Research on Soil Microbial Communities and Enzymatic Activity in Tropical Soils in Puerto Rico.

Limited land area and high population pressure on the island of Puerto Rico create unique agricultural challenges where agricultural intensification is necessary. Greater understanding of tropical soil biology and the functions they provide is needed to mitigate the effects of intensive agriculture such as soil degradation and enhance soil sustainability and ecosystems resilience. Soil microbial community composition using fatty acid methyl ester profiles (FAME) and enzyme activities (EAs) involved in C, N, P, and S cycling were assessed across a variety of land use patterns, soil types (five different soil orders), and climate (semi-arid to tropical). In Vertisols and Mollisols of the semiarid southern coast, fungal FAME markers were higher under pasture and tropical fruit trees (quenepas and mangoes) compared to agricultural soils under vegetable production. Microbial communities differentiated land under pasture compared to vegetable land and the pasture and treed areas supported up to 4-fold higher EAs. Higher EAs were measured in the acidic, low fertile Oxisols and Ultisols of the central-mountainous area compared to the less weathered tropical Inceptisols. However, Inceptisols managed for pasture and forested areas were found to reach similar levels of EAs as found in Oxisols and Ultisols. We also investigated soil C and EA changes following a 26-year conversion from sugarcane (Saccharum officinarum) to forest (Eucaliptus robusta or Leucaena leucocephala), pasture (mixture of tropical grasses), or vegetable cropping (agriculture) in a Vertisol.  Agricultural soils had the lowest C cycling EA, lowest proportion of SOC as biomass C, and highest proportion of SOC present as mineralizable C. The C-cycling EA decreased in the order: Leucaena = Eucalyptus > pasture > agriculture, whereas the N-cycling EA decreased in the order: Eucalyptus >Leucaena = pasture > agriculture. This differential response was likely attributed to differences in available substrates and cycling from the different tree species. Our studies contribute to the database of soil health attributes across Puerto Rico and will help identify sustainable land management practices to meet growing demands.