The Effects Of Warming And Nitrogen Deposition On Litter Decomposition And Flammability In Stipa Breviflora Desert Steppe

Litter is not only an important component of ecosystem decomposition but also the basic material that could cause grassland fire.Here we investigated the effects of long-term warming and nitrogen（N）addition on litter decomposition and flammability for the dominant species,Stipa breviflora,in a desert steppe.We conducted a field litter bag experiment to investigate the temporal dynamics of the litter mass remaining and the stoichiometry of leaf litter carbon（C）,N,phosphorus（P）and trace elements（Fe,Ca,Mn,Na,Zn and Cu）.We also measured litter flammability in the study area.Our study will provide scientific support for grassland fire prevention and sustainable management.The main results are as follows:（1）The decomposition process of Stipa breviflora litter includes three phases:rapid decomposition stage（0～371 d）,slow decomposition rate stage（371～738 d）and accelerated decomposition rate stage（738～939 d）.In the first stage of decomposition,on the 183rd day,compared to the control,warming and N addition significantly promoted litter decomposition with significant interactions（P<0.05）.In the second stage of decomposition,on the 559th day,N addition significantly decreased litter decomposition（P<0.05）,but the warming effect was not significant.On the 371st day,the 738th day and the 939th day,warming and N addition did not influence litter decomposition.In the whole decomposition process,litter mass remaining was 24.3%in the control.Warming,N addition and combined warming and N addition reduced litter mass remaining by 3.7%,2.2%and 2.1%,respectively.（2）In a 3-year decomposition experiment,litter N concentration initially increased,sequently decreased and then increased with time.N addition resulted in a three-phase pattern of litter N,including increases,decreases and increases（P<0.05）,but these effects did not occur under warming.In contrast with the N dynamics,litter C and P concentrations initially decreased and then increased with time in the control,whereas litter C showed initial increase and sequent decrease with warming and N addition（P<0.01）.Litter mass remaining was positively correlated with litter C,P and the C/N ratio（P<0.001）,but it was negatively associated with litter N concentration（P<0.001）.（3）The litter trace element concentration pattern was Fe(1.89mg·g^-1)>Ca(0.81mg·g^-1)>Mn(0.36mg·g^-1)>Na(0.27mg·g^-1)and Zn(0.27mg·g^-1)>Cu(0.02mg·g^-1).N addition caused decreases,increases and decreases in Fe concentration with time（P<0.0001）,while warming first increased and then decreased Fe concentration（P<0.01）.Under N addition,Mn concentration first decreased and then increased with time（P<0.01）.N addition significantly reduced Zn concentration（P<0.0001）,and the interaction between warming and N on Zn concentration was significant（P<0.0001）.N addition resulted in decreases and increases in Cu concentration with time（P<0.05）.Litter Ca and Na concentrations did not respond to warming and N.Litter mass remaining was significantly negatively correlated with Cu concentration.No correlation was found between litter mass remaining and the concentrations Fe,Ca,Mn,Na and Zn.（4）In the present study,litter includes standing litter and falling litter.Standing litter was the dead but not fallen part of plants,which were mainly concentrated in the upper layer with the height of approximately 20 cm.Falling litter consisted of fallen dead plant tissues,which accumulated in the lower layer with the thickness of 2 cm.The results showed that the loading capacity,water content,crude fat,crude protein concentration and calorific value of standing litter were significantly higher than those of falling litter（P<0.05）,but the crude ash concentration of standing litter was significantly lower than that of falling litter(P<0.05.The flammability of standing litter was higher than that of falling litter.The flammability of litter was further found to increase from the lower layer to the upper layer.Compared to other regions in Inner Mongolia,the litter in Siziwang desert grassland had lower crude fat concentration and calorific value,higher crude ash concentration,smaller surface litter accumulation and weaker flammability.These findinngs revealed that litter decomposition in grassland ecosystem was affected by litter quality（C,N,P and trace elements）and environmental factors such as temperature and nitrogen supply.Increased temperature and nitrogen deposition enhanced litter decomposition by improving litter quality.Our results also suggested that the flammability of standing litter（upper layer）was higher than that of falling litter（lower layer）in Siziwang desert grassland.The flammability of surface litter in this area is weak.Litter,as the main component of combustibles in grassland ecosystems,is the material condition of grassland fire.The present study will provide a scientific basis for grassland fire prevention and management.