Holocene PARs and CARs at high altitudes
The records of PARs and CARs at high altitudes in China during the Holocene reveal slightly different patterns. We found an overall long-term decreasing trend in PARs and CARs at altitudes higher than 2400 m (a.s.l.). PARs range from 0.27 to 0.70 mm yr− 1, with an average of 0.52 mm yr− 1 (Fig. 2a), and the composite CAR record range from 16.62 to 35.86 g m− 2 yr− 1, with an average of 24.98 g m− 2 yr− 1 (Fig. 2b). Different to the persistently long-term decreasing trend in PARs, the long-term decreasing trends in CARs and dry bulk densities (DBDs) are both superimposed by short-term increasing trends from 6 to 4 kyr BP (Figs. 2a–c). According to the frequency distribution and cumulative percentage curve of radiocarbon dates from high-altitude archaeological sites in China (Fig. 2d), prior to 6 kyr BP, human activity was limited, the declining trends in both PARs and CARs are mainly response to climate change (Figs. 2a–b). From 6 to 4 kyr BP, the increased human activities influenced the DBDs and CARs, but had no significant effect on PARs (Figs. 2a–d). After 4 kyr BP, the levels of human activity stabilized, and the PAR and CAR primarily responded to climate change once again, showing a downward trajectory (Figs. 2a–b). It is noteworthy that despite short-term fluctuations in CAR/DBD, both PAR and CAR/DBD show a long-term decline at high altitudes throughout the Holocene (Figs. 2a–c). Therefore, the results obtained from high-altitude peatlands enable us to assess the impact of Holocene climate change on PARs and CARs, with human activities playing a relatively minor role.
To assess the impact of climate change on PARs and CARs, a composite Holocene temperature record was constructed based on seven paleotemperature records from the Altai Mountains in Northwest China (Sahara sand peatland, SSP) and the Qinghai-Tibet Plateau (Hongyuan peatland, HY). These records are derived from seven cores with absolute dating33–39. The composite temperature record for the SSP and HY peatlands indicates an overall long-term warming trend throughout the Holocene (Fig. 2e). Furthermore, to assess changes in moisture conditions, 30 pollen records from the Qinghai-Tibet Plateau and two pollen records from the Altai Mountains were used to calculate regionally-averaged standardized moisture indices (RA-SMI) and a regionally-averaged aridity-index curve, respectively40,41. The composite record reveals a gradual drying trend in high-altitude regions (> 2400 m a.s.l.) during the Holocene (Figs. 2f–g). These temperature and moisture records provide insights into Holocene climatic changes in high-altitude areas.
The composite records of PARs and CARs in the high-altitude peatlands in China consistently demonstrate a long-term decreasing trend during the Holocene, in accord with composite temperature and moisture records that indicate a sustained warming and drying pattern. These findings support the conclusion that climatic warming and drying have negative impacts on PARs and CARs in high-altitude peatlands in China. Specifically, as temperatures rise, the drying effect likely contributes to decreased peat formation and carbon sequestration within these regions. Previous studies have also highlighted the significance of climatic warming and drying as primary factors contributing to carbon emissions from peatlands42–44. In theory, warmer conditions can simultaneously promote the growth of peatland plants (resulting in carbon sequestration and a negative feedback to the climate system), and the decomposition of peat/carbon (leading to carbon emissions and a positive feedback to the climate system)45. Our results suggest that within the relatively warm and dry climatic context of the late Holocene, the increase in microbial respiration rates outbalanced the growth in plant net primary productivity, resulting in decreased peat formation and net peat carbon sequestration in high-altitude peatlands. Essentially, under warm and dry conditions, peat decomposition is favored above the growth of peatland plants. As a result, the increased peat decomposition caused by warmer and drier conditions results in decreased PARs and CARs. Within this climatic context, peatlands function as a weak carbon sink, thus providing a positive feedback between carbon accumulation and climatic warming.
