Straw Amendment Research Articles

DOM derivations determine the distribution and bioavailability of DOM-Se in selenate applied soil and mechanisms

Straw amendment and plant root exudates modify the quality and quantities of soil dissolved organic matter (DOM) and then manipulate the fractions of soil selenium (Se) and its bioavailability. Two typical soils with distinct pH were selected to investigate the effect of different contributors on DOM-Se in soil. The mechanisms relying on the variation in DOM characteristics (quality, quantity and composition) were explored by UV–Vis, ATR-FTIR and 3D-EEM. Straw amendment significantly (p < 0.05) suppressed the selenate bioavailability. The reduction in wheat Se content was greater in krasnozems than in Lou soil, as more HA fraction appeared in krasnozems. The root exudates of wheat mainly elevated the low molecular hydrophilic compounds (Hy) in soil, which contributed to the SOL-Hy-Se fractions and thus grain Se in soils (p < 0.01). However, straw amendment promoted DOM transforming from small molecules (Hy and FA) to aromatic large molecules (HA), when accompanied with the reduction and retention of Se associated with these molecules. As a result, selenium bioavailability and toxicity reduced with DOM amendment and DOM-Se transformation.

Yield reduction of direct-seeded rice under returned straw can be mitigated by appropriate water management improving soil phosphorus availability

The yield of direct-seeded rice has been shown to decrease after straw amendment. However, the reasons for the yield decrease, and any measures to alleviate it, are currently unknown. We hypothesised that straw return exerts negative effects on soil fertility and on root growth of direct-seeded rice, which subsequently reduces the remobilisation of reserves to grains under continuous flooding (CF); and that alternate wetting and drying (AWD) irrigation can alleviate these negative impacts. Field and greenhouse experiments were conducted to test the hypotheses, by comparing CF and AWD in combination with two wheat-straw treatments (incorporation and mulching). Under CF, wheat-straw incorporation decreased soil available phosphorus by 23–79%, root biomass by 10%, leaf biomass by 13%, and leaf area by 15% compared with the control with no straw incorporation; negative effects on these characteristics were lessened if the straw was mulched. The AWD treatment alleviated the negative effects of straw incorporation compared with CF, and straw mulching with AWD had no negative effects or resulted in positive effects. The results suggest that CF along with straw incorporation limits soil phosphorus availability, root growth and grain yield by affecting photosynthate accumulation and remobilisation. AWD irrigation mitigates these undesirable effects by decreasing soil total reductants, which subsequently increases soil pH and plant-available phosphorus. The proposed AWD treatment could be a promising strategy for the sustainable production of direct-seeded rice.

Manure amendment increased the abundance of methanogens and methanotrophs but suppressed the type I methanotrophs in rice paddies

Methane (CH4) emission is the consequence of CH4 production and consumption performed by methanogens and methanotrophs, respectively. Fertilization is an important factor that regulates the behavior of methanogens and methanotrophs; however, the effect of manure and rice straw addition combined with inorganic fertilizers on these communities is not well understood. This study aimed to explore how manure and rice straw amendments together with inorganic fertilizers influenced the methanogenic and methanotrophic communities in a 31-year fertilized rice paddy. Manure amendment significantly increased the abundance of mcrA and pmoA genes by 61.2% and 63.3% compared with the unfertilized control, whereas inorganic NPK fertilization alone or rice straw addition did not affect their abundances. Manure and rice straw amendments greatly decreased the Shannon index and ACE index of the methanogenic communities, whereas inorganic NPK fertilization alone increased the ACE index of the methanotrophic communities compared with the unfertilized control. Methanosarcinaceae and Methylococcaceae dominated at the family level, representing 23.1-35.0% and 48.7-67.2% of the total reads, for the methanogenic and methanotrophic communities, respectively. Application of manure together with inorganic fertilizers suppressed the Methanocellales methanogens and the type I methanotrophs (Methylococcus and Methylobacter). Fertilization greatly altered the community structure of methanogens and methanotrophs, and manure addition had more apparent effects than rice straw. Moreover, total nitrogen, soil organic carbon, available phosphorus, and available potassium correlated significantly to the abundance, composition, and community structure of methanogens and methanotrophs. In conclusion, our study revealed that long-term manure amendment in combination with inorganic fertilizers significantly increased the abundance of methanogens and methanotrophs, but suppressed the type I methanotrophs in rice paddies.

