Field <sup>13</sup>CO<sub>2</sub> pulse labeling reveals differential partitioning patterns of photoassimilated carbon in response to livestock exclosure in a <i>Kobresia</i> meadow

Abstract

Abstract. Livestock exclosure has been widely used as an approach for grassland restoration. However, the effects of exclosures on grasslands are controversial and can depend on many factors, such as the grassland ecosystem types, evolutionary history and so on. In this study, we conduct field experiments to investigate the variations of the ecosystem function in response to livestock exclosure in a Kobresia humilis meadow with 6 years of grazing exclosure on the QinghaiTibetan Plateau. We focused on two ecosystem functions: plant community structure and ecosystem carbon cycling. The plant aboveground productivity, plant diversity and the composition of plant functional groups of the meadow were addressed as the indicators of the plant community structure. The 13 C isotope pulse labeling technique was applied to evaluate the alterations of ecosystem carbon cycling during a short term. The results showed that the plant community structure was changed after being fenced in for 6 years, with significantly decreased aboveground productivity, species loss and varied composition of the four plant functional groups (grasses, sedges, legumes and forbs). Using the pulse labeling technique, we found a lower cycling rate of 13 C in the plant-soil system of the fenced plots compared with the grazed sites during the first 24 h after labeling. A higher proportion of recovered 13 C in the plant-soil system migrated into the soil as root exudates immediately after labeling at both fenced and control grazed sites, with a significantly lower proportion in the fenced site, coinciding with the lower proportion of 13 C lost from soil respiration. Thirtytwo days after labeling, 37 % of the recovered 13 C remained in the soil of the fenced plots, with significant differences compared to in the grazed plots (47 %). In addition, less 13 C (5 vs. 7 %) was lost by soil respiration in the fenced plots during the chase period of 32 days. Overall, our study suggests that livestock exclosures have negative effects on the plant community structure and partitioning patterns of the photoassimilated carbon in the Kobresia meadow, and the effects on photoassimilated carbon cycling are likely to result from the variations of community structures in the ecosystem.

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“…Percentage of recovered 13 C in the shoots in this study (35.75-38.11%) was similar to that reported by Hafner et al (2012) (38.5-42.5%) and Wu et al (2010) (28.9%), but was greater than the 8.14-19.6% reported by Gao et al (2015); Ingrisch et al (2015), and Zou et al (2014). Percentage of recovered 13 C in the roots in this study (8.77-13.52%) was similar to previous results of Zou et al (2014) (6-10%) and Gao et al (2015) (5-6.3%), greater than those reported by Hafner et al (2012) (0.5-2.4%) and significantly lower than the results of Ingrisch et al (2015) (67.1%) and Wu et al (2010) (34.2%). The different partitioning observed for the shoots and roots was related to the shoot and root biomass, as shown by the root:shoot ratio (Hafner et al, 2012;Ingrisch et al, 2015;Zou et al, 2014).…”

“…Percentage of recovered 13 C in the roots in this study (8.77-13.52%) was similar to previous results of Zou et al (2014) (6-10%) and Gao et al (2015) (5-6.3%), greater than those reported by Hafner et al (2012) (0.5-2.4%) and significantly lower than the results of Ingrisch et al (2015) (67.1%) and Wu et al (2010) (34.2%). The different partitioning observed for the shoots and roots was related to the shoot and root biomass, as shown by the root:shoot ratio (Hafner et al, 2012;Ingrisch et al, 2015;Zou et al, 2014). Percentage of recovered 13 C in soil respiration and shoot respiration in this study were also related to biomass and were within the ranges of previous studies (Hafner et al, 2012;Ingrisch et al, 2015;Wu et al, 2010;Zou et al, 2014).…”

“…Although there were significant differences in the m E ( 13 C) in the C pools of both grasslands, partitioning of photoassimilate (% of recovered 13 C) in the plant-soil system 25 days after labeling was similar to that from various other previous studies (Hafner et al, 2012;Ingrisch et al, 2015;Kuzyakov, 2001;Street et al, 2013;Wu et al, 2010;Zou et al, 2014). Percentage of recovered 13 C in the shoots in this study (35.75-38.11%) was similar to that reported by Hafner et al (2012) (38.5-42.5%) and Wu et al (2010) (28.9%), but was greater than the 8.14-19.6% reported by Gao et al (2015); Ingrisch et al (2015), and Zou et al (2014).…”

“…The 13 C pulse labeling made it possible to explore the fate of newly assimilated C, including its partitioning within plants and to the soil, its storage within ecosystems and losses into the atmosphere in the natural and GfGP grasslands under in situ conditions (Gao et al, 2015;Hafner et al, 2012;Ingrisch et al, 2015;Street et al, 2013;Wu et al, 2010;Zhao et al, 2015;Zou et al, 2014). The results indicated that, although the 13 CO 2 uptake rate was greater in the GfGP grassland than in the natural grassland (Figs.…”

“…13 C pulse labeling can be used to explore the SOC sequestration rate, i.e., newly assimilated C partitioned into SOC, under in situ conditions (Gao et al, 2015;Hafner et al, 2012;Ingrisch et al, 2015;Street et al, 2013;Wu et al, 2010;Zhao et al, 2015;Zou et al, 2014). Using the method from Street et al (2013) that is based on the robust positive linear relationship between gross primary productivity (GPP) and annual net primary productivity (ANPP) reported by Litton et al (2007), the product of GPP and % of recovered 13 C in the soil provides estimates of the SOC sequestration rate and total photosynthesized C partitioned into the soil (Litton et al, 2007;Roper et al, 2013;Street et al, 2013).…”