Holocene PARs and CARs at mid-altitudes
The composite Holocene PAR record for mid-altitudes in China shows an overall asymmetrical “U”-shape, with the values ranging between 0.29 and 0.43 mm yr− 1 with an average of 0.35 mm yr− 1 (Fig. 3a). Specifically, PARs show the following temporal trends: decreasing values, ranging from 0.41 to 0.29 mm yr− 1, during ~ 11–8 kyr BP; low PARs, ranging from 0.29 to 0.30 mm yr− 1, during ~ 8–5 kyr BP; and pronounced increasing values, ranging from 0.31 to 0.43 mm yr− 1, during 5–0.5 kyr BP (Fig. 3a). Whereas the composite CAR record at mid-altitudes shows a downward trend, ranging from 35.47 to 20.57 g m− 2 yr− 1, with an average of 29.50 g m− 2 yr− 1 (Fig. 3a). Specifically, the CAR record for mid-altitudes shows the following temporal trends: decreasing values, ranging from ~ 35.47 to 31.95 g m− 2 yr− 1, during ~ 11.5–9 kyr BP; increasing values, ranging from ~ 31.95 to 34.18 g m− 2 yr− 1, during ~ 9–6 kyr BP; and decreasing values, ranging from ~ 34.18 to 20.81 g m− 2 yr− 1, ~during 6–0.5 kyr BP (Fig. 3a). These trends in PAR and CAR indicate significant fluctuations in peat accumulation and carbon storage in the mid-altitude peatlands in China during the Holocene.
It is worth noting that there is a wide distribution of peat-core-based PAR records at mid-altitudes in China, including regions in subtropical southern China, northern, northeastern, and northwestern China (Fig. 1a). In contrast, CAR records are fewer and mainly concentrated in the subtropical regions of southern China (Fig. 1b). Considering the different spatial distributions of these peat cores, we paired the PAR and CAR cores from the same peatlands at mid-altitudes (Fig. 1c). The paired results of composite PARs and CARs at mid-altitudes reveal a similar trend during ~ 11.5–6 kyr BP. This demonstrates that when PARs and CARs are mainly influenced by climatic conditions, their trends are essentially consistent (Fig. 3b). However, the significantly increased human activities after ~ 6 kyr BP led to contrasting trends in PARs and CARs: PARs show an upward trend, while CARs show a downward trend (Fig. 3b). The composite record of SARs also indicates a gradual increase since 6 kyr BP21, consistent with the trend observed in PARs (Fig. 3c). Similarly, the frequency distribution and cumulative percentage curve of Holocene radiocarbon ages from archaeological sites, based on 766 radiocarbon dates from mid-altitude sites, also show an exponential increase since 6 kyr BP22 (Fig. 3d). These findings suggest that the increase in PARs in China's mid-altitude peatlands during this period can be attributed primarily to intensified human activity.
It has been proposed that increased human activities and associated vegetation destruction can lead to enhanced soil erosion in peat-covered watersheds20,21,46,47. This, in turn, could have caused the increased PARs since ~ 6 kyr BP. Reference to the composite records of total organic carbon (TOC) in peat cores from the mid-altitude peatlands (Fig. 3e) shows that since ~ 6 kyr BP, the composite TOC content first decreased gradually and then stabilized at a low level (~ 35%). This can be attributed to intensified soil erosion, which supplied more soil, with a low TOC content, to the peatlands, which was mixed with plant residues with a higher TOC content. This caused an overall decrease in the TOC content of the peat cores (Fig. 3e), leading to a decrease in CARs (Figs. 3a–b). Based on these observations, we propose that the increased PARs and decreased CARs at mid-altitudes since ~ 6 kyr BP were primarily driven by intensified human activities. These activities, including vegetation destruction and increased soil erosion, had a considerable influence on carbon accumulation in peatlands. However, further research is needed to investigate the specific mechanisms and to quantify the influence of human activities have contributed to the observed changes in PARs and CARs in these mid-altitude peatlands.