Earthworms offset straw-induced increase of greenhouse gas emission in upland rice production

Increasing water scarcity and rapid socio-economic development are driving farmers in Asia to transform traditionally flooded rice cropping systems into non-flooded crop production. The management of earthworms in non-flooded rice fields appears to be a promising strategy to support residue recycling and mitigate greenhouse gas (GHG) emissions triggered by residue amendment. We conducted a field experiment on non-flooded rainfed rice fields, with and without residue amendment. In-situ mesocosms were inoculated with endogeic earthworms (Metaphire sp.), with either low (ET1: 150 individuals m−2), or high density (ET2: 450 individuals m−2), and a control (ET0: no earthworms). We measured GHG emissions (methane (CH4); nitrous oxide (N2O); carbon dioxide (CO2)) twice a week during the cropping season with static chambers. Effects of earthworms on yield and root growth were additionally assessed. Earthworms offset the enormous increase of CH4 emissions induced by straw amendment (from 4.6 ± 5 to 75.3 ± 46 kg CH4-C ha−1 in ET0). Earthworm activity significantly reduced CH4 release, particularly at ET2, by more than one-third (to 22 ± 15 kg CH4-C ha−1). In contrast, earthworm inoculation did not affect N2O emission. Straw amendment more than doubled the global warming potential (GWP). Earthworms reduced GWP by 39% at low (ET1) and 55% at high densities (ET2). Earthworm activity reduced root mass density under conditions of straw amendment but did not affect yield. Earthworms can significantly reduce detrimental effects of rice crop residue amendment on GHG release under upland rice production. Organic carbon (C) might be preserved in earthworm casts and thereby limit C availability for CH4 production. At the same time, earthworm activity might increase methanotrophic CH4 consumption, due to improved soil aeration or less root exudates. Consequently, earthworms have a strong potential for regulating ecosystem functions related to rice straw decomposition, nutrient allocation and thus GHG reduction.

Comparing the Effects of Biochar and Straw Amendment on Soil Carbon Pools and Bacterial Community Structure in Degraded Soil

The degradation of soil often occurs due to the depletion of organic matter. Both biochar and straw have value as two primary means of remediating soil. In this study, we compared the effects of biochar and straw on soil organic carbon pools and on microbial communities in degraded soil. We added straw (maize) and biochar (maize-derived) to the degraded soil at the ratios of total soil weight (0, 1, 2, or 4%), respectively. Soil samples were collected after 180 days. We determined that upon straw addition, there was an increase in microbial biomass carbon (MBC), dissolved organic carbon (DOC), and easily oxidizable carbon (EOC). Upon biochar addition, there was an increase in soil total organic carbon and recalcitrant organic carbon. Analysis of bacterial community was conducted via 16SrDNA sequencing with an Illumina MiSeq platform and revealed that the relative abundance of Actinobacteria increased, but the Bacteroidetes and Chloroflexi decreased with straw addition. We further found that the addition of straw altered the Shannon and Simpson diversity index values, whereas biochar did not impact soil bacterial diversity. Nonmetric multidimensional scaling and canonical correspondence analyses revealed that bacterial community structure was influenced by both biochar and straw addition. Mantel testing further revealed that this community structure was associated with soil EOC (r = 0.286, P = 0.001), DOC (r = 0.174, P = 0.002), and MBC (r = 0.299, P = 0.002) upon straw treatments. These results suggested that straw can improve soil labile organic carbon pools and soil bacterial communities better than biochar, whereas biochar (via pyrolysis at 550 °C, for 2 h) can improve carbon sequestration better than straw in degraded soil.

