Dynamics of grassland carbon sequestration and its coupling relation with hydrothermal factor of Inner Mongolia

Abstract

Grassland carbon (C) sink/source evaluation is important to terrestrial ecosystem C cycling research. In this paper, boreal ecosystem productivity simulator (BEPS) model was used to simulate the net ecosystem productivity (NEP) of Inner Mongolia (IM) from 1982 to 2008. The coupling relation between NEP and climate factors was then explored. Results showed that the mean annual value of grassland NEP is 12.2 gC/m2/yr, and regions with NEP of >0 (C sink) accounts for 73.2% of the total grassland area of IM. Additionally, the total C sequestration reaches 0.35 Pg C yearly. NEP was positively correlated with precipitation (R = 0.31), and the positive correlation percentage accounted for 90% of total grassland area. But NEP was negative correlated with temperature (R = −0.11), and the negative correlation percentage was 72% of case. And the partial correlation coefficient between NEP and precipitation and between NEP and temperature was 0.30 and −0.06 respectively. Meanwhile, the monthly NEP of the grassland is obviously lagging behind the precipitation and temperature, and the lag time is both three months. We defined a precipitation differential (PD) parameter to explore the coupling relation between grassland NEP and precipitation. Generally, areas with positive PD are typically a C sinks (72% of the grassland area of IM). However, regions with negative PD are likely a C source (28% of case). The obtained mean PD value of IM is 18 mm, indicating that IM is a C sink.Further analysis showed that 69% of regions have positive PD and positive NEP, and 20% of regions have negative PD and negative NEP. This result confirmed that precipitation deficit restrains C sequestration. However, the rest of 11% of regions was sensitive area of carbon sink and carbon source transition. Among 8% of the regions (with negative PD but positive NEP) have insufficient precipitation, although other conditions (i.e., ecological restoration program) are favorable to C sink. Moreover, 3% of the regions (with positive PD but negative NEP) have sufficient precipitation, but have negative NEP because of inappropriate management or low temperature. Thus, appropriate measures that can convert a C source to a C sink are necessary. This paper can serve as a reference for policy makers for the efficient targeted implementation of ecological engineering.