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Abstract: Nitrogen amendment followed by flooding irrigation is a general management practice for a wheat–maize rotation in the North China Plain, which may favor nitrification and denitrification. Consequently, high emissions of nitrous oxide (N2O) and nitric oxide (NO) are hypothesized to occur. To test this hypothesis, we performed year-round field measurements of N2O and NO fluxes from irrigated wheat–maize fields on a calcareous soil applied with all crop residues using a static, opaque chamber measuring system. To interpret the field data, laboratory experiments using intact soil cores with added carbon (glucose) and nitrogen (nitrate, ammonium) substrates were performed. Our field measurements showed that pulse emissions after fertilization and irrigation/rainfall contributed to 73% and 88% of the annual N2O and NO emissions, respectively. Soil moisture and mineral nitrogen contents significantly affected the emissions of both gases. Annual emissions from fields fertilized at the conventional rate (600 kg N ha−1 yr−1) totaled 4.0 ± 0.2 and 3.0 ± 0.2 kg N ha−1 yr−1 for N2O and NO, respectively, while those from unfertilized fields were much lower (0.5 ± 0.02 kg N ha−1 yr−1 and 0.4 ± 0.05 kg N ha−1 yr−1, respectively). Direct emission factors (EFds) of N2O and NO for the fertilizer nitrogen were estimated to be 0.59 ± 0.04% and 0.44 ± 0.04%, respectively. By summarizing the results of our study and others, we recommended specific EFds (N2O: 0.54 ± 0.09%; NO: 0.45 ± 0.04%) for estimating emissions from irrigated croplands on calcareous soils with organic carbon ranging from 5 to 16 g kg−1. Nitrification dominated the processes driving the emissions of both gases following fertilization. It was evident that insufficient available carbon limited microbial denitrification and thus N2O emission. This implicates that efforts to enhance carbon sink in calcareous soils likely increase their N2O emissions. [Copyright &y& Elsevier]

Abstract: One-year winter wheat–summer maize rotation is the most popular double cropping system in north-central China, and this highly productive system is an important source of nitrous oxide (N2O) and nitric oxide (NO) emissions due to the high fertilizer N and irrigation water inputs. To sustain the high crop production and mitigate the detrimental impacts of N2O and NO emissions, improved management practices are extensively applied. The aim of this study is therefore to evaluate the effects of an improved management practice of irrigation, fertilization and crop straw on grain yield and N2O and NO emissions for a wheat–maize rotation field in northern China. Using automated and manual chamber measuring systems, we monitored N2O and NO fluxes for the conventional (CT, 2007–2008), improved (IT, 2007–2008), straw-amended (WS, 2008–2009), straw-not-amended (NS, 2008–2009), and no N-fertilizer treatments (WS–NN, 2008–2009), respectively, for one rotation-year. The grain yields were determined for CT and IT for three rotation-years (2005–2008) and for WS, NS and WS–NN for one rotation-year (2008–2009). The improved management of irrigation and fertilization reduced the annual N fertilization rate and irrigation amount by 17% and 30%, respectively; increased the maize yield by 7–14%; and significantly decreased the N2O and NO emissions by 7% (p <0.05) and 29% (p <0.01), respectively. The incorporation of wheat straw increased the cumulative N2O and NO emissions in the following maize season by 58% (p <0.01) and 13%, respectively, whereas the effects of maize straw application were not remarkable. The N2O and NO emission factors of applied N were 2.32±2.32% and 0.42±1.69% for wheat straw and 0.67±0.23% and 0.54±0.15% for chemical N-fertilizers, respectively. Compared to conventional management practices using high application rates of irrigation water and chemical N-fertilizer as well as the field burning of crop straw, the improved management strategy presented here has obvious environmentally positive effects on grain yield and mitigation of N2O and NO emissions. [Copyright &y& Elsevier]

Abstract: Annual paddy rice–winter wheat rotation constitutes one of the typical cropping systems in southeast China, in which various water regimes are currently practiced during the rice-growing season, including continuous flooding (F), flooding-midseason drainage-reflooding (F-D-F), and flooding-midseason drainage-reflooding and moisture but without waterlogging (F-D-F-M). We conducted a field experiment in a rice–winter wheat rotation system to gain an insight into the water regime-specific emission factors and background emissions of nitrous oxide (N2O) over the whole annual cycle. While flooding led to an unpronounced N2O emission during the rice-growing season, it incurred substantial N2O emission during the following non-rice season. During the non-rice season, N2O fluxes were, on average, 2.61 and 2.48mg N2O–Nm− 2 day−1 for the 250kg N ha−1 applied plots preceded by the F and F-D-F water regimes, which are 56% and 49% higher than those by the F-D-F-M water regime, respectively. For the annual rotation system experienced by continuous flooding during the rice-growing season, the relationship between N2O emission and nitrogen input predicted the emission factor and background emission of N2O to be 0.87% and 1.77kg N2O–Nha−1, respectively. For the plots experienced by the water regimes of F-D-F and F-D-F-M, the emission factors of N2O averaged 0.97% and 0.85%, with background N2O emissions of 2.00kg N2O–Nha−1 and 1.61kg N2O–Nha−1 for the annual rotation system, respectively. Annual direct N2O–N emission was estimated to be 98.1Gg yr−1 in Chinese rice-based cropping systems in the 1990s, consisting of 32.3Gg during the rice-growing season and 65.8Gg during the non-rice season, which accounts for 25–35% of the annual total emission from croplands in China. [Copyright &y& Elsevier]

