Translocation of nitrogen and carbon integrates biotic crust and grass production in desert grassland

1. Arid ecosystems are a patch mosaic of plants and biological soil crusts that have been described as islands and mantles of fertility, respectively. To determine whether these patches are metabolically linked by a fungal network of dark septate and arbuscular mycorrhizal fungi (AMF), we measured translocation of nitrogen (N) and carbon (C) in a desert grassland using N-15-NO3 and C-13(5), N-15-glutamic acid as substrates. 2. Substrates were applied as point sources to either small patches of biotic crust or to a subset of leaves within a bunch grass tussock. 3. Both substrates were translocated over approximately 1 m(2) areas (approximately 20 patches) at rates up to 100 cm day(-1) during a 4-day period following a natural rainfall event. Foliar uptake of N-15 from glutamate was initially more rapid than N-15 uptake from nitrate, and translocation from foliage was initially more rapid than translocation from soil crust. Rates of N-15 translocation between patches were similar to rates of atmospheric N deposition and denitrification. 4. C-13 from C-13(5), N-15-glutamic acid applied to leaves was translocated to crust suggesting that plant C may provide metabolic support to biological soil crusts during periods of active growth. C-13 from C-13(5), N-15-glutamic acid applied to soil crusts did not enter plants, in contrast to the N-15 from this substrate, indicating that glutamate was not translocated intact. 5. The dominant fungi of roots, rhizosphere soil and biological soil crusts are dark septate ascomycetes, most classified as Pleosporales, and AMF are rare. Phylogenetic analyses indicate substantial overlap in fungal community composition between roots and crusts, which may facilitate nutrient transfers. 6. Synthesis. This study supports the hypothesis that the spatial structure of semi-arid ecosystems, a patch mosaic of grasses and biological soil crust described, respectively, by the island of fertility and mantle of fertility paradigms, is functionally integrated by exchanges of C and N through a symbiotic fungal network dominated by dark-septate fungi.