Agriculture changes soil properties on the Galápagos Islands – two case studies

The Galapagos Islands have faced a huge rise in the number of inhabitants and tourists over recent decades. As a consequence, natural forest vegetation has been converted to arable land to meet the increasing demand for agricultural produce. However, studies on soils and soil use change in the Galapagos Islands are scarce. In this study, we assessed the impacts of agricultural land-use on the islands of Santa Cruz and San Cristobal. The site on Santa Cruz was called El Cascajo (EC, Leptosol, 5 years after conversion to intensive agriculture) and that on the oldest island of the Galapagos archipelago, San Cristobal, Cerro Verde (CV, Ferralsol, 15 years after conversion). At both sites, we compared arable soils to soils under forests within the adjacent Galapagos National Park, which had formed on the same parent materials. Several soil properties showed a significant impact of the arable use. Soil organic carbon (Corg) stocks were relatively high at both sites (between 94 ± 28 and 142 ± 10 Mg ha–1). At CV, the stocks were 25% lower in arable plots compared with natural forest, but the observed decrease at EC was non-significant. Many other parameters like total nitrogen and several Mehlich-III extractable nutrients were closely correlated with the changes in Corg. Microbial biomass carbon (Cmic) and dissolved organic carbon were significantly lower in arable plots compared with forest at both sites. At EC, Cmic/Corg dropped by a factor of 1.9. Changes in pH and the use of agrochemicals, evidenced by pesticide residues detected in the soil, might be a possible explanation. Ammonium-N decreased and nitrate-N increased tremendously in the arable plots on both islands. Our research shows that present agricultural practices have potential for improvements towards a more sustainable agricultural use of soil resources on the Galapagos Islands.