Soil carbon sequestration under long-term rice-based cropping systems of purple soil in Southwest China

Carbon sequestration in agricultural soils is a complex process controlled by farming practices, climate and some other environment factors. Since purple soils are unique in China and used as the main cropland in Sichuan Basin of China, it is of great importance to study and understand the impacts of different fertilizer amendments on soil organic carbon（SOC） changes with time. A research was carried out to investigate the relationship between soil carbon sequestration and organic carbon input as affected by different fertilizer treatments at two long-term rice-based cropping system experiments set up in early 1980 s. Each experiment consisted of six identical treatments, including（1） no fertilizer（CK）,（2） nitrogen and phosphorus fertilizers（NP）,（3） nitrogen, phosphorus and potassium fertilizers（NPK）,（4） fresh pig manure（M）,（5） nitrogen and phosphorus fertilizers plus manure（MNP）, and（6） nitrogen, phosphorus and potassium fertilizers plus manure（MNPK）. The results showed that annual harvestable carbon biomass was the highest in the treatment of MNPK, followed by MNP and NPK, then M and NP, and the lowest in CK. Most of fertilizer treatments resulted in a significant gain in SOC ranging from 6.48 to 2 9.13% compared with the CK, and raised soil carbon sequestration rate to 0.10–0.53 t ha–1 yr-1. Especially, addition of manure on the basis of mineral fertilizers was very conducive to SOC maintenance in this soil. SOC content and soil carbon sequestration rate under balanced fertilizer treatments（NPK and MNPK） in the calcareous purple soil（Suining） were higher than that in the acid purple soil（Leshan）. But carbon conversion rate at Leshan was 11.00%, almost 1.5 times of that（7.80%） at Suining. Significant linear correlations between soil carbon sequestration and carbon input were observed at both sites, signifying that the purple soil was not carbon-saturated and still had considerable potential to se questrate more carbon.

Formation of high-silica adakites and their relationship with slab breakoff:Implications for generating fertile Cu-Au-Mo porphyry systems

In recent years,the characteristics and sources of fertile adakites has received considerable attention.As well,most recently the geodynamic environment of convergent margins subducting oceanic crust aiding arc formation,evolving to slab rollback,then slab break-off after collision(i.e.late-to post-collisional slab failure(arc-like magmatism)and transpression)has gained more recognition,although their relationship to each other has yet to be explored.The geochemical characteristics imply that adakites/adakite-like,in particular high-silica adakites(HSA),can form by partial melting of subducting hydrothermally altered oceanic crust in convergent plate boundary settings during the terminal stages of subduction,lithosphere thickening,and then failure(all late to post collisional),while the melting of the mantle wedge during subduction-related dehydration creates more typical calc-alkaline basalt-andesite-dacite-rhyolite series(ADR)to form intraoceanic island arc to intracontinental margin arc systems,before the collisional stage.HSAs are characterized by high-silica(SiO2>67 wt.%),Al2O3>15 wt.%,Sr>300 ppm,Y<20 ppm,Yb<1.8 ppm,and Nb≤10 ppm,and MgO<3 wt.%,with high Sr/Y(>50),and La/Yb(>10).Some specific geochemical features,such as high Mg#(ave 0.51),Ni(ave 924 ppm),and Cr(ave 36 ppm),in HSAs are typical,in contrast to calc-alkaline arcs,although both groups display similar but less pronounced negative anomalies of Nb,Ta,and Ti in primitive mantle-normalized trace element spider diagram profiles.These unique geochemical features are likely ascribed to the involvement of garnet,hornblende,and titanite either during partial melting of hydrous MORB-like oceanic crust with only minor assimilation and fractional crystallization(AFC)within the mantle and crustal during ascent in a transpressional collisional environment.Hypotheses for origin of HSA derivative from melting in convergent margins from young,hot oceanic plates subducting into the mantle is applicable to only some adakitic systems.The difference in geochemical characteristics of adakites compared to ADR,such as relative higher MgO,Cr,Cu,and Ni,are due to their slab source,as well as interaction of the slab-derived adakitic melts with overlying hot lithospheric mantle;altered oceanic slabs are also relatively rich in siderophile and other chalcophile elements,as well as sulfates and sulfides.HSA magmas related to slab failure have special geochemical properties,such as Sr/Y>20,Nb/Y>0.4,Ta/Yb>0.3,La/Yb>10,Gd/Yb>2,and Sm/Yb>2.5.Slightly higher Nb+Ta is due to high T melting of rutile.Varieties of Nb/Ta compared to silica are also significant in HSA as a result of slab failure(roll back to break-off).High T-P partial melting of the hydrothermally altered oceanic slab produces HSA with quite high activities of H2O,SO2,HCl,with chalcophile metals that remain incompatible at higher fO2(low fH2);these situations happen in late-to post-collisional settings where the subducting oceanic crust experienced slab failure,resulting in advective heat addition to the system from upwelling asthenosphere.In such a slab failure setting,transpression and transtension play a significant role in the rapid emplacement of a high amount of fertile adakitic magmas through the subduction-modified lithosphere and crust into the upper crust.When oxidized slab melts interact with the subduction-modified lithospheric mantle,the resulting magmas stay oxidized,potentially contributing to the special conditions conducive to formation of porphyry Cu-Au mineralization.