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

In the central highlands of Mexico, heavily eroded soils are often colonize d by catclaw (Mimosa buincifiera): an N-2-fixing shrub. An experiment was c arried out to investigate how this shrub affected characteristics of the so il and its biological functioning. Soil was sampled from outside and under the canopy of catclaw at three sites characterized by different degrees of erosion and an increase in plant density. The soil microbial biomass C, tot al amounts of bacteria, fungi, actinomycetes and free-living N-2-fixing mic ro-organisms were measured, while production of CO2 and dynamics of nitrate (NO3-), nitrite (NO2-) and ammonium (NH4+) were monitored in an aerobic in cubation at 22+/-1 degreesC for 35 days. The C content was 1.6 times greate r in the area with the largest density of plants and the least erosion (REC UP) compared with the site with the lowest density and greatest erosion (DE GR), while it was 1.2 times greater under the canopy of the catclaw than ou tside it (average of the three sites). The incorporation of N into the soil organic matter was greater under the canopy of the catclaw than outside it as the C:N ratio was on average 8.4 and 9.1, respectively. The microbial b iomass C, as a percentage of soil. organic matter, was 1.5 times greater in the RECUP than in the DEGR site. Greatest total number of colony-forming b acteria and fungi (mean of organisms found under and outside the canopy) we re found in the RECUP treatment and lowest in the DEGR treatment. Free-livi ng N-2-fixing organisms and actinomycetes showed opposite trends. Greater t otal numbers of colony-forming bacteria, fungi, actinomycetes and free-livi ng N-2-fixing organisms (mean of the three treatments) were found under the canopy of catclaw than outside of it, Production of CO2 was 1.8 times grea ter in the RECUP than in the DEGR and 1.6 times greater under the canopy of catclaw than outside. Production of NO3- was 1.3 times greater in the RECU P than in the DEGR and 3.5 times greater under the canopy of catclaw than o utside. There was no significant effect of location or canopy on NO2- and N H4+ concentrations. It is concluded that the natural vegetation of catclaw increased microbial biomass and soil organic matter content under, but also outside its canopy, and preserved N better, releasing greater amounts of i norganic N upon mineralization. Catclaw can serve as a first colonizer of h eavily eroded soil and be replaced by other vegetation, natural or crops, w hen fertility is restored.