Effect of grazing on carbon stocks and assimilate partitioning in a Tibetan montane pasture revealed by ¹³CO₂ pulse labeling

Abstract

Since the late 1950s, governmental rangeland policies have changed the grazing management on the Tibetan Plateau (TP). Increasing grazing pressure and, since the 1980s, the privatization and fencing of pastures near villages has led to land degradation, whereas remote pastures have recovered from stronger overgrazing. To clarify the effect of moderate grazing on the carbon (C) cycle of the TP, we investigated differences in below-ground C stocks and C allocation using in situ 13 CO 2 pulse labeling of (i) a montane Kobresia winter pasture of yaks, with moderate grazing regime and (ii) a 7-year-old grazing exclosure plot, both in 3440 m asl. Twenty-seven days after the labeling, 13 C incorporated into shoots did not differ between the grazed (43% of recovered 13 C) and ungrazed (38%) plots. In the grazed plots, however, less C was lost by shoot respiration (17% vs. 42%), and more was translocated below-ground (40% vs. 20%). Within the below-ground pools, <2% of 13 C was incorporated into living root tissue of both land use types. In the grazed plots about twice the amount of 13 C remained in soil (18%) and was mineralized to CO 2 (20%) as compared to the ungrazed plots (soil 10%; CO 2 9%). Despite the higher contribution of root-derived C to CO 2 efflux, total CO 2 efflux did not differ between the two land use types. C stocks in the soil layers 0-5 and 5-15 cm under grazed grassland were significantly larger than in the ungrazed grassland. However, C stocks below 15 cm were not affected after 7 years without grazing. We conclude that the larger below-ground C allocation of plants, the larger amount of recently assimilated C remaining in the soil, and less soil organic matter-derived CO 2 efflux create a positive effect of moderate grazing on soil C input and C sequestration.

More than a century of Grain for Green Program is expected to restore soil carbon stock on alpine grassland revealed by field 13 C pulse labeling

“…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).…”

“…Soil and root samples were taken from three soil layers: 0-5, 5-15, and 15-30 cm using a 5 cm diameter root auger. Living roots were chosen for subsequent 13 C analysis when the inside was white and moist, which indicated intact transport tissue (Hafner et al, 2012). The same soil sampling protocol without pulse labeling was used on the control cropland (cultivated in the 1950s from the natural grassland and partly converted to the GfGP in 2002) to estimate the baseline SOC stocks.…”

“…Following Kuzyakov (2006) and Hafner et al (2012), the static alkali absorption method was used to determine CO 2 released from soil and absorbed in alkali (NaOH) over a specific time period within a closed chamber (10 cm diameter, 10 cm high) that was installed on the bare soil surface just after the shoots were cut. A plastic bottle containing 1 M NaOH was placed inside the chamber to absorb CO 2 released from the soil between adjacent plant and soil sampling dates at time intervals of 3, 3, and 18 h, and 3, 7, 7, and 7 days.…”

“…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).…”