Long-term effects of fertilizer managements on crop yields and organic carbon storage of a typical rice–wheat agroecosystem of China

The present paper summarizes the results from a long-term experiment setup in 1980 in the Taihu Lake region, China, to address the yield sustainability, the dynamic changes of soil organic carbon (SOC) storage, and soil fertility in the rice–wheat ecosystem. Treatments in three replicates comprising manure-treated and chemical fertilizer-treated groups (two factors), each having seven sub-treatments of different combinations of inorganic nitrogen (N), phosphorus (P), potassium (K), and rice straw, were randomly distributed. Results showed that the treatments of manure (pig manure from 1980 to 1996 and oil rape cake thereafter) + N + P + K (MNPK) and chemical fertilizer + N + P + K (CNPK) produced the highest and the most stable yields for both rice and wheat within the respective fertilizer treatment group. Potassium fertilization was necessary for yield sustainability in the ecosystem. Treatments of straw (as rice straw) + N (CRN) and manure + straw + N (MRN) produced more stable yield of rice but less stable of wheat. It was therefore recommended that straw should be only incorporated during the rice season. SOC contents in all treatments showed increasing trends over the period, even in the control treatment. Predicted SOC in chemical fertilizer-treated plots (mostly yet attainable) ranged from 16 to 18 g C kg⁻¹, indicating the high carbon (C) sequestration potential of the soil as compared to the initial SOC. SOC in manure- or straw-treated plots ranged from 17 to 19 g C kg⁻¹, which had been attained roughly 10 years after the experiment was initiated. Nutrient balance sheet showed that there was P surplus in all P-treated plots and a steady increase in Olsen-P over a 24-year period in 0–15 cm soil, which contributed little to crop yield increases. It was therefore suggested that P fertilization rate should be decreased to 30–40 kg P ha⁻¹ year⁻¹. Comparison of yields among the treatments showed that wheat was more responsive to P fertilizer than rice. Thus P fertilizer should be preferably applied to wheat. Soil pH decrease was significant over the 24-year period and was not correlated with fertilizer treatments. The overall recommendation is to incorporate straw at 4,500 kg ha⁻¹ year⁻¹ during the rice season only, with additional 190 kg N ha⁻¹ year⁻¹, 30–40 kg P ha⁻¹ year⁻¹ mainly during the rice season, and 150–160 kg K ha⁻¹ year⁻¹. Further research on the unusual P supply capacity of the soil is needed.     

Einfluss differenzierter Stickstoffdüngung auf Kornertrag und Backqualität von Winterweizen: (Effect of different nitrogen fertilization on yield and bread-making quality of winter wheat)

Abstract

In the “International Long-term Organic Nitrogen Fertilization Experiment” (IOSDV) the organic and mineral fertilization showed a distinct influence on yield formation and quality parameters of winter wheat, though modified by annual weather conditions. Grain yield was affected by annual weather conditions in the first place, followed by mineral N-fertilization and organic fertilization respectively. This result was due to soil conditions at the experimental site. Dry periods during the stages of spikelet formation and flowering formation, which were caused by water deficiency, reduced the yield components strongly. The yield optimum was reached at the level of 110 and 160 kg ha^?1 mineral N on the sandy soil at the location of Berlin-Dahlem. The highest nitrogen level caused non effective or slight yield depressions, especially in the “green, straw and sugar beet leaf manure” fertilization system. Organic N-fertilization (farm yard manure, straw, green manure and beet leaf fertilization) led to significant increases of winter wheat yield in the course of which biggest differences were observed when combined mineral N-fertilization was missing. The bread-making quality of B-wheat variety Flair was primarily dependent on mineral N-fertilization. Necessary values for bread wheat were predominantly obtained only, when mineral N-fertilization rates of 110?–?160 kg ha^?1 were applied. Organic N-fertilization?–?especially farmyard manure?–?improved the bread-making quality of wheat as well in treatments without mineral-N as in combination with mineral-N-fertilization.     

Nitrogen and carbon mineralization of surface-applied and incorporated winter wheat and peanut residues

Laboratory incubation experiments were conducted to study the C and N mineralization dynamics of crop residues (fine roots and straw) of the two main crops (winter wheat and peanut) in the Chinese Loess Plateau under different ways of incorporation. The C mineralization patterns of the soil amended with winter wheat residues differed greatly, and the highest C mineralization was observed in the treatment with winter wheat straw incorporated (39% of the total added C mineralized). The way of straw placement had only a minor effect on the pattern of C mineralization for peanut. Generally, winter wheat residues showed a stronger immobilization than peanut residues during the incubation period, without any net N release. Winter wheat straw incorporated showed the strongest N immobilization with 35 mg kg⁻¹ (equivalent to 27% of added N) immobilized at the eighth week. This study indicated that retaining crop residues at the soil surface in the dry land soils of the Chinese Loess Plateau is beneficial for C sequestration. It also showed that N immobilization occurs only during a limited period of time, sufficient to prevent part of the mineral N pool from leaching, and that net N mineralization can be expected during the subsequent cropping season, thus enhancing synchronization of N supply and demand.     

