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Legumes mitigate ecological consequences of a topographic gradient in a northern Mongolian steppe

Casper, B.B., R. Goldman, A. Lkhagva, B.R. Helliker, A.F. Plante, L.A. Spence, P. Liancourt, B. Boldgiv and P.S. Petraitis. 2011. Oecologia 169:85-94

Abstract

Topography should create spatial variation in water and nutrients and play an especially important role in the ecology of water-limited systems. We use stable isotopes to discern how plants respond both to ecological gradients associated with elevation and to neighboring legumes on a south-facing slope in the semi-arid, historically grazed steppe of northern Mongolia. Out of three target species, Potentilla acaulis, Potentilla sericea, and Festuca lenensis, when >30 cm from a legume, all showed a decrease in leaf δ15N with increasing elevation. This, together with measures of soil δ15N, suggests greater N processing at the moister, more productive, lower elevation, and more N fixation at the upper elevation, where cover of legumes and lichens and plant-available nitrate were greater. Total soil N was greater at the lower elevation, but not lichen biomass or root colonization by AMF. Leaf δ13C values for P. acaulis and F. lenensis are consistent with increasing water stress with elevation; δ13C values indicated the greatest intrinsic water use efficiency for P. sericea, which is more abundant at the upper elevation. Nearby legumes (<10 cm) moderate the effect of elevation on leaf δ15N, confirming legumes' meaningful input of N, and affect leaf δ13C for two species, suggesting an influence on the efficiency of carbon fixation. Variation in leaf %N and %C as a function of elevation and proximity to a legume differs among species. Apparently, most N input is at upper elevations, pointing to the possible importance of grazers, in addition to hydrological processes, as transporters of N throughout this landscape.

Key Words

C isotopes; Montane steppe; Moisture gradient; Nitrogen fixation; N isotopes