Nitrogen(N) isotope ratio(δ^(15)N) of soil extractable NO_3^- plays a pivotal role in the study of N biogeochemical circulation in ecosystems. However, the NO_3^-concentration and its isotope composition of soil samples are unstable, making sample storage critical for preserving the N isotope composition of extracted soil NO_3^-. Nevertheless, studies on the appropriate selection of storage methods after soil sampling are scarce. In this study, we compared two commonly used methods for storing soil samples and investigated the stability of N isotopes of soil NO_3^-. The results demonstrated that no significant changes in the NO_3^-concentration and δ^(15)N value occurred in the samples stored at-18?C. However, the soil NO_3^-concentration markedly increased, and NO_3^-δ^(15)N value significantly changed after air-drying storage. Meanwhile, we also found that NO_3^-and its δ^(15)N were well preserved in the filtered soil extracts after 1 month. In contrast, the NO_3^-concentration gradually decreased and the^(15)N in NO_3^-was gradually enriched in the bactericidal agent-containing soil mixture solution during the storage period. Overall, our results indicated that N isotopes of NO_3^-could be effectively preserved in frozen-stored soil samples or filtered soil extracts. For field investigations conducted in remote areas and continued for a long-time period(and lacking a refrigerant supply), soil extraction/filtration using a CaSO_4-saturated solution may be a superior preparation and storage method for analyzing N isotopes of soil NO_3^-.
The large-scale syntheses of global mean temperatures in IPCC fourth report suggested that the Northern Hemisphere temperature in the second half of the 20th century was likely the highest in at least the past1,300 years and the 1990s was likely the warmest decade.However,this remains debated and the controversy is centered on whether temperatures during the recent half century were higher than those during the Medieval Climate Anomaly(MCA,AD 800–1300)and the Roman Warm Period(RWP,BC 200–AD 400),the most recent two natural warm periods of the late Holocene.Here the high resolution sea surface temperatures(SSTs)of two time windows around AD 990(±40)and AD 50(±40),which located in the MCA and RWP respectively,were reconstructed by the Sr/Ca ratio and d18O of Tradacna gigas shells from the northern South China Sea.The results suggested that the mean SSTs around AD 990(±40)and AD 50(±40)were 28.1°C and 28.7°C,0.8°C and 1.4°C higher than that during AD 1994–2005,respectively.These records,together with the tree ring,lake sediment and literature records from the eastern China and northwest China,imply that the temperatures in recent decades do not seem to exceed the natural changes in MCA,at least in eastern Asia from northwest China to northern SCS.
