Effect Of Invasive Plants (Conyza Sumatrenss And Alternanthera Philoxeroides) On Soil Carbon And Nitrogen Processes

Alien species invasion has become one of the three global environmental problems due to its tremendous effects on community structure and ecosystem function. Recently, many studies have devoted to documenting the impact of exotic plant invasions on ecosystem processes with the deeply research of biological invasion. To examine the effects of alien plants invasion on soil properties and soil processes, we collected rhizosphere soil samples under invasive plants, conyza sumatrensis (Cs) and alternanthera philoxeroides (Ap), and the neighboring indigenous species, Digitaria sanguinalis (Ds) in a deserted paddy field. The research includes the following two elements:(1) The shifts in soil properties and soil C, N process, as well as the dynamics of plant available N supply and soil CO2, N2O emission rate in growing season under exotic plant invasions. (2) In non-growing season, rates of nutrients leaching from plant residue and the C, N process as well as its induced accumulation of soil nutrients, relative changes in CO2, N2O emission rate driven by soil biological characteristic were investigated. On the above understanding, we studied the feedbacks between invasive species and soil, in addition, the effect of invasion on greenhouse gas emission. The results are as follows:1. The effects of the conyza sumatrensis (Cs) and alternanthera philoxeroides (Ap) invasion on soil properties and C, N process in the growing seasonThe results showed that alien invasive plants relative to neighboring indigenous plants increased soil organic carbon and nitrogen contents. Compared with the Ds, the soil organic carbon content, total nitrogen, nitrate and ammonium contents were increased by 106%,63%,97% and 71% for the invasive Cs, and 27%,97%,94% and 243% for the invasive Ap, respectively. Soil microbial carbon and nitrogen biomass were increased by 123% and 225% for the Cs, and 225% and 339% for the Ap, respectively. Under similar laboratory incubation conditions, soil mineralization and denitrification rates were increased by 210% and 100%, or 380% and 80% due to the Cs or Ap invasion relative to the Ds, respectively. In contrast with the Ds, soil CO2 emission and N2O emission rates were increased for the Cs and Ap under aerobic conditions, respectively. Similarly, the Cs and Ap invasion significantly increased rhizosphere soil enzyme activity and nitrification rates.2. Variations in soil properties and soil C, N process under the effect of Conyza sumatrensis (Cs) and Alternanthera philoxeroides (Ap) in non-growing seasonShifts in soil properties and C, N process in non-growing season are in consistent with the trend in growing season. Compared with the Digitaria sanguinalis (Ds), the soil organic carbon content, total nitrogen, nitrate and ammonium contents were increased by 134%,87%,49% and 20% for the invasive Cs, and 57%,115%,71% and 89% for the invasive Ap, respectively. Soil microbial carbon and nitrogen biomass were increased by 48% and 62% for the Cs, and 128% and 166% for the Ap, respectively. Soil mineralization and nitrification rates were increased by 190% and 300%, or 120% and 150% due to the Cs or Ap invasion relative to the Ds, respectively. Similarly, the Cs and Ap invasion significantly increased rhizosphere soil enzyme activity and denitrification rates. Under aerobic conditions, soil collected under invasive species Cs and Ap both expressed significant higher CO2, N2O emission rate in comparison with soil collected under indigenous species Ds. The soil enzyme activity and microbial biomass as well as the microbial communities that meet the requirement of litter decomposition may be the reasons that contribute the most to the accelerated decomposition velocity. The consequent result would be the fast nutrient leaching rate from plant residue, which provides enough soil nutrients for the forthcoming growing season. The accumulated emission of CO2, N2O could be another indication of the facilitated soil C and N process. The sufficient soil nutrients supply in growing season induced by the accelerated litter decomposition and nutrients leaching from plant residue in non-growing season give an impetus to the fast spread of the invasive plants.Such being the case, a positive feedback come into being between the invasive species and soil, while the induced elevated emission amount of CO2, N2O will facilitate the green house effect substantially.