Field undisturbed tension-free monolith lysimeters and 15 N-labeled urea were used to investigate the fate of fertilizer nitrogen in paddy soil in the Taihu Lake region under a summer rice-winter wheat rotation system. We determined nitrogen recovered by rice and wheat, N remained in soil, and the losses of reactive N (i.e., NH 3 , N 2 O, NO 3 − , organic N and NH 4 + ) to the environment. Quantitative allocation of nitrogen fate varied for the rice and wheat growing seasons. At the conventional application rate of 550 kg N ha −1 y −1 (250 kg N ha −1 for wheat and 300 kg N ha −1 for rice), nitrogen recovery of wheat and rice were 49% and 41%, respectively. The retention of fertilizer N in soil at harvest accounted for 29% in the wheat season and for 22% in the rice season. N losses through NH 3 volatilization from flooded rice paddy was 12%, far greater than that in the wheat season (less than 1%), while N leaching and runoff comprised only 0.3% in the rice season and 5% in the wheat season. Direct N 2 O emission was 0.12% for the rice season and 0.14% for the wheat season. The results also showed that some dissolved organic N (DON) were leached in both crop seasons. For the wheat season, DON contributed 40-72% to the Nleaching, in the rice season leached DON was 64-77% of the total N leaching. With increasing fertilizer application rate, NH 3 volatilization in the rice season increased proportionally more than the fertilizer increase, N leaching in the wheat season was proportional to the increase of fertilizer rate, while N 2 O emission increased less in proportion than fertilizer increase both in the rice season and wheat season.

Biochar has been increasingly used as a method for C sequestration and soil improvement. To understand how feedstock and pyrolysis conditions affect biochar characteristics, we investigated two wood-based biochars (bamboo and elm) and five crop-residue-based biochars (wheat straw, rice straw, maize straw, rice husk and coconut shell), which were pyrolyzed at 500°C or 700°C and remained at that temperature for 4 h, 8 h, and 16 h under oxygen-limited conditions. For a given feedstock, increasing pyrolysis temperature from 500°C to 700°C resulted in increases in ash content, BET surface area, pH, total P and Ca contents (P<0.05) and decreases in yield, cation exchange capacity (CEC), total acid and total N (P<0.05). Prolonging residence time (from 4 h to 8 h or 16 h), the BET surface area and ash content of biochars increased (P<0.05), whereas the yield decreased (P<0.05). Fourier-transform infrared spectroscopy (FTIR) analysis showed that more recalcitrant and aromatic structures were formed in the biochars with increased temperature. The three straw-based biochars consistently exhibited far greater ash percentage (14.5-40.3 wt%), CEC (14.1-34.8 cmol kg -1 ), and the contents of total N (0.24-2.81 wt%), P (0.60-8.41 wt%), Ca (0.60-8.41 wt%) and Mg (0.24-0.60 wt%), and generally higher yield (19-37.6 wt%), pH (9.7-11.1), and contents of total acid (0.15-0.53 mmol g -1 ), C (42.1-55.1 wt%), Na (0.27-6.72 wt%) and K (7.07-28.1 wt%) than the two wood-based biochars. The BET surface area of straw-based biochars with 700°C pyrolysis temperature could be as high as 112-378 m 2 g -1 , a comparable level with that of wood-based biochars. Despite the high variability in biochar properties, these results demonstrate that biochars from crop straw may be more effective and desirable for improving soil fertility and C sequestration in Chinese vast soils.

Using soil slurry-based (15)N tracer combined with N2/Ar technique, the potential rates of denitrification, anaerobic ammonium oxidation (anammox), and dissimilatory nitrate reduction to ammonium (DNRA), and their respective contributions to total nitrate reduction were investigated in 11 typical paddy soils across China. The measured rates of denitrification, anammox, and DNRA varied from 2.37 to 8.31 nmol N g(-1) h(-1), 0.15 to 0.77 nmol N g(-1) h(-1) and 0.03 to 0.54 nmol N g(-1) h(-1), respectively. The denitrification and anammox rates were significantly correlated with the soil organic carbon content, nitrate concentration, and the abundance of nosZ genes. The DNRA rates were significantly correlated with the soil C/N, extractable organic carbon (EOC)/NO3(-) ratio, and sulfate concentration. Denitrification was the dominant pathway (76.75-92.47%), and anammox (4.48-9.23%) and DNRA (0.54-17.63%) also contributed substantially to total nitrate reduction. The N loss or N conservation attributed to anammox and DNRA was 4.06-21.24 and 0.89-15.01 g N m(-2) y(-1), respectively. This study reports the first simultaneous investigation of the dissimilatory nitrate reduction processes in paddy soils, highlighting that anammox and DNRA play important roles in removing nitrate and should be considered when evaluating N transformation processes in paddy fields.