SOIL NUTRIENTS AND SALINITY AFTER LONG-TERM GRAZING EXCLUSION IN A FLOODING PAMPA GRASSLAND

Soil organic C, total N, extractable P, and salinity were evaluated af ter 12-16 years of protection from grazing in 2 native grassland sites which differed in frequency of soil waterlogging in the Flooding Pamp a of Argentina, We tested the hypothesis that flooding regime would af fect the impact of grazing on soil chemical properties, We sampled soi l to 10-cm depth in adjacent grazed and ungrazed plots in each site, a nd assessed the percentage dissimilarity (PD) in vegetation compositio n among pastures. Grazing condition significantly interacted with site (P<0.001) in affecting topsoil C, N, and salinity, Soil C and N were higher in grazed grassland (C = 4.8%; N = 0.42%) than in long-term exc losure (C = 3.7%; N = 0.35%) for the more frequently flooded, lowland site, but did not vary between grassland plots in the upland site (C = 3.1%; N = 0.29%), Soil electrical conductivity (E.C.) was low in both ungrazed plots (< 2 dS/m), yet in grazed condition salinization was h igher in the upland (E.C. = 6.85 dS/m) than in the lowland site (3.88 dS/m), Soil extractable P did not change in any consistent way with gr azing treatment, Grazing apparently amplified differences in soil chem istry between lowland and upland sites, while differences in botanical composition between topographical positions were smaller for grazed ( PD = 44%) than for ungrazed (64%) grassland, Moreover, contrasting res ponses between sites occurred for various soil parameters, whereas com positional differences between grazed and ungrazed plots were similar in each site (PD = approximate to 65%). Thus, soil-vegetation changes in response to grazing appeared to be loosely coupled in this rangelan d ecosystem.