Abstract:

【Objective】 The Greater Khingan Mountains area is the only frigid temperate region in China with abundant forest resources; however, in the Greater Khingan Mountains, the soil layer is thin and permafrost exists. The Greater Khingan Mountains has a great influence on the soil structure and nutrient circulation in this area. The structure and composition of the soil aggregates, and the content and distribution of organic carbon and nitrogen in this area were discussed. Further, the effects of aggrgates with different particle sizes on the retention and protection of organic carbon and nitrogen in the soil were understood, which provided a theoretical basis for further study of soil structure and the carbon-nitrogen cycle in frigid temperate regions of China.【Method】 In the Greater Khingan Mountain area of Heilongjiang Province, four main forest types in the frigid temperate zone of China (Larix gmelinii, Pinus sylvestris var. mongolica, Populus davidiana and Betula platyphylla) were studied. In the growing season, we determined the proportions of water-stable aggregates with particle sizes of >1.000, ≥0.500-1.000, ≥0.250-0.500, ≥0.053-0.250 and <0.053 mm, as well as the contents of organic carbon and nitrogen bound by aggregates in the soil layers of 0-5, ≥5-10 and ≥10-20 cm. The contribution rates of organic carbon and nitrogen to total soil organic carbon and total nitrogen were analyzed, and multi-factor analysis of variance was performed.【Result】 ① The 0-10 cm soil layer of Pinus sylvestris var. mongolica, Populus davidiana and Betula platyphylla, as well as the 0-5 cm soil layer of Larix gmelinii, were dominated by macroaggregates (≥0.250 mm), accounting for more than 50% of the soil. However, as the soil deepens the macroaggregate content declines. The macroaggregate content in the middle of the growing season of each forest type was higher than that in the early and late growth seasons, and the content of macroaggregates in broadleaved forests was higher than that in coniferous forests. ② The organic carbon and total nitrogen contents of the aggregates showed roughly the same change rule. The contents of organic carbon and nitrogen in aggregates with a diameter of ≥0.500 mm were higher in the four forest types, which showed a decrease as the size decreased. The content of organic carbon and nitrogen in the aggregates of broadleaved forests was higher than that of coniferous forests. The content of broad-leaved forest is also higher than that of coniferous forest in the middle and late growing season, whereas the deciduous pine forest of Larix gmelinii showed a fluctuating trend, and the Pinus sylvestris var. mongolica had higher content during the early growth season. ③ In the 0-10 cm soil layer of the four forest types, the contribution rate of organic carbon and nitrogen aggregates with a ≥0.500 mm particle size to soil total carbon and nitrogen was high, up to 90%. As the soil layer deepened, the contribution rate of macroaggregates decreased and the contribution rate of aggregates of <0.250 mm in the ≥10-20 cm soil layer was the highest. ④ The forest types, soil layers, and time of year had significant effects on the composition of soil aggregates, as well as the organic carbon and total nitrogen contents of aggregates. Moreover, the contents of organic carbon and nitrogen in aggregates with a ≥0.500 mm particle size were positively correlated with the contents of the aggregates of the corresponding particle size. Further, the contents of organic carbon and nitrogen in aggregates with a >1.000 mm particle size were significantly and positively correlated with the contents of the aggregates of the corresponding particle size.【Conclusion】 Changes in forest type, soil layer, and month all have an impact on the composition of soil aggregates, and their combined organic carbon and nitrogen content. The content of macroaggregates, along with the contents of organic carbon and nitrogen in macroaggregates in broadleaved forests were higher than those in coniferous forests. Depending on the month, four forest types had higher concentrations of macroaggregates in the middle of the growing season, the contents of organic carbon and nitrogen in broadleaved forest aggregates was higher in the middle and late growing seasons, and the coniferous forest showed a fluctuating trend during the growing season. With the deepening of the soil layer, the contents of macroaggregates, organic carbon, nitrogen and the contribution rate of macroaggregates all decreased gradually. Aggregates measuring >1.000 and ≥0.500-1.000 mm in size were the main carriers of organic carbon and total nitrogen in the study area. Thus, it can be seen that the aggregate composition and the combined organic carbon and nitrogen characteristics of the four forest types in the frigid temperate zone are different. To some extent, this reflects the soil structure, and the carbon and nitrogen fixation characteristics of the main forest types in the frigid temperate zone.

Key words: frigid temperate zone, forest type, soil structure, growing season, water-stable aggregate, soil organic carbon(SOC), total nitrogen(TN), Greater Khingan Mountains