|Other Abstract||Soil organic carbon (SOC) and inorganic carbon (SIC) are important reservoirs of carbon in the terrestrial ecosystems. Assessments of both SOC and SIC are lacking in arid regions. We carried out a study in the Yanqi Basin, located in central Xinjiang, to evaluate the dynamics of SOC and SIC. Twenty one soil profiles were sampled from three types of land: desert, shrub, and agricultural soils. We determined SOC and SIC, and the stable carbon isotopic compositions (δ13C). In order to investigate the effects of calcium source and cropping on soil CO2 concentration and CO2 efflux, field measurements were also conducted in the cropland of Yanqi basin during the maize growing season. The main parts of this study are as follows:
(1) Evaluation of Loss-on-ignition (LOI) methods: There were strong linear relationships (r > 0.94, P < 0.001) between the traditional method and the LOI technique for both SOC and SIC measurements. We used one set of soil samples to develop relationships between LOI and SOC (by the Walkley-Black method), and between LOI and SIC (by the pressure calcimeter method). Then the other set of soil samples was used to evaluate the derived equations. By comparing predicted SOC and SIC with measured values, we found that the mean absolute errors were small for both SOC (1.7 g C kg-1) and SIC (1.22 g C kg-1). Our study demonstrated the capacity of the LOI methods for accurate estimates of SOC and SIC in arid soils
(2) Effects of calcium source and cropping on soil CO2 and CO2 efflux: Soil CO2 concentrations showed an increase with depth during the maize growing season, which followed an order: bare ground with Ca addition (BG+Ca) < bare ground (BG) < maize field (MF). There was a significant diurnal variation in CO2 efflux, with the lowest and highest values appeared at 5:00-7:00 and 12:00-14:00, respectively. As for the seasonal variation, the lowest CO2 efflux appeared in October whereas the highest was in June under the MF treatment and in July under the BG and BG+Ca treatments. During the growing season, CO2 efflux was 273, 241 and 688 g C m-2 for the BG, BG+Ca and MF treatments, respectively.
(3) Impacts of land use change on SOC and SIC: Our data showed SOC decreased with depth in all soil profiles, however, SIC increased with depth in all soil profiles except in the desert soils. Both SOC and SIC stocks (over the 0–30-cm depth) were lowest in the desert soils (1.0±0.3 and 4.0±0.8 kg C m-2 for SOC and SIC, respectively), but greatest in the agricultural soils (4.6 ±0.6 and 11.0 ±2.0 kg C m-2 for SOC and SIC, respectively). Total soil carbon stocks in the 0–100 cm were 11.6 ±4.8, 45.1±10.4 and 51.2±5.6 kg C m-2 in the desert soils, shrub soils and agricultural soils, respectively. On average, SIC accounted for more than 80% of the total carbon in this region. There were no significant differences in δ13C of SOC among land use types. In contrast, the δ13C of SIC was different: desert soils (-0.6‰) > shrub soils (-2.2‰) > agricultural soils (-3.4‰). The depletion of 13C in SIC of the agricultural soils indicates enhancement of pedogenic carbonate (PIC) by cropping. Our study suggested that converting shrub land to agricultural land in arid regions may lead to an increase not only in SOC stock, but also in SIC stock.
(4) Accumulation rate of soil carbonate: In the Yanqi Basin, SIC stock was 9.7, 36.8 and 42.0 kg C m-2 in desert soil, shrub soil and agricultural soil, respectively. On average, PIC was 18.3%, 32.5% and 44.5% of SIC for desert soil, shrub soil and agricultural soil, respectively. The accumulation rate of SIC in the agricultural soil was 23.3, 85.0 and 113.3 g C m-2 yr-1 for SOC, SIC and PIC, respectively.
In summary, converting shrub land to agricultural land in arid regions may lead to an increase not only in SOC stock, but also in SIC stock. The accumulation rate of PIC exceeds that of SOC in the agricultural soil. These analyses suggest that SIC accumulation rate may be underestimated previously. Further studies are needed to investigate the fluxes and transformations of various carbon forms in arid regions.|