Quantifying soil N pools and N2O emissions after application of chemical fertilizer and straw to a typical chernozem soil

An incubation experiment with equivalent N rates was conducted for 56 days using a typical black soil amended with chemical fertilizer with or without straw amendment using a 15N cross-labeling technique. Compared with the chemical fertilizer treatment (15NCF), chemical fertilizer combined with straw treatment (CF + S) showed a significantly higher (P < 0.05) contribution from applied N to microbial biomass N (BN) in the first 14 days and to particulate organic N (PON) and mineral-associated total N (MON) throughout the incubation. Straw application in the CF + S treatment significantly (P < 0.05) decreased the recovery of chemical fertilizer N as soil inorganic N except at day 3 but increased the recovery of chemical fertilizer N as BN before day 14 and as PON and MON from day 14 to the end of the incubation period. At the end of the incubation period, the total N2O-N emissions in the CF + S treatment increased significantly (P < 0.05) compared with the CF treatment, and the increase in N2O-N emissions was 73% from chemical fertilizer and 27% from straw N individually. Our results suggest that the combined application of chemical fertilizer and straw increased soil fertility together with an increase in N2O emissions in the typical black soil and the N2O emissions from straw cannot be ignored.

Effects of different corn straw amendments on humus composition and structural characteristics of humic acid in black soil

ABSTRACT Straw returning to the field has become a common way to utilize crop residues. This method does not only solve the problem of rational utilization of straw but also improves soil organic carbon (SOC) quality and quantity. So far, various methods of straw return exist, but the comparison of these methods has not been sufficiently studied, especially in the black soil of Northeast China. The purpose of this study is to evaluate the effects of different corn straw amendments applied under equal carbon (C) mass on soil humus composition and structural characteristics of humic acid (HA). The pot experiment was carried out by simulating corn straw return, which included four treatments: no corn straw applied (CK), corn straw (CS), humidified corn straw (HCS) and corn straw biochar (Bc). Soil HA was characterized by Fourier transform infrared spectroscopy (FTIR), elemental analyzer and thermogravimetric analysis (TG). The results showed that CS, HSC, and Bc treatments increased SOC content by 7.51%, 31.03%, and 33.65%, respectively, compared to CK. Bc treatment showed a greater degree of humification (79.16%) and increased the C content of HA by 10.87% and 7.37% compared with CS and HCS, respectively. The FTIR spectra and elemental results revealed that CS and HCS treatments increased the aliphaticity of the HA structure, while Bc increased the aromaticity of C in HA structure, rendering HA structure complicated. It can, therefore, be inferred that the application of biochar will improve humus compositions, SOC content and enhances the stability of HA structure.

Straw and optimized nitrogen fertilizer decreases phosphorus leaching risks in a long-term greenhouse soil

Phosphorus (P) mobility in soil is controlled by its forms and soil sorption capacity. The P forms and soil sorption capacity are both affected by nitrogen (N) and carbon (C) addition. This paper aimed to (i) analyze effects of N and straw application on the different forms and content of P in the soil and its leachates in greenhouse soil, and (ii) explain variations in soil P transformation and transport in terms of contributing soil factors. In this study, the impacts of N and straw application on the transformation and transport of soil P were investigated after 17 years in a greenhouse. Four fertilization regimes were implemented: farmer standard fertilization practice (CK), straw incorporation treatment (SC), optimized N fertilizer application (ON), and combined straw and optimized N fertilizer application (SN). P forms and its contents were determined in the selected leached water and its related soil samples. Compared with CK, ON treatment significantly (p < 0.05) decreased total phosphorus (Pt) and the proportion of organic phosphorus (Po) in soil, while straw amendment did not affect soil Pt content. SC, ON, and SN all decreased soil available phosphorus (Pa) but enhanced P transformation, as evidenced by the increase in the ratio of Pa to Pt and microbial phosphorus (MBP) and alkaline phosphatases (ALP) in soil. After SN implementation, soil P adsorption capacity increased significantly and was associated with higher soil organic matter (SOM) and CaCO3. ON showed lower Pt in the leachate than CK, but SC did not lead to significantly different. Under the SN regime, Pt loss by leaching decreased by 29.4% compared with CK and significantly reduced proportion of total dissolve P (DPt) in leachate. Our study highlights that straw and optimized N fertilizer in SN treatment not only generates lower P loss by leaching but also promotes the transformation of soil P, which were attributed to higher soil Pa in greenhouse soil. This finding further indicates that P transformation and transport in the different fertilizer regimes was primarily linked to pH and SOM in greenhouse soil.

