| Soil organic carbon(SOC),total nitrogen(TN),and soil water content(SWC)are the most critical elements in the ecosystem,which determine the ecological stability and sustainability.In the process of vegetation restoration for the arid and semi-arid Loess Plateau,SOC,TN,and SWC are the limiting soil resources,and their availability is the key to the success or failure of ecological construction in this region.Therefore,understanding the dynamics,coupling interactions and resilience of SOC,TN,and SWC during revegetation is an important link to clarify the stability and sustainability of the ecosystem in the Loess Plateau.Focusing on the ecological construction needs and frontier issues of arid and semi-arid regions in China,the soil-based multi-factor database related to SOC,TN,and SWC was constructed through historical data collection and field survey sampling,in which the planted forest,planted shrub and abandoned grassland after cropland conversion were selected as the research objectives.Then,the dynamics,coupling interaction and resilience of SOC,TN and SWC were studied,and we discussed how to realize the sustainable use and coordinated development of such resources,thereafter proposed planted vegetation management countermeasures based on the limiting resources which may provide scientific basis for sustainable management of planted vegetation in the Loess Plateau.(1)Revegetation exerted profound impacts on the SOC sequestration in deep soils horizons,and the extent varied with soil depths.Specifically,converting croplands to planted forest had higher SOC sequestration potential in the overall profiles,and the restoration age was the key factor.SOC stock increased significantly in the 0-100 and 100-200 cm horizons regardless land-use conversion types while SOC loss occurred in the 200-400 cm due to cropland conversion.Furthermore,the changes in SOC stock varied with restoration age,except for converting cropland to grassland.Specifically,SOC stock increased with restoration age in the upper 200 cm layers,whereas that in the 200-400cm horizons first increased and then decreased in the middle to later stages when converted to planted forest and shrub.Moreover,initial SOC stock and rainfall zones had significant effects on the changes of deep SOC stock.Interestingly,an accumulation of 1 Mg ha-1 in the upper 100 cm was associated with an approximately0.45 Mg ha-1 increase in the 100-400 cm depths.These results indicate that land-use conversion,particularly converting cropland to planted forest,changes deep(>100 cm)SOC stock,and restoration age should be taken into consideration when assessing deep SOC sequestration.(2)Revegetation enhanced TN stock in both top and deep soils.Planted forest and shrubs exhibited higher advantages than grassland in sequestering TN in entire soil profiles.The TN stock increased significantly in the 0-200 cm horizons especially by converting cropland to planted forest or shrub,and increased with time since land-use conversion,with higher increment occurring later(>30yr);Moreover,the increments in the TN stock correlated tightly with SOC,SWC and initial TN stock;Furthermore,the changes in TN stock were influenced by the land-use conversion types,time since cropland conversion,mean annual precipitation and temperature and their interactions.Specifically,the changes in TN stock correlated significantly with that in topsoil.Our results suggest TN stock can be increased by converting croplands into deep-rooted perennials(e.g.,planted forest or shrub)in arid and semi-arid environments.Finally,we argue that subsoil N stock accumulation after cropland conversion should be taken into consideration due to its potential to alleviate N limitation and sustain plant growth when in droughts.(3)Revegetation significantly reduced the SWC in the 0-1000 cm soil profile,and the limiting effect of SWC strengthened with the increase of the restoration ages,especially after 20 years;the interactive effect of vegetation type and precipitation gradient strongly affected the dynamics of deep SWC;deep SWC limited the sustainability of ecological restoration in this region.(1)Planted forests and shrubs exerted a strong negative impact on deep SWC within the 1000 cm depth in which the average soil water content in 100-500 cm was lower than the average permanent wilting point(8.6%),the choice of tree species greatly influenced the deep SWC depletion extent,and Platycladus orientalis and Hippophae rhamnoides were recommended to maintain a better deep SWC condition.(2)The temporal pattern of deep SWC dynamics within the 1000 cm depth under planted forests and shrubs exhibited a significant and continuous decrease in first 20years and remained relatively stable after 20 years,in which the soil water content was lower than the average permanent wilting point,and grassland showed no obvious trends among different restoration periods(0-30 yr).(3)The limiting effects of precipitation changed greatly along the soil profile under planted forests and shrubs,and the restrictions may begin to appear when the mean annual precipitation is less than 480 mm when considering the deep soil layer(200-1000 cm).