Some Environmental Chemistry And Molecular Ecology Characteristics As Affected By Long-term Green Manures In A Red Paddy Soil In Southern China

Long-term application of green manure has been proved to be one of the most important methods toachieve stable and high yields of rice, the long-term effects and mechanism of green manure are worthfurther studying. Based on a long-term rice-rice-winter green manure cropping experiment which wasestablished in1982in a red paddy soil in south China, soils were sampled at different typical stages,i.e., flowering stage of winter green manure, before transplanting of early rice, tillering stage of earlyrice, jointing stage of early rice, mature stage of early rice and after the harvest of late rice. Using thetechnologies of UV-Vis adsorption spectroscopy, FT-IR adsorption spectroscopy, real-time qRCR and454pyrosequencing, etc., some soil fertility characteristics, microbial biomass properties, DOMparameters were studied, the contents of HCl-extractable total Fe (TFeHCl), ferrous iron [Fe(II)HCl] andferric iron [Fe(III)HCl] were measured, the abundances of the functional genes of nitrification(arch-amoA) and denitrification (narG and nosZ) were tested, and the effects of green manure onmicrobial communities were studied as well. The treatments included rice-rice-milk vetch (RRV),rice-rice-winter rape (RRP) rice-rice-ryegrass and rice-rice-winter fallow (RRF). The main resultswere as follows:After turned over of the green manures, the contents of soil available nutritions were improvedrapidly during decomposition. The incorporation of ryegrass had the most obvious effects on theincrease of mineral nitrogen, but after the harvest of late rice, the contents of inorganic nitrogen in thetreatments with green manures were lower than that in RRF treatment. The application of wintergreen manure decreased the contents of soil available P, there were differences between differentgreen manures, the contents of soil available P in RRG were the highest among treatments with greenmanures. The incorporation of winter rape and ryegrass could increase the contents of soil available K,after the harvest of late rice, there were no differences among different treatments, and the contents ofsoil available K in RRF treatment were a little higher than that in the3green manure treatments. Thecontents of HCl-extractable Si in all the treatments showed the trends of increasing gradually duringthe growth period of early rice but decreasing to the lowest after the harvest of late rice. Theapplication of winter green manures decreased the contents of available Si in paddy soils and nosignificant differences were found among different green manures. The contents of soil microbialbiomass carbon (SMBC), soil microbial biomass nitrogen (SMBN) and soil microbial quotient had thesame trends, i.e., they decreased smoothly during the growth period of early rice and then rose torelatively high values after the harvest of late rice. Green manures increased the contents of SMBC,SMBN and soil microbial quotient compared with RRF treatment, the leguminous green manure milkvetch had the most remarkable effects. RRG treatment increased the values of SMBC to SMBN ratioduring the growth period of early rice significantly.The incorporation of green manure could increase the contents of soil DOM, the RRV and RRPtreatments increased the contents of DOC and DON. Long-term application of green manures could increase the contents of easily degradable organic carbon, and could enhance its’ proportion in soilDOC. The RRP treatment at the stage of before transplanting of early rice and the RRV treatment atthe stage of the mature stage of early rice had the highest SUVA254and SUVA280values, the lowestE2/E3values, indicating that winter green manures could increase the humification degree, thecontents of aromatic compounds and the molecular weight of DOM. The RRV treatment decreasedthe value of reduction potential, suggesting that the RRG treatment increased the redox ability and thebio-availability of DOM. Different treatments had little different effects on DOM components, butchanged the contents of each component.Long-term application of green manure had some effects on the ability of ferric iron reduction, anddifferences were found among treatments. TFeHCldecreased significantly after the introduction ofgreen manures. The Fe(II)HClincreased rapidly after the incorporation of green manure and keptincreasing during the growth period of early rice then turned out to be the lowest after the harvest oflate rice. During the sampling stage, Fe(II)HClin the treatment with ryegrass increased rapidly at theearly stages and were higher than others while increased slowly at the late stages and became thelowest one, at the mature stage of early rice the Fe(II)HClwas lowest in winter fallow. Fe(III)HClandFe(II)/Fe(III)HClhad the opposite and same trends with Fe(II)HCl, respectively. The maximum Fe(II)HClaccumulation capacity in RRF was significant higher than that in green manure treatments, but themaximum reaction rate (Vmax) of Fe(II)HClaccumulation was found in RRG treatment. Significantcorrelations were detected between Fe(II)HClaccumulation and soil pH, Eh and TOAs, respectively.The abundances of narG, nosZ and arch-amoA had no significant differences in different treatments,they decreased at the mature stage of early rice compared with the stages of before and afterincorporation of green manure, and the abundances of narG and nosZ decreased significantly. Highsignificant positive correlation relationships were found among the three genes, also the significantand even high significant correlation relationships between the3genes and the contents of Fe(II)HCl,SMBC and SMBN were found, this might provide a theoretical guide for the further study of therelationship between nitrogen transformation and Fe(III) reduction in paddy soil.Long-term application of green manure had some effects on the characteristics of microbialcommunity and diversity. The Chao1richness and non-parametric Shannon’s index increased in allthe soils with green manure treatments. Community structures with green manure applications weresignificantly dissimilar with that under the winter fallow. By a Metastats test, numerous genera inRRG, RRP and RRV soils were significantly different from that in the RRF soil, including severalfunctional genera. Analyses of this and the other functional genera revealed the shifts in communitiesfunctions after long-term green manuring. Increasing ammonium supply and decreasing soilacidification were concluded to be implemented by the microbial communities in the greenmanure-treated soils. Together these data pointed towards the great effects on the microbialcommunities and on the biogeochemical cycle driven by the functional microbes.