| The effect of soil fauna is one of the most important biotic factors moderating litter decomposition process, which plays an important role in material cycling and maintaining elements balances in terrestrial ecosystems. The effect of soil fauna on litter decomposition can be affected by a wealth of environmental factors such as temperature, soil moisture, freeze-thaw cycles and cycles of drying and wet etc., and the initial litter quality. However, we still know little about the effect of soil fauna on litter decomposition in the typical ecosystems in the upper reaches of the Minjiang River, which is one of the most important ecological barriers in China. We conducted a field experiment using litterbags with different mesh sizes to investigate the effects of soil fauna on litter decomposition process. Briefly, during the period of November 2013 and October 2014, foliar litter from two dominant species in seven typical ecosystems were used to determine the effects of soil fauna on litter mass loss, nutrient release pattern, dynamics of litter quality and metallic metals. The specific litter species were Ajania nubigena and Carex atrofusca in alpine meadow, Salix paraqplesia and Sabina saltuaria in alpine forest, Betula albo-sinensis and Abies faxoniana in montane forest, Quercusbaronii and Cupressus chengiana in ecotone, Campylotropis delavayi and Cupressus chengiana in dry valley, Quercus acutissima and Cryptomeria fortune in north subtropical evergreen broadleaf forest, and Chnnamomum longepaniculatum and Pinus massoniana in subtropical evergreen broadleaf forest. After one year's incubation, the main findings of our study are as follows:1. Foliar litter mass-loss rates were in the order of dry valley (45.98%-61.23%)> north subtropical evergreen broad-leaved forest (47.73-49.31%)> ecotone (28.27%-48.86%)> alpine forest (34.1%-37.13%)> subtropical evergreen broad-leaved forest (27.78%-38.24%)> montane forest (22.53%-24.55%). Soil fauna promoted moss loss of C. delavayi, C. chengiana, C. longepaniculatum, Q. acutissima, B. albo-sinensis and S. paraqplesia, with contribution rates of 1.61%-34.12%; while mass loss of other litter were inhibited, with contribution rates ranging from-16.04% to-2.85%;2. Soil fauna showed remarkable effects on C concentration in decomposing litter except that in ecosystems of alpine meadow and north subtropical evergreen broad-leaved forest. Soil fauna increased C concentration for A. faxoniana litter, with a contribution rate of -7.89%, but decreased C concentration for the other litter species, and the contribution rates ranging from 8.32% to 58.39%. Similarly, N concentration dynamics were also affected by soil fauna remarkably except that in dry valley. Soil fauna increased N concentration for A. nubigena, S. paraqplesia, and C. chengiana litter ranging from-13.97% to-0.48%, but decreased N concentration for other litter species, with contribution rates 16.0%-38.31%. The concentration dynamics of P in all litter species were affected by soil fauna as well except for those in montane forest, soil fauna increased P concentration for A. faxoniana, Q. baronii, C. fortunei, C. chengiana(ET) and C. delavayi litter, with contribution rates ranging from -6.12% to-59.28%, but decreased P concentration for the other litter species, with contribution rates ranging from 4.78% to 23.72%;3. The concentrations of lignin and cellulose in decomposing litter were affected significantly by soil fauna. Soil fauna increased lignin concentrations for S. saltuaria, A. faxoniana, S. paraqplesia and C. chengiana, with contribution rates ranging from-29.18% to -5.27%, but decreased lignin concentration for the other litter species, the contribution rates were 1.21%-29.37%. Cellulose concentrations in decomposing litter incubated in alpine meadow, montane forest, ecotone, dry valley and subtropical evergreen broad-leaved forest were affected significantly (P< 0.05). Soil fauna decreased cellulose concentration for C. delavayi, Q. acutissima, C. chengiana, C. fortunei, S. paraqplesia and P. massoniana, the contribution rates were 4.01%-70.62%, but increased cellulose concentration for the other litter species, with the contribution rates ranging from -3.86% to-24.51%;4. Soil fauna showed remarkable effects on K concentration in decomposing litter except that in alpine meadow. Soil fauna increased K concentration for C. delavayi, C. longepaniculatum, A. nubigena, B. albo-sinensis and Q. acutissima litter, with the contribution rates ranging from -18.62% to -112.98%, but decreased K concentration for the other litter species, the contribution rates were 1.34%-35.31%. Concentration of Ca was influenced by soil fauna in decomposing litter except central subtropical evergreen broad-leaved forest, soil fauna increased Ca concentration for A. faxoniana, Q. baronii, C. delavayi and C. chengiana{DV) litter, with the contribution rates ranging from -4.66% to -38.25%, but decreased Ca concentration for the other litter species, the contribution rate were 2.35% -31.93%. Concentration of Na was influenced by soil fauna in each ecosystem. Soil fauna increased Na concentration for A. nubigena, S. paraqplesia, Q. baronii, C. delavayi and C. chengiana(DV) litter, with the contribution rates ranging from-1.46% to-42.51%, but decreased Na