Distribution of soil carbon and nitrogen in two typical forests in the semiarid region of the Loess Plateau, China

- We examine C and N distribution along soil profiles of two semiarid forests.
- Organic C and N contents are higher in surface layers and remain steady under 1 m.
- Inorganic C content is substantial in the loess and relatively stable along profiles.
- In this region the soil layer of 1-2 m still stores great amounts of C and N.
- Natural oak forest has advantage over black locust plantation in C and N accumulation.

Forest ecosystems on the Loess Plateau are receiving increasing attention for their special importance in carbon sequestration and in the conservation of soil and water in the region. To evaluate soil carbon and nitrogen accumulation in two typical forests in the region, we conducted plot surveys in three near-mature stands for each forest type. Soil samples within 0-200 cm were collected from different depths, and the soil organic carbon (SOC), inorganic carbon (SIC), and total nitrogen (STN) were measured. The results indicate that the oak (Quercus liaotungensis) forests had greater SOC and STN contents and lower soil buck density than the black locust (Robinia pseudoacacia) plantations within top soil layers. Both SOC and STN contents decreased with soil depth, and the differences between the forest types became insignificant in layers beneath 50 cm. The total accumulations of SOC and STN were larger in the oak forests than in the black locust plantations, particularly in the upper soil layers. The results suggest that the oak forest has advantages over the plantation forest regarding SOC and nitrogen accumulation in the upper soil layer, and both types of forest soils have acted as substantial carbon sinks. However, in the loess soil in this region, there were considerable amounts of SIC storage (4-5 times of SOC in deep layers), and the amounts of soil total carbon (STC) did not differ obviously between the two ecosystems. Relative to the conventional soil survey of 100-cm depth, the SOC, STN and STC stored in 100-200 cm deep soil account for 0.45, 0.49 and 0.91 times in the oak forests, respectively, compared to 0.65, 0.65 and 1.01 times in the black locust plantations for SOC, STN and STC, respectively. Thick loess soil coupled with dry climate may be attributable to the development of root system and consequent SOC and STN distribution into deeper layer. This suggests that deep soil layers contribute considerable amounts and should not be omitted in soil carbon estimates.

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