An arbuscular mycorrhizal inoculum enhances root proliferation in, but notnitrogen capture from, nutrient-rich patches in soil

Most work on root proliferation to a localized nutrient supply has ignored the possible role of mycorrhizal fungi, despite their key role in nutrient acquisition. Interactions between roots of Plantago lanceolata, an added ar buscular mycorrhiza (AM) inoculum and nitrogen capture from an organic patc h (Lolium perenne shoot material) dual-labelled with N-15 and C-13 were inv estigated, to determine whether root proliferation and nitrogen (N) capture was affected by the presence of AM fungi. Decomposition of the organic pat ch in the presence and absence of roots peaked in all treatments at day 3, as shown by the amounts of (CO2)-C-13 detected in the soil atmosphere. Plan t N concentrations were higher in the treatments with added inoculum 10 d a fter patch addition, but thereafter did not differ among treatments. Plant phosphorus concentrations at the end of the experiment were depressed by th e addition of the organic residue in the absence of mycorrhizal inoculum. A lthough uninoculated plants were also colonized by mycorrhizal fungi, colon ization was enhanced at all times by the added inoculum. Addition of the AM inoculum increased root production, observed in situ by the use of minirhi zotron tubes, most pronouncedly within the organic patch zone. Patch N capt ure by the end of the experiment was c. 7.5% and was not significantly diff erent as a result of adding an AM inoculum. Furthermore, no C-13 enrichment s were detected in the plant material in any of the treatments showing that intact organic compounds were not taken up. Thus, although the added. AM f ungal inoculum benefited P. lanceolata seedlings in terms of P concentratio ns of tissues it did not increase total N capture or affect the form in whi ch N was captured by P. lanceolata roots.