Biocrusts are communities of microorganisms within the top centimeter of soil, often dominated by phototrophic nitrogen-fixing (N-fixing) organisms. They are common globally in desert ecosystems and have recently been identified in agroecosystems. However, the N-fixation activity of these biocrusts and their relationship with soil nutrients in perennial agroecosystems is unknown, and of particular interest given management requirements. Six biocrust samples paired with adjacent bare soil were collected seasonally at a Florida citrus grove and vineyard. N-fixation activity was measured in field incubations using acetylene reduction assays. Samples were also analyzed for microbial and salt-extractable carbon (C), nitrogen (N), and phosphorus (P).
Results/Conclusions
In both agroecosystems, biocrusts had significantly greater microbial C and extractable C compared to bare soil. Microbial N only significantly differed from bare soil at the vineyard in the fall, while extractable N was greater in biocrusts in both locations. Microbial P content in grape and citrus biocrusts was significantly higher in biocrusts at both sites only during the summer, when the soil moisture was the highest. The citrus and grape biocrust N-fixation rates were all within the range of those reported for natural arid and mesic systems, from 5 - 401 µmol of ethylene m-2 h-1. Soil moisture, particularly via precipitation, may have impacted N-fixation rates for the grape biocrusts, as the greatest N-fixation rates were recorded for grape biocrusts during the summer rainy season. However, this moisture influence was less evident in the citrus biocrusts. Higher available P strongly correlated with higher N-fixation activity. Microbial C:N did not correlate with N-fixation activity, while microbial C:P and N:P did. Overall, biocrust presence, moisture, and season had significant impacts on microbial C, N, P, and N-fixation activity. The activity of these agroecosystem biocrusts indicates the possibility to enhance N cycling in perennial agroecosystems, with potential benefits to crop production.