Below and aboveground responses to lupines and litter mulch in a California grassland restored with native bunchgrasses

Goals of ecosystem restoration in California grasslands include the reestablishment of plant communities with a high proportion of native species, and simultaneously improve soil nutrient cycling. Addition of annual lupines and a litter mulch layer were hypothesized to be factors that would promote the growth of the native perennial bunchgrass, Nassella pulchra, in a restored California grassland. To test this hypothesis, field mesocosms were installed, each encircling a Nassella plant, at a perennial grassland restoration site in Carmel Valley, California. Two sets of treatments were imposed: (1) seeding of the annual N-fixing legume, Lupinus bicolor; and (2) exchanging the grassland litter for a thicker mulch layer of C₄ grass litter (C/N=99). Stable isotope analysis allowed the tracking of fates of N fixed by the legume and the C₄-litter derived C. Treatments continued for 28 months, from December 2002 to April 2005, when most of the destructive measurements were taken. In 2005, neither treatment had significantly increased the biomass of the annuals or the perennial bunchgrass, and there was little effect on total soil C and N. Lupinus decreased the δ¹⁵N content, but did not affect the biomass, N and P content of the litter, which was largely composed of annual plants from the previous year. Lupinus resulted in higher soil microbial biomass carbon (SMB-C), and distinct effects on soil microbial communities, especially soil fungi, as measured by phospholipid fatty acid analysis (PLFA) and ergosterol. The high C/N litter mulch tended to increase biomass of Nassella, despite its lower P concentration, and it reduced SMB-C, presumably due to lower decomposition rates compared to the ambient litter. Using a high C/N litter mulch thus is ambiguous for grassland restoration. Repeated increases of legumes over a longer time frame may potentially increase soil fertility and soil C pools in California grasslands, but this study suggests that native perennial grasses may be slow to benefit.