(4)The change rate of deep SWC within 1000 cm in planted forests(-0.08 to-0.11 g 100 g-1yr-1)and planted shrubs(-0.05 to-0.20 g 100 g-1 yr-1)was much higher than that in natural grassland(-0.02 to-0.07 g 100 g-1 yr-1).Our results demonstrate that in arid and semiarid regions,the deep SWC in planted forests and shrubs was lower than or approached the average permanent wilting point and might limit the sustainability of vegetation restoration.When considering the joint effects between precipitation and vegetation type in deep soil layers,planted forests and shrubs are acceptable only when precipitation is greater than 480 mm,while grassland may be the best restoration type if precipitation is less than 480 mm.(4)Revegetation changes the soil carbon-nitrogen coupling interactions(soil C-N coupling)in both top and deep soil horizons,and deep soil was more sensitive;The soil C-N coupling was relatively stable before 20 years while there was strong decoupling trend after 20 years,particularly during 20-30 years;The soil C-N coupling was influenced profoundly by precipitation and temperature changes,in which natural grassland exhibited a stable soil C-N coupling.The SOC and TN correlated positively and significantly in different vegetation types along the 200 cm soil profile,however,there were various temporal patterns for SOC,TN and C/N ratios.Overall,the C/N ratios remained stable in the first 20 years,indicating a stable soil C-N coupling,while the decoupling trends occurred in 20-100cm and 100-200 cm depths for 20-30 years,and the decoupling trend in the topsoil was observed after 30 years.Along the restoration ages,soil C-N coupling in the entire profile remained stable under climax communities including climax forest and grassland while revegetated lands altered such coupling interaction indicating an unstable status.Moreover,the soil C-N coupling was influenced greatly by climatic factors,for example,precipitation and temperature.In detail,SOC and TN showed a first increase and then a decrease along the precipitation gradients.The turning points for C/N ratios in 0-20 cm and 20-100 cm were detected in which C/N ratio kept stable before the points,while the ratios in 100-200 cm increased linearly and significantly.The responses of SOC,TN and C/N ratios were more sensitive to temperature than precipitation and the C/N ratios in the entire profiles showed a linear increase.Compared with planted forest and shrub,natural grassland exhibited higher ability in maintaining a stable soil C-N coupling and therefore showed higher stability and adaptability in future climate change scenarios.(5)Revegetation changes the soil carbon-water coupling interaction(soil SOC-SWC coupling)in both top and deep soil horizons.Overall,the soil SOC-SWC coupling showed decoupling trends before 30 years since revegetation,with the strongest trends for 10-20 years.Specifically,natural grassland exhibited higher sensitivity to climatic factors than planted forest and shrubs,indicating higher abilities in controlling SOC,SWC and their interactions.Vegetation types affected soil SOC-SWC coupling profoundly in both top and deep soil horizons,in which soil SOC-SWC coupling in planted forest,shrub and natural grassland was significantly lower than that of climax forest and grassland.Overall,the general soil SOC-SWC coupling in the Loess Plateau ranged from nearly to mildly decoupling.The temporal changes of soil SOC-SWC coupling varied among different vegetation types,and the overall trends showed that soil SOC-SWC coupling decoupled before 30 years since revegetation,with the strongest for 10-20 years.Moreover,the sensitivity of soil SOC-SWC coupling to precipitation and temperature differed among vegetation types and soil depths.Specifically,natural grassland exhibited higher sensitivity to climatic factors than planted forest and shrubs,indicating higher abilities in controlling SOC,SWC and their interactions.(6)Revegetation promoted the resilience of SOC and TN,however,there were still great gap between revegetated land and zonal climax communities.The overall resilience of SOC and TN ranged from 22%to 46%in the 200 cm profile.In contrast,the SWC resilience in planted forest and shrub showed opposite trends,particularly the deep soil horizons,while that of natural grassland was better in SWC resilience.The resilience of SOC and TN in the entire 200 cm profile exhibited synergetic changes,in which the resilience index for SOC and TN were 0.23?0.46 and 0.22?0.45,respectively,indicating a relatively low recovery degree.In contrast,revegetation degraded the SWC resilience,especially in deep soil horizons.Natural grassland showed higher ability in promoting SWC resilience than planted forest and shrub.Moreover,there were strong interaction effects among different vegetation types,restoration periods and climatic zones.Along the age gradients,the resilience of SOC and TN in both top and deep soil depths increased significantly with ages while SWC showed an opposite trend.Along the precipitation and temperature gradients,the resilience of SOC,TN and SWC showed various trends while that of planted shrub showed increasing trends compared with planted forest and natural grassland.Overall,SOC and TN exhibited recovery trend which was less than 50%while SWC showed degraded trend,indicating that it is urgent to take relevant measures to alleviate soil water stress. |
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