Plant functional groups, grasses versus forbs, differ in their impact on soil carbon dynamics with nitrogen fertilization

Nitrogen (N) addition in N-limited grasslands often increases aboveground productivity, decreases species richness and leads to changes in species composition. In contrast to these consistant results in aboveground vegetation parameters, there is no consistant pattern in how N fertilization affects soil organic carbon (SOC) dynamics. Our objectives were to test how plant functional group changes caused by N fertilization affect soil C dynamics and determine if different plant functional groups respond similarly. We conducted a two-factorial experiment to examine soil C dynamics with N fertilization and soil inoculation with field microbial communities in a greenhouse pot experiment. We used six plant species (two grasses and four forbs) that are dominant within sub-alpine meadows on the east part of the Qinghai-Tibetan plateau. For both grasses and forbs, N fertilization and soil inoculation, alone or in combination, decreased SOC by 4-10% and increased soil microbial biomass. For grasses, N fertilization combined with inoculation caused much lower SOC content and higher soil microbial biomass carbon (MBC) as compared to inoculation alone. In contrast to forbs, grass aboveground biomass was significantly negatively correlated with SOC change and positively correlated with MBC change. Nitrogen fertilization combined with inoculation significantly increased basal respiration and cumulative C mineralization rates for both grasses and three of the four forbs as compared to inoculation alone. Grasses had higher basal respiration rates than forbs under these two treatments. Despite higher aboveground grass biomass, N-fertilization lowered the SOC pool by increasing soil MBC and basal respiration rates, thus increasing C decomposition. Overall, in these sub-alpine meadows, grasses and forbs impact on soil C dynamics differs and grasses, but not forbs, may reduce soil C sequestration in response to N fertilization.