Mechanisms Of Soil Organic Carbon Stability And Its Response To Nitrogen Deposition In Subtropical Evergreen Broad-leaved Forest Of Different Ages

Global and regional climate change caused by global warming and doubled CO₂concentration poses a severe challenge to the global ecosystem.Land soil is the largest carbon pool on the earth's surface.Soil carbon pool plays an important role in the global carbon cycle and carbon balance.Small changes in soil carbon pool may cause huge fluctuations in atmospheric CO₂ concentration.Different studies have often ignored the effects of soil physical and chemical,nutrient and site differences brought by forest age on the response of soil carbon pool to nitrogen accumulation.The response of soil carbon sequestration to nitrogen deposition is still unknown.In this paper,the change of soil carbon sequestration in evergreen broad-leaved forest and its response to nitrogen deposition were studied.Main conclusions:1.With the increase of forest age,soil bulk density,sand,silt,and water contents increased significantly,and p H decreased significantly(P<0.05).Soil nutrients(NH₄⁺-N,NO₃^--N,TP)showed a"concave"distribution.N treatment resulted in soil acidification.The available nitrogen in soil mainly existed in the form of NO₃^--N.NH₄⁺-N increased in mid-aged and mature forests while decreased in the oldgrowth forest under N+P treatment.NO₃^--N increased significantly in the mid-aged forest(P<0.05).TP in mid-age forest was increased,but was decreased in mature and oldgrowth forests.TP of different stands was increased in N+P treatment,and there was significant difference in oldgrowth forest(P<0.05).2.Soil organic carbon(SOC)was significantly affected by forest age(P<0.05).The SOC content was significantly higher in mid-aged forest than in mature and oldgrowth forests(P<0.05).The soil carbon density in oldgrowth forest(2.74 kg·m^-2)was higher than that in mature(1.61 kg·m^-2)and mid-aged forests(2.42 kg·m^-2).N+P treatment only increased SOC in oldgrowth forest.The soil C/N ratio decreased significantly with the increase of forest age.The soil C/N in mid-age forest decreased,while that of mature and oldgrowth forests increased.Compared with the mid-age forest,the soil carbon of mature and oldgrowth forest is more stable under nitrogen deposition.3.DOC content of different stands increased with the increase of stand age,and the distribution was"convex",ranging from 170.15 to 333.22 mg·kg^-1.The contents of DOC in mature and oldgrowth forests were significantly higher than that in mid-age forest(P<0.05).The content of DON increased significantly with the increase of forest age(P<0.05).N treatment increased DOC and DON in different forests,and the oldgrowth forest reached a significant level(P<0.05).DOC/SOC in mature(0.97%)and oldgrowth forests(0.73%)was significantly higher than that in mid-aged forest(0.31%).N and N+P treatments increased DOC/SOC of mid-aged and mature forests,but had no significant effect on oldgrowth forest.Soil MBC and MBN increased significantly with the increase of forest age(P<0.05),among which the oldgrowth forest was significantly higher than the mid-aged forest(P<0.05).N treatment increased MBC of different stands,but decreased MBN.4.The content of ROC in different stands was between 6.12?7.22 g·kg^-1,which was"concave"distribution with the increase of stand age.ROC/SOC showed as mature forest(17.49%)>oldgrowth forest(14.86%)>mid-aged forest(12.62%),which indicated that the change of soil carbon activity with forest age resulted in the lower stability of soil carbon components in mature forest.The ROC contents with high,medium and low activities in different CK stands were 1.02?1.42 g·kg^-1,2.82?3.82 g·kg^-1 and 1.86?2.74g·kg^-1,respectively.The highly active ROC increased significantly with the increase of forest age,and the oldgrowth forest was significantly higher than the other two stands(P<0.05);the middle living ROC was the highest in the mid-age forest,followed by the aged forest,while the lowly active ROC decreased with the increase of forest age.N treatment increased the ROC content of different stands.5.Soil organic carbon was mainly composed of the heavy component(HC).With the increase of forest age,the light component(LC),the LC/HC ratio and the carbon and nitrogen content in LC showed a"convex"distribution.The HC content decreased with forest age.The nitrogen content of HC was significantly lower in mature forest than in mid-aged and oldgrowth forests(P<0.05).N treatment increased the content of carbon and nitrogen in LC in different stands,and reached a significant level in the oldgrowth forest(P<0.05).The HC decreased in different stands.N and N+P treatments increased the proportion of LC in different stands,in which the