Effects of long-term straw return on soil organic carbon fractions and enzyme activities in a double-cropped rice paddy in South China

Long-term straw return is an important carbon source for improving soil organic carbon(SOC) stocks in croplands, and straw removal through burning is also a common practice in open fields in South China. However, the specific effects of long-term rice straw management on SOC fractions, the related enzyme activities and their relationships, and whether these effects differ between crop growing seasons remain unknown. Three treatments with equal nitrogen, phosphorus, and potassium nutrient inputs, including straw/ash and chemical nutrients, were established to compare the effects of straw removal(CK), straw return(SR), and straw burned return(SBR). Compared to CK, long-term SR tended to improve the yield of early season rice(P=0.057), and significantly increased total organic carbon(TOC) and microbial biomass carbon(MBC) in double-cropped rice paddies. While SBR had no effect on TOC, it decreased light fraction organic carbon(LFOC) in early rice and easily oxidizable organic carbon(EOC) in late rice, significantly increased dissolved organic carbon(DOC), and significantly decreased soil p H. These results showed that MBC was the most sensitive indicator for assessing changes of SOC in the double-cropped rice system due to long-term straw return. In addition, the different effects on SOC fraction sizes between SR and SBR were attributed to the divergent trends in most of the soil enzyme activities in the early and late rice that mainly altered DOC, while DOC was positively affected by β-xylosidase in both early and late rice. We concluded that straw return was superior to straw burned return for improving SOC fractions, but the negative effects on soil enzyme activities in late rice require further research.

Effects of different long-term crop straw management practices on ammonia volatilization from subtropical calcareous agricultural soil

Ammonia(NH3)volatilized from agricultural production and its secondary aerosols contribute greatly to air pollution.Different long-term crop straw management practices may significantly affect the soil fertility and soil nitrogen cycle,however,the effect on NH3 volatilization has not been well studied.Therefore,a one-year field experiment was conducted to evaluate the effect of straw incorporation on NH3 volatilization from subtropical calcareous agricultural soil from a longterm perspective,including four treatments:synthetic fertilizer(CK);synthetic fertilizer incorporation with 100%or 50%of the previous season’s crop straw(SI1 and SI2,respectively);and synthetic fertilizer incorporation with 50%burned crop straw(SI2B).Soil NH3 volatilizations were monitored through a wheat–maize rotation year by using a dynamic chamber method.The results demonstrated that NH3 volatilization primarily occurred within 38 days and 7–10 days following nitrogen fertilization events for the wheat and maize seasons,respectively.Different crop straw management practices mainly impacted the NH3 flux of the basal fertilization rather than the topdressing fertilization;long-term crop straw incorporation effectively lowered NH3 loss(35.1%for SI1 and 16.1%for SI2 compared to CK;and the inhibiting effect increased with increasing straw amount,possibly contributed by the high straw carbon/nitrogen ratio,and enhanced microbial activity,which contributed to inorganic nitrogen immobilization and lower ammonium content in the topsoil.However,SI2B significantly increased(29.9%)the annual NH3 flux compared with SI2,indicating that long-term 100%straw incorporation could be a promising straw management practice for mitigating NH3 loss and increasing soil fertility.

Emissions of parent, nitrated, and oxygenated polycyclic aromatic hydrocarbons from indoor corn straw burning in normal and controlled combustion conditions

Emission factors （EFs） of parent polycyclic aromatic hydrocarbons （pPAHs）, nitrated PAHs （nPAHs）, and oxygenated PAHs （oPAHs） were measured for indoor corn straw burned in a brick cooking stove under different burning conditions. The EFs of total 28 pPAHs, 6 nPAHs and 4 oPAHs were （7.9 ±3.4）, （6.5 ±1.6）×10^-3, and （6.1 ±1.4）×10^-1mg/kg, respectively. Fuel charge size had insignificant influence on the pollutant emissions. Measured EFs increased significantly in a fast burning due to the oxygen deficient atmosphere formed in the stove chamber. In both restricted and enhanced air supply conditions, the EFs of pPAHs, nPAHs and oPAHs were significantly higher than those measured in normal burning conditions. Though EFs varied among different burning conditions, the composition profiles and calculated isomer ratios were similar, without significant differences. The results from the stepwise regression model showed that fuel burning rate, air supply amount, and modified combustion efficiency were the three most significant influencing factors, explaining 72%-85% of the total variations.

