Influence of a Commercial Fertilizer and Tropic Sunn (Crotalaria juncea) As a Cover Crop On Soil Microbial Communities Under Organic Sweet Pepper Production in Puerto Rico.

Organic production in Puerto Rico is at an early stage and research is needed to validate the sustainability of different management practices.  This research initiated evaluation of selected soil properties including the microbial communities and enzyme activities in order to evaluate the effects of Tropic sunn (Crotalaria juncea) as a cover crop and a commercial fertilizer on soil quality under an organic sweet pepper production system.  The study was conducted at the Agricultural Experimental Station of Lajas, University of Puerto Rico under a Vertisol during one growing season, starting on April 2012.  The field experiment was a randomized complete block design with four replications of a 2x4 factorial arrangement of treatments. Treatments were cover crop and no cover crop, and four N fertilization rates [(0, 56, 112 and 168 kg N ha^-1; supplied by the commercial fertilizer BioFlora Dry Crumbles™ (6-6-5)].  Soil samples were analyzed using a high throughput phospholipid fatty acid (PLFA) and the ester-linked (EL-FAME) extraction procedures.  ANOVA revealed no significant effect of cover crop and N fertilization rates on FAME markers for different microbial groups or in the microbial community size (according to total FAMEs).  Although, it was not possible to compare throughly the two methods (PFLA and EL-FAME) due to lack of significant differences among treatments, this study elucidated three important findings: i) the arbuscular mycorrizal fungal (AMF) indicator (16:1w5c) showed similar abundance with both methods; (ii) the fungal:bacterial (F:B) ratios were also similar with both methods, and (iii) the PCAs from both methods showed a trend of separation in the microbial community structure of the cover crop vs. no cover crop.  This trend of early shifts in microbial community structure due to cover crops is of ecological significance due to potential future benefits on biogeochemical cycling in this soil under organic farming.  The selected soil properties evaluated in this Vertisol can provide background data to establish a soil quality index under organic farming production.