The Mineralization Of Soil C And N And It’s Response To N Deposition In The Temperate Forests Of Northern China

Carbon and nitrogen mineralization are the centre of the biogeochemical cycle of carbon and nitrogen,which maintain the structure and function of ecosystem.However,increased atmospheric temperature and nitrogen deposition will affect these processes of terrestrial ecosystem,in turn the affected carbon and nitrogen mineralization will also give feedback to the global climate change.Forest is the largest carbon and nitrogen pool in terrestrial ecosystem,thus it is necessary to study the carbon and nitrogen mineralization respond to climate changes.In our research,we select the typical forests in northern China,through simulating nitrogen deposition in the field,we study the different response of soil carbon mineralization of different vegetation types to the nitrogen addition in a natural condition.At the same time,carbon and nitrogen cycles are closely coupled;we also simulate the increased atmospheric temperature in laboratory in order to exclude influences from other factors.We expect to study the mineralization of soil carbon and nitrogen,their temperature sensitivity and the intrinsic coupled relationship in response to changes in vegetation types.(1)We conducted the simulation of increasing atmospheric nitrogen(N)deposition in three typical temperate forests in Dongling mountain,which were oak forest(broadleaf forest),larch forest and pine forest(coniferous forest),and were dominant forests in North China.In each forest,we set control and N addition treatments(10 g N·m–2·a–1,five times of current atmospheric N deposition)and four replicates(15 m × 15 m).The main objectives of this study were to investigate the short-term effect of N addition on soil respiration of different temperate forests and to explore the underlying mechanisms.N addition(NH4NO3)and soil respiration measurement were conducted in growing season.The results showed that N addition decreased soil respiration rate of broadleaf forest but enhanced that in coniferous forest.Under N enriched conditions,the accumulation emission of soil respiration in oak forest reduced 6.13 g C·m–2 during the growing season while increased 1.78 g C·m–2 and 5.64 g C·m–2 in larch and pine forest,respectively.However,the short-term effect of N addition was not significant among three temperate forests.Moreover,soil respiration rate(P < 0.001)and the accumulation emission of soil respiration in growing season(P < 0.001)varied significantly among different forests,with the similar trends of oak forest > pine forest > larch forest(or broadleaf forest > coniferous forest).Soil temperature is the key factor regulating soil respiration in control and N addition treatment,which resulted in a apparently seasonal dynamic of soil respiration.Thetwo-factor model,involving soil temperature and moisture,can well fit the variation of soil respiration in response to the temperature and moisture in the three temperate forests,and can explain the 47%-87% variation of soil respiration.Furthermore,N addition can regulated to some extent the response of soil respiration to soil temperature and moisture,and soil respiration would increased faster and be more sensitive in relative higher and lower moisture under N addition.These findings explored the short-term effects of N addition on soil respiration in different temperate forest in North China,its underlying mechanisms however is still unclear.(2)Decomposition of soil organic matter(SOM)is sensitive to vegetation and climate change.Here,we investigated the influence of changes in forest types on the mineralization of soil carbon(C)and nitrogen(N),and their temperature sensitivity and coupling relationships by using a laboratory soil incubation experiments with 4temperatures(10,15,20 and 25 ℃).We sampled soils from four forest types,namely,a primary Quercus liaotungensis forest(QL),Larix principis-rupprechtii plantation(LP),Pinus tabulaeformis plantation(PT),and secondary shrub forest(SS)in temperate northern China.The results showed that soil C(F = 710.83,p < 0.0001)and N(F = 30.922,p <0.0001)mineralization differed significantly among forest types.Cumulative soil C mineralization was highest in SS(264.675),followed(in decreasing order)by LP(166.449),QL(159.217),and PT(151.079 ug C·g-1).Cumulative soil N mineralization was highest in LP(40.712),followed(in decreasing order)by SS(29.726),QL(28.504),and PT(19.540 ug N·g-1).Soil C and N mineralization were closely coupled in all plots(p <0.0001),The C:N ratios of decomposed SOM were 3.67,3.29,4.12,and 2.54 in the QL,LP,TP,and SS soils,respectively.The C:N ratios of SOM decomposition showed a significant negative linear correlation with soil pH(R2 = 0.99,p = 0.0043).Vegetation type significantly influenced the Q10 values of soil C and N mineralization(p < 0.0001).The activation energy(Ea)of soil C and N mineralization was negatively related to the SOM quality index in all forest types.The reverse relationships suggested that the carbon quality-temperature(CQT)hypothesis was simultaneously applicable to soil C and N mineralization.Our findings show that the coupled relationships of soil C and N mineralization can be affected by vegetation change.