certain conditions, 13C and 15N signatures of recalcitrant nuttalliana. The corresponding r 2 value was 0.90. FlanaSOM fractions may be used to reconstruct historic salinity patterns. gan and Jefferies (1989) reported a significant shift from 32.4 to 28.3‰ because of salinity in the C3 halophyte Plantago maritima L. under controlled conditions. C stress related to soil salinity is very common Chmura et al. (1987) found a range of 13C values in in irrigated agroecosystems. It is estimated that sediment from 27.9‰ in fresh water marshes to globally, 50% of all irrigated land experience some 16.2‰ in salt marshes over a variety of plant species. degree of salinity stress. Close to 10 million ha has to However, these differences were probably primarily bebe taken out of production yearly because of salinitycause of a corresponding shift from C3 plants to C4 related problems (Rhoades and Loveday, 1990). Alplants. Walker and Sinclair (1992) reported r 2 values though 15% of all agricultural land is irrigated, this around 0.66 between electromagnetic measurements land contributes to 40% of food production. Relatively and 13C values of leaves of the C4 halophytes Atriplex little is known about the effects of salinity stress on vesicaria and A. stipitata, although they did not detect C and (especially) N cycling in agroecosystems. Such an effect in whole plant 13C. knowledge is crucial in devising remedial action, either Initially, the observed relation was attributed to a by improving crop management or by developing new shift in the use of the primary enzyme for CO2 fixation cultivars. from ribulose 1,5 bisphosphate (RuBP) carboxylase toSince many biochemical and physiochemical reacwards phosphoenolpyruvate (PEP) carboxylase because tions have the potential for some sort of isotopic discrimof high levels of salinity (Guy et al., 1980). However, ination, natural variations in stable isotopes of C and Farquhar et al. (1982) and Guy and Reid (1986) showed N could elucidate many nutrient cycling processes in that the positive correlation was more likely an indirect agroecosystems. However, because isotopic concentraconsequence of reduced stomatal conductance under tions are almost always the result of a combination of stress conditions. This results in a lower intercellular processes, it is often difficult to interpret results. The CO2 pressure. Since 12C is preferentially assimilated be13C concentrations in SOM, for example, can be the cause of discrimination during diffusion and carboxylaresult of photosynthetic activity in past vegetation (C3 tion, the reduced CO2 pressure will lead to an enrichment of the assimilated material with 13C. Farquhar et J.-W. van Groenigen, currently at: Alterra, Dep. of Water and Environment, P.O. Box 47, 6700 AA Wageningen, the Netherlands; J.-W. Abbreviations: CLF, C from light fraction; Cplant, C from plants; CSOM, van Groenigen and C. van Kessel, Dep. of Agronomy and Range C from soil organic matter; EC, electrical conductivity; EC1:5, electrical Science, University of California-Davis, 1 Shield Avenue, Davis, CA conductivity of a 1:5 soil/water paste; LF, light fraction; NLF, N from 95616. *Corresponding author ([email protected]. LF; Nplant, N from plants; NSOM, N from soil organic matter; SOM, soil nl). organic matter; 13C, natural abundance of C; 15N, natural abundance of N. Published in Soil Sci. Soc. Am. J. 66:489–498 (2002).