| To explore the influence of mass ecological restoration on soil carbon balance in the Loess plateau, this study as the objects of different plant types-arbor, shrub, grass and wheat, different vegetation restoration stages and different plantation structures of pure and mixed forests, on the base of field investigation and laboratory analysis, by measure the typical soil and it’s aggregates carbon fractions and their relationship, had been done to research the trend of carbon variations. The main research results as follows:1. Effects of vegetation restoration on soil organic carbon(SOC) and total carbon(STC)(1) Vegetation restoration could influence on SOC and STC greatly.Soil carbon content could be improved greatly in forestland and grassland, especially in shrub forestland and natural grassland in the whole profile. The obvious difference of SOC and STC was in 0-5 cm layer, SOC under forestland and grassland was markedly higher than that under farmland by 70%-107%, their STC by 34%-66%.(2) There were some difference on SOC and STC during vegetation restoration stages.During short time planting Hippophae reamnoides, soil carbon could been increased distinctly, while long time planting Robinia pseudoacacia, soil carbon could been enhanced notably. The obvious differences of SOC and STC during different stages were in 0-5 cm layer, with the extension of planting time, STC and SOC increased very slowly under Robinia pseudoacacia but constantly(22-28 year STC increased by 14%, 22-41 year SOC increased by 7%-68%). SOC and STC under Hippophae reamnoides increased promptly during 15-21 years(STC and SOC increased by 94% and 158% individually), while that during 21-26 years decreased obviously(STC and SOC decreased by 32%, 43% individually). The rate of change of SOC and STC under Pinus tabuliformis was between those under Robinia pseudoacacia and under Hippophae reamnoides, 30 years ago SOC and STC slowly increased with the etension of planting time, while 30 years later they decreased slowly.(3) The accumulation of SOC and STC were influenced greatly by plantation structure.The accumulation of carbon under mixed forestland was more than that under pure forestland. The obvious difference of SOC and STC was in 0-5 cm layer, SOC content under mixed forestland was higher than that under their pure forestland for 16%-160%, except that under the mixed forestland of Pinus tabuliformis-Hippophae reamnoides-Acer mono Maxim was lower than that under Hippophae reamnoides pure forestland for 3%, that under mixed forestland of Pinus tabuliformis-Hippophae reamnoides-Robinia pseudoacacia was lower than that under Hippophae reamnoides and Robinia pseudoacacia pure forestland for 19%, 21% individually. STC content under mixed forestland was higher than that under their pure forestland for 8%-115%, except that under mixed forestland of Pinus tabuliformis-Hippophae reamnoides-Robinia pseudoacacia was lower than that under Hippophae reamnoides and Robinia pseudoacacia pure forestland for 15% and 10% individually.2. Effects of vegetation restoration on soil aggregates and their organic carbon content(1) Vegetation restoration could affect soil aggregates and their carbon concent greatly.Vegetation restoration could improve effectively big size aggregate content, especially by shrub forest and natural grass, while big size aggregates could been destroyed by conventional tillage. The obvious difference of >0.25 size aggregate content was in 0-5 cm layer, soil MWD under forestland and grassland was higher than that under farmland by 417-811%, total big size aggregate content were higher than that under farmland by 29%-51%. 48% aggregates under farmland was in 0.25-0.5mm size, that under grassland and forestland was less than 20% in 0.25-0.5mm size, while more than 50% aggregate was in 0.5-2mm size.Vegetation restoration also could improve effectively soil aggregate organic carbon content of big size, especially by shrub forest. For each plant type, among aggregate of >0.25mm size, aggregate organic carbon content in 0.25-0.5mm size was the lowest, while that in 2-5mm and 1-2mm size was higher. The obvious difference of aggregate organic carbon content was in 0-5 cm layer, the difference of aggregate organic carbon in 0.5-1mm size (that under forestland and grassland markedly higher than that under farmland for 53%-129%) and 0.25-0.5mm(that under forestland and grassland markedly higher than that under farmland for 194%-196%) were more obvious than others among different plant types.(2) There were certain different in soil aggregates and their organic carbon during different vegetation restoration stages.During short time of planting Pinus tabuliformis, it’s soil aggregates could been increased distinctly. While long time of planting Robinia pseudoacacia, it’s soil aggregates could been enhanced greatly. The obvious difference of aggregate content was in 0-5 cm layer, aggregate content under Pinus tabuliformis forestland during 16-36 years decreased with the extension of planting time, during 16-26 years soil aggregates in 2-5mm, 1-2mm and 0.5-1mm decreased by 41%-79%, during 26-36 years soil aggregates in 1-2mm, 0.5-1mm and 0.25-0.5mm decreased by 43%-57%. during 28-41 years under Robinia pseudoacacia forestland increased differently by 64%-143% (except >5mm size). That under 16-26 years Hippophae reamnoides forestland decreased 36%-90% (except 0.25-0.5mm).During vegetation restoration years, aggregate organic carbon in 2-5mm, 1-2mm and 0.5-1mm size were higher than that in other sizes. 