concentration for the other litter species, the contribution rates were 0.34%-14.44%. Concentration of Mg was influenced by soil fauna in ecotone, dry valley and subtropical evergreen broad-leaved forest. Soil fauna increased Mg concentration for C. delavayi, Q. acutissima, C. fortunei and C. chengiana(DV) litter, with the contribution rates ranging from -43.23% to -75.79%, but decreased Mg concentration for the other litter species, the contribution rates were 0.42%-26.22%. Soil fauna decreased Al concentration for B. albo-sinensis, Q. acutissima, C. fortunei and C. chengiana(ET) litter, the contribution rates were 6.20%-35.32%, but increased Mg concentration for the other litter species, with the contribution rates ranging from-10.49% to -65.83%;5. Concentrations of Fe, Cu and Mn were influenced by soil fauna as well in all the ecosystems. Specifically, the contribution rates ranged from -300% to 43.53% for Fe, from -102.64% to 18.14% for Cu, from -158.65% to 47.56% for Mn, depending on ecosystems type and litter species. Soil fauna showed remarkable effects on Zn concentration in decomposing litter except that in alpine meadow and ecotone. Soil fauna decreased Zn concentration for Q. acutissima, A. nubigena, C. chengiana (ET) and B. albo-sinensis litter, the contribution rates were 9.13%-27.05%, but increased Zn concentration for the other litter species, with the contribution rates ranging from-17.93% to-102.64%;6. Concentrations of Pb and Cr during litter decomposition were influenced remarkably by soil fauna. Soil fauna increased Pb concentration for all the litter species, with the contribution rates ranging from -10.16% to -151.12%. Soil fauna decreased Cr concentration for A. nubigena, B. albo-sinensis, C. delavayi and C. longepaniculatum litter, the contribution rates were 15.84%-70.97%, but increased Cr concentration for the other litter species. Soil fauna showed remarkable effects on Cd concentration in decomposing litter except that in ecotone and central subtropical evergreen broad-leaved forest. Soil fauna decreased Cd concentration for A. nubigena, B. albo-sinensis, A. faxoniana, Q. baronii, S. paraqplesia and Q. acutissima litter, the contribution rates were 3.38%-70.93%, but increased Cd concentration for the other litter species, with the contribution rates ranging from -0.62% to-506.25%;7. Concentrations of C/P, N/P and lignin/cellulose were influenced by soil fauna as well in all the ecosystems, while soil fauna showed remarkable effects on C/N in decomposing litter except that in north subtropical evergreen broad-leaved forest, soil fauna increased C/N of foliar litter for A. faxoniana and P. massoniana, the contribution rates were-9.67% and -3.45%, respectively, but decreased C/N for the other litter species, the contribution rate were 1.51%-39.81%. Soil fauna increased C/P of foliar litter quality for C. chengiana, S. paraqplesia, C. longepaniculatum and A. faxoniana litter, with the contribution rates ranging from-2.44% to-18.01%, but decreased C/P for the other litter species, the contribution rates were 0.56%-73.84%. Soil fauna increased N/P of foliar litter for C. chengiana, A. nubigena, S. paraqplesia, S. saltuaria and C. longepaniculatum litter, with the contribution rates ranging from-4.16% to-32.33%, but decreased N/P for the other litter species, the contribution rates were 2.38%-56.48%. Soil fauna showed remarkable effects on lignin/N of foliar litter. Soil fauna decreased lignin/N of foliar litter for A. nubigena, C. fortunei, S. paraqplesia, Q. baronii and C. chengiana (DV) litter, the contribution rates were 0.90%-35.55%, but increased lignin/N for the other litter species, with the contribution rates ranging from-2.91% to -30.19%. Soil fauna decreased lignin/cellulose of foliar litter for A. nubigena, C. longepaniculatum, Q. baronii, B. albo-sinensis and S. saltuaria litter, the contribution rates were 1.35%-43.26%, but increased lignin/cellulose for the other litter species, with the contribution rates ranging from -2.71% to -152.26%;In summary, during the decomposition of foliar litter from the dominant species in the upper reaches of the Minjiang River, soil fauna showed significant effects on litter mass-loss rate, elements release, degradations of lignin and cellulose, and the releases of multiple metallic metal elements. Soil fauna contribution rates in alpine ecosystem, ecotone between dry valley and montane forest show consistent decrease with increase in elevation, but showed a contrast trend in subtropical evergreen broad-leaved forests. Such effects were correlated strongly with climate, environmental factors such as precipitation, soil physiochemistry and temperature in different ecosystems. Meanwhile, litter quality was also affected by soil fauna during litter decomposition, with decreases in C/N, C/P and N/P ratios, which indicated that soil fauna promoted litter decomposition. In addition, litter initial quality and temperature were significantly correlated with the effects of soil fauna on litter decomposition process. These results provided necessary data for our better understanding on the effects of soil fauna on litter decomposition process and the related ecological processes in the upper reaches of the Minjiang River. |
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