REPORT Protein accumulation and distribution in floodplain soils and river foam Mary J. Harner, 1 * Philip W. Ramsey 2 and Matthias C. Rillig 2 1 Department of Biology, The University of New Mexico, Albuquerque, NM 87131, USA 2 Microbial Ecology Program, Division of Biological Sciences, The University of Montana, Missoula, MT 59812, USA *Correspondence: E-mail: mharner@unm.edu Abstract Many processes contribute to nutrient transfer from terrestrial to aquatic systems, but in most cases the contribution of particular organisms is unknown. In this study, we explore how a Bradford-reactive soil protein (BRSP) produced by arbuscular mycorrhizal fungi may provide nutrients to river ecosystems. Along a floodplain in Montana, we extracted BRSP from soils and related the protein concentrations to the age of soil surfaces. We identified BRSP in surface soils, as well as to a depth of 1.4 m, and found that the protein accumulates through time. We also detected BRSP in foam from five rivers in the western United States. Experiments were conducted that demonstrate that the protein may be leached or washed from soils and become a constituent of foam when mixed into turbulent water. We propose that terrestrially derived soil protein may enter rivers via erosion and leaching and serve as a nutrient source for aquatic organisms. Keywords Arbuscular mycorrhiza, Bradford-reactive soil protein, floodplain, foam, glomalin, Populus, soil carbon, soil structure. Ecology Letters (2004) 7: 829–836 INTRODUCTION The flow of nutrients between terrestrial and aquatic habitats in riparian zones is facilitated by flooding (Junk et al. 1989), percolation of water through sediments and soil (Wondzell & Swanson 1996), and groundwater flow (Brunke & Gonser 1997; Hill 2000). It is known that allochthonous sources of organic matter support high levels of benthic and hyporheic metabolism in rivers (Pusch et al. 1998; Tockner et al. 1999; Baker et al. 2000), but little is known about the sources or the amount of organic matter that is transported between floodplain soils and rivers. In terrestrial ecosystems the movement of nutrients between above- and below-ground environments is often mediated by mycorrhizal fungi that exchange phosphorus and nitrogen for plant carbon (Smith & Read 1997). Arbuscular mycorrhizal fungi associate with c. 80% of plant taxa (Allen 1991) and may utilize up to 45% of the carbon fixed by photosynthesis (Grayston et al. 1997) and subsequently store a fraction of this carbon in soil (Rillig et al. 2001; Lovelock et al. 2004a). The central role that arbuscular mycorrhizal fungi play in the transfer of carbon between plants and the soil ecosystem led us to investigate their role in transferring nutrients from floodplain soils to rivers. Arbuscular mycorrhizal fungi (AMF) contribute to nutrient storage in soil directly via the formation of mycelial networks, as well as indirectly by affecting the structure of soil (Miller & Jastrow 2000). AMF produce extensive hyphae in soil that may exceed 100 m of hyphal length per gram of soil (Miller et al. 1995). Hyphae, along with plant roots, enmesh soil particles and contribute to the binding of soil particles (Tisdall et al. 1997; Jastrow et al. 1998). The contribution of AMF to soil structure, defined as the size and arrangements of particles and pores in soil (Hartge & Stewart 1995), is important because the formation of stable aggregates increases the storage of organic matter in the soil (Jastrow et al. 1996; Jastrow 1996), improves water infiltra- tion and soil porosity (Paul & Clark 1989), and provides resistance to erosion (Sumner 2000). AMF also produce biochemical compounds that are important in soil aggrega- tion. Of particular interest is the production of glomalin (Wright et al. 1996), a putative glycoprotein of as yet unknown biochemical structure that is operationally defined as Bradford-reactive soil protein (BRSP) (Rillig 2004). BRSP supplies nutrients to soil through the structure of the molecule, which in tropical soils has been measured as c. 37% C and 4% N, representing 3 and 5% of soil carbon and nitrogen pools, respectively (Lovelock et al. 2004a). BRSP also enhances soil nutrient storage via its contribution to soil Ecology Letters, (2004) 7: 829–836 doi: 10.1111/j.1461-0248.2004.00638.x Ó2004 Blackwell Publishing Ltd/CNRS