Holocene PARs and CARs at low altitudes
The composite Holocene PAR and CAR records for low-altitude areas in China are consistent, showing an overall asymmetrical “V” shape, with PARs ranging between 0.36 and 0.76 mm yr− 1, with an average of 0.47 mm yr− 1, and CARs ranging from 26.31 to 49.22 g m− 2 yr− 1, with an average of 37.87 g m− 2 yr− 1 (Fig. 4a). Specifically, the PARs and CARs show a trend of decreasing values, with PAR ranging from 0.50 to 0.37 mm yr− 1, and CAR ranging from 45.43 to 26.31 g m− 2 yr− 1, during ~ 11–7.5 kyr BP, followed by a pronounced long-term increase, with PARs ranging from ~ 0.36 to 0.76 mm yr− 1, and CARs ranging from 26.31 to 49.22 g m− 2 yr− 1, during ~ 7.5–0.5 kyr BP (Fig. 4a). These findings provide insights into changes in precipitation and carbon dynamics in low-altitude areas of China during the Holocene. The "V" shape suggests changes in the environmental conditions and processes influencing the accumulation of peat and carbon in these regions. Further analysis and interpretation of these records can help understand the underlying factors driving these changes and their implications for ecosystem dynamics and climate interactions.
The analysis of various records from low-altitude peatlands in Northeast China provides insights into the trends of PAR and CAR in this region. Holocene CARs were reconstructed based on 46 cores from the Sanjiang Plain15,16 (Fig. 4b). The frequency distribution and cumulative percentage curve of Holocene radiocarbon dates from 236 archaeological sites in Northeast China provide information on the timing and intensity of human activities in this region22 (Fig. 4c). The composite SARs record of 51 lakes and 25 peatlands provides insights into SARs, which can be influenced by soil erosion21 (Fig. 4d). Finally, the composite TOC records of 7 cores from four peatlands provide information on the temporal variations in TOC content (Fig. 4e). During ~ 11–7.5 kyr BP, PARs and CARs both show a decreasing trend in the low-altitude peatlands, which can be attributed to the strong East Asian summer monsoon, which led to the formation of shallow lakes in depressions and limited the growth of peatland vegetation16. Consequently, PARs and CARs decreased during this period (Fig. 4a). Human activities have played a significant role in influencing soil erosion during the Holocene. The radiocarbon dates of archaeological sites in Northeast China indicate an abrupt increase in human activities after ~ 7.5 kyr BP (Fig. 4c), which led to soil erosion and a subsequent rapid increase in SARs (Fig. 4d). Additionally, there was a relatively high TOC content (~ 41%) during this period (Fig. 4e). These factors together contributed to the increases in PARs and CARs in the low-altitude peatlands. The CAR results for the Sanjiang Plain in Northeast China support the CAR trends observed in the composite records (Fig. 4b). Furthermore, the difference in TOC content between the subtropical and northern peatlands further emphasizes the influence of human activities on the trends in CARs. The CARs in the mid-altitude peatlands, mainly distributed in subtropical regions, show a decreasing trend (Figs. 3a–b), while the CARs in the low-altitude peatlands in the northeast show an increasing trend (Fig. 4a), primarily due to the disparity in TOC content (Fig. 3e and Fig. 4e), which is supported by recent research results48. Overall, the analysis of these records highlights the complex interplay between natural factors, human activities, and carbon dynamics in low-altitude peatlands in Northeast China during the Holocene.
The evidence presented herein suggests that human activities have substantially affected the trends in PAR and CAR at low and mid-altitudes, whereas their effects at high altitudes were short-term, minor, and insignificant. These observations indicate that the impact of human activities on carbon accumulation varied according to the altitude and associated environmental conditions, and they highlight the spatial and temporal variations of enhanced human activities across different altitudinal zones in China. The earlier significant intensification of human activities in low-altitude areas can be attributed to factors like favorable environmental conditions, resource availability, or cultural and socioeconomic factors that were specific to those regions.
