Nitrogen fate and environmental consequence in paddy soil under rice-wheat rotation in the Taihu lake region, China

Abstract

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.

Nitrogen Loss through Anaerobic Ammonium Oxidation Coupled to Iron Reduction from Paddy Soils in a Chronosequence

“…For the examined chronosequence, which was composed of an uncultivated soil (RC10 soil) and three cultivated paddy soils at the top (RC11 soil), middle (RC12 soil), and bottom (RC13 soil) of a slope with increasing cultivation age, the measurements of microbially reducible Fe(III), total Fe, and other chemical properties including pH, total organic carbon (TOC), total nitrogen (TN), NH 4 + , NO x − (i.e., NO 2 − and NO 3 − ), and dissolved organic carbon (DOC) are shown in Table 1. A gradient of microbially reducible Fe(III) levels was an important characteristic of the chronosequence, varying considerably from the lowest (0.54 ± 0.03 g kg −1 ) in the RC10 soil to the highest (4.5 ± 0.16 g kg −1 ) in the RC12 soil.…”

“…It is noteworthy that, 29 N 2 also significantly accumulated in all soils amended with 15 NH 4 + (Figure 1b), though the added 15 NH 4 + pool (90 mg N kg −1 ) was much larger than the indigenous 14 NH 4 + pool in all soils (7.0−21 mg N kg −1 after the preincubation), indicating that the indigenous 14 NH 4 + and added 15 NH 4 + pools were completely mixed. Several possible pathways exist for 29 N 2 production: the utilization of indigenous 14 NH 4 + in combination with added 15 NH 4 + in Feammox to N 2 , Feammox to NO 2 − followed by anammox or codenitrification to N 2 , or Feammox to NO 2 − or NO 3 − followed by denitrification to N 2 (Table 2).…”

“…The headspace of soil slurries was flushed with ultrahigh purity He. Three treatments (n = 6 per treatment) were established: (i) control treatment with sterile anoxic deionized water instead of 15 4,17 For the C 2 H 2 treatment, 23 mL headspace gas in each vial was removed and replaced with 23 mL C 2 H 2 to reach 30% (v/v) C 2 H 2 in the headspace. All vials were shaken vigorously to homogenize the treatment solutions and dissolve the C 2 H 2 .…”

“…Several possible pathways exist for 29 N 2 production: the utilization of indigenous 14 NH 4 + in combination with added 15 NH 4 + in Feammox to N 2 , Feammox to NO 2 − followed by anammox or codenitrification to N 2 , or Feammox to NO 2 − or NO 3 − followed by denitrification to N 2 (Table 2). Nevertheless, all of these pathways are based on anaerobic NH 4 + oxidation. As with the potential Feammox rates (i.e., 30 N 2 production rates), the 29 N 2 production rates were also positively correlated with Fe(III) reduction rates (P < 0.0001) and microbially reducible Fe(III) (P < 0.01) levels (Figure 3b and SI Table S1), this further indicates that Feammox played an important part in anaerobic NH 4 + oxidation, irrespective of the ultimate pathway for 29 N 2 production.…”

“…Nevertheless, all of these pathways are based on anaerobic NH 4 + oxidation. As with the potential Feammox rates (i.e., 30 N 2 production rates), the 29 N 2 production rates were also positively correlated with Fe(III) reduction rates (P < 0.0001) and microbially reducible Fe(III) (P < 0.01) levels (Figure 3b and SI Table S1), this further indicates that Feammox played an important part in anaerobic NH 4 + oxidation, irrespective of the ultimate pathway for 29 N 2 production. Considering the generation of both 30 N 2 and 29 N 2 , anaerobic NH 4 + oxidation consumed 0.40−0.60% and 2.1−9.1% of the added 15 NH 4 + label in the uncultivated and paddy soils after the incubations, respectively.…”