Temperature sensitivity of soil organic carbon (SOC) decomposition is important in understanding terrestrial C dynamics under warming. Variations in temperature sensitivity of the decomposition of plant litter across different species (inter-species) with contrasting litter quality are well understood. However, the effect of changed litter quality of a species (intra-species) grown under different CO₂ concentration ([CO₂]) and/or air temperature (T_air) on temperature sensitivity (Q₁₀) of the decomposition of litters in soils has not been explored. In this study, temperature sensitivity of decomposition of litters of needle-leaved pine (Pinus densiflora) and broadleaf oak (Quercus variabilis) leaves as well as rice (Oryza sativa L.) root and shoot that were produced under different [CO₂] and T_air conditions was investigated in incubation experiments under four temperature (5, 15, 25, and 35°C) levels. Regardless of [CO₂] levels, both tree and rice litters, which had decreased lignin/N via increased N concentrations, that were produced under elevated T_air showed a lower Q₁₀ compared to those under ambient T_air. These results are in agreement with the “litter quality-temperature theory”; i.e., high-quality litters have low Q₁₀ and vice versa. Therefore, it is expected that global warming may decrease the temperature sensitivity of plant litters decomposition due to decreased lignin/N. However, it still doubts that lower temperature sensitivity of decomposition of litter grown under warming may translate into increased SOC content as CO₂ emissions from the soils amended with high-quality litters was greater than those with low-quality litters at a given T_air. Nevertheless, this study provides a novel insight into the changes in temperature sensitivity of decomposition of plant litters under climate change in relation with changed litter chemistry (Funding sources: BK21 project and NRF).

Soils contain nearly 3 times the amount of carbon (C) as the atmosphere and about 2 times as terrestrial plant biomass and the ratios of soil organic C (SOC) and total nitrogen (TN) in rice paddies are stable. Increasing soil organic matter (both SOC and TN) not only helps to mitigate climate change, but also improves soil N fertility for crop growth. Some results showed organic farming enhanced top soil carbon stocks under in uplands, but the report from submerged rice paddies was a few. For understanding whether and how long natural rice farming affected SOC stocks and N fertilities in submerged rice paddies, we investigated an organic rice farmer fields in Japanese Andosols, located in Tochigi Prefecture Japan. The fields were divided into 4 treatments based on the years after organic farming, which are conventional (0 years), organic 4~5 years, 8~9 years and 12 years. The soil samples were divided to 2 layers of 0-15 cm and 15-20 cm depth. Anaerobic incubation was performed for 2, 4, 6 and 8 weeks and 30 ^oC to measure decomposed C (CO₂ and CH₄ productions) and mineralized N. The results showed that SOC and TN on topsoil increased 13.8% and 15.6% respectively, after 12 years of application organic farming. The decomposed C and mineralized N on topsoil significantly increased by 23.9% and 24.7% after 8~9 years organic farming. In contrast, on subsoil there is no significant difference between conventional and organic paddy fields. The percentages of decomposed C to SOC and mineralized N to TN were increased with the organic farming years. These results indicated that organic rice farming long-term organic rice farming enhanced soil carbon sequestration and nitrogen fertility in Japanese Andosols.

The Qinghai-Tibetan Plateau (QTP) is the highest plateau on Earth and has a large area of alpine marshland and wet meadows. Artificial drainage, overgrazing and climate change have caused severe desiccation and degradation of the alpine wetlands. However, little is known about the effects of wetland degradation on soil organic carbon (SOC) stock, and studies only focused on the Zoige marshland. Direct SOC observations from the extensively distributed wet meadows remain scarce. In this study, SOC in the soil surface layer (0–50 cm), were investigated at four wetland sites where degradation has continued for decades. One site is in marshland, and three are in wet meadows of the QTP, which improved the representativeness of available observations across the entire QTP. The results showed that initially, marshland degradation to wet meadows prompted the accumulation of SOC; however, grazing in wet meadows reduced SOC accumulation. Wetland degradation to dried meadows led to a much greater SOC loss than that in the initial degradation stage, and grazing exacerbated the loss of SOC. An exponential decay rate of SOC was found in the grazed dried meadows. Using datasets from the literature and the field measurements of the present study, we estimated the loss of alpine wetland SOC. The wetlands of the QTP, i.e., the marshland and wet meadows, have lost 140.52±25.10 Tg in 1966–2016, representing 14.7% of the SOC stock. The loss of SOC is the dominant indicator of severe degradation of vegetated wetlands within the plateau, although there are reports showing the extension of open water as a result of glacial melting. The present results made with direct observations, though limited compared to the vast area of the QTP, warn of the deteriorating situation of the wetlands, especially the extensively distributed wet meadows.