The role of resource transfer in positive, non-additive litter decomposition

Naturally occurring, mixed litter decomposes at unpredictable rates when individual components do not decompose in mixtures as they do individually. Consequently, nutrient, carbon and energy fluxes associated with decomposition may be difficult to predict. However, predictability is improved when we understand the mechanisms responsible for such non-additive decomposition. In this study, we explored mechanisms to explain our previous observation that an approximately 30% increase in oat straw decomposition due to the presence of clover litter is associated with a significant increase in the activity of cellobiohydrolase, an enzyme involved in litter decomposition. We hypothesized that resources limiting decomposer microbe enzyme activity in oat straw can be supplied by clover litter. Amendment of oat straw with water, NH4Cl, glucose, or NH4Cl combined with glucose did not account for the significant, positive effect of clover litter on oat straw decomposition and cellobiohydrolase activity. However, amendment of oat straw with a complete set of mineral nutrients for plant growth did account for the entire effect of clover litter, and the addition of the complete set of mineral nutrients without N accounted for the majority of the clover effect. In our system, therefore, the majority of the positive effect of clover litter on oat straw decomposition and cellobiohydrolase activity was unexpectedly not attributable to the transfer from clover to oat straw of labile N. We found that mineral soil could substitute for the mineral nutrients other than N. This highlights the role of soil as a potential source of limiting resources for microbes decomposing litter. It may also explain why positive, non-additive decomposition has been observed in some previous studies but not in others depending on whether the soil supplied a resource that limited decomposer activity.

Effects of corn straw on dissipation of polycyclic aromatic hydrocarbons and potential application of backpropagation artificial neural network prediction model for PAHs bioremediation

In order to provide a viable option for remediation of PAHs-contaminated soils, a greenhouse experiment was conducted to assess the effect of corn straw amendment (1%, 2%, 4% or 6%, w/w) on dissipation of aged polycyclic aromatic hydrocarbons (PAHs) in contaminated soils. Backpropagation artificial neural network (BP-ANN) was applied to model the relationships between soil properties and PAHs concentration in soils. The removal rate of PAHs, enzyme activity (catalase and dehydrogenase), dissolved organic carbon (DOC) and microbial biomass carbon (MBC) in soils were investigated to evaluate the dissipation of PAHs under different ratio of corn straw amendment. The present study showed that corn straw amendment apparently accelerated the dissipation of PAHs after incubation of 112 days, especially under 4% and 6% treatments. Compared with non-amended soil, corn straw amendment significantly (p < 0.05) increased the removal rate of low molecular weight (LMW) PAHs and significantly (p < 0.05) enhanced the dissipation of high molecular weight (HMW) PAHs only under 6% treatment. Moreover, corn straw amendment increased activities of catalase and dehydrogenase, concentrations of DOC and MBC in soils, which are beneficial to the degradation of PAHs in soils. The performance of the BP-ANN model was assessed through the root mean square error (RMSE) and determination coefficient (R2). The results indicated that BP-ANN model could provide satisfactory prediction of PAHs concentration in soils during incubation period at R2 and RMSE values of 0.948, 187.4 μg kg−1, respectively. The results indicated that high amendment of corn straw was a potential option for remediation of PAHs-contaminated soils and that the BP-ANN model could successfully provide prompt prediction of PAHs concentration in soils.

Comparison of methylmercury accumulation in wheat and rice grown in straw-amended paddy soil

Paddy soil is a key area of methylmercury (MeHg) production and is dominated by fluctuating redox conditions following rice plant growth or rice-wheat rotation planting in eastern and southern Asia. The role of organic matter in the formation of MeHg under these biogeochemical redox cycles remains poorly understood, especially in certain mercury (Hg)-contaminated paddy soils. Here, we provide a detailed understanding of the formation of MeHg and its accumulation in crops (i.e., wheat and rice) in rice-wheat rotation systems under straw return. Two series of experiments, pot and microcosm experiments, were performed using Hg-contaminated paddy soil with 1% (w/w) wheat or rice straw addition under aerobic or anoxic conditions. The results showed that straw amendments increased MeHg levels in wheat (by 225%) and rice (by 20%) grains, most likely due to the elevated soil MeHg following straw amendment. Microcosm experiments further confirmed that fresh straw-derived organic matter enhances MeHg net production in soil through an overall increase in the activity of sulfate-reducing bacteria (SRB), particularly under anoxic conditions. Our study clearly demonstrated that straw amendment enhanced MeHg accumulation in wheat and rice grains and highlighted that straw return in Hg-contaminated soils may increase the health risk of MeHg exposure to local residents via crop consumption. Thus, some guidance should be provided for crop residue return in rice-wheat rotation system farming practices.