oldgrowth stand reached a significant level(P<0.05).The treatment of N and N+P decreased the C/N ratio of LC in different stands,and the difference between different treatments and different stands was not significant(P>0.05).The C/N of HC in mid-aged and mature forests decreased,but increased in oldgrowth forest.6.The soil is mainly composed of macroaggregates,with the content between 78%and 84%.With the increase of forest age,the macroaggregate increased(P>0.05),and the microaggregate in mature and oldgrowth forests was significantly lower than that in mid-aged forest(P<0.05).MWD and MGD increased with forest age(P<0.05).In CK stand,the carbon contents of macroaggregate and microaggregate were the highest in the mid-aged forest,which presented a"concave"distribution with the forest age.According to the SOC distribution in different particle sizes,N treatment was beneficial for the physical protection of SOC in the mid-aged forest,but decreased the SOC stability in the mature and the old-growth forests as a whole.The C/N of macroaggregate and microaggregate in different stands decreased with forest age.Nitrogen deposition increased the decomposibility of aggregate organic matter in mature and oldgrowth forests,while fresh organic matter with low C/N was enriched in aggregate in mid-aged forest.7.With the increase of forest age,soil?¹³C increased.N treatment increased the?¹³C abundance in mid-aged and mature forests,and decreased the?¹³C abundance in oldgrowth forest.The distribution of?¹⁵N abundance in different forests was contrary to?¹³C,and decreased significantly with forest age.N and N+P treatments increased the?¹⁵N abundance of mid-aged and oldgrowth forests,and decreased the?¹⁵N abundance in mature forest.The?¹³C abundances in different forests were ranked as total soil>macroaggregate>microaggregate;the?¹³C abundances in macroaggregate and silty were the highest in mature forest and the lowest in oldgrowth forest;the microaggregate was the highest in mature forest and the lowest in mid-aged forest.N and N+P treatments increased the?¹³C in macroaggregates in oldgrowth and mature forests,decreased the?¹³C in macroaggregates in mid-aged forests,increased the?¹³C abundance of microaggregates in mature forests,decreased the?¹³C abundance in fine clays in mid-aged and oldgrowth forests,and increased the?¹³C abundance of fine clay in mature forest.N treatment increased the?¹⁵N in macroaggregate in mid-aged forest,decreased the?¹⁵N in macroaggregate in mature forest and oldgrowth forest,and increased the?¹⁵N in microaggregate in different aged forest.8.The bacterial abundance was 46 a>200 a>78 a,but the bacterial?diversity of different forests was on the rise.After N treatment,the soil bacterial abundance of 46 year old stands decreased,78 year old stands increased,and 200 year old stands did not change significantly.At the same time,the diversity of soil bacteria and dominant phylum increased with forest age.There was no significant difference in soil fungal richness among different forests under N treatment,and the response of fungal community diversity to nutrient addition was different among different forests.Compared with bacteria,fungi had more niche advantage.The distribution of fungi/bacteria is different in different forests.Mature forest was higher than other forest stands,and the difference between mature and oldgrowth forests was smaller.Among different stands,the gene abundance of lignin decomposition was the highest,followed by hemicellulose and starch.The distribution of C-decomposing gene abundance and SOC in different forests were the same,which was shown as mid-aged forest>oldgrowth forest>mature forest,in which mature forest was significantly smaller than mid-age and oldgrowth forests(P<0.05),while the C-fixing gene abundance was the highest in mature forest,and mid-aged and mature forests were significantly higher than oldgrowth forest(P<0.05).The changes of C-decomposing gene abundance and bacterial abundance were the same in different forests.The soil microbial community in oldgrowth forest under N treatment was stable without significant change,but the abundance of carbon decomposition gene was significantly reduced(P<0.05).The long-term N deposition in subtropical evergreen broad-leaved forest of different forest age series may induce the increase of its active carbon component and decrease the stability of soil carbon;soil nutrient characteristics dominate the distribution of SOC among different forests,while soil physical and chemical characteristics affect the difference of SOC response to the outside world.Based on the changes of soil aggregates and carbon-cycle genes in different stands,the changes of carbon stability in different stands may be different in the future.