| In order to understand the quantity and regularity of dissolved organic matter(DOM)release from fresh leaves and litter layer and their stoichiometric characteristics of carbon(C),nitrogen(N)and phosphorus(P),an investigation of ecological mechanism of DOM leaching was conducted in three different subtropical forests.Fresh foliage,undecomposed litter(L layer)and semi-decomposed litter(F layer)were collected from the mid-aged Castanopsis sclerophylla,old-growth Castanopsis sclerophylla forests and Cunninghamia lanceolata plantation.A simulated leaching experiment was conducted in the laboratory.The objectives of the study was to reveal the variation patterns of DOM,SUVA and pH of leaching solution from the different components in the three forest stands.The results obtained are as follows.(1)In mid-aged Castanopsis sclerophylla stand,the average concentrations of DOC,DON and DOP in fresh leaves were,respectively,1197.43 mg·kg-1,25.62 mg·kg-1,and 1.58 mg·kg-1 within 11 times of leaching,and the cumulative amounts were 13171.71 mg·kg-1 for DOC,281.77 mg·kg-1 for DON and 17.42 mg·kg-1 for DOP.In the litter layer,the average concentrations of DOC,DON and DOP were,respectively,479.58 mg·kg-1,20.29 mg·kg-1 and 1.97 mg·kg-1,with the accumulations were 10550.68 mg·kg-1,20.29 mg·kg-1 and 1.97 mg·kg-1.After the leaching of Castanopsis sclerophylla forest through different layers,The mean concentrations and accumulations were decreased for DOC and DON,while was increased for mean DOP concentration and decreased significantly for DOP accumulations.The contents of DOC and DON in the leaching solution were significantly different in the early leaching period than in the middle and late leaching periods(p<0.05),and the contents of DOP were significantly different in the early and late leaching periods than in the middle leaching periods(p<0.05).The average value of SUVA was F layer(2.56)>L layer(2.03)>fresh leaves(1.76),and the maximum value of DOC and SUVA did not appear in the same leaching times.The SUVA value of the seventh leaching solution of fresh leaves,L layer and F layer was significantly different from other leaching times(p<0.01).The average pH values of leaching solution from fresh leaves,L layer and F layer were 6.55,6.66 and 6.63,respectively.The pH values of the leaching solution in the later leaching stage of fresh leaves,L layer and F layer was significantly different from that in other leaching times(p<0.01).(2)In old-gowth Castanopsis eyrei stand,the average concentrations of DOC,DON and DOP from fresh leaves were 1292.75 mg·kg-1,22.96 mg·kg-1 and 0.24 mg·kg-1,within 11 times of leaching,and the cumulative amounts were 14220.23 mg·kg-1,252.58 mg·kg-1 and 2.64 mg·kg-1,respectively.The average concentrations of DOC,DON and DOP from litter layers were 570.89 mg·kg-1,31.28 mg·kg-1,1.96 mg·kg-1,and the accumulations were 570.89 mg·kg-1,688.15 mg·kg-1,and 43.06 mg·kg-1,respectively.The average concentration and the cumulative amount of DOC decreased during successive leaching,while DON and DOP were increased.Under the same leaching conditions,DOCs from fresh leaves and F layer in the early leaching stage were significantly different from those in the middle and late leaching stages(p<0.05).However,in the middle stage of leaching,DOC from L layer was significantly different from that in the earlier and later stages(p<0.05).The DON and DOP from fresh leaves,L layer and F layer were significantly different in the middle stage than in early and late stages of leaching(p<0.05).The average value of SUVA was F layer>L layer.The SUVA value of fresh leaf leaching solution was significantly different from that of other leaching time(p<0.01).The average value of SUVA was F layer>L layer>fresh leaves,the SUVA value of fresh leaf leaching solution in the second leaching time was significantly different from other leaching times(p<0.01),the SUVA values for L and F layers in the seventh leaching time was significantly different from other leaching times(p<0.05).The pH values in fresh leaves,L and F layers were all slightly acidic at the beginning of leaching(pH<6.0)and neutral at the end of leaching(pH=7.0 or so).The pH values of fresh leaves,L and F layers at the later stage of leaching were significantly different than at middle stage(p<0.05).(3)In Cunninghamia lanceolata plantation,the average concentrations of DOC,DON and DOP in the fresh leaves were 1855.97 mg·kg-1,6.51mg·kg-1 and 0.44 mg·kg-1,the accumulations were 21405.69 mg·kg-1,71.63 mg·kg-1 and 4.84 mg·kg-1,respectively.The average concentrations of DOC,DON and DOP in litter layers were 1531.85 mg·kg-1,18.95 mg·kg-1,and 0.40 mg·kg-1,and the accumulations were 33700.75 mg·kg-1,416.86 mg·kg-1,and 8.88 mg·kg-1,respectively.There was a significant difference in DOC between the middle stage and the earlier and the latter stages(p<0.05).However,the difference in DOC in L and F layers were significant betweem the latter stage and other stages(p<0.05).In the leaching process,SUVA value of F layer was much higher than the others.SUVA values of fresh leaves and F layer at the later stage of leaching were significantly different with other leaching stages(p<0.01).The SUVA value in L layer in the middle stage was significantly different with other stages(p<0.05).The average pH value was:L layer(6.55)>F layer(6.35)>fresh leaves(6.00).The pH values were significantly different between the later stage and other stages(p<0.05).(4)The loss rate of N and P in C.sclerophylla forest was higher than that of C loss.The loss rate of P and N was higher than that of C in L and F layers.This indicates that N is easier to be leached in fresh leaves in C.sclerophylla forest.However,the P had strong retention capacity.In L and F layers,P is easier to be leached out and N has stronger retention capacity.In C.eyrei forest,the P loss rate of fresh leaves was higher than C and N loss rates.The N loss rate of L layer was approximately the same as P loss rate,while was greater than C loss.The C,N and P loss rates in F layer were approximately similar.This indicated that P was more likely to be leached out in fresh leaves,while N and P in litter layers were more likely to be leached out.In Cunninghamia lanceolata forest,N loss rate from fresh leaves was greater than C,and P loss rate was greater than its mass loss rate.The P loss rate in L layer was higher than C and N loss rate.The N and P loss rate in F layer is greater than C loss rate.(5)The average content of C in the three stands was C.lanceolata(505.34 g·kg-1)>C eyrei(453.29 g·kg-1)>C.sclerophylla(452.76 g·kg-1),and the average content of N was C.eyrei(11.69 g·kg-1)>C.sclerophylla(11.29 g·kg-1)>C.lanceolata(11.11 g·kg-1).The average P content showed C.lanceolata(0.46 g·kg-1)>C.sclerophylla(0.44 g·kg-1)and C.eyrei(0.43 g·kg-1).The average ratios of C:N,C:P,N:P in fresh leaves and L layer were basically expressed as C:P>C:N>N:P.C:N ratio was C.lanceolata forest>C.eyrei>C.sclerophylla;C:P and N:P ratios,all are C.sclerophylla>C.eyrei>C.lanceolata forest.The C:N ratio of L layer is C.lanceolata>C.sclerophylla>C.eyrei,the C:P ratio is C.lanceolata>C.eyrei>C.sclerophylla,the N:P ratio is C.eyrei>C.sclerophylla>C.lanceolata.The average ratios of C:N,C:P,N:P in F layer were basically expressed as C:P>C:N>N:P.However,in C.eyrei forest,expressed as N:P>C:N.The C:N ratio is C.sclerophylla>C eyrei>C.lanceolata,C:P and N:P ratio were both as C.eyrei>C.sclerophylla>C.lanceolata. |
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