Contrasting microbial community responses to salinization and straw amendment in a semiarid bare soil and its wheat rhizosphere

Soil salinization is a major constraint of agriculture in semiarid ecosystems. In this study soil microcosms were applied to evaluate the impact of a lower- and higher-level salinization treatment of a pristine scrubland soil on the abundance of Bacteria, Archaea, and Fungi, and on prokaryotic diversity in bare soil and the rhizosphere of wheat assessed by qPCR and high-throughput sequencing of 16S rRNA gene amplicons. Furthermore, the impact of soil straw amendment as a salt-stress alleviation strategy was studied. While the low-level salinity stimulated plant growth, the seedlings did not survive under the higher-level salinity unless the soil was amended with straw. Without the straw amendment, salinization had only minor effects on the microbial community in bare soil. On the other hand, it decreased prokaryotic diversity in the rhizosphere of wheat, but the straw amendment was effective in mitigating this effect. The straw however, was not a significant nutrient source for the rhizosphere microbiota but more likely acted indirectly by ameliorating the salinity stress on the plant. Members of Proteobacteria, Actinobacteria, and Firmicutes were abundant among the bacteria that reacted to soil salinization and the straw amendment but showed inconsistent responses indicating the large physiological diversity within these phyla.

Soil organic carbon associated with aggregate-size and density fractions in a Mollisol amended with charred and uncharred maize straw

Straw return has been strongly recommended in China, whereas applying biochar into soil is considered to provide more benefits for agriculture as well as the environment. In this study, a five-year (2011–2015) field experiment was conducted to evaluate the effects of uncharred maize straw amendment (MS) and charred maize straw amendment (charred MS) on organic carbon (C) contents in bulk soil and in various soil aggregate-size and density fractions. Compared to no amendment (CK), the bulk soil organic C content significantly improved by 9.30% for MS and by 23.4% for charred MS. Uncharred and charred maize straw applied annually at a consistent equal-C dosage resulted in 19.7 and 58.2% organic C sequestration efficiency in soil, respectively, after the five years of the field experiment. The percentages of macroaggregates (>0.25 mm) and occluded microaggregates (0.25–0.053 mm) obviously increased by 7.73 and 18.1% for MS and by 10.7 and 19.6% for charred MS, respectively. Moreover, significant incremental increases of 19.4 and 35.0% in macroaggregate-associated organic C occurred in MS and charred MS, respectively. The occluded microaggregates associated organic C significantly increased by 21.7% for MS and 25.1% for charred MS. Mineral-associated organic C (<0.053 mm) inside the macroaggregates and the occluded microaggregates obviously improved by 24.7 and 33.3% for MS and by 18.4 and 44.9% for charred MS. Organic C associated with coarse particulate organic matter (POM) within the macroaggregates markedly increased by 65.1 and 41.2% for MS and charred MS, respectively. Charred MS resulted in a noteworthy increment of 50.4% for organic C associated with heavy POM inside the occluded microaggregates, whereas charred MS and MS observably improved organic C associated with heavy POM inside the free microaggregates by 36.3 and 20.0%, respectively. These results demonstrate that uncharred and charred maize straw amendments improve C sequestration by physically protecting more organic C in the macroaggregates and the occluded microaggregates. Compared to the feedstock straw amendment, charred maize straw amendment is more advantageous to C sequestration.

