Impact Of Natural Forest Converted To Plantation On Soil C,N And ¹³C,¹⁵N Isotope Abundance Along North-south Transect In Eastern China

Soil is the major carbon?C?sink of global terrestrial ecosystems.Soil C sequestration is closely linked to the soil nitrogen?N?because it regulates soil C cycles in forests.A better understanding of soil C and N balance in forests and how on a wider regional or global conversion of natural forests to plantations does change soil C and N cycles could be the key bottleneck in order to improve the prediction and feedbacks of soil C and N mitigation related to the changes of forests conversion.Although forest soils store a large portion of terrestrial C,there are ample evidences showing it may release large CO₂ when they undergo land use change?LUC?.The conversion from natural forests?NF?to intensively managed plantations?PF?is known to strongly affect soil aggregation and aggregate stabilizing C and N pools.However,there is still some uncertain of the positive,negative or negligible effects on soil C and N stocks after LUC.Thus,it is necessary to better understand the mechanisms and control factors controlling the changes of soil aggregate proportion,soil C and N concentrations,and natural abundance of ¹³C and ¹⁵N after LUC particularly in soil aggregates.In this study,soil samples were collected from natural forests and paired plantations at six sites:Mohe?MH?,Qingyuan?QY?,Xinyang?XY?,Huitong?HT?,Dinghushan?DH?,Jianfengling?JF?along an approximately 4000 km North-South Transect of Eastern China.We measured soil C and N concentration,isotope abundances of ¹³C and ¹⁵N in bulk soils and seven soil aggregate size fractions.Our objectives were to:i)Assess the changes in soil carbon and nitrogen stocks after conversion of natural forest to plantations in Eastern China;ii)Study how land use change from natural forest to plantation could impact on the levels of C,N and natural abundance of ¹³C and ¹⁵N along a climate gradient;iii)Assess stable isotopic signatures of carbon and nitrogen in soil aggregates following conversion of natural forests to managed plantations in Eastern China.?1?Natural forests?NF?have a better capacity to accumulate soil C and N.The soil C and N decreased from the southeast to the northwest and increased from tropical to cold-temperate zone in the eastern part of the study area.The C and N stock mainly occurred in the topsoil and decrease significantly with depth layer.In addition,soil C and N stocks increased as plantation age increased.?2?Forest floor had higher C contents and lower N contents in PF compared to NF,resulting in high forest floor C/N ratios and a decrease in the quality of organic materials in forest floor under plantations.In general,soil C,N contents and their isotope changed significantly in the forest floor and mineral soil after land use change?LUC?.Soil?¹³C was significantly enriched in forest floor after LUC while both?¹³C and?¹⁵N values were enriched in mineral soils.Linear and non-linear regressions were observed for MAP and MAT in soil C/N ratios and soil?¹³C,in their changes with NF conversion to PF while soil?¹⁵N values were positively correlated with MAT.?3?Conversion of NF to PF generally lead to a decline of soil macro-aggregate mass,but its effects on soil aggregate C and N concentrations varied among sites.Soil aggregate?¹³C and?¹⁵N were enriched after NF was converted to PF,although isotope abundances of ¹³C and ¹⁵N uniformly distributed among soil aggregate fractions.Topsoil C and N were more sensitive than subsoil and thus more influenced by LUC while soil?¹³C and?¹⁵N strongly increased with soil depth.Mean annual temperature?MAT?and precipitation?MAP?were important regulators of soil aggregate?¹³C and?¹⁵N in NF,with higher values associated with increasing MAT and declining MAP.In summary,this manuscript has highlighted the impact of land use change from natural forest to plantation forest on soil carbon and nitrogen dynamics including SOC stock,soil aggregation,stable isotope.Specifically,this study indicated that,although mean annual temperature?MAT?and mean annual precipitation?MAP?are the key factors affecting the variations in soil C and N,their vertical and horizontal distribution differed in various aspects.LUC alters soil and aggregate C and N concentrations and the relationships between climatic factors?i.e.,MAT and MAP?and soil C and N turnover.The proportional majority of new soil aggregate C and N was in plantations derived from the litter inputs after LUC.Our findings implied also that the conversion of natural forests to plantations alters soil C turnover and contents,but the extent of these effects varied among sites and were related to climate factors?i.e.temperature and precipitation?.The soil C content was generally reduced,¹³C values increased,whereas N and ¹⁵N content were not?strongly?affected by the conversion of natural forest to plantation along the transect covering the six climate zones in Eastern China.