The Soil Fertility Maintenance And Water Conservation Functions Of The Forest Ecosystems In The Xinlong Mountain,China

The forest ecosystem is one of the important components of the terrestrial ecosystem,which has various ecosystem service functions,such as soil fertility maintenance,water conservation,water quality improvement,and biodiversity protection and so on.In this study,three plantation forests（Larix principis-rupprechtii,Picea crassifolia and Pinus tabuliformis）and four natural forests(（Picea wilsonii,Betula platyphylla,Populus davidiana and Rhododendron community）in Xinglong Mountain,were used to evaluate the relationship between soil fertility maintenance,water conservation and tree water consumption characteristics and environmental factors in different forest ecosystems.We studied the nutrient cycling process between different forests of“leaf-litter-soil”and the interaction between soil physical and chemical properties,to understand the soil fertility maintenance function of different forest stands;We examined the changes of acid anions(NO₃^-,SO₄^2-and Cl^-)and water-soluble metal cations contents(K⁺,Ca²⁺,Na²⁺,Mg²⁺,Fe³⁺,Pb²⁺,Cu²⁺and Cd²⁺)in rainfall,throughfall,stemflow and forest floor leachate of different forest stands,to evaluate the water conservation function of different forest ecosystems.We analysed the response relationship between the water consumption characteristics of Larix principis-rupprechtii and environmental factors to understand the water balance of watershed ecosystems.The main research results are as follows:（1）The results revealed that the leaf and litter layer strongly influences soil nutrient availability through biogeochemical processes:P.tabuliformis has higher organic carbon,C:N ratio and C:P ratio in the leaves and litter layer than L.principis-rupprechtii or P.crassifolia,suggesting that higher C:N ratios and C:P ratios hinder litter decomposition.For the three plantation forests,the soil organic carbon（SOC）,C:N and C:P ratio in the leaf and litter layers of P.tabuliformis were higher than those in L.principis-rupprechtii and P.crassifolia.However,among the four natural forests,the organic carbon（OC）,C:N and C:P ratio in the leaf and litter layers of B.platyphylla and P.davidiana were lower than those in P.wilsonii and Rhododendron community.Therefore,the L.principis-rupprechtii and P.crassifolia can significantly improve soil nutrient contents than P.tabuliformis among the three plantation forests,and the P.wilsonii and Rhododendron community can significantly improve soil nutrient contents than B.platyphylla and P.davidiana among the four natural forests.Moreover,the SOC,TN and available nutrients（NH₄⁺-N,NO₃^--N,AP and AK）significantly increase the soil water holding capacity,total porosity and capillary porosity,and decrease the soil density,which make the soil have a larger porosity and enhance the water storage capacity.（2）The acid anions and metal cations enrichment capacity of coniferous forests（L.principis-rupprechtii,P.wilsonii,P.crassifolia and P.tabuliformis）in the vegetation canopy were stronger than those of broad-leaved forests（B.platyphylla,P.davidiana and Rhododendron community）.The concentrations of NO₃^-,Cl^-,K⁺,Mg²⁺,Fe³⁺,Pb²⁺,Cu²⁺and Cd²⁺in the leachate of all forests showed a general decreasing trend from the topsoil layer to deep soil layer,and these ions were also concentrated in the topsoil layer(except for Cu²⁺and Cd²⁺concentrated in the deep soil layer).The concentrations of SO₄^2-,Ca²⁺and Na⁺in the leachate of all forests showed an increasing trend from the topsoil to the deep soil layer.SO₄^2-and Na⁺were concentrated in the deep soil,while Ca²⁺was concentrated in the topsoil layer.Except for P.tabuliformis and P.davidiana forests,the SOC and TN contents of other coniferous forests were higher than those of the broad-leaved forests,indicating that the soil and litter layer of the coniferous forests had a stronger capacity of the ion removal and adsorption.Therefore,the adsorption and enrichment ability of the coniferous tree species to atmospheric pollutants is stronger than that of the broad-leaved tree species in the vegetation layer,and in the soil layer,the pollutant enrichment ability of L.principis-rupprechtii,P.wilsonii and P.crassifolia forests is stronger than those of P.tabuliformis,P.davidiana,B.platyphylla and Rhododendron community forests.（3）For the L.principis-rupprechtii,within a daily timescale,the net radiation（Ra）,air temperature（AT）,air humidity（AH）,vapor pressure deficit（VPD）and wind speed（WS）showed a significantly positive correlation with sap flow velocity in day-time,but in the night-time sap flow had no significant correlation.Within a monthly timescale,the Ra,AT,WS,and VPD showed a significantly positive correlation with stand water use,whereas precipitation（Pr）showed a significantly negative correlation with stand water use.Our results also showed that the daily transpiration of L.principis-rupprechtii continued to increase with the increase of net radiation（Ra）,until the late stages of growth season（except rainy and cloudy days）,and the sap flow velocity showed an obvious hysteresis response to meteorological factors to alleviate plant water stress from June to October,while there was no significant correlation between day-time and night-time water use during the stage of germination and rapid growth in May.Besides,due to the relatively low temperature,the water consumption of forest stands was only about 151.05 mm during the whole growing season,and there was no obvious‘noon break’phenomenon in the whole growing season.In conclusion,the soil fertility maintenance and water conservation functions of the plantation forests are as follows:L.principis-rupprechtii>P.crassifolia>P.tabuliformis,and natural forests are P.wilsonii>Rhododendron community>P.davidiana>B.platyphylla.Thus,we recommend planting L.principis-rupprechtii,P.wilsonii and P.crassifolia can be used as suitable plantation tree species to improve the ecological service function of the forest ecosystems,especially in the mountain ecosystems of the semi-arid regions.The results of this study can provide the theoretical guidance for the implementation of soil fertility maintenance,artificial afforestation,water resource management and forest resource restoration measures in the semi-arid mountain forest ecosystem.