Online single particle analysis of chemical composition and mixing state of crop straw burning particles： from laboratory study to field measurement

Fresh straw burning （SB） particles were generated in the laboratory by the combustion of rice straw and corn straw. The chemical composition and mixing state of the fresh SB particles were investigated by an Aerosol Time-of-Flight Mass Spectrometer （ATOFMS）. Based on the mass spectral patterns, the SB particles were clustered into four major types： Salt, Organic Carbon （OC）, Elemental Carbon （EC）, and internally mixed particles of EC and OC （EC-OC）. In addition, particles containing ash, polycyclic aromatic hydrocarbons, heavy metals or nicotine were also observed. Physical and chemical changes of the SB particles immediately after the emission were analyzed with highly time-resolved data. During the aging processes, the average particle size increased steadily. Freshly emitted organic compounds were gradu- ally oxidized to more oxygenated compounds in the OC- containing particles. Meanwhile, an important displace- ment reaction （2KCI＋ SO24- KzSO4 ＋ 2C1-） was observed. The marker ions for SB particles were optimized and applied to identify the SB particles in the ambient atmosphere. The fluctuation of the number fraction of ambient SB particles sorted by ATOFMS agrees well with that of water soluble K＋ measured by an online ion chromatography, demonstrating that the optimized marker ions could be good tracers for SB particles in field measurements.

Influence of fuel mass load,oxygen supply and burning rate on emission factor and size distribution of carbonaceous particulate matter from indoor corn straw burning

The uncertainty in emission estimation is strongly associated with the variation in emission factor （EF）,which could be influenced by a variety of factors such as fuel properties,stove type,fire management and even methods used in measurements.The impacts of these factors are complicated and often interact with each other.Controlled burning experiments were conducted to investigate the influences of fuel mass load,air supply and burning rate on the emissions and size distributions of carbonaceous particulate matter （PM） from indoor corn straw burning in a cooking stove.The results showed that the EFs of PM （EFPM）,organic carbon （EFOC） and elemental carbon （EFEC） were independent of the fuel mass load.The differences among them under different burning rates or air supply amounts were also found to be insignificant （p 〉 0.05） in the tested circumstances.PM from the indoor corn straw burning was dominated by fine PM with diameter less than 2.1 μm,contributing 86.4%±3.9% of the total.The size distribution of PM was influenced by the burning rate and air supply conditions.On average,EF PM,EF OC and EF EC for corn straw burned in a residential cooking stove were （3.84±1.02）,（0.846±0.895） and （0.391±0.350） g/kg,respectively.EF PM,EF OC and EF EC were found to be positively correlated with each other （p 〈 0.05）,but they were not significantly correlated with the EF of co-emitted CO,suggesting that special attention should be paid to the use of CO as a surrogate for other incomplete combustion pollutants.

Secondary organic aerosol formation from straw burning using an oxidation flow reactor

Herein,we use an oxidation flow reactor,Gothenburg:Potential Aerosol Mass(Go:PAM)reactor,to investigate the secondary organic aerosol(SOA)formation from wheat straw burning.Biomass burning emissions are exposed to high concentrations of hydroxyl radicals(OH)to simulate processes equivalent to atmospheric oxidation of 0-2.55 days.Primary volatile organic compounds(VOCs)were investigated,and particles were measured before and after the Go:PAM reactor.The influence of water content(i.e.5%and 11%)in wheat straw was also explored.Two burning stages,the flaming stage,and non-flaming stages,were identified.Primary particle emission factors(EFs)at a water content of 11%(~3.89 g/kg-fuel)are significantly higher than those at a water content of 5%(~2.26 g/kg-fuel)during the flaming stage.However,the water content showed no significant influence at the non-flaming stage.EFs of aromatics at a non-flaming stage(321.8±46.2 mg/kg-fuel)are larger than that at a flaming stage(130.9±37.1 mg/kg-fuel).The OA enhancement ratios increased with the increase in OH exposure at first and decreased with the additional increment of OH exposure.The maximum OA enhancement ratio is~12 during the non-flaming stages,which is much higher than~1.7 during the flaming stages.The mass spectrum of the primary wheat burning organic aerosols closely resembles that of resolved biomass burning organic aerosols(BBOA)based on measurements in ambient air.Our results show that large gap(0%-90%)still remains to estimate biomass burning SOA if only the oxidation of VOCs were included.

Rice straw recycling: A sustainable approach for ensuring environmental quality and economic security

One of the major challenges in Asian countries is the effective management of rice straw.To ensure food security for their ever-growing population,Asian countries grow more rice,leading to increasing rice straw generation.Burning of rice straw,a common practice,is detrimental to both environmental and human health.However,if managed effectively,rice straw has the potential to safeguard the sustainability of agricultural ecosystems and to uplift the economic security of the population depending on rice farming.Judicious management of rice straw involving minimum soil disturbance along with retention of residues alters the soil carbon cycle through reduced carbon emissions and increased levels of total soil organic carbon.Several alternative uses of rice straw,such as production of livestock feed,bioethanol,biochar,biogas,electricity,mushroom,and paper,can add prosperity to rice farmers’life by fetching extra income.It is evident that efficient management of rice straw is of enormous economic value.Therefore,it is essential to create awareness among the different rice stakeholders of these alternative economic avenues associated with rice straw.This review is an attempt to provide effective options for sustainable rice straw management and rice straw value chains for harnessing its economic potential.It further identifies gaps in our understanding of the effects of rice straw on ecological sustainability,particularly concerning the multifaceted connections between the diverse mechanisms of rice agro-ecosystems,which may greatly influence food security in the 21st century.

