Soil nutrients and salinity after long-term grazing exclusion in a Flooding Pampa grassland.

Abstract

Soil organic C, total N, extractable P, and salinity were evaluated after 12-16 years of protection from grazing in 2 native grassland sites which differed in frequency of soil waterlogging in the Flooding Pampa of Argentina. We tested the hypothesis that flooding regime would affect the impact of grazing on soil chemical properties. We sampled soil to 10-cm depth in adjacent grazed and ungrazed plots in each site, and assessed the percentage dissimilarity (PD) in vegetation composition 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 exclosure (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 higher 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 grazing treatment. Grazing apparently amplified differences in soil chemistry 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 responses between sites occurred for various soil parameters, whereas compositional differences between grazed and ungrazed plots were similar in each site (PD approximately 65%). Thus, soil-vegetation changes in response to grazing appeared to be loosely coupled in this rangeland ecosystem.