| In recent years,series of problems,including declination of forest productivity,reduction in plant diversity and soil acidification,which caused by persistent high nitrogen deposition,has had a profound impact on the biogeochemical cycling in forest ecosystem.In this study,nitrogen and phosphorus addition experiment combined with ecological stoichiometry theory was used to reveal the relationships among C,N,P contents and their mass ratios between litterfall and mineral soil in middle-aged,mature and old-growth forest stands,and to find out the restrictive nutrient elements on the regional forest growth and the effect of exogenous nitrogen and phosphorus additions on litterfall-soil system across the age sequence.The overall objective was to understand the distribution pattern of nutrient and the intrinsic circulation mechanism in forested ecosystems.The following results were obtained.(1)From April 2016 to April 2017,the seasonal variations of the C and N contents in foliar litter were basically the same for different aged stands(mid-aged stand,mature stand,and old-growth),showing increase and then decrease with the peak value in July.The N content in woody litter and P content in foliar litter changed instability.The peak P content appeared in April and July for foliar litter and in April for woody litter.The peak N content in woody litter occurred in different months.The results from the variance analysis showed that N,P additions had significant effects on C and N content in litterfall in mid-aged stand,with significant levels in some months(p < 0.05).However,different treatments had no significant(p > 0.05)effects on litterfall C and N contents in mature stand and old-growth stand.In the mid-aged stand,the P content of litterfall under different treatments had significant difference(p < 0.05),however,the P contents of mature stand and old-growth stands was significantly different in only a few months(p < 0.05).(2)The ratios of C : N,C : P and N : P in litterfall appeared seasonal variations for different treatments and stand ages.The minimum ratios of C : N in foliar litter in different treatments for three stands were mostly observed in July.The C : N ratio only in N treatment in mature stand was the smallest in January,while the C : N ratio in branch litter had no obvious peaks.In different treatments and ages,the seasonal variations of C : P and N : P ratios in litterfall were inconsistent,with the peak values in different months.The variance analysis showed that the ratios of C : N,C : P and N : P of litterfall in mature stand had no significant difference among different treatments(p > 0.05),while were significantly different in some seasons for mid-aged stand and old-growth stands(p < 0.05).(3)The treatments had no significant effect on C and N contents of litterfall for different ages,while had an effect on P content to a certain extent,which in turns affected the C,N,P mass ratios of litterfall,especially the C: P and N : P ratios.The N treatment increased the mass ratio of C: P and N : P in litterfall,while N+P treatment showed the opposite results.It indicates that the litterfall production at our sites was probably limited by P avaibility.(4)During July 2016 and January 2017,the soil N content was lower in the growing season(July)than in the dormant period(January)except for the old-growth stand.In the annual cycle,except for mature stand,the soil C content was less in July than in January.Soil P contents in the three chronosequent stands were mostly greater in January than in July.The results from variance analysis showed that there was a significant difference(p < 0.05)in C content in surface soil among old-growth stand in January.There was no significant difference(p > 0.05)in soil C content among other stands in other seasons.Among different treatments,there were no significant difference(p > 0.05)in N contents among soil layers in three different stands.The P content in the topsoil in the mid-aged stang was significantly different(p < 0.05).The soil P in mature stand showed significant difference(p < 0.05)only in the 10-20 cm in July.And in old-growth stand,P content in 0-10 cm soil was significantly different(p < 0.05)in July among three treatments.(5)The seasonal variation of soil C : N ratios in different aged stands was mostly greater in July than in January.The topsoil C : P ratios in the three chronosequent stands showed lower in July than in January.Soil C : P ratios in mature stand changed little among treatments and seasons,with a stable value(ca 130).Soil N : P ratios had different seasonal variations in different stands,treatments and soil layers.In mid-aged stand,the topsoil N : P rato in CK was greater in July than in January.Under the three treatments,soil N : P ratios in the mature stand was higher in January than in July.The soil N : P ratios in CK of old-growth stand was greater in July than in January,whereas it was the opposite in the N and N+P treatments.The results from variance analysis showed that ratios of soil C : N,C : P,N : P in the mature stand had no significant difference(p > 0.05),while were significantly different in the mid-aged and old-growth stands in some seasons(p < 0.05).(6)The nitrogen and phosphorus additions affected the contents of C,N and P in soil and their mass ratios,and differences were found among forest ages,soil layers and seasons.The N and N+P treatment increased soil C : N ratio,and decreased soil N:P ratio,with the trend ranked as N > CK > N+P for soil C : P ratio.In the three treatments,the soil C : P ratios were higher at our sites than that of the average national value.This indicates that soil P may be deficient at our study sites,and the N+P treatment made a certain improvement effect to the soil P deficiency.(7)There existed some correlation and differences as well among contents of C,N,P and their stoichiometric ratios in foliar litter and mineral soil.In particular,the correlation between foliar litter and soil N contents was not significant.The significant negative correlation was observed between foliar litter and soil C : N ratios.There existed significant differences in C and P contents,C : P and N : P ratios between foliar litter and soil.These differences represent the competition and interaction between litter decomposition and soil nutrient supply. |
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