Effect Of Litter On Grassland Primary Productivity Under The Background Of Nitrogen Deposition

Litter plays an important role in grassland, influencing ecosystem structure, processand functioning. Litter accumulation can change the nutrient availability and modifylocal physical environmental conditions, e.g. quantity and quality of light conditions,soil temperature and water dynamics. Together with such environment change,phytotoxins produced by litter can affect seed germination and seedling establishment,and regulate interspecies interactions and community structures. Consequently, plantproductivity and diversity may respond to the change of litter levels. There has beensome study that investigates effects of litter on plant production in grassland. However,nitrogen deposition is rarely considered.In this study, we explored the interactive impacts of nitrogen and litter on grasslandproduction, with a factorially designed (split-plot) field experiment in meadow steppesof the Songnen Plains, Northeast China. The study was performed in three years (2009-2011). There were two factors, nitrogen (no nitrogen, nitrogen added), and litter (nolitter, mean amount of litter, double the mean amount of litter). For the three years, weexamined influence of nitrogen and litter on plant production in August, litter productionin October, and belowground root biomass. In2011we investigated the response ofproduction of functional group. We address three major hypotheses:(1) nitrogenaddition increased plant production,(2) effects of litter vary with time, and (3) therewere interactive effects between nitrogen and litter. The results and conclusions arepresent as following:(1) Nitrogen addition increased plant production and litter production significantly.Response of plant production was mainly contributed by change in grass production,whereas legume or forb did not respond to fertilization. Nitrogen did not affectbelowground production, although it tended to increase root biomass in treatments withintact and double litter in2009while in2010and2011there was a slight increase inroot production in the treatments with no or intact litter.(2) Without nitrogen, litter had no significant impacts on plant or litter production. In2011, legume production tended to be higher in treatments with litter than that in thetreatment without litter and forb production was highest in the plot with intact litter. In2009, root production in the plot without litter was much higher than that in other littertreatments. With nitrogen addition, plant production decreased with increasing litterlevels in2009, and in2010and2011, plant production in the treatment with intact litter was higher than that in other treatments. The change in plant production was mainlycontrolled by response in grass production, and production of legume and forb increasedwith litter levels. There was no significant difference in litter production among littertreatments. In2009, there was no difference in root production among litter treatmentwith nitrogen addition, while in2010and2011, root production decreased withincreasing litter levels.(3) The different response of plant production to litter, without versus with nitrogenaddition, suggested the interactive effects of nitrogen and litter on plant growth.Especially in2009, the first year of this study, with nitrogen addition, plant productiondeclined with litter levels, while without nitrogen, there was no difference in plantproduction among litter treatments. However, in2010and2011, the positive effect oflitter on plant production emerged in intact litter treatment. The temporal change of littereffects suggested the physical and chemical impacts of litter on plant growth. In the firstyear, litter had not decomposed and there was only physical environmental response tolitter treatment, mainly the decrease in light environment, which was negative to plantgrowth. In the following years, change in soil chemical conditions emerged asconsequent of litter decomposition, which was positive to plant growth and improveplant production.Results of this study showed that nitrogen addition increased plant production inthe middle of growing season and litter production in the end of growing season.Without fertilization, plant production did not differ among litter treatments. Withnitrogen, plant production decreased with litter levels in the first year, and the positiveeffects of litter emerged since the second year, which was the compromise of positiveand negative effects of litter on plant growth. Grass response to litter attributed to thechange in plant production, while legume and forb showed different responses to litterfrom grass. Effects of litter treatment on plant production disappeared in the end ofgrowing season, and belowground production varied differentially from abovegroundproduction. Decrease in light condition due to light interception caused by litter andstronger light competition among plant individuals caused by nitrogen addition was themechanism behind the interactive effects of litter and nitrogen. This study improved ourknowledge about the role of litter in grassland ecosystem under the background ofnitrogen deposition, and provide guide to scientific grassland management.