RESUMO
Vegetation changes can affect soil organic carbon (SOC) content and storage by altering the inputs of plant biomass and the catabolism and anabolism of soil microorganisms. However, influence of vegetation degradation on aggregate associated carbon fractions and the contribution of different aggregates to total SOC in bulk soil remains poorly understood. In this study, undisturbed soil samples were collected from three types of grassland in Songnen grassland: an undegraded grassland (LEY, Leymus chinensis), a moderately degraded grassland (CHL, Chloris virgata), and a severely degraded grassland (SUA, Suaeda heteroptera). Three soil aggregates including macroaggregate (> 0.25 mm), microaggregate (0.053-0.25 mm) and silt and clay fraction (< 0.053 mm) were separated using wet sieving. Contents of total SOC, soil labile and stable carbon in bulk soil and different soil aggregates were measured. Compared with LEY, the mean weight diameter and geometric mean diameter under the degraded vegetation communities reduced by 39.42 % and 28.47 %, respectively. The reduction in SOC contents in bulk soil, macroaggregate, microaggregate and silt and clay fraction resulting from vegetation degradation was 49.81 %, 26.00 %, 76.17 % and 43.65 %, respectively. Under the degraded vegetation communities, contents of soil labile and stable carbon in bulk soil (45.73 % and 52.61 %, respectively), macroaggregate (17.38 % and 31.61 %, respectively), microaggregate (77.83 % and 74.18 %, respectively), and silt and clay fraction (21.20 % and 53.45 %, respectively) were significantly lower than those under LEY. The contribution of macroaggregate, microaggregate and silt and clay fraction to total SOC was 13.27 %, 23.61 % and 63.12 %, respectively. The contribution of soil aggregates to total SOC following vegetation degradation reduced by 53.63 % for microaggregate, but increased by 47.10 % for silt and clay fraction. These findings collectively indicate that vegetation degradation reduces the aggregate associated carbon content by reducing both labile and stable carbon fraction in Songnen grassland, and sustainable vegetation restoration strategies are need to enhance soil carbon storage in Northeast China.
RESUMO
Climate change inevitably affects vegetation growth in the Tibetan Plateau (TP). Understanding the dynamics of vegetation phenology and the responses of vegetation phenology to climate change are crucial for evaluating the impacts of climate change on terrestrial ecosystems. Despite many relevant studies conducted in the past, there still remain research gaps concerning the dominant factors that induce changes in the start date of the vegetation growing season (SOS). In this study, the spatial and temporal variations of the SOS were investigated by using a long-term series of the Normalized Difference Vegetation Index (NDVI) spanning from 2001 to 2020, and the response of the SOS to climate change and the predominant climatic factors (air temperature, LST or precipitation) affecting the SOS were explored. The main findings were as follows: the annual mean SOS concentrated on 100 DOY-170 DOY (day of a year), with a delay from east to west. Although the SOS across the entire region exhibited an advancing trend at a rate of 0.261 days/year, there were notable differences in the advancement trends of SOS among different vegetation types. In contrast to the current advancing SOS, the trend of future SOS changes shows a delayed trend. For the impacts of climate change on the SOS, winter Tmax (maximum temperature) played the dominant role in the temporal shifting of spring phenology across the TP, and its effect on SOS was negative, meaning that an increase in winter Tmax led to an earlier SOS. Considering the different conditions required for the growth of various types of vegetation, the leading factor was different for the four vegetation types. This study contributes to the understanding of the mechanism of SOS variation in the TP.