[Effects of Combined Application of Biogas Slurry and Straw on the Migration and Fractions of Soil Heavy Metals in Rice-wheat Rotation System in Coastal Reclamation Areas].

In order to reveal the impact of the application of biogas slurry instead of chemical nitrogen fertilizer on the environmental risk of heavy metals in the soil by returning straw to the field, four treatments, without biogas slurry and without straw applications (CK), biogas slurry application without straw (B), straw application without biogas slurry (S), and biogas slurry combined with straw applications (BS), were applied in a typical coastal reclaimed farmland (rice-wheat rotation) in Jiangsu province. The migration and morphological characteristics of Cu, Zn, Cd, and Pb in different soil layers were observed, and the potential environmental risks were estimated. The results showed that:① The total amounts of Zn and Pb in the surface soils (0-20 cm) in the rice and wheat fields under the BS treatment decreased significantly (P<0.05). The four heavy metals in the paddy soils migrated 6%-11% from the surface to the middle and lower layers (20-60 cm), and Cu, Cd, and Pb in the wheat soils migrate down from the surface by 25% to 33%. This indicated that the combined use of biogas slurry and straw accelerates the vertical downward movement of heavy metals in the surface soil. ② Under the BS treatment, the contents of the weak acid extraction of Cu in the surface soil of the paddy field decreased by 8.8%, and the residual state of Zn, Cd, and Pb decreased by 7.0% to 14.2%. This revealed that Cu was passivated, but Zn, Cd, and Pb tended to be activated. In comparison, the reduction in Cu residues in wheat field surface soil was 2.8 times that of the weak acid extraction, indicating that Cu was activated. Furthermore, the residue state of Cd increased, the weak acid extraction state of Pb decreased, and Cd and Pb were passivated. ③ The ecological risk assessment of heavy metals showed that there is no ecological risk in the soils under the BS treatment, and the risk indices were significantly lower than those of the B and S treatments (P<0.05). Therefore, the combined use of biogas slurry and straw helps to significantly reduce the risk of heavy metal pollution in the soils in the coastal reclamation areas.

[Effects of drying-rewetting alternation on urease activity in Chongming east intertidal flat: results of a simulation study].

Sediment and sea water samples were taken from Chongming east intertidal flat, a typical coastal wetland in China, to define the responses of urease activity to periodic alternation of drying and rewetting. Sediment cores were incubated under 30, 35 and 40 degrees C with simulated spring tides (approximately twice a month) and semi-diurnal tides (two almost equal high tides and two low tides in a lunar day). Urease activities in sediments were determined during the incubations as well as dissolved organic nitrogen (DON) and ammonium nitrogen (NH+4-N) contents. The apparent temperature sensitivity index (Q'10) of urease activity was calculated based on the observations of urease activities under 30, 35 and 40 degrees C. Urease activities rapidly increased following the rewetting of dried sediments (1%-3% ) and dropped with decreasing sediment moisture during simulated spring tides. Repeated drying and rewetting cycles lowered the sensitivity of urease to the changes of moisture. In contrast, semi-diurnal tides slightly affected urease activities, in which the enzyme maintained relatively stable activities with (0.067 +/- 0. 018 ) , (0. 143 +/- 0. 027 ) and ( 0. 028 +/- 0. 011) g (10 g.h)-1 under 30, 35 and 40 degrees C , respectively. Urease activities during spring tides were found to be much lower than those of semi-diurnal tides under both 30 and 35 degrees C , implying that repeated drying and rewetting cycles possibly lessened urease activities in sediments. In addition, the highest Q'10 of urease activity occurred happened between 35 degrees C and 40 degrees C during the spring tides, but happened between 30 degrees C and 35 degrees C during semi-diurnal tides. This indicated that drying and rewetting events helped to increase the temperature regime, where the Q'10 of urease activity achieved to the highest. Moreover, NH+4 -N contents depended on the urease activities in sediments during spring tides under 30 degrees C and 35 degrees C. However, No significant relationship between urease activities and DON contents was found. In contrast, changes in DON and NH+4 -N contents hardly affected urease activities during semi-diurnal tides.

[Effects of drying-rewetting alternation on nitrogen, dynamics in a typical coastal wetland: a simulation study].

The coastal wetland, a key transitional zone between land and ocean, is a complex and sensitive ecosystem with special environmental processes and functional values. Sediment and sea water samples were taken from Chongming east intertidal flat, a typical coastal wetland in China. Sediment cores were incubated under simulated spring tides (approximately twice a month) and semi-diurnal tides (two almost equal high tides and two low tides in a lunar day), to define the responses of nitrogen to periodic alternation of drying and re-flooding. The contents of NO3(-) -N, NO2(-) -N, NH4(+) -N, dissolved organic N (DON), and the activities of nitrate reductase (Nar), nitrite reductase (Nir), and hydroxylamine reductase (Hyr) in sediments were determined during the incubation. Nitrification stood in a dominant position during the drying period with sediment moisture decreasing from 35% to 5% -7% in simulated spring tides. The NO3(-) -N and NO2(-) -N were found to be substantially converted into DON following a further exsiccation (the sediment moisture decreased from 5%-7% to 0%-3%). However, the conversion rates decreased with the increase of drying and rewetting cycles. Following the rewetting of dried sediments (from 0% -3% to 37% -45%), the contents of NO3(-) -N, NO2(-) -N, NH4(+) -N and DON increased to 1 to 3 times that of the dry sediments. Nar and Nir activities in sediments rapidly increased after the rewetting, indicating that the reduction of NO3(-) -N and NO2(-) -N were substantially enhanced. Significant positive correlations were found between Nar and Nir activities, and Hyr activities and NH4(+) -N contents during the simulated spring tides. Moreover, the decreases of NO3(-) -N and NO2(-) -N contents were observed to be significantly correlated to the increases of NH4(+) -N contents. Combined with the fact that the sediment had a high organic carbon content but a very low NO3(-) -N content, it can be concluded that the reduction of NO3(-) -N and NO2(-) -N following the rewetting was dominated by nitrate ammonification. In contrast, NO3(-) -N, NO2(-) -N, NH4(+) -N and DON contents in sediments were relatively stable during semi-diurnal tides, which were (3.0 +/- 0.3), (1.2 +/- 0.1), (133.3 +/- 4.3) and (41.1 +/- 10.6) mg x kg(-1), respectively. This indicated that semi-diurnal tides slightly affected the variations of nitrogen contents in the wetlands.