For Online Publication Only
Appendix A: Patagonian Steppe Patches
Vegetation of the Patagonian steppe is distributed in two contrasting microsites, dense vegetated patches formed by shrubs surrounded by a ring of grasses (vegetated patches) and scattered tussocks in a bare soil matrix (bare soil). The proportion of the two types of vegetation patches did not differ significantly among the 30 experimental plots (F2,9=2.19, P = 0.15), (Table A1).
Pulse manipulation effects on soil water content in Patagonian steppe patches
Water pulses caused consistent changes in soil water content after the manipulation events in the three dates, October, December, and January, and was significantly higher in bare soil than in vegetated patches (ANOVA RM terms: irrigation pulses F2,117 =7.32, P =0.001; vegetation patches: F1,117 =5.48, P=0.02; time F2,117 =2.28, P=0.13) (Figure A1). Precipitation pulses applied as three small pulses of 5 mm each did not cause significantly different volumetric soil water content at 0-20 cm depth from precipitation applied as one single 15 mm pulse, neither in vegetated patches or bare soil (Figure A1). Interactions among irrigation treatments, vegetation patches and time were not statically significant.
Soil nitrogen content in the Patagonian steppe patches
Soil inorganic nitrogen in the 0-10 cm depth was significantly (P<0.001) higher in vegetated patches composed of shrub surrounded by a ring of grasses than in the bare soil with a scattered tussock matrix in the three sampling dates, October, December, and January (Figure A2). These differences were accounted for by significant differences in soil ammonium concentration, the predominantly inorganic nitrogen species in the upper soil (Figure A2). Nitrate concentration did not differ among patches (P>0.05). This pattern was consistent during the three dates evaluated along the growing season (October, December, and January). The NO3:NH4 ratio was significantly higher (P<0.05) in bare soil than in vegetated patches, where we found significantly (P<0.05) lower pH, lower %C and lower %N (Table A1). Bulk density did not differ significantly between patch types (Table A1). In vegetated patches, the identity of the shrub within the patch significantly influenced (P<0.05) soil inorganic N content, being higher under the legume Adesmia volkmanni (1.43 ±1.23 µN-NO3-.g-1 of dry soil and 13.8 ± 5.22 µN-NH4+.g-1 of dry soil; mean ± SE for n=3) than under Mulinum spinosum (0.36 ± 0.10 µN-NO3-.g-1 and 5.43 ± 0.72 µN-NH4+.g-1; mean ± SE for n=3) or Senecio filaginoides (0.30 ± 0.90 µN-NO3-.g-1 and 5.28 ± 1.47 µN-NH4+.g-1; mean ± SE for n=3).
Table A1: Percent Cover and Soil Physical and Chemical Characteristics of the Two Types of Vegetation Patches in the Patagonian Steppe
32.7 % ± 3.8 / Bare soil
67.3 % ± 3.8
pH / 7.71 ± 0.07 a / 6.87 ± 0.06 b
NO3:NH4 / 0.10 ± 0.01 a / 0.37 ± 0.05 b
Gravimetric soil water content
(% g water.g-1 dry soil) / 2.64 ± 0.10 a / 2.24 ± 0.08 a
Dry bulk density (g.cm-3)
Soil C (%)*
Soil N (%)* / 1.49 ± 0.05 a
1.04 ± 0.06 a
0.09 ± 0.003 a / 1.51 ± 0.04 a
0.37 ± 0.01 b
0.04 ± 0.004 b
(*) data from Gonzalez-Polo and Austin (2009).
Vegetated patches are composed of shrubs surrounded by a ring of grasses and bare soil is composed of scattered tussocks in a bare soil matrix (percent cover for each patch type ± SE; n=10). Values are averages (± SE; n=10) for the 0-10 cm soil-depth measured in January. Different letters show significant differences between patch types at P<0.05.
Figure legends
Figure A1: Effects of irrigation treatments on volumetric soil water content in bare soil (BS) and vegetated patches (VEG) at 0-20 cm depth. Volumetric soil water content was measured after precipitation pulse additions in October, December, and January. Bars represent mean values for the three dates (+ SE) for n = 30. Different uppercase letters show significant differences among pulse treatments at P<0.001, and lowercase letters show significant differences between patch types at P<0.05.
Figure A2: Soil inorganic nitrogen concentration in contrasting patch types in the Patagonian steppe along three dates during the growing season. Vegetated patches (VEG) are formed by a shrub surrounded by grasses and bare soil patches (BS) are composed of scattered tussocks in a bare soil matrix. Bars represent mean values (+ SE) for n = 10. Different lower-case letters show significant differences in soil inorganic nitrogen concentration for patches within a sampling date (P<0.01) and different capital-letters show differences among dates at P<0.001. Differences for soil nitrate content were not statistically significant (P>0.05).
Figure A1
Figure A2
Literature Cited
Gonzalez-Polo, M., and A. T. Austin. 2009. Spatial heterogeneity provides organic matter refuges for soil microbial activity in the Patagonian steppe, Argentina. Soil Biol Biochem 41:1348-1351.
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