Holocene PARs and CARs throughout China
The Holocene composite PARs and CARs records from China (Fig. 5a) show similar temporal trends. PARs range from 0.37 to 0.54 mm yr− 1, with an average of 0.43 mm yr− 1. CARs range from 28.48 to 37.04 g m− 2 yr− 1, with an average of 31.82 g m− 2 yr− 1. Specifically, during ~ 11.5–8 kyr BP, both PARs and CARs show a decreasing trend. PARs range from 0.48 to 0.39 mm yr− 1, while CAR values range from 37.04 to 28.48 g m− 2 yr− 1. From 8 to 4 kyr BP, PARs and CARs show minimal changes. PARs remained at ~ 0.38 mm yr− 1, with a range of 0.37 to 0.39 mm yr− 1, while CARs remained at ~ 30.87 g m− 2 yr− 1, with a range of 28.96 to 31.98 g m− 2 yr− 1. During ~ 4–0.5 kyr BP, there was a significant increasing trend in both PARs and CARs. PARs increased from 0.41 to 0.54 mm yr− 1, while CARs increased from 30.68 to 34.62 g m− 2 yr− 1. These findings indicate significant variations in PARs and CARs during the Holocene in China, with changes in vegetation cover and other environmental factors potentially responsible.
According to the frequency distribution and cumulative percentage curve of Holocene radiocarbon dates from 4119 archaeological sites in China (Fig. 5b), indeed, during the early Holocene (~ 11.5–8 kyr BP), human activities were at a low intensity and had negligible environmental effects, indicating that climate change was the main factor driving the changes in PARs and CARs. During this period, the overall impact of climate change on PARs and CARs in China was to reduce them (Fig. 5a). Intriguingly, during the middle Holocene (~ 8–4 kyr BP), PARs and CARs remained relative stability (Fig. 5a). However, it's important to note that this period marked a transitional phase when anthropogenic influences gradually intensified on a continental scale in China (Fig. 5b). Pollen-based reconstructions of the Holocene plant cover in China indicate the beginning of human-induced land-cover changes at ~ 8 kyr BP in temperate deciduous forest49. This indicates that human activities were becoming more significant during this period, leading to changes in land use and vegetation patterns. Archaeological evidence22 and demographic studies50 also suggest the influence of human activities since ~ 8 kyr BP. Given the low-level human activities during the early Holocene of ~ 11.5–8 kyr BP and the corresponding overall negative impact of climate change on PARs and CARs in China, we therefore speculate the relatively stable PARs and CARs during the middle Holocene of ~ 8–4 kyr BP evident the balance between the negative impact of climate change and positive impact of human activities on PARs and CARs. However, it is difficult to absolutely distinguish the impacts of natural climate variations and human activities on peatland PARs and CARs during ~ 8–4 kyr BP period. The relatively stable PARs and CARs during this period emphasizes the difficulty of separating the effects of anthropogenic activities from those caused by natural climate shifts51. Early anthropogenic impacts were generally considered to be relatively weak, and the timing of deviations from the natural background variability likely varied between different regions52,53. Therefore, understanding the specific contributions of human activities versus natural climate variations during the middle Holocene requires further investigation and regional analysis.
We have provided compelling evidence to support a transition in the dominant drivers of PARs and CARs in China. After ~ 4 kyr BP, human activities became the dominant influence on PARs and CARs in China, coinciding with significant historical changes. The collapse of Neolithic cultures at ~ 4 kyr BP and the emergence of Bronze Age cultures was an important historical boundary in China54. Various indicators have been adopted to understand the impacts of historic human activities. For examples, records of total population (Fig. 5c) and the area of cultivated land (Fig. 5d) in China demonstrate an apparent increase after ~ 4 kyr BP, highlighting a notable change in land use50,55. Charcoal records from throughout China also show a sharp and sustained increase in fire frequency since ~ 4 kyr BP (Fig. 5e), implying land cover changes56. Furthermore, the declines in tree pollen percentages in northern and southern China and in the forest cover throughout China since ~ 4 kyr BP (Fig. 5f) suggest extensive land clearance for cultivation and settlement57–59. The SARs throughout China provide further evidence that enhanced human activities have caused intensified soil erosion since ~ 5 kyr BP (Fig. 5g), with a stronger upward trend in SARs observed from ~ 4 kyr BP21. Collectively, these pieces of evidence strongly demonstrate that since ~ 4 kyr BP, the primary driver of the increased PARs and CARs in China was human activities.