0-5cm layer, aggregate organic carbon content under 16-26 years’Pinus tabuliformis forestland decreased by 14%-36%, that under that of 26-36 years increased a little bit. That under 28-41 years Robinia pseudoacacia forestland increased differently by 23%-71%. That under 16-26 years Hippophae reamnoides forest increased firstly and then decreased, that of 16-21 years increased by 46%-126%, 21-26 years decreased by 24%-52%.(3) Soil aggregates and their carbon were influenced greatly by plantation structure. >0.25mm aggregate content under pure forest was higher than that under mixed(except that under Robinia pseudoacacia pure forest). While aggregate organic carbon was on the contrary, in the 0-5 cm layer, that under Biota orientalis pure forest was lower than that under it’s mixded by 40%-58% (except that in >5mm size higher by 30%); that under Hippophae reamnoides pure forest was lower than that under mixed Pinus tabuliformis-Hippophae reamnoides-Robinia pseudoacacia forest by 23%-39% (except that in 0.25-0.5mm size higher by 21%); that under Pinus tabuliformis pure forest was lower than that under mixed forest of Pinus tabuliformis-Hippophae reamnoides-Robinia pseudoacacia by 7%-30%; that under Robinia pseudoacacia pure forest was lower than that under it’s mixed by 1%-51%.3. Effects of vegetation restoration on soil labile organic carbon(LOC)(1) Different plant type leaded to the big difference on soil LOC.Soil LOC fractions’content and their proportions under forestland and grassland were higher than those under farmland in the 0-100 cm profile, especially under shrub forestland and natural grassland. The obvious difference of soil LOC was in 0-5cm layer, compared to farmland, EOC of forestland and grassland was higher by 179%-204%, LFOC of those was higher by 250%-1303%, CPOC of those was higher for 865%-1409%, FPOC of those was higher by 65%-239%, DOC was higher by 86%-155%.(2) Soil LOC during different vegetation restoration stages varied greatly.During short time of planting Hippophae reamnoides, soil LOC could been increased distinctly. While long time of planting Robinia pseudoacacia, soil LOC could been improved greatly. The obvious difference of soil LOC was in 0-5 cm layer, soil LOC under Robinia pseudoacacia forestland increased very slowly with the extension of planting time but constantly increased, EOC increased by 60%, LFOC increased by 18%-137%, CPOC increased by 67%-210%, FPOC increased by 27%-243%, DOC increased by 69%. In 0-5 cm layer, soil LOC under Hippophae reamnoides forestland increased promptly during 15-21 years, EOC increased by 25%-67%, LFOC increased by 82%, CPOC increased by 128%, FPOC increased by 57%, DOC increased by 90%; while those of during 21-26 years decreased obviously, EOC decreased by 31%, LFOC decreased by 58%, CPOC decreased by 65%, FPOC decreased by 40%, DOC decreased by 35%.The rate of change of soil LOC under Pinus tabuliformis forestland was between that under Robinia pseudoacacia and that under Hippophae reamnoides, 30 years ago with the etension of planting time, LOC slowly increased, while 30 years later they decreased slowly.(3) There were obvious difference on soil LOC among different plantation structure.Compared to pure forestland, mixed forestland could boost effectively soil LOC, for example, in the 0-5 cm layer, EOC increased by 16%-304%, LFOC increased by 17%-1399% (while that under mixed forest of Hippophae reamnoides was lower than that under it’s pure forest by 7%-42%), CPOC increased by 7%-639% (while that under mixed forest of Robinia pseudoacacia-Hippophae reamnoides was lower than that under Hippophae reamnoides pure forest by 8%,that under mixed forest of Pinus tabuliformis-Hippophae reamnoides-Robinia pseudoacacia was lower than that under their pure forest by 21%-54%), FPOC increased by 16%-633% (while was lower than that under Hippophae reamnoides forest by 5%-53%), DOC increased by 17%-170% (while that under mixed Hippophae reamnoides forest was lower than that under it’s pure forest for 10%).4. The relationship among different carbon fractionsThe correlation coefficients among carbon fractions showed: the composition of the substance and its material source of EOC, LFOC and POC were basically the same, while those of heavy fraction organic carbon and stable organic carbon were basically the same, DOC mainly came from EOC and FPOC, the increase of DOC could decrease soil inorganic carbon content. Alkaline-phosphatase, Invertase and Urease activities related directly with the transformation of soil LOC and their availibility; while Catalase activity could reflect the characteristics of soil carbon amount.The correlations between STC, organic carbon fractions and aggregates, aggregate organic carbon showed: organic carbon fractions were mainly concentrated in aggregates of 1-2mm and 0.5-1mm; in addition, the correlation coefficients between 1-2mm, 0.5-1mm size aggregate and four enzyme activities were higher than those of other sizes, this also could indirectly explain that soil enzyme activity would affect the formation of aggregate. |
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