Field sup13/supCOsub2/sub pulse labeling reveals differential partitioning patterns of photoassimilated carbon in response to livestock exclosure in a iKobresia/i meadow

摘要

Livestock exclosure has been widely used as an approach for grasslandrestoration. However, the effects of exclosures on grasslands are controversialand can depend on many factors, such as the grassland ecosystem types,evolutionary history and so on. In this study, we conduct field experimentsto investigate the variations of the ecosystem function in response to livestockexclosure in a Kobresia humilis meadow with 6 years of grazingexclosure on the Qinghai–Tibetan Plateau. We focused on two ecosystemfunctions: plant community structure and ecosystem carbon cycling. Theplant aboveground productivity, plant diversity and the composition of plantfunctional groups of the meadow were addressed as the indicators of the plantcommunity structure. The 13C isotope pulse labeling technique wasapplied to evaluate the alterations of ecosystem carbon cycling during ashort term. The results showed that the plant community structure was changedafter being fenced in for 6 years, with significantly decreased abovegroundproductivity, species loss and varied composition of the four plantfunctional groups (grasses, sedges, legumes and forbs). Using the pulselabeling technique, we found a lower cycling rate of 13C in theplant–soil system of the fenced plots compared with the grazed sites duringthe first 24 h after labeling. A higher proportion of recovered 13C inthe plant–soil system migrated into the soil as root exudates immediatelyafter labeling at both fenced and control grazed sites, with a significantlylower proportion in the fenced site, coinciding with the lower proportionof 13C lost from soil respiration. Thirty-two days after labeling,37% of the recovered 13C remained in the soil of the fenced plots, withsignificant differences compared to in the grazed plots (47%). In addition,less 13C (5 vs. 7%) was lost by soil respiration in thefenced plots during the chase period of 32 days. Overall, our study suggests thatlivestock exclosures have negative effects on the plant community structureand partitioning patterns of the photoassimilated carbon in theKobresia meadow, and the effects on photoassimilated carbon cyclingare likely to result from the variations of community structures in theecosystem.