Vegetation influence on soil quality in a highly degraded tropical ...
Vegetation influence on soil quality in a highly degraded tropical ...
Vegetation influence on soil quality in a highly degraded tropical ...
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18 SOTOMAYOR-RAMÍREZ ET AL./TROPICAL SOILS<br />
may have c<strong>on</strong>tributed to the observed trend for higher <strong>soil</strong> pH, greater<br />
percentage base saturati<strong>on</strong>, and lower exchangeable Al because of presumably<br />
lower nutrient losses due to less leach<strong>in</strong>g and runoff. Further<br />
proof of the excellent aerial coverage provided by the grasses is that<br />
dur<strong>in</strong>g the course of the study sediment yields from runoff were 0, 621,<br />
and 2,525 g/plot/year for the grasses, A. glabrata, and A. procera, respectively<br />
(Ramos-Santana et al., 2000).<br />
The effects of vegetat<strong>in</strong>g a bare <strong>soil</strong> is apparent, as bare <strong>soil</strong> had significantly<br />
lower SOM and TON than groups of legumes, grasses, and<br />
both comb<strong>in</strong>ed (Table 2). Legumes <strong>in</strong> general had lower SOM and TON<br />
than grasses. The SOM and TON c<strong>on</strong>centrati<strong>on</strong>s <strong>in</strong> the beg<strong>in</strong>n<strong>in</strong>g and<br />
at the end of the experiment were similar with<strong>in</strong> species and groups of<br />
species (c<strong>on</strong>trasts). C<strong>on</strong>versi<strong>on</strong> of forests of the humid tropics <strong>in</strong>to pastures<br />
and agricultural fields is expected to produce <strong>in</strong> the l<strong>on</strong>g term a<br />
decl<strong>in</strong>e <strong>in</strong> SOM and <strong>soil</strong> fertility. In a study <strong>in</strong> Costa Rica, the largest<br />
decrease (about 50%) <strong>in</strong> SOM occurred between six and 15 m<strong>on</strong>ths follow<strong>in</strong>g<br />
removal of 20-yr-old sec<strong>on</strong>dary vegetati<strong>on</strong> of ferns, grasses, and<br />
scattered stands of trees, after which time the rates of SOM decrease<br />
TABLE 2. <str<strong>on</strong>g>Vegetati<strong>on</strong></str<strong>on</strong>g> effects <strong>on</strong> <strong>soil</strong> organic matter, total organic N and extractable N<br />
pools. Values for each <strong>in</strong>dividual species and c<strong>on</strong>trol are the means averaged<br />
across depth and time.<br />
Species Group<br />
Soil organic<br />
matter<br />
Total<br />
organic N<br />
Extractable<br />
N<br />
g/kg g/kg mg N/kg<br />
A. procera Legume trees 15.2 1.6 3.45<br />
A. <strong>in</strong>ermis 9.6 1.2 3.18<br />
B. humidicola Grasses 15.8 1.7 5.33<br />
H. altissima 17.9 1.7 4.10<br />
A. glabrata Legume shrubs 9.3 1.2 3.53<br />
C. acutifolium 15.0 1.4 4.54<br />
C<strong>on</strong>trol Bare <strong>soil</strong> 6.0 1.2 3.66<br />
F-test NS1 C<strong>on</strong>trasts<br />
NS NS<br />
<str<strong>on</strong>g>Vegetati<strong>on</strong></str<strong>on</strong>g> vs. bare <strong>soil</strong> 12.72 1.45<br />
NS<br />
6.0<br />
1.21<br />
Legumes vs. grasses 12.3<br />
1.36<br />
3.68<br />
16.9<br />
1.76<br />
4.72<br />
Grasses vs. bare <strong>soil</strong> 16.9<br />
1.76<br />
4.72<br />
6.0<br />
1.21<br />
3.66<br />
Legumes vs. bare <strong>soil</strong> 12.3<br />
1.36<br />
NS<br />
6.0<br />
1.21<br />
1 NS denotes n<strong>on</strong> significance at P < 0.05.<br />
2 C<strong>on</strong>trast means are significantly different at P < 0.05.