Dynamics of major air pollutants from crop residue burning in China's Mainland,2000–2014

Based on satellite image data and China＇s Statistical Yearbooks（2000 to 2014）, we estimated the total mass of crop residue burned, and the proportion of residue burned in the field vs.indoors as domestic fuel. The total emissions of various pollutants from the burning of crop residue were estimated for 2000-2014 using the emission factor method. The results indicate that the total amount of crop residue and average burned mass were 8690.9 Tg and4914.6 Tg, respectively. The total amount of emitted pollutants including CO2, CO, NOx,VOCs, PM（2.5）, OC（organic carbon）, EC（element carbon） and TC（total carbon） were 4212.4–8440.9 Tg, 192.8–579.4 Tg, 4.8–19.4 Tg, 18.6–61.3 Tg, 18.8–49.7 Tg, 6.7–31.3 Tg, 2.3–4.7 Tg, and8.5–34.1 Tg, respectively. The emissions of pollutants released from crop residue burning were found to be spatially variable, with the burning of crop residue mainly occurring in Northeast, North and South China. In addition, pollutant emissions per unit area（10 km ×10 km） were mostly concentrated in the central and eastern regions of China. Emissions of CO2, NOx, VOCs, OC and TC were mainly from rice straw burning, while burning of corn and wheat residues contributed most to emissions of CO, PM（2.5） and EC. The increased ratio of PM（2.5） emissions from crop residue burning to the total emitted from industry during the study period is attributed to the implementation of strict emissions management policies in Chinese industry. This study also provides baseline data for assessment of the regional atmospheric environment.

Atmospheric pollution of agriculture-oriented cities in Northeast China: A case in Suihua

Agriculture-oriented cities in Northeastern China have experienced frequent atmospheric pollution events.Deeper understandings of the pollution characteristics,haze causes and effects of management on local air quality are crucial for conducting integrated management approaches for the sustainable development of agriculture-oriented cities.Taking a typical agriculture-dominant city(i.e.,Suihua)in Northeast China,we analyzed in detail the characteristics and causes of atmospheric pollution and evaluated the straw-burning prohibition using multisource data.The results showed a clear temporal pattern of air quality index(AQI)on an annual scale(i.e.,2015-April 2019),with two typical pollution periods occurring in late autumn and early spring.The large areas of concentrated straw burning at local and regional scales accounted for the first period(i.e.,October and November),while dust emissions and farming disturbances comprised the second period.The interannual variation in pollution periods among these years was large,showing similar trends from 2015 to 2017 and the postponed late-autumn pollution period in 2018.Our evaluation has shown that the prohibition effect of straw burning significantly improved air quality in 2018,with a reduction of 59%±88%in the PM2.5 concentrations in October and November compared to 2015–2017.However,From October to April of the following year,the improvement effect was not significant due to postponement of straw burning to February or March.Our analysis also highlighted the roles of meteorological conditions,Therefore,combined with the promotion of straw utilization,scientifically prescribed burning considering the burning amount and location,meteorological conditions and regional transportation should be implemented.

Comparisons of two serious air pollution episodes in winter and summer in Beijing

Characteristics of two serious air pollution episodes（9–15 January, as the winter case; and30 June to 1 July, as the summer case）, which occurred in Beijing in 2013 were investigated and compared using multi-method observations and numerical simulations. During these two air pollution episodes, PM2.5 concentrations varied significantly within Beijing, with PM2.5 concentrations in southern parts of Beijing being significantly higher than in northern areas. Typically, heavy air pollution episodes begin in the southern parts and disperse towards the northern parts of Beijing. Clearly, synoptic patterns and the stability of atmospheric circulation patterns were the main factors controlling air pollution in Beijing.During the winter case, a warm center above 900 h Pa occurred over Beijing. Meanwhile, in the summer case, although there was only a weak inversion, the convective inhibition energy was strong（over 200 J/k G）. This clearly influenced the duration of the air pollution event. Except for the local accumulation and secondary atmospheric reactions in both cases, regional straw burnings contributed a lot to the PM2.5 concentrations in summer case.Using the CAMxmodel, we established that regional transport contributed almost 59% to the PM2.5 averaged concentration in Beijing in the winter case, but only 31% in the summer case. Thus, the winter case was a typical regional air pollution episode, while the summer case resulted from local accumulation straw burnings transportation and strong secondary atmospheric reactions. Given that air pollution is a regional problem in China, consistent and simultaneous implementation of regional prevention and control strategies is necessary to improve regional air quality.