Differential Effect And The Potential Mechanisms Of Root And Mycelium Derived Carbon On Soil Nitrogen Transformation In Soil Of The Cryptomeria Fortunei Plantation In Southwest Karst

Plant roots and associated mycelia play an important role in soil nitrogen（N）cycle.Roots and mycelia secrete a series of carbohydrates such as organic acids,amino acids and sugars,which promote the activities of rhizosphere microorganisms and extracellular enzymes,thus accelerating the decomposition of soil organic matter（SOM）and driving the soil N cycle.However,the chemical components of root-derived carbon（C）and mycelium-derived C compounds are different and their production rates are quite different.The root-derived C and mycelium-derived C compounds are supposed to differentially affect the microbial and extracellular enzyme activities in soil.Therefore,the differential effect of root and mycelium on N transformation in karst soil remains unclear.In addition,most of the previous studies on the mechanism of root deposition driving N transformation mainly emphasized the microbial mechanisms,with few studies on the abiotic mechanism driving N transformation.Therefore,the underlying mechanisms by which roots and mycelia affect soil N transformation in karst soil remain unclear.This study took Cryptomeria fortunei plantation which are widely distributed in southwest China as the research object.Taking rhizosphere processes as the entry point,we focused on the differential effects of roots and mycelia on soil N pool and N mineralization rate,soil microbial parameters and metal/mineral organic complexes.The relative contributions of the two pathways to the soil N transformation process were compared and calculated basing on an ingrowth core experiment.The main experimental results are as follows:（1）Effects of roots and mycelia on soil nitrogen availability in soil of the cryptomeria fortunei plantationIn the non-growing season,compared with the control,the combination of root and mycelium and mycelium alone significantly increased the concentration of nitrate nitrogen（NO₃^--N）(29.02±3.71 mg·kg^-1 and 25.9±1.53 mg·kg^-1),ammonium nitrogen（NH₄⁺-N）(130.84±4.87 mg·kg^-1 and 126.49±3.82 mg·kg^-1),and total inorganic nitrogen（Tin）(159.86±11.53 mg·kg^-1 and 152.40±5.28 mg·kg^-1)in soil of the Cryptomeria fortunei plantation.The concentration of NO₃^--N,NH₄⁺-N,and Tin in soil of the Cryptomeria fortunei plantation significantly increased by 33.8%,3.7%,and 9.3%under the root treatment,while the concentration of NO₃^--N,NH₄⁺-N,and Tin in soil of the Cryptomeria fortunei plantation significantly increased by 24.6%,8.2%,and 10.9%under the mycelium alone treatment.In the growing season,the similar increasing trends of NO₃^--N,NH₄⁺-N and Tin under different ingrowth core treatments were observed.The combination of root and mycelium and mycelium alone also significantly increased the concentration of NO₃^--N(43.82±2.44 mg·kg^-1 and 43.35±3.09 mg·kg^-1),NH₄⁺-N(138.21±2.36mg·kg^-1 and 133.54±3.12 mg·kg^-1)and Tin(182.03±4.71 mg·kg^-1 and 176.89±6.11 mg·kg^-1)in soil of the Cryptomeria fortunei plantation.The concentration of NO₃^--N,NH₄⁺-N,and Tin in soil of the Cryptomeria fortunei plantation significantly increased by 3.9%,and 2.4%under the root alone,while the concentration of NO₃^--N,NH₄⁺-N,and Tin in soil of the Cryptomeria fortunei plantation significantly increased by 24.3%,12.8%,and 15.2%under the mycelium alone.（2）Effects of roots and mycelia on soil nitrogen mineralization rate in soil of the cryptomeria fortunei plantationIn the non-growing season,compared with the control,the combination of root and mycelium and mycelium alone significantly increased the concentration of N mineralization in soil of the Cryptomeria fortunei plantation(7.55±0.55 mg·kg^-1 and7.02±0.29 mg·kg^-1).In the growing season,the combination of root system and mycelium and mycelium alone also significantly increased the soil N mineralization rate in soil of the Cryptomeria fortunei plantation(8.03±0.10 mg·kg^-1 and 7.11±0.39 mg·kg^-1).The N mineralization rate significantly increased soil of the Cryptomeria fortunei plantation soil by 13.4%under root alone,while the N mineralization rate significantly increased of soil the Cryptomeria fortunei plantation by under 20.3%mycelium alone.