Catclaw (Mimosa buincifiera): a pest or a means to restore soil fertility in heavily eroded soil from the central highlands of Mexico?

Abstract

Abstract In the central highlands of Mexico, heavily eroded soils are often colonized by catclaw (Mimosa buincifiera): an N2-fixing shrub. An experiment was carried out to investigate how this shrub affected char­ acteristics of the soil and its biological functioning. Soil was sampled from outside and under the canopy of cat­ claw at t hree sites characterized by different degrees of erosion and an increase in plant density. The soil mi­ crobial biomass C, total amounts of bacteria, fungi, acti­ nomycetes and free-living N2"fixing micro-organisms were measured, while production of C02 and dynamics of nitrate (N03), nitrite (NOi) and ammonium (NHt) were monitored in an aerobic incubation at 22 ± 1oc for 35 days. The C content was 1.6 times greater in the a rea with•the largest density of plants and the least ero­ sion (RECUP) compared with the site with the lowest density and greatest erosion (DEGR), while it was 1.2 t im es greater under the canopy of the catclaw than out­ side i t ( average of the three sites). The incorporation of N i nto the soil organic matter was greater under the ca­ nopy of the catclaw than outside it as the C:N ratio was on average 8.4 and 9. 1, respectively. The microbial bio­ m ass C, as a percentage of soil•organic matter, was 1.5 ti mes greater i n the RECUP than in the DEGR site. G rea test total number of colony-forming bacteria and fungi (mean of organisms found under and outside the canopy) were found in the RECUP treatment and low­ est in the DEGR treatment Free-living Nz-fixing or­ ganisms and actinomycetes showed opposite trends. Greater total numbers of colony-forming bacteria, fun­ gi, actinomycetes and free-living N2"fixing organisms (mean of the three treatments) were found under the canopy of catclaw than outside of it, Production of C02 was 1.8 times greater in the RECUP than in the DEGR and 1.6 times greater under the canopy of catclaw than outside. Production of N03 was 1.3 times greater in the RECUP than in the DEGRand 3.5 times greater under the canopy of catclaw than outside. There was no sig­ nificant effect of location or canopy on NOi and NHt concentrations. It is concluded that the natural vegeta­ tion of catclaw increased microbial biomass and soil or­ ganic matter content under, but also outside its canopy,;: • and preserved N better, releasing greater amounts of inorganic N upon minenilization. Catclaw can serve as a first colonizer of heavily eroded soil and be replaced by other vegetation, natural or crops, when fertility is restored.