Variations in soil bacterial taxonomic profiles and putative functions in response to straw incorporation combined with N fertilization during the maize growing season

Integrating nitrogen (N) fertilization with crop residue amendments is usually used to enhance soil nutrient levels and promote crop growth in regions with intensive agriculture. However, the interactive effects of straw incorporation and N fertilization on the abiotic and biological properties of soil are not fully understood, especially under field conditions at different crop growth stages. In this study, we estimated the dynamic changes in the soil physicochemical properties and bacterial communities in response to N fertilization, straw incorporation, and their combinations during the maize growing season in black soil region in Harbin, China. N fertilization increased the relative abundance of the typical copiotrophic bacterial taxa, Alphaproteobacteria, but reduced that of the oligotrophic group, Acidobacteria. The straw incorporation enhanced the relative abundance of Bacteroidetes and Gammaproteobacteria, within which a variety of cellulolysis-related species phylogenetically fell. Interestingly, the straw amendments changed the effects of N fertilization on the overall soil bacterial community composition, possibly by altering the soil total carbon, C/N ratio, fixed ammonium, and moisture level. N fertilization stimulated most of the carbon (C) transformation and N cycling processes, and the straw incorporation increased the cellulolysis, N fixation, ureolysis, ammonia oxidation and denitrification functions. There were no differences in the soil chemical properties, microbial taxa profiles or predicted functions between the conventional N fertilizer usage (high N level) and the optimized N fertilization management scheme (low N level), irrespective of straw incorporation, implying that moderately reducing the usage of N fertilizer will not change the ecosystem service and function in this organic matter-rich black soil region. The relative abundance of the copiotrophic bacterial taxa decreased at the mid-growing season, which might be caused by the low soil C and N availability. In contrast, the oligotrophic bacterial groups showed predominance at the mid-growing season, which might aim to mineralize soil organic matter to meet the growth needs of both the crops and the microbes. The nitrifiers (Nitrospirae) and nitrification process were sharply activated at the mid-growing season, especially under the straw amendment condition, inferring that the combination of straw application with N fertilizer could serve as an effective measure for improving soil fertility in Northeast China. The present study enhanced our knowledge on the dynamic changes in soil biotic and abiotic parameters in response to straw incorporation and N fertilization during the maize growing season. Still, this study provides a reference for designing an environmentally friendly fertilization strategy that both alleviates environmental stress and maintains a high crop yield in Northeast China.

Substrate quality and concentration control decomposition and microbial strategies in a model soil system

Soil carbon models typically scale decomposition linearly with soil carbon (C) concentration, but this linear relationship has not been experimentally verified. Here we investigated the underlying biogeochemical mechanisms controlling the relationships between soil C concentration and decomposition rates. We incubated a soil/sand mixture with increasing amounts of finely ground plant residue in the laboratory at constant temperature and moisture for 63 days. The plant residues were rye (Secale cereale, C/N ratio of 23) and wheat straw (Triticum spp., C/N ratio of 109) at seven soil C concentrations ranging from 0.38 to 2.99%. We measured soil respiration, dissolved organic carbon (DOC) concentrations, microbial biomass, and potential enzyme activities over the course of the incubation. Rye, which had higher N and DOC contents, lost 6 to 8 times more C as CO2 compared to wheat residue. Under rye and wheat amendment, absolute C losses as CO2 (calculated per g dry soil) increased linearly with C concentration while relative C losses as CO2 (expressed as percent of initial C) increased with C concentration following a quadratic function. In low C concentration treatments (0.38–0.79% OC), DOC decreased gradually from day 3 to day 63, microbial C increased towards the end in the rye treatment or decreased only slightly with straw amendment, and microbes invested in general enzymes such as proteases and oxidative enzymes. At increasing C levels, enzyme activity shifted to degrading cellulose after 15 days and degrading microbial necromass (e.g. chitin) after 63 days. At the highest C concentrations (2.99% OC), microbial biomass peaked early in the incubation and remained high in the rye treatment and decreased only slightly in the wheat treatment. While wheat lost C as CO2 constantly at all C concentrations, respiration dynamics in the rye treatment strongly depended on C concentration. Our results indicate that litter quality and C concentration regulate enzyme activities, DOC concentrations, and microbial respiration. The potential for non-linear relationships between soil C concentration and decomposition may need to be considered in soil C models and soil C sequestration management approaches.

