Effects Of Nitrogen And Phosphorus Addition On Litter Production,Nutrient Return And Stoichiometry Across A Chronosequence Of Subtropical Evergreen Broad-leaved Forests

Forest litter has important effects on nutrient balance in forest ecosystem and plays a key role in maintaining forest productivity.The continuous increase of nitrogen deposition poses a serious threat to the healthy development of terrestrial ecosystems and seriously affects the structure and function of forest ecosystems.Three subtropical evergreen broad-leaved forest stands(including Castanopsis sclerophylla middle-aged stand of 46-year-old,Castanopsis eyrei old-growth stand of about 200-year-old,Castanopsis eyrei mature stand of 78-year-old)were selected to conduct a simulated nitrogen(N)deposition experiment.The experimental design was random block with treatments including high N[HN,100 kg N/(hm2·a)],low N,[LN,50 kg N/(hm2·a)],phosphorus(P)[P,50 kg P/(hm2·a)],high N+P[HN+P,100 kg N/(hm2·a)+50 kg P/(hm2·a)]and control[CK,no N and P additions].The litterfall was collected monthly using four or five litter-traps in each sampling plot.In the laboratory,the litterfall collected was oven-dried at 600 C,and then separated as leaves,woody component(twig and bark),sexual organs(flower and fruit)and miscellaneous.The different litterfall components were ground for chemical analysis.The annual litter production and their nutrient returns were calculated-The results are as follows.1.Annual litterfall productionIn Castanopsis sclerophylla middle-aged stand,the annual litterfall productions were P(9.63 t·hm-2·a-1)>HN+P(9.50 t·hm-2·a-1)>HN(8.39 t·hm-2·a-1)>CK(7.72 t·hm-2·a-1).The HN+P and P treatments were significantly different from CK(p<0.05).In C.eyrei old-growth stand,the annual litterfall productions were HN+P(7.12 t·hm-2·a-1)>LN(7.09 t·hm-2·a-1)>HN(7.04 t·hM-2·a-1)>CK(6.70 t·hm-2·a-1),and the differences between treatments were not significant(p>0.05).In C eyrei mature stand,the annual litterfall productions were HN+P(4.96 t·hm-2·a-1)>CK(4.81 t hm-2·a-1)HN(4.49 t·hm-2·a-1),and the difference between treatments was not significant(p>0.05).The additions of N and P increased the annual litterfall productions,and the P addition could alleviate the suppression caused by high N deposition on the litterfall production.The proportions of each component of litterfall in three stands were leaf litter[50.1%-55.4%(middle-aged stand),68.1%-69.9%(old-growth),75.5%-77.5%(mature stand)]>woody component[20.8%-22.0%(middle-aged),16.6%-21.0%(old-growth),7.8%-10.5%(mature stand)].2.Monthly dynamics of litterfallThe monthly dynamics of the annual litterfall showed an fluctuation pattern with increasing and then decreasing,but the overall trend was upward.The monthly dynamics of leaf litter was consistent to the pattern of total litterfall,with fluctuation between increase and decrease was obvious.There was a certain difference in the time when the peak appeared among stands.The monthly dynamic peak of reproductive organs was obvious.3.Annual average nutrient content and nutrient return in litterfallIn CK,the nutrient contents of litterfall in C.sclerophylla middle-aged stand,C.eyrei old-growth,C.eyrei mature stand were:C(465.80 g·kg-1,486.43 g·kg-1,479.65 g·kg-1)>N(13.38 g·kg-1,13.09 g·kg-1,13.33 g·kg-1)>P(0.76 g kg-1,0.46 g·kg-1,0.40 g·kg-1).The corresponding nutrient retuens were C(3600.84 kg hm-2·a-1,3132.00 kg·hm-2·a-1,2314.92 kg·hm-2·a-1)>N(103.50 kg·hm-2·a-1,84.58 kg·hm-2·a-1,64.78 kg·hm-2·a-1)>P(6.04 kg·hm-2·a-1,2.95 kg·hm-2·a-1,1.91 kg·hm-2·a-1).4.Monthly dynamics of nutrient content and restitution of littersLitterfall production and nutrient dynamics varied irregularly.The monthly changes in nutrient returns were great.Their trends were consistent to litterfall production,with unimodal and bimodal,multimodal and irregular dynamic models.The returns of C,N and P were controlled by litterfall production.The monthly dynamics of nutrient contents and returns in litterfall tended to be consistent in different treatments.5.Nutrient stoichiometric ratio of litterfallThe ratios of C:N,N:P and C:P in leaf litter and woody component were negatively correlated with the increase of N and P contents.The N content to C:N and P content to C:P were extremely significantly correlated(p<0.01).The results from this study showed that the N and P additions had somewhat effect on the C,N and P stoichiometric ratio in litterfall.