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Calcium (Ca) and phosphorus (P) are essential nutrients for plant growth, with consequences on ruminant diet and health. Their balance in forage, as indicated by Ca:P ratio, is important for the absorption and utilisation of both elements by livestock. While nitrogen (N) deposition has been globally observed, it is declining or is projected to decline due to controls on N emissions in many countries. It is an open question whether N inputs would have legacy effects on forage Ca and P nutrition in grasslands. Here, we examine the changes of Ca and P concentrations and Ca:P ratio in plants and soils in a temperate steppe for three continuous years (2016 ~ 2018) following historically 6 years N inputs (2008 ~ 2013) with a gradient ranging from 0 to 20 g N m−2 year−1. Soil extractable Ca concentrations significantly decreased with increasing N addition rates in all the 3 years, but soil available P concentrations increased and Ca:P ratio decreased only in 2016. Ca and P nutrition varied greatly amongst the four plant functional types, but they were generally conservative across the historical N addition rates, due to the high stoichiometric homeostasis. Furthermore, plant Ca and P concentrations and Ca:P ratios at the community level showed no significant variation along the N addition gradient. Our results highlighted the role of plant Ca and P homeostasis in ensuring stable Ca and P nutrition in forage following historical N inputs in temperate grasslands. [ABSTRACT FROM AUTHOR]

Although numerous Paleo-Tethyan ophiolites with mid-oceanic ridge basalts (MORB) and/or oceanic-island basalt (OIB) affinities have been reported in the central Tibetan Plateau (CTP), the origin and tectonic nature of these ophiolites are not well understood. The petrogenesis, mantle sources and geodynamic setting of the mafic rocks from these ophiolites are unclear, which is the main reason for this uncertainty. In this paper, we present new geochronological, mineralogical and Sr-Nd isotopic data for the Chayong and Xiewu mafic complexes in the western Garzê-Litang suture zone (GLS), a typical Paleo-Tethyan suture crossing the CTP. Zircon LA-ICP-MS U–Pb ages of 234 ± 3 Ma and 236 ± 2 Ma can be interpreted as formation times of the Chayong and Xiewu mafic complexes, respectively. The basalts and gabbros of the Chayong complex exhibit enriched MORB (E-MORB) compositional affinities except for a weak depletion of Nb, Ta and Ti relative to the primitive mantle, whereas the basalts and gabbros of the Xiewu complex display distinct E-MORB and OIB affinities. The geochemical features suggest a probable fractionation of olivine ± clinopyroxene ± plagioclase as well as insignificant crustal contamination. The geochemical and Sr-Nd isotopic data reveal that the Chayong mafic rocks may have been derived from depleted MORB-type mantle metasomatized by crustal components and Xiewu mafic rocks from enriched lithospheric mantle metasomatized by OIB-like components. The ratios of Zn/Fe t , La/Yb and Sm/Yb indicate that these mafic melts were produced by the partial melting of garnet + minor spinel-bearing peridotite or spinel ± minor garnet-bearing peridotite. We propose that back-arc basin spreading associated with OIB/seamount recycling had occurred in the western GLS at least since the Middle Triassic times, and the decompression melting of the depleted MORB-type asthenosphere mantle and partial melting of sub-continental lithosphere were metasomatized by plume-related melts, such as OIB s, which led to the generation of the Chayong and Xiewu mafic melts. [ABSTRACT FROM AUTHOR]

An integrated study of the geochronology, mineralogy, geochemistry and Sr–Nd isotope compositions of the upper Permian Yushu mafic rocks in the central Tibetan Plateau (CTP) was conducted to evaluate the interaction between the Emeishan mantle plume and the Paleo-Tethyan subduction system. These mafic rocks can be geochemically subdivided into three groups. Group 1 rocks yielded a weighted mean 206 Pb/ 238 U age of 258 ± 2 Ma. They have relatively high TiO 2 , FeO t , Nb/Y and high ε Nd (t) values and display oceanic island basalts (OIB)-like rare earth and trace element patterns, with obvious enrichment of Nb and Ta. Group 2 and 3 rocks yield weighted mean 206 Pb/ 238 U ages of 258 ± 1 Ma and 257 ± 1 Ma, respectively. Both have relatively low TiO 2 , FeO t , Nb/Y and ε Nd (t) values and island arc tholeiites (IAT)-like rare earth and trace element patterns with obvious depletion of Nb and Ta. However, the Group 2 rocks have relatively low FeO t /MgO ratios, but high MgO, Mg # , Cr and Ni contents, resembling primitive magmas. They have lower light rare earth element (LREE), Nb, Ti and Zr contents and higher ε Nd (t) values than the Group 1 rocks. The geochemical and isotopic variations suggest that the Group 1 rocks might be derived from a plume-related mantle source, whereas Group 2 and Group 3 rocks originated from subduction-modified asthenospheric mantle and lithospheric mantle. Moderate degrees of olivine and clinopyroxene fractionation under low oxygen fugacities ( f O 2 ) appear to be responsible for the Fe-Ti enrichment in the Group 1 rocks. We propose a model involving plume-subduction interaction to explain the geodynamics and generation of the late Permian mafic magmatism in the CTP. The study region was rifted by the Emeishan mantle plume activity in association with rollback of Longmuco-Shuanghu oceanic lithosphere in late Permian times. Deep melting of the plume source led to the generation of the Group 1 plume-related magmas, whereas shallower melting of the subduction-metasomatized asthenospheric and lithospheric mantle resulted in the generation of the Group 2 and Group 3 subduction-related magmas. [ABSTRACT FROM AUTHOR]