There was lack of message about how changes in winter snow can affect soil respiration and its compositions under temperate forest stands in northeastern China. In this study, a mature broadleaf and Korean pine mixed forest (BKPF) and an adjacent white birch forest (WBF) in northeastern China were selected to study the effects of removal of winter snow on soil respiration, heterotrophic and autotrophic respiration during non-growth and growth seasons in the years from 2018 to 2020. Soil total respiration and heterotrophic respiration were measured using a portable greenhouse gas analyzer together with soil cores inserting the soil at different soil depths. Soil autotrophic respiration was calculated by using differences between soil total respiration and heterotrophic respiration. Soil total respiration, heterotrophic and autotrophic respiration under the BKPF stand ranged from 0.05 to 4.98, from 0.04 to 3.55 and from 0.01 to 1.37 CO₂-C μmol/m²/s, respectively, and those under the WBF stand ranged from 0.05 to 6.10, from 0.04 to 4.09 and from 0.01 to 2.01 CO₂-C μmol/m²/s, respectively. The changes in soil respiration and its compositions upon the removal of winter snow varied with forest stands, seasons, and years. Compared with control plots, the removal of winter snow appeared to increase the temperature sensitivity of soil autotrophic respiration under the WBF stand, which was different from that under the BKPF stand. The results can improve the understanding of responses of soil respiration and its compositions to decreasing winter snow depth under temperate forest stands in northeastern China. Acknowledgement: This work was funded by the National Natural Science Foundation of China (41775163 and 41975121).

Paddy soils are widely considered as the main sources of methane (CH₄) and nitrous oxide (N₂O)_. However, it hasn’t been fully understood the combined effects of water management and fertilizer application on CH₄ and N₂O emissions in double-rice system in tropical region, China. Therefore, an in-situ experiment with two water regimes (conventional irrigation (D), continuous flooding (F)) and four fertilizer applications (PK, NPK, NPK+M, and M) were conducted in double-rice field, Hainan Island. The results showed that 1) CH₄ emissions in early rice (10.3-78.9 kg·hm^-2) were significantly lower than those in late rice (84.6-185.5 kg·hm^-2). Irrigation and fertilization had extremely significant effects on CH₄ emission in early rice season. CH₄ emissions under D condition was significantly lower than that in F regime in both seasons. 2) N₂O emissions across all treatments were 0.18-0.76 kg·hm^-2 in early rice season and 0.15-0.58 kg·hm^-2 in late rice season, respectively. Compared with F-PK, F-NPK significantly increased the cumulative N₂O emission. However, compared with D-PK, D-NPK and D-M treatments significantly increased the cumulative N₂O emissions. Irrigation and fertilization had significant impacts on N₂O emission in both seasons. 3) The yields of early and late rice were 7310.7-9402.4 and 3902.8-7354.6 kg·hm^-2, respectively. N fertilizer treatments under the same irrigation condition significantly increased late rice yield. The GWP and GHGI in early rice season were 580.8-2818.5 kg·hm^-2 and 0.08-0.30 kg·kg^-1, which were significantly lower than those in late rice season (3091.6-6334.2 and 0.50-1.23 kg·kg^-1). F-NPK+M and F-M treatments significantly promoted GWP and GHGI. Irrigation significantly affected GWP and GHGI in both rice seasons. Given rice yield, GWP and GHGI, D-NPK+M can be recommended as the best model for local water and fertilizer management to reduce greenhouse gas emissions and maintain rice yield.

Based on time-series data of MODIS-NDVI from 2000 to 2017, we extracted four remote sensing phenological parameters of vegetation, including the Start of Season (SOS), the End of Season (EOS), the Middle of Season (MOS) and the Length of Season (LOS), in Tundra-taiga transitional zone in the Eastern Siberian, using Asymmetric-Gaussian function and dynamic threshold method. Meanwhile, we analyzed the responses of the four phenological parameters to the temperature change, combined with the temperature change data from Climate Research Unit (CRU). The results show that: in the south of 64 °N, along with temperature risen in April and May, the SOS in the corresponding area was 5-15 days ahead of schedule; in the area between 64 °N and 72 °N, along with temperature risen in May and June, the SOS in the corresponding area was 10-25 days ahead of schedule; in the northernmost of the study area where is on the coast of the Arctic Ocean, along with temperature dropped in May and June, the SOS in the corresponding area was 15-25 days behind of schedule; in the northwest of the study area in August and the Southwest in September, along with temperature dropped, the EOS in the corresponding area was 15-30 days ahead of schedule; in the south of 67 °N, along with temperature risen in September and October, the EOS in the corresponding area was 5-30 days behead of schedule; the change of the EOS in autumn was more sensitive to the change of the SOS in spring, because the smaller temperature fluctuation can cause the larger change of the EOS; the growth season of vegetation in this area was generally moving forward, and the LOS in the northwest was shortened, while the LOS in the middle and south of this area is prolonged.

