Land management effects on soil nutrients, biomass, and fauna and fungal communities in Costa Rica

Background/Question/Methods

Forty years of clear-cutting and generation of grasslands has caused damage to lowland forests of Costa Rica, yet little is known of how these practices have impacted soil ecosystems, and critical fungal and faunal decomposers and detritivores. Our study was conducted in adjacent unharvested primary and 18 year old secondary forests, and 28 year old grasslands in Costa Rica to identify differences in fungal and faunal communities between habitats. We tested whether such differences could be linked to differences in organic C and N components; and if these metrics could serve as indicators of soil recovery from disturbance. Soil samples from each habitat type were analyzed for rates of production of dissolved organic C (DOC) by the modified Walkly-Black method; total N by the potassium thiosulfate oxidation method; rates of biomass C and N development by chloroform-fumigated and unfumigated methods; and CO₂ flux using the LI-COR LI-8100 CO₂Flux System. Initial soil biota diversity was estimated by sequence analysis of amplicons generated from soil DNA extracts by PCR-based amplification of the eukaryotic faunal cytochrome oxidase gene (primers: HCO2198 and LCO1490) and the internal transcribed space of fungi (Primers: ITS 1F and ITS4).

Results/Conclusions

Rates of DOC and C biomass production (54 to 82 µgDOC/cc/24h) were greater in primary and secondary forest than grassland soils (10 to 33 µgDOC/cc/24h). Rates of total N and N biomass production (0.149 to 0.164 mgN/cc/24h) were greatest in primary and intermediate in secondary forests. The CO₂ flux was lower in both forest (0.4 to 0.78 µmolCO₂/m²/s) compared to grassland soils (2.24 µmolCO₂/m²/s). These data corresponded with the greater diversity (9.22 and 10.59 vs. 7.02) and richness (25.7 and 28.3 vs. 18.5) of important consumer and detrivore fungal and eukaryotic faunal groups in primary and secondary forest soils, with greater abundance of Coleoptera (33.33%), Collembola (14.29%), Trichoptera (4.76%), and Isoptera (4.76%), and fungal groups Tremellales (70%) and Hypocreales (10%) in primary, and Hemiptera (42.90%) and fungal group Helotiales (45.0%) in the secondary forest soils. Confirmation of these patterns by metagenomic analysis of soil DNA using 454 pyrosequencing is in-process. The data suggest the use of secondary forest development as a remediation tool in Costa Rican lowland forests will result in soil biotic communities better able to incorporate C and N into biomass; and that the metrics presented can serve as indicators of environmental integrity in these soils.