| Forest soil is an important place of forest ecosystem carbon(C)and nitrogen(N)cycles,can sequester C and N into various pools that are mostly immobile in soil,which is considered as an effective way to the mitigation of global climate change.The accumulation of soil organic carbon and nitrogen is closely coupled and forest type conversion is an important driving factor for the change of soil organic carbon and nitrogen pool.In this study,four typical forest types under similar site conditions were investigated including mixed broad-leaved Korean pine forest(Pinus koraiensis Sieb.et Zucc.),secondary birch(Betula platyphyllla Suk.)forest,spruce-fir plantation and Dahurian larch(Larix gmelinii Rupr.)plantation in Xiaoxing'an Mountains,Northeast China.We measured the concentrations and stocks of soil organic carbon(SOC)and total nitrogen(TN)and the decomposition dynamics of organic carbon in the bulk soil and water stable aggregates in 0-5 cm and 5-15 cm soil layer to assess the effects of forest type conversion(primary forest—secondary forest and primary forest—plantations)on SOC and N pools and OC decomposition dynamics;quantified the distribution characteristics of SOC and TN in soil physical fractions(i.e.,free light fraction(FLF),occluded light fraction(OLF),sand fraction(SF),un-aggregated fraction(un-agg)and silt plus clay fraction(S&C)),and analyzed the chemical composition of SOC and TN by solid-state 13C and 15N NMR spectroscopy to explore the accumulation pathways and stability mechanism of SOC and TN and the relationships between C and N in the stable soil C pool,which can provide reference for the accurate evaluation of temperate forest soil C source/sink function and Csequestration potential.The main results were as follows:(1)The concentrations and stocks of SOC and TN in the bulk soil and water stable aggregates in 0-15 cm soil layer were significantly affected by forest type conversion(P<0.05).Compared with the SOC and TN stocks of mixed broadleaved-Korean pine forest,those of spruce-fir plantation increased by 34.0%and 18.6%,those of secondary birch forest increased by 14.9%and 33.7%,and those of Dahurian larch plantation decreased by 9.6%and 14.0%.The contents and stocks of SOC and TN decreased with the size of water stable aggregates.The C/N ratios of mixed broadleaved-Korean pine forest and secondary birch forest were significantly lower than those of spruce-fir plantation and Dahurian larch plantation.Correlation analysis showed that litter and fine root traits(eg N content)were important factors affecting the changes of soil organic carbon and nitrogen stocks.(2)During the 195 days of incubation,the SOC mineralization rates of four forest types were higher at the beginning of the incubation period but decreased progressively over time.The cumulative C mineralization in the bulk soil and water stable aggregate fractions decreased in the following order:spruce-fir plantation>secondary birch forest>mixed broadleaved-Korean pine forest>Dahurian larch plantation.When normalized to the initial amount of SOC,the cumulative C mineralization proportions were increased in the order of secondary birch forest<mixed broadleaved-Korean pine forest<Dahurian larch plantation<spruce-fir plantation.In a word,mixed broadleaved-Korean pine forest conversion significantly altered the decomposition dynamics and stability of soil organic carbon.Although the decomposition of SOC increased,the stability of SOC increased when primary forest conversion to secondary birch forest,while the stability of SOC decreased when primary forest conversion to plantations.(3)Although the SOC and TN concentrations in the FLF and OLF were very high,their contributions to the bulk soil SOC and TN were quite small.Conversely,the S&C had relatively low SOC and TN concentrations but accounted for a large proportion of the bulk soil SOC and TN(about 71%of the SOC and 80%of the TN),suggesting SOC and TN were mainly sequestered in the soil silt and clay.In forest soils 95-98%of the total nitrogen is organic nitrogen,and the significant positive relationships exhibited between SOC and TN concentrations for the bulk soil and physical fractions(P<0.01),suggesting a co-accumulation relationship between SOC and TN in these forest soils.The determination coefficients of TN to SOC in the S&C(R2=0.93)were significantly higher than those in the FLF and OLF(R2=0.70 and 0.41),indicating that SOC accumulation in the S&C was more dependent on TN accumulation than that in the light fractions.The SOC and TN have two accumulation pathways:aggregate physical protection and associated with soil clay and silt minerals,in which the latter plays a leading role.The distribution and accumulation of SOC and TN were not affected by mixed broadleaved-Korean pine forest conversion.(4)The solid-state 13C NMR spectra of the bulk soil and physical fractions under four forests showed that the O-alkyl C was the highest(36-47%),followed by aromatic C(18%-37%)and alkyl C(16-26%),and carbonyl C(11-15%)was least.Compared to those of the FLF,the alkyl C,aromatic C and alkyl/O-alkyl C ratios of OLF increased at the expense of a decrease in O-alkyl C,indicating that OLF was in an advanced stage of decomposition.The alkyl/O-alkyl C ratio and alkyl/carboxyl C ratio of S&C were larger than those of the bulk soil and other physical fractions,indicating that the organic matter in S&C was the most stable short chain organic matter.Mixed broadleaved-Korean pine forest conversion significantly changed the OC chemical composition of bulk soil and physical fractions(P<0.05),but had little effect on the alkyl/O-alkyl C and alkyl/carboxyl C ratios(P>0.05).(5)Mixed broadleaved-Korean pine forest conversion significantly changed the concentrations of mineral associated SOC and TN(P<0.05),but had little effect on C/N ratios(P>0.05).The chemical structures of SOC and N are mainly O-alkyl C and amide N,respectively.Peptides contributed 57-82%of the signal intensity of carboxyl C and,48-67%of the signal intensity of alkyl C.Mixed broadleaved-Korean pine forest has little effect on the chemical composition of soil mineral associated OC,but significantly changed the chemical composition of TN in mineral associated carbon pool.The OC chemical compositions of soil mineral associated carbon pool were significantly correlated with the SOC concentration(P<0.01),indicating that the higher the concentration of SOC,the higher the content of each chemical structure of organic carbon,and the higher the signal intensity in solid-state 13C NMR spectrum.In forest surface soil,more than 95%of the nitrogen is organic nitrogen,and the N chemical compositions were significantly correlated with TN content(P<0.01),which indicated that organic nitrogen determined the chemical composition of mineral associated organic carbon.(6)As a conclusion,mixed broadleaved-Korean pine forest conversion in Xiaoxing'an Mountains significantly changed the SOC and N pool,and primary mixed broadleaved-Korean pine forest conversion into secondary birch forest significantly increased the SOC and TN stocks,while conversion into spruce fir plantation increased the SOC stock while decreased the TN stocks,and conversion into larch plantation decreased the SOC and TN stocks.Mixed broadleaved-Korean pine forest conversion significantly changed SOC chemical composition,but did not affect the accumulation pathway and stability mechanism of SOC and TN.SOC and TN were co-accumulated in the soil,which had two stable mechanisms:aggregate physical protection and associated with soil clay and silt minerals.The litter and fine roots traits(N content)and the carbon-nirogen coupling mechanisms in soil stable organic matter formation were important driving mechanisms underlying the changes of soil organic carbon and nitrogen pools after mixed broadleaved-Korean pine forest conversion.Therefore,the results of this study can provide an important basis for temperate forest management and soil carbon sequestration potential assessment from the perspective of ecosystem carbon sequestration. |
PDF Full Text Request