Biological Soil Crust Occurrence and Nitrogen Cycling in an Agroecosystem.

Biological soil crusts (BSCs), organosedimentary groups of microorganisms in the top centimeters of soil, have been identified and examined in dryland ecosystems on all continents except Antarctica. In dryland ecosystems, BSCs are dominated by primary production of edaphic microbial phototrophs, and serve as a primary source of fixed nitrogen (N). Similar assemblages of microorganisms have recently been identified in citrus groves of Southwest Florida. These BSCs are in groves with 1-4-year-old trees, and generally occur in the space between the tree canopies. To our knowledge, this is the first documented report of BSCs in an agroecosystem. Initial characterization studies were performed on samples collected in February 2017. Samples of BSCs and soil without BSCs were collected to determine the microbial community composition, N-fixation, ammonia-oxidation, and denitrification rates, and soil chemistry. These BSCs from a citrus grove were composed of 20-40% cyanobacteria, including similar cyanobacteria found in desert BSCs such as Nostocales, Oscillatoriales, and Pseudanabaenales. Proteobacteria, particularly Alphaproteobacteria Sphingomonadales and Rhizobiales, were also present in both dryland BSCs and those found in citrus. All N-cycling rate measurements were significantly greater in the BSCs compared to soil without BSCs. Potential ammonia-oxidation and N-fixation rates were significantly greater in BSCs compared to potential denitrification rates. The concentration of carbon (C), N, and phosphorous (P) was over 100 times greater in BSCs than soil without BSCs. This greater nutrient concentration appears to impact the soil beneath BSCs, as concentrations of total organic C and total N were significantly greater in the soil below BSCs compared to soil without BSCs. In addition, soil moisture was significantly greater in BSCs compared to non-BSC soils. Additional studies are being performed to determine the contribution of BSCs to tree growth and the overall soil nutrient cycle throughout the year.