Soil thawing can affect the turnover of soil carbon (C) and nitrogen (N) and their release into the atmosphere. However, little has been known about the release of C and N during the thawing of alpine soils in the Qinghai-Tibet Plateau. This study investigated the effects of soil thawing on the release of CO 2 , CH 4 , and N 2 O from alpine peatland soils and alpine meadow soils through an indoor experiment and determined the changes in the dissolved organic C (DOC), dissolved organic N (DON), NO 3 − -N, NH 4 + -N, and NO 2 − -N concentrations in the soils after soil thawing. The freezethaw treatments were performed by incubating the soil columns at mild (−5°C) and severe (−15°C) for 14 days, and then at 5°C for 18 days. The control columns were incubated at 5°C. During thawing, the cumulative CO 2 emissions from the severely frozen alpine peatland soils and alpine meadow soils were 36 and 85 % higher than those from the control soils, and the cumulative N 2 O emissions were 3.9 and 5.8 times higher than those from the control soils. However, the thawing after mild freezing produced no significant effects. The two freezing temperatures significantly increased the release of CH 4 from the alpine peatland soils, but the thawing of the severely frozen soils reduced the CH 4 uptake of the alpine meadow soils by 27 %. After the severely frozen alpine peatland soils thawed, the concentrations of DOC, DON, NO 3 − -N, NH 4 + -N, and NO 2 − -N increased significantly, but NO 2 − -N showed no significant changes for the alpine meadow soils. After thawing with mild freezing, DOC in the alpine peatland soils and NH 4 + -N, NO 2 − -N, and DOC in the alpine meadow soils showed no significant changes. This study indicates that the potential for release of C and N from alpine soils during thawing periods strongly depends on the freezing temperature and soil types.
The Qinghai-Tibet Plateau (QTP) holds massive freshwater resources and is one of the most active regions in the world with respect to the hydrological cycle. Soil moisture (SM) plays a critical role in hydrological processes and is important for plant growth and ecosystem stability. To investigate the relationship between climatic factors (air temperature and precipitation) and SM during the growing season in various climate zones on the QTP, data from three observational stations were analyzed. The results showed that the daily average (Tave) and minimum air temperatures (Tmin) significantly influenced SM levels at all depths analyzed (i.e., 10, 20, 30, 40, and 50 cm deep) at the three stations, and Tmin had a stronger effect on SM than did Tave. However, the daily maximum air temperature (Tmax) generally had little effect on SM, although it had showed some effects on SM in the middle and deeper layers at the Jiali station. Precipitation was an important factor that significantly influenced the SM at all depths at the three stations, but the influence on SM in the middle and deep layers lagged the direct effect on near-surface SM by 5–7 days. These results suggest that environment characterized by lower temperatures and higher precipitation may promote SM conservation during the growing season and in turn support ecosystem stability on the QTP.
Ensuring stable crop yield increases to meet rising demand is an important issue globally, particularly when accounting for climate change. In this study, using observations, reanalysis datasets, and the Hodrick and Prescott filter method, we find that changes in a distinct pattern of Indian Ocean–Pacific five-pole (IPFP) SST (sea surface temperature) are strongly linked to the ensuing year’s winter wheat climatic yield (the part of yield that fluctuation caused by climatic factors change) in the North China Plain (NCP), which is the main production region of winter wheat in China. Here we define a normalized IPFP index (IPFPI) and demonstrate that the autumn IPFPI (1948–2014) is well correlated with the ensuing year’s winter wheat climatic yield (1949–2015), particularly for October (r = 0.69; n = 67; P < 0.001). A composite analysis shows that the October IPFP is correlated with sowing-period and emergence-period climate factors in the NCP. When the October IPFP is in a positive phase, the atmosphere geopotential height fields and water vapor flux are bebefitial to rainfall formation in NCP, and the precipitation and soil moisture are higher in NCP and benefit winter wheat growth, thus increasing the climatic yield. In addition, accumulated rainfall and soil water content might influence winter wheat growth from sowing and emergence (autumn) to the returning green stage (following spring).
A critical shortage of spectrum resources and the underutilization of the licensed spectrum urge people to discover new approaches to efficiently utilize the spectrum resources. Opportunistic spectrum access (OSA) has been considered to be a good approach to exploit spectral resources and improve licensed spectral utilization. In this paper, based on this idea, we model and analyze one type of network (i.e., main network) overlaid by another type of network. The main network is defined as primary network while the other type of network is called secondary network. A centralized network access protocol is adopted in primary network and secondary network uses a distributed network access protocol. The system performance is investigated through extensive simulations. The obtained results show that channel utilization can be improved by about 50% compared with traditional sharing spectrum model in a single network. We also observe that there is tradeoff between the blocking probability of calls in primary network and the throughput of OSA network. Therefore, the system performance can be dynamically controlled.
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