Effects of native Lupinus bicolor on plant and soil microbial communities in a perennial California grassland restored with Nassella pulchra

Does planting a native annual legume, Lupinus bicolor, contribute to the successful establishment of native bunchgrasses and how does this manipulation in plant community structure affect the soil? This investigation was carried out at a restored grassland in Carmel Valley, CA. In addition to lupine treatments, litter of a C4 perennial grass was added to this C3 plant community. In field-mesocosms that encircled a Nassella pulchra plant, four treatments were applied: (1) NIL, (2) litter exchange from C3 to C4 litter, (3) planting of Lupinus, and (4) both litter exchange and Lupinus addition. Plant community responses to Lupinus were only apparent in the first year after planting Lupinus seeds whereas the soil microbial community, based on phospholipid fatty acids, was affected more persistently. Ergosterol, a fungal sterol, increased due to Lupinus addition. The incorporation of C4 litter-derived C into the microbial biomass accounted for about 25% after 4 months and 10% after 28 months. The corresponding values for the litter-derived C in the soil total carbon increased with time, from about 2.5% to 7% after 4 months and 28 months, respectively. Plant available P increased in both the C4 litter and Lupinus addition treatments. Yet there was no increase in the aboveground biomass of N. pulchra in any treatment, indicating that other factors, possibly moisture, limited growth more than nutrient availability. Greater effects of Lupinus addition or mulches may show up in sites with lower fertility and/or higher water holding capacity.