Wheat straw decomposition stage has little effect on the removal of inorganic N and P from wastewater leached through sand-straw mixes

ABSTRACT Wheat straw amendment to sandy soil can remove nitrogen (N) and phosphorus (P) from wastewater but it is unclear whether prior decomposition affects removal. Sand mixed with finely ground wheat straw at 12.5 g straw kg−1 was placed in leaching columns. Wastewater was added either immediately after mixing with straw (fresh straw) or after the sand-straw mix had been incubated moist for 7 or 14 days (7D or 14D straw). Sand alone was considered as control. Leaching was carried out 4, 8 or 16 days after addition of wastewater and inorganic N and P were analysed after leaching in both leachate and sand. In the amended treatments, nitrate and available P in the sand-straw mix were not detectable throughout the experiment. On day 16, inorganic N in the sand-straw mix was highest in fresh straw where it was three-fold higher than in 14D straw and 30% higher than in sand alone and 7D straw on day 16. Straw decomposition stage had no consistent effect on microbial biomass N and P. Released CO2 was lower in 14D straw than in fresh straw and 7D straw. With straw amendment, > 95% of inorganic N added with wastewater was removed compared to 40–50% with sand alone. Inorganic P leaching was reduced by about 30% compared to sand alone on day 16. In conclusion, wheat straw addition reduced leaching of N compared to sand alone, but the decomposition stage of the straw had little effect on the removal of N and P from wastewater.

Carbonaceous Greenhouse Gases and Microbial Abundance in Paddy Soil under Combined Biochar and Rice Straw Amendment

Little is known about the carbonaceous greenhouse gases and soil microbial community linked to the combination of biochar (BC) and rice straw (RS) in paddy soils. The objectives of this research were to evaluate the effects of combining BC and RS on (1) CH4 and CO2 production from paddy soil, (2) archaeal and bacterial abundance, and (3) rice grain yield. The experiments consisted of a pot trial and an incubation trial, which had a completely randomized design. The experiments included five treatments with three replications: (a) the control (without BC, RS, and chemical fertilizer (CF)); (b) CF; (c) BC 12.50 t ha−1; (d) RS 12.50 t ha−1; and (e) combined BC 6.25 t ha−1 + RS 6.25 t ha−1 + CF. In the sole RS treatment, CH4 production (0.0347 mg m−2 season−1) and the archaeal and bacterial abundance (5.81 × 108 and 4.94 × 1010 copies g−1 soil dry weight (DW)) were higher than outcomes in the sole BC treatment (i.e., 0.0233 mg m−2 season−1 for CH4 production, and 8.51 × 107 and 1.76 × 1010 copies g−1 soil DW for archaeal and bacterial abundance, respectively). CH4 production (0.0235 mg m−2 season−1) decreased significantly in the combined BC + RS + CF treated soil compared to the soil treated with RS alone, indicating that BC lessened CH4 production via CH4 adsorption, methanogenic activity inhibition, and microbial CH4 oxidation through bacterial methanotrophs. However, the archaeal abundance (3.79–5.81 × 108 copies g−1 soil DW) and bacterial abundance (4.94–5.82 × 1010 copies g−1 soil DW) in the combined BC+ RS + CF treated soil and the RS treated soil were found to increase relative to the treatments without RS. The increase was due to the easily decomposable RS and the volatile matter (VM) constituent of the BC. Nevertheless, the resultant CO2 production was relatively similar amongst the BC, RS, and BC + RS treated soils, which was indicative of several processes, e.g., the CO2 production and reduction that occurred simultaneously but in different directions. Moreover, the highest yield of rice grains was obtained from a combined BC + RS + CF treated soil and it was 53.47 g pot−1 (8.48 t ha−1). Over time, the addition of BC to RS soil enhanced the archaeal and bacterial abundance, thereby improving yields and reducing CH4 emissions.

Three-Year Field Observation of Biochar-Mediated Changes in Soil Organic Carbon and Microbial Activity.