Foxtail (Alopecurus aequalis) and milk vetch (Astragalus sinicus) are the species of winter weeds those grow in the organic rice fields during the fallow season. These winter weeds are incorporated into soil during ploughing before rice planting as manure. Considering the amount of weeds biomass, the carbon (C) decomposition and nitrogen (N) mineralization potential from winter fallow weeds need to be understood in the organic rice farming fields. Firstly, to determine the role of foxtail and milk vetch on organic rice farming, 1% pulverized of young and old age foxtail (YF and OF), and milk vetch (YM and OM) were mixed with the soil samples taken from 15 years of organic farming field. As the results, more than half of C from weeds decomposed during 4 weeks anaerobic incubation. In addition, the C decomposition potential significantly higher in YF and YM than OF and OM. Percentage of mineralized N from weeds from milk vetch were higher than foxtail. Lower CN ratio in milk vetch contributed to 8.4 times higher net N mineralization rate than foxtail. Secondly, we examined the effect of foxtail addition among different durations of organic farms. It showed that the longer duration of organic farming tended to increase total N mineralization from the soil and foxtail. However, the net N mineralization from foxtail only did not show significant differences among different durations of organic farms. It should be to note that N immobilization was found in second week incubation, while it was changed to mineralization after four weeks incubation. Therefore, as winter fallow weed, foxtail incorporation could play an important role as N nutrient for organic rice farming. However, four weeks duration between weeds ploughing and rice transplanting is recommended.

Green manure use in rice production may influence methane (CH₄) or nitrous oxide (N₂O) emission. Results from our previous pot experiments showed that the utilization of Azolla as a biofertilizer for rice cultivation has the potential to increase rice yield and significantly reduce CH₄ and/or N₂O emissions. To confirm these findings under field conditions, we set up a lowland rice field experiment in Tsuruoka, Yamagata, Japan. Azolla was either grown as a dual crop (herein Cover) in the standing water or incorporated as green manure plus dual cropping (herein AGM + Cover) at the beginning of the experiment along with the rice crop. Compared with the control (chemical fertilizer; herein NPK), NPK + Cover did not influence the seasonal cumulative CH₄ emission but significantly increased N₂O emission by 252.4%. AGM + Cover significantly effected high CH₄ emission by 21.7% (NPK), 30.3% (NPK + Cover), and N₂O emission by 472.0% (NPK), with no significant differences compared with NPK + Cover. No significant grain yield differences were observed between NPK and NPK + Cover treatments, but AGM + Cover significantly decreased yield by 39.7% (NPK) and 38.0% (NPK + Cover). Unlike in the pot experiments, Azolla as green manure significantly decreased rice biomass and yield, and significantly increased the cumulative CH₄ and N₂O emissions. This was largely ascribed to its faster decomposition in the early rice growth stages. Also, dead masses of Azolla cover were conspicuously observed in the NPK + Cover and AGM + Cover treatments throughout the middle rice growth stages due to the hot summer conditions during the rice-growing period. The results indicated that the application of Azolla as green manure in this field study has significant effects on the increase of CH₄ and N₂O emission, which was not consistent with our previous pot findings.

Future climatic change is likely to increase the occurrence of soil freeze-thaw (FT) events in high latitude and/or high altitude zones. Soil respiration in those cold regions tends to be more sensitive to temperature change than that in a warm climate, but it is not clear how the temperature sensitivity of soil respiration responds to changing FT patterns under different soil moisture conditions in the northern temperate forest stand. In this study, soils were sampled from a mature broadleaf and Korean pine mixed forest and an adjacent secondary white birch forest in northeastern China. By using packed soil-core incubation experiments, the forest soils with two soil moisture levels (40% and 80% water-filled pore space) were subjected to -2, -8, and -18°C freezing treatments for 60 days, and then respectively incubated at 2, 4, 8, 12 and 16°C for 8 days. Forest soils with the two soil moisture levels incubated at 5°C for 68 d were served as control. We quantified heterotrophic respiration, microbial biomass, inorganic nitrogen, and water-extractable organic matter in soils at the beginning and end of the incubation experiments. With decreasing freezing temperature, heterotrophic respiration during thaw increased for soils with the low moisture level but changed little for those with high moisture. Heterotrophic respiration increased with increasing thaw temperature for both forests soil with all the freezing temperature and soil moisture treatments. Comparing with thaw temperature ranging from 2°C to 16°C, the temperature sensitivity of heterotrophic respiration was significantly higher with a narrow range of thaw temperature (2-4°C), especially for soils with high moisture. The redundancy analysis results showed that the variations in temperature sensitivity of heterotrophic respiration were attributed to changes in the content and bio-availability of dissolved organic matters in soils.