Impact of forest organic farming change on soil microbial C turnover using 13C of phospholipid fatty acids

Situated in the fast-developing country of Brazil, the Atlantic rainforest Mata Atlântica faces risks generated by population growth-derived problems. Conversion of forest to agriculture has led to a mosaic landscape with fragments of secondary forest in agricultural land. This disturbance to a naturally well-adapted ecosystem prompts rapid soil degradation. Therefore, here we compared soil C incorporation into soil microorganisms and their turnover in typical land-use systems such as primary forest, secondary forest, and agricultural land at the Atlantic Plateau of São Paulo, Brazil. In C3 and C4 plants having different 13C/12C compositions, a C3–C4 vegetation change was induced using maize, a C4 plant. We measured the δ13C composition of individual phospholipid fatty acids (PLFA) because PFLA are specific of typical microbes. Results show that statistical analysis of soil PLFA allow differentiation of four microbial units: (1) Gram-positive bacteria; (2) anaerobic Gram-positive bacteria; (3) fungi, vesicular–arbuscular mycorrhizal fungi, and Gram-positive bacteria; and (4) actinomycetes and Gram-positive bacteria. We also found that soil organic matter is cycled for longer time in primary forest ecosystems, of mean turnover time of 28 years, than in agricultural ecosystems with mean turnover time of 4 years for organic farming and 8 years for conventional farming. Calculation of maize-derived carbon of each microbial unit suggested that fungi and vesicular–arbuscular mycorrhizal fungi dominate microbial activity in primary forest whereas Gram-negative bacteria are prominent in the agricultural sites. To conclude, we found that PLFA profiles are sensitive to land-use conversion, and their compound-specific stable-isotope analysis can strongly discriminate between different managements.