（3）Effects of roots and mycelia on soil microbial biomass carbon and nitrogen concentrations in soil of the cryptomeria fortunei plantationIn the non-growing season,compared with the control,the combination of root and mycelium and mycelium alone significantly increased the concentration of microbial biomass carbon（MBC）(237.84±4.75 mg·kg^-1 and 134.59±0.630 mg·kg^-1)and the concentration of soil microbial biomass nitrogen（MBN）(22.34±0.77 mg·kg^-1and 11.95±0.11 mg·kg^-1)in soil of the Cryptomeria fortunei plantation.The concentration of MBC and MBN increased by 104%and 120%under root alone,while the concentration of MBC and MBN increased by 47%and 38%under mycelia alone,respectively.In the growing season,the combination of root and mycelium and the mycelium alone also significantly increased the concentration of soil microbial biomass carbon(319.69±11.64 mg·kg^-1 and 277.54±9.59 mg·kg^-1)and the concentration of microbial biomass nitrogen(44.11±0.24 mg·kg^-1 and 18.88±0.73 mg·kg^-1)in soil of the Cryptomeria fortunei plantation.The concentration of MBC and MBN increased by 30%and 141%under root alone,while the concentration of MBC and MBN increased by 100%and 19%under mycelia alone,respectively.（4）Effects of roots and mycelia on extracellular enzyme activity in soil of the cryptomeria fortunei plantationThe combination root and mycelium,as well as mycelium alone,significantly increased N-acetylglucosidase activity(0.012±0.002 nmol·g^-1 h^-1 and 0.008±0.000nmol·g^-1h^-1),peroxidase activity(0.569±0.035 nmol·g^-1h^-1 and 0.566±0.022nmol·g^-1h^-1),and polyphenol oxidase activity(0.176±0.013 nmol·g^-1h^-1 and 0.158±0.017 nmol·g^-1h^-1)in the soil of Cryptomeria fortunei plantation.The concentrations of NAG and PPO increased by 61%and 20%under root alone,respectively,while the concentrations of NAG and PPO increased by 24%and 90%under mycelia alone,respectively.（5）Effects of roots and mycelia on the concentrations of iron,aluminum,and calcium-metal organic complexes in soil of the cryptomeria fortunei plantationIn the non-growing season,compared with the control,the combination of root and mycelium and mycelium alone significantly decreased the concentration of calcium-organic complexes（Ca-MOCs）(0.56±0.04 mg·kg^-1 and 0.52±0.02 mg·kg^-1),the concentration of aluminum-organic complexes（Al-MOCs）(9.51±0.09 mg·kg^-1and 8.72±1.03 mg·kg^-1)and iron-organic complexes（Fe-MOCs）(2.10±0.21mg·kg^-1 and 2.15±0.09 mg·kg^-1)in the soil of Cryptomeria fortunei plantation.Specifically,the concentrations of Ca-MOCs,Al-MOCs,and Fe-MOCs increased by6%,8%,and 8%under root alone,respectively,while the concentrations of Ca-MOCs,Al-MOCs,and Fe-MOCs decreased by 17%,16%and 18%under mycelia alone,respectively.In the growing season,the combination root and mycelium and mycelia alone also significantly decreased the concentration of Ca-MOCs(0.65±0.06 mg·kg^-1 and1.01±0.08 mg·kg^-1),Al-MOCs(5.95±0.21 mg·kg^-1 and 6.30±0.46 mg·kg^-1)and Fe-MOCs(8.72±0.26 mg·kg^-1 and 6.90±0.14 mg·kg^-1)in soil of the Cryptomeria fortunei plantation.The concentrations of Ca-MOCs and Fe-MOCs increased by 32%and 32%under root alone,respectively,while the concentrations of Al-MOCs and Fe-MOCs increased by 32%and 23%under mycelia,respectively.（6）Effects of roots and mycelia on the concentrations of iron,aluminum,and calcium-short range orders in soil of the cryptomeria fortunei plantationIn the non-growing season,compared with the control,the combination of root and mycelium and mycelium alone significantly decreased the concentration of calcium short range ordered（Ca-SROs）(0.10±0.01 mg·kg^-1 and 0.09±0.01 mg·kg^-1),aluminum short range ordered states（Al-SROs）(1.29±0.13 mg·kg^-1 and 1.26±0.07mg·kg^-1)and iron short range ordered states（Fe-SROs）(2.10±0.21 mg·kg^-1 and 2.15±0.09 mg·kg^-1)in soil of the Cryptomeria fortunei plantation.Specifically,the concentrations of Ca-SROs and Al-SROs increased by 5%and 2%under root alone,respectively,while the concentrations of Ca SROs,Al SROs,and Fe SROs decreased by 20%,4%and 11%under mycelia alone,respectively.In the growing season,the combination of root and mycelium and mycelia alone also significantly decreased the concentration of Ca-SROs(0.17±0.01 mg·kg^-1 and0.14±0.08 mg·kg^-1),Al-SROs(2.33±0.12 mg·kg^-1 and 1.41±0.04 mg·kg^-1)and Fe-SROs(4.32±0.07 mg·kg^-1 and 4.75±0.35 mg·kg^-1)in soil of the Cryptomeria fortunei plantation.The concentrations of Ca-SROs and Al-SROs increased by 10%and 56%under root alone,while the concentrations of Ca-SROs,Al-SROs,and Fe-SROs decreased by 38%and 12%under mycelia alone,respectively.To sum up,plant roots and associated mycelia enhanced soil N transformation through different mechanisms.Specifically,mycelium enhanced soil N transformation through biological processes（microbial mineralization process）and non-microbial processes（destabilizing metal-organic mineral phases）,while root mainly through biological processes.Given root and mycelium are underground symbiosis,the biotic and abiotic processes play a joint role in promoting soil N transformation,which has an important impact on the N cycle of karst ecosystem in southwest China.