Although biochar is considered a promising C sequestration agent, long-term field experiments are lacking to assess the effects of biochar addition on the soil organic C (SOC) and microbial community. Corn ( L.) straw feedstock and biochar were applied to a sandy loam soil for three consecutive years to investigate the SOC distribution within various fractionations, responses of soil microbial biomass, enzyme activity, and community structure. In comparison with straw amendment, higher levels of biochar (6.0 and 12 t ha) significantly increased soil pH, SOC, total N (TN), available P (AP) and available K. Biochar (12 t ha) decreased the fulvic acid fraction by 15.9% and increased the free-light C (FLC), intra-aggregate, and organomineral fractionations by 6.05-, 2.52-, and 0.22-fold, respectively. There was no significant influence of straw or biochar application on the soil microbial biomass C (MBC) contents or the activities of soil enzymes. A phospholipid fatty acids assay suggested that 6.0 t ha straw slightly enriched the abundance of Actinobacteria in soil, whereas biochar (6.0 and 12 t ha) mainly promoted the growth of Gram-positive bacteria, fungi, and general bacteria groups. Canonical correspondence analysis indicated that soil pH, SOC, TN, AP, FLC, and fulvic acid significantly influence the structure of soil microbial community ( < -0.50, < 0.05 for both MBC and the ratio of MBC to SOC; > 0.50 for microbial biomass N, basal respiration [BR], and the ratio of BR through incubation to MBC). Established quantitative relationships provided evidence for understanding the effects of biochar amendment on soil environment after a long-term field application.

Maize production and field CO&lt;sub&gt;2&lt;/sub&gt; emission under different straw return rates in Northeast China

In order to understand and clarify the impacts of straw return on maize production and field CO&lt;sub&gt;2&lt;/sub&gt; emission in Northeast China, the most important agricultural base of the nation, a field experiment was conducted in 2012–2015, including no straw return (CK), straw amendment at 4000 kg/ha (S&lt;sub&gt;4&lt;/sub&gt;), and at 8000 kg/ha (S&lt;sub&gt;8&lt;/sub&gt;). The average grain yield was found significantly promoted by the two straw treatments, with comparably increased magnitudes of 11.0% and 12.8% for S&lt;sub&gt;4&lt;/sub&gt; and S&lt;sub&gt;8&lt;/sub&gt;, respectively, and the benefits were gradually enlarged with increasing experimental duration. Although straw return tends to reduce slightly the harvest index, it was detected that it exerted significantly positive impacts on nitrogen harvest index. These results implied that added straw could lead to raising grain yield and enhancing nitrogen use efficiency simultaneously. In 2015, our monitoring showed that CO&lt;sub&gt;2&lt;/sub&gt; emission was elevated with intensified use of straw, and S&lt;sub&gt;4&lt;/sub&gt; and S&lt;sub&gt;8&lt;/sub&gt; decreased carbon emission efficiency by 7.3% and 13.6%, respectively. However, there was no statistical difference between S&lt;sub&gt;4&lt;/sub&gt; and CK. Overall, straw addition at the rate of 4000 kg/ha accompanied with inorganic fertilizer was recommended to be adopted in Northeast China, which was considered as a sustainable and relatively environment-friendly agricultural technique during maize production.

Increased Methylmercury Accumulation in Rice after Straw Amendment.

Consumption of rice has been shown to be an important route of dietary exposure to methylmercury (MeHg, a neurotoxin) for Asians having a low fish but high rice diet. Therefore, factors that increase MeHg production and bioaccumulation in soil-rice systems, could enhance the risk of MeHg exposure. On the basis of a national-scale survey in China (64 sites in 12 provinces) and rice cultivation experiments, we report that straw amendment, a globally prevalent farming practice, could increase MeHg concentrations in paddy soils (11-1043%) and rice grains (95%). By carrying out a series of batch incubation, seedling uptake and sand culture experiments, we demonstrate that these increases could be attributed to (1) enhanced abundances/activities of microbial methylators and the transformation of refractory HgS to organic matter-complexed Hg, facilitating microbial Hg methylation in soils; (2) enhanced MeHg mobility, and increased root lengths (35-41%) and tip numbers (60-105%), increasing MeHg uptake by rice roots; and (3) enhanced MeHg translocation to rice grains from other tissues. Results of this study emphasize fresh organic matter-enhanced MeHg production and bioaccumulation, and highlight the increased risk of MeHg after straw amendment and thus the need for new policies concerning straw management.