Dependence of carbon sequestration on the differential responses of ecosystem photosynthesis and respiration to rain pulses in a semiarid steppe

Abstract

Precipitation pulses play an important role in regulating ecosystem carbon exchange and balance of semiarid steppe ecosystems. It has been predicted that the frequency of extreme rain events will increase in the future, especially in the arid and semiarid regions. We hypothesize that large rain pulses favor carbon sequestration, while small ones cause more carbon release in the semiarid steppes. To understand the potential response in carbon sequestration capacity of semiarid steppes to the changes in rain pulse size, we conducted a manipulative experiment with five simulated rain pulse sizes (0, 5, 10, 25, and 75 mm) in Inner Mongolia steppe. Our results showed that both gross ecosystem productivity (GEP) and ecosystem respiration (R e ) responded rapidly (within 24 h) to rain pulses and the initial response time was independent of pulse size. However, the time of peak GEP was 1-3 days later than that of R e , which depended on pulse size. Larger pulses caused greater magnitude and longer duration of variations in GEP and R e . Differences in the response time of microbes and plants to wetting events constrained the response pattern of heterotrophic (R h ) and autotrophic (R a ) components of R e following a rain event. R h contributed more to the increase of R e in the early stage of rain pulse response, while R a played an more important role later, and determined the duration of pulse response, especially for large rain events of 410 mm. The distinct responses of ecosystem photosynthesis and respiration to increasing pulse sizes led to a threshold in rain pulse size between 10 and 25 mm, above which post wetting responses favored carbon sequestration. The disproportionate increase of the primary productivity of higher plants, compared with those in the activities of microbial decomposers to larger pulse events suggests that the carbon sequestration capacity of Inner Mongolia steppes will be sensitive to changes in precipitation size distribution rather than just precipitation amount.

Comparison of evapotranspiration components and water-use efficiency among different land use patterns of temperate steppe in the Northern China pastoral-farming ecotone

“…The ETs of grazed (3.29 kg m −2 day −1 ), grazing excluded (3.52 kg m −2 day −1 ), and cultivated (3.74 kg m −2 day −1 ) steppe observed in this study were comparable with those found by related research conducted on adjacent Inner Mongolia steppe (Chen et al 2009b;Miao et al 2009). …”

“…According to the Chinese classification, soils of the study site are chestnut or chernozem soil (Chen et al 2009b). Xilinhot is located at the northern edge of the Chinese pastoral-farming ecotone (Miao et al 2009;Wu et al 2013), where grazing is the primary land use.…”

“…All the plants in this plot withered in 2 days, and the canopy of this treatment was kept standing and dry for the observation period. The aboveground tissue of another plot (CL) in each block was removed by clipping following the procedure used in previous studies (Chen et al 2009b;Niu et al 2011). The third plot was the control plot (CK) without any treatment.…”

“…The observation days were distributed alternatively among different ecosystems to enhance the comparability of data measured in similar weather conditions. ET, NEE, and Re were determined in control plots using standard methods (Chen et al 2009b;Niu et al 2013).…”

“…Consequently, it is necessary to separate T from ET accurately and reliably (Hu et al 2009;Béziat et al 2013), when assessing WUE and its controlling mechanism in these special regions. Net ecosystem exchange (NEE) reveals net CO 2 exchange at ecosystem level, and gross ecosystem productivity (GEP) could estimate the potential carbon absorption ability of canopy photosynthesis (Chen et al 2009b). The gap between NEE and GEP depends on the ecosystem respiration (Re) level.…”