Effects Of Nitrogen Addition And Mowing On Key Processes Of Belowground Carbon Cycling In Natural Grassland Ecosystem On The Loess Plateau

Grassland is an important component of terrestrial ecosystems,and plays a key role in global carbon cycling.Under the context of global change,grassland ecosystem is being exposed more and more human disturbance.The Loess Plateau is characterized by an arid and semiarid climate,with an increasing atmosphere N deposition in this area.In addition,clipping or mowing for hay,as a prevalent land-use practice,is considered to be an important component of global change.Whether this grassland ecosystem is still be a carbon sink under the further N deposition and clipping practice is dependent on the rate of carbon input and release.Therefore,understanding the underground carbon cycling leading by fine root dynamics is essential for accurate estimation of the soil carbon budget and forecast the carbon distribution under N deposition and clipping practice in this grassland ecosystem.In this paper,we selected a long-term fenced grassland in Yunwu Mountain National Grassland Nature Reserve as research objects,and combined the field investigation and laboratory analysis to investigate the effect of N addition and mowing on several key processes of underground carbon cycling,including the soil carbon input processes,such as the fine root production,mortality,turnover and decomposition,as well as the soil carbon release processes,for example,soil respiration,microbial respiration and root respiration.The study explicated the magnitude and directions of response of these underground carbon processes to N addition and clipping,analysis the leading factors that affect these process,and lastly to discover the inherent mechanism.The main conclusions were as follows:?1?Root production and mortality showed obvious seasonal dynamic in all soil plots.In control plots,root production and mortality in 2014 primarily occurred in June,however,in 2015 the root production and mortality mainly occurred both in June and September.Correlation analysis showed that the variation of root production for both years were significantly and positively related to soil moisture,but had no relationship with soil temperature.The root mortality in 2014 was significantly and positively related to soil temperature,but had no significant effect on soil moisture,however,the root mortality in2015 exhibited opposite.The bivariate correlation analysis showed soil temperature and moisture could explain the 52.4%and 82.2%variation of root production,and 24.6%and53.0%variation of root mortality.?2?N addition had not changed the seasonal variation of root production and mortality,but the root production and mortality in N addition plots showed lower values compared to control.N addition significantly reduced both years annual root production,as well as the root mortality in 2015,however,had no significant effect on root mortality in 2014.The effect of clipping on root production and mortality was different with time.Clipping sharply reduced root production and increased root mortality immediately after treatment,however,with the plant regrowth,root production increased and mortality decreased to minimize the gaps with control.Clipping significantly reduced both years root production and significantly increased root mortality in 2014,but had no significant effect on root mortality in 2015.?3?The root production and mortality both decreased with soil depth.In control plots,root production in 2014 and 2015 respectively accounted for 73.8%and 79.0%in 0-20 cm of soil layer,and root mortality accounted for 73.1%and 74.2%.N addition mainly decreased the root production and mortality in 0-20 cm of soil layer,but had little effect in2014,as well as positive effect in 2015 in 30-50 cm of soil layer.Clipping effect on root production and mortality both mainly occurred in shallow soil layers,but the root production and mortality in deeper soil layers response showed little,or showed opposite tread.?4?We calculated root turnover by averaged fine root production and mortality verse the annual root biomass.The results showed the fine root turnover rates in 2014 and 2015were 1.30 a^-11 and 0.97 a^-1,respectively,and the root turnover rates in different treatments varied between 0.86-1.90 a^-1.N addition decreased root turnover rates for both years,but had not reached significant level.Clipping increased both years fine root turnover rates,and the effect reached significant level in 2014.The response of root turnover rates to N addition and clipping mainly occurred in shallow soil layers.?5?Fine root decomposition process of Stipa grandis,Stipa przewalskyi and Artemisia sacrorum could be divided into rapid?0-60 d?and slow?60-719 d?decomposition stages.Compared within species,the fine root of S.grandis decomposed fastest and A.sacrorum decomposed lowest.Correlation analyses showed the root mass remaining rate was positively related with N and K content,as well as N/P ratio,and negatively correlated with the C content and C/N ratio.N addition significantly reduced fine root decomposition constant of three species,however clipping significantly increased them,and significant interaction effect of N addition and clipping was found in fine root decomposition.?6?With fine root decomposition,the transfer models for C and K of the three species fine roots were direct releasing,and the transfer models for N showed an enriching model,and P exhibited enriching-releasing model,with the trend of a decrease generally.N addition suppressed the fine roots C release of three species,as well as the fine root K release of S.przewalskyi and A.sacrorum,but had no significant effect on the fine root N and P release of three species,as well as fine root K release of S.grandis.Clipping promoted fine root C,P and K release of three species,but had no significant effect on the fine root N release of three species.?7?Soil respiration and root respiration showed obvious seasonal dynamic in all soil plots.In 2014,the peak values of soil and root respiration occurred in June,however,the soil and root respiration exhibited two obvious maximum values respectively in June and September.Soil microbial respiration had no obvious seasonal variation.The root respiration ratio in 2014 and 2015 were respectively 54.1%?41.3%in control plots.N addition had not changed the root respiration ratio for both years.Clipping significantly reduced root respiration ratio by 9.6%in 2014,but had no effect in 2015.?8?N addition had not changed the seasonal variation of soil respiration and its components.The effects of clipping on soil and root respiration were different with time.Clipping sharply decreased soil and root respiration in short term after treatment,but with plant regrowth,the soil and root respiration increased to recover.N addition and clipping,as well as their interactive effect had significant effect on soil and root respiration.N addition alone decreased soil and root respiration significantly,but N addition increased soil and root respiration significantly under clipping plots.Clipping alone decreased soil and root respiration,but week the reduction in N addition soil plots in 2014,increase the soil and root respiration in 2015 in N addition soil plots.The seasonal soil microbial respiration were not altered by N addition and clipping treatments.N addition significantly reduced microbial respiration in 2015,but had no effect in 2014.Clipping significantly decreased microbial respiration for both years.?9?Soil temperature and soil moisture could explain 43.1-66.0%variation of soil respiration,42.2-81.5%variation of microbial respiration,and only 14.1-22.8%variation of root respiration.Both N addition and clipping decreased the correlation of soil temperature and moisture with soil respiration and its components.The temperature coefficients?Q10?of root respiration were obvious higher than that of microbial respiration almost in all soil plots.N addition decreased the Q10 values of root and microbial respiration,with the reduction more pronounced in root respiration.Following N addition,the decreased rates of root respiration were significantly and positively related to the monthly root production.The decreased rates of microbial respiration were significantly and positively related to the soil microbial biomass C.Overall,N addition reduced the fine root production,reduced the root litter input into soil,inhibited the soil C losses from microbial respiration.Clipping reduced the fine root production,but promoted the root litter input into soil,inhibited the soil C losses from microbial respiration.