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Soil organic carbon (SOC) and soil inorganic carbon (SIC) are important C pools in the Loess Plateau of Northwest China, however, variations of SOC and SIC stocks under different cultivation practices and nitrogen (N) fertilization rates are not clear in this area. A long-term field experiment started in June 2003 was conducted to investigate the SOC and SIC stocks in a calcareous soil of the Chinese Loess Plateau under four cultivation practices, i.e., fallow (FA), conventional cultivation (CC), straw mulch (SM), and plastic film-mulched ridge and straw-mulched furrow (RF), in combination with three N fertilization rates, i.e., 0 (N0), 120 (N120), and 240 (N240) kg N/hm2. Results indicate that the crop straw addition treatments (SM and RF) increased the contents of soil microbial biomass C (SMBC) and SOC, and the SOC stock increased by 10.1%–13.3% at the upper 20 cm soil depth in comparison to the 8-year fallow (FA) treatment. Meanwhile, SIC stock significantly increased by 19% at the entire tested soil depth range (0–100 cm) under all crop cultivation practices in comparison to that of soil exposed to the long-term fallow treatment, particularly at the upper 60 cm soil depth. Furthermore, moderate N fertilizer application (120 kg N/hm2) increased SOC stock at the upper 40 cm soil depth, whereas SIC stock decreased as the N fertilization rate increased. We conclude that the combined application of crop organic residues and moderate N fertilization rate could facilitate the sequestrations of SOC and SIC in the calcareous soil.

In this study, we built a submerged macrophyte model (GBAEDM) that could simulate several macrophyte species simultaneously. The model was calibrated and validated using pilot-scale experimental data, with satisfactory results. The coefficient of determination (R2) and root-mean-square error (RMSE) values were 0.86 and 0.03 for TP, 0.71 and 0.38 for TN, 0.93 and 6.78 for Chl a, and 0.99 and 27.61 for submerged macrophyte biomass. The GBAEDM was then used to choose appropriate species and suitable initial plant biomass submerged macrophyte recovery in Gonghu Bay. The appropriate species at 1 m depth were Vallisneria spinulosa (1150 g/m2), Myriophyllum spicatum (525 g/m2), and V. spinulosa–M. spicatum mixed (600 g/m2; 3:1) communities. At 2 m depth, the appropriate species were M. spicatum (1450 g/m2) and V. spinulosa–M. spicatum mixed (1950 g/m2; 12:1) communities. Only M. spicatum (1608 g/m2) was recommended at 3 m depth. Finally, the GBAEDM was used to determine the critical water transparency thresholds for submerged macrophytes in Gonghu Bay. A transparency value of 0.62 m at 2 m (the WT/H value of 0.31) was determined as the critical underwater light threshold for the recovery of submerged macrophytes in Gonghu Bay.