Abstract: Field operations of tillage and residue incorporation could have potentially important influences on N-trace gas fluxes, though poorly quantified. Here we studied the effects of straw incorporation in the preceding rice season and no-tillage prior to wheat sowing on nitric oxide (NO) and nitrous oxide (N2O) emissions during the non-rice period of a typical rice-wheat rotation in the Yangtze River Delta. Compared to conventional management practice (no straw incorporation along with rotary harrowing tillage to 10 cm before wheat sowing), straw incorporation alone decreased cumulative N2O emissions over the entire non-rice period by 32% (1.53 vs. 2.24 kg N ha-1, P < 0.05) but did not affect NO emissions (0.88 vs. 0.87 kg N ha−1). In contrast, no-tillage alone increased N2O emissions by 75% (P < 0.05) while reducing NO emissions by 48% (P < 0.01). Combination of no-tillage and straw incorporation led to no change in N2O emissions but a reduction in NO emissions compared to the conventional management regime. The direct N2O emission factors (EFds) of applied nitrogen fertilizers during the non-rice season ranged from 0.29% to 1.35% with a coefficient of variation (CV) as large as 68% among the investigated management regimes. The EFds for NO ranged from 0.13% to 0.32% with a CV of 50%. Adoption of these new EFds will allow us to account for management effects on N-trace gas emissions when calculating emission inventories. Nevertheless, it is noteworthy that the uncertainty remains high, since the effects of soil properties such as texture or pH on management practices are not yet well defined. [Copyright &y& Elsevier]

Emission factors (EFs) of pollutants from post-harvest agricultural burning are required for predicting downwind impacts of smoke and inventorying emissions. EFs of polycyclic aromatic hydrocarbons (PAH), methoxyphenols (MP), levoglucosan (LG), elemental carbon (EC) and organic carbon (OC) from wheat and Kentucky bluegrass (KBG) stubble burning were quantified in a US EPA test burn facility. The PAH and MP EFs for combined solid+gas phases are 17±8. 2mgkg−1 and 79±36mgkg−1, respectively, for wheat and 21±15mgkg−1 and 35±24mgkg−1, respectively, for KBG. LG, particulate EC and artifact-corrected OC EFs are 150±130mgkg−1, 0. 35±0. 16gkg−1 and 1. 9±1. 1gkg−1, respectively, for wheat and 350±510mgkg−1, 0. 63±0. 056gkg−1 and 6. 9±0. 85gkg−1, respectively, for KBG. Positive artifacts associated with OC sampling were evaluated and remedied with a two-filter system. EC and OC accounted for almost two-thirds of PM2. 5 mass, while LG accounted for just under 3% of the PM2. 5 mass. Since EFs of these pollutants generally decreased with increasing combustion efficiency (CE), identifying and implementing methods of increasing the CEs of burns would help reduce their emissions from agricultural field burning. PAH, OC and EC EFs are comparable to other similar studies reported in literature. MP EFs appear dependent on the stubble type and are lower than the EFs for hard and softwoods reported in literature, possibly due to the lower lignin content in wheat and KBG. [Copyright &y& Elsevier]

Emission factors (EFs) of PM2. 5, CO, elemental carbon (EC), particulate organic carbon (OC), polycyclic aromatic hydrocarbons (PAHs) and methoxyphenols (MPs) from post-harvest burning of wheat and Kentucky bluegrass (KBG) stubble were evaluated in a series of field burns. Integrated smoke samples were collected at ground level, upwind and downwind of the fires, and EFs were determined with the carbon balance method (validated during previous chamber experiments). These EFs were compared against EFs evaluated from previously conducted chamber burns, to determine how well the latter represent field scenarios. In general, when combustion efficiency (CE) differences were taken into account, a reasonable degree of agreement was observed between emission factors measured in the field and in the chamber, except for EC and solid+vapor phase PAHs, both from wheat burns. EC and PAHs from wheat burns were seen in higher amounts in the chamber, although the PAH data are in agreement at CEs>90%. EC overestimates might be due to a misassigned EC–OC split in the heavily loaded quartz filters from chamber burns. Poor EC and OC EF–CE correlations in KBG chamber data make the comparison with field data difficult. The particulate organic matter/OC ratios (2. 1±1. 3 for wheat and 1. 9 for KBG) were higher than those observed during chamber experiments (1. 5 for both wheat and KBG). Overestimates of EC in the chamber and possibly the condensation of oxygenated species in the field may be responsible for this difference. Though CO and CH4 EFs evaluated from ground-based samples differed from those collected on board a light aircraft, EF–CE relationships were similar. This underscores the importance of determining both the CEs and EFs simultaneously. [Copyright &y& Elsevier]