On the scale of the whole of China, the trends of increasing PAR and CAR since ~ 4 kyr BP can be primarily attributed to the intensification of human activities, including agriculture, land use change, and other anthropogenic factors. These findings suggest that human activities had a significant positive impact on PARs, CARs, and carbon sequestration in Chinese peatlands. As proposed in previous studies20,21, intensified human activities like land clearance for cultivation and settlement, could have led to vegetation destruction on a large scale, which in turn would have increased the susceptibility of peat-covered watersheds to soil erosion. The northern low altitudes, characterized by a higher soil TOC content, may have been especially prone to these processes. The combined impacts of vegetation destruction and increased soil erosion likely contributed to the observed rises in PARs and CARs in China since ~ 4 kyr BP. This conclusion provides a plausible explanation for the increased PARs and CARs in China since ~ 4 kyr BP, linking them to intensified human activity, vegetation destruction, and enhanced soil erosion in peat-covered watersheds.
Overall significance of the results
Our results provide valuable insights into the temporal and spatial patterns of PARs and CARs in peatlands at different altitudes in China during the Holocene. They highlight the dual roles of climate change and human activities in shaping carbon accumulation dynamics. The adverse impact of climatic warming on high-altitude peatlands underscores the vulnerability of these ecosystems to ongoing and future climate change. Moreover, the changes in PARs and CARs in peatlands at middle and low altitudes demonstrate the major influence of human activities on peat/carbon accumulation. On the scale of the whole of China, the increased PARs and CARs since ~ 4 kyr BP have several important implications, including the following. a) Human impact on peatland dynamics: The results highlight the substantial influence of human activities on peatland dynamics in China. The observed increase in PARs and CARs demonstrate the profound impact of intensified human activity, especially in terms of vegetation destruction and soil erosion. Understanding the extent and consequences of human-induced changes in peatland systems is crucial for developing effective land management and conservation strategies. b) Historical transition and cultural shift: The identification of ~ 4 kyr BP as an important historical boundary in China adds to our knowledge of cultural and societal shifts. The collapse of Neolithic cultures and the rise of Bronze Age cultures during this period coincide with the intensification of human activities and changes in PARs and CARs. These findings provide insights into the interplay between human civilization, environmental changes, and land-use practices in ancient societies. c) Implications for environmental change studies: The results contribute to our understanding of long-term environmental changes in China during the Holocene. By examining PARs and CARs as proxies for peatland productivity and carbon accumulation, researchers can reconstruct past environmental conditions and track the impacts of climate change and human activities. These findings contribute to broader discussions on global environmental change and its drivers. d) Conservation and sustainable land management: Our findings underscore the importance of conservation and sustainable land management practices for peatlands in China. Given the significant role of peatlands in carbon sequestration, water regulation, and biodiversity conservation, it is crucial to mitigate the impacts of human activities on these sensitive ecosystems. The results emphasize the need for implementing measures to protect and restore peatland areas and promote sustainable land use practices. e) Interdisciplinary research integration: The significance of the results lies in their integration of multiple scientific disciplines, including archaeology, paleoecology, and environmental science. Combining data from archaeological sites with radiocarbon, pollen, and peatland records can provide a comprehensive understanding of the interactions between human activities, environmental changes, and peatland dynamics. This interdisciplinary approach can serve as a model for future research on complex socio-environmental systems. In summary, by comprehensively assessing the interplay between climate change and human activities on peat and carbon accumulation, our study contributes to an improved understanding of carbon cycle dynamics and the evaluation of future changes in peatland carbon reservoirs in China. The findings have implications for understanding historical transitions, informing land management practices, and contributing to broader discussions on environmental change and sustainability.