Contribution of termites to the breakdown of straw under Sahelian conditions

 The effect of termites on the breakdown of Pennisetum pedicellatum straw under Sahelian conditions was studied in northern Burkina Faso during the dry and wet seasons of 1995. Litterbags were laid on plots treated with drieldrin at a rate of 500 g a.i. (active ingredient) ha^–1 to exclude termite activity and on untreated plots. Termites were the only macroscopically visible consumers which were observed in the litterbags during the dry and wet season. In litterbags with no termites present the rate of breakdown of straw was lower during both the dry or wet seasons than in litterbags with termites. In the litterbags with termites, much of the mulch was taken off and replaced by termite-made sheeting. From April to September over 60% of the added material disappeared from termite litterbags, while only 18% of the added straw disappeared from litterbags without termites. From October to December, 28% of the straw disappeared from termite-infested litterbags versus 8% from litterbags without termites. The contribution of termites to the breakdown of straw was estimated to be over 70% for each of the two experiments. Under Sahelian conditions, the breakdown of organic residues is strongly influenced by termites. Received: 6 May 1998     

Dynamics of soil organic carbon and nitrogen associated with physically separated fractions in a grassland-cultivation sequence in the Qinghai-Tibetan plateau

This study is aimed at quantifying organic carbon (C) and total nitrogen (N) dynamics associated with physically separated soil fractions in a grassland-cultivation sequence in the Qinghai-Tibetan plateau. Concentrations of organic C and N of soil, free and occluded particulate organic matter (OM), and aggregate- and mineral-associated OM in different land uses are increased in the following order: 50 years cultivation < 12 years cultivation ≤ native grassland. The prolonged cropping of up to 50 years markedly affected the concentrations of free and occluded particulate OM and mineral-associated OM. After wet-sieving, 43% of native grassland soil mass was found in >1−10 mm water-stable aggregates that stored 40% of bulk soil organic C and N; only 16% and 7% of soil mass containing 16% and 7% of bulk soil organic C and N was >1−10 mm water-stable aggregates of soils cultivated for 12 years and 50 years, respectively. This indicated that losses of soil organic C and N following cultivation of native grassland would be largely related to disruption of >1–10 mm size aggregates and exposure of intra-aggregate OM to microbial attack. Organic C and N concentrations of soil aggregates were similar among aggregate size fractions (>0.05−10 mm) within each land use, suggesting that soil aggregation process of these soils did not follow the hierarchy model. The increase of the C-to-N ratio of free and occluded particulate fractions in the cultivated soils compared to the grassland soil indicated a greater loss of N than C.     

Results of the long-term field experiment IOSDV Jable at the beginning of the 21st century

The impact of organic fertilization and the level of mineral nitrogen fertilization on organic nitrogen and humus balance was studied in a long-term field experiment IOSDV Jable in central Slovenia (sub-Alpine climate, average annual precipitations 1345 mm, average annual temperature 9.5°C, heavy hydromorphic silty loam, umbric Planosols). During the period 1993–2010, precipitation increased significantly at an average rate of 19 mm a^?1; over the same period, average annual temperature increased by 0.025°C (l.f.). The increase in annual precipitation had a negative impact on the yield of all crops (maize, wheat and oats). Increases in mineral N rate led to increased yields. At the highest mineral nitrogen rate, farmyard manure (FYM) did not have a positive impact on yield in the investigated crops, although there was a positive effect of straw incorporation on the yield of maize and oats, and a negative impact on the yield of winter wheat. The organic carbon (Corg) level in the soil increased for all treatments, including FYM or straw and mineral N fertilization. The N content in the soil decreased in the treatment with no organic fertilization and no mineral N, and in the treatment with straw and no mineral N. Corg was increased in treatments with organic fertilization and the highest N rates and remained the same in treatments with moderate N fertilization. All C balances were negative.     

Long-term straw management and N fertilizer rate effects on quantity and quality of organic C and N and some chemical properties in two contrasting soils in Western Canada

Crop residue and fertilizer management practices alter some soil properties, but the magnitude of change depends on soil type and climatic conditions. Field experiments with mainly barley (and canola, wheat, triticale, or pea in a few years) under conventional tillage were conducted from 1983 to 2009 at Breton (Gray Luvisol (Typic Haplocryalf) loam) and Ellerslie (Black Chernozem (Albic Argicryoll) clay loam), Alberta, Canada, to determine the effects of straw management (straw removed (S _Rem) and straw retained (S _Ret)) and N fertilizer rate (0, 25, 50, and 75 kg N ha⁻¹) on total organic C (TOC) and N (TON), light fraction organic C (LFOC), and N (LFON) in the 0–7.5 and 7.5–15 cm, pH in the 0–7.5, 7.5–15, and 15–20 cm and extractable P, ammonium-N, and nitrate-N in the 0–15, 15–30, 30–60, and 60–90 cm soil layers. The S _Ret and N fertilizer treatments usually had higher mass of TOC, TON, LFOC, and LFON in soil at Breton, but only of LFOC and LFON in soil at Ellerslie compared with the corresponding S _Rem and zero-N control treatments. The responses of soil organic C and N to management practices were more pronounced for N fertilization than straw management. There were significant correlations among most soil organic C or N fractions, especially at Breton. Linear regressions between crop residue C or N input, or rate of fertilizer N applied and soil organic C or N were significant in most cases at Breton, but only for LFOC and LFON at Ellerslie. At Breton, compared with zero-N rate, the C sequestration efficiency of additional crop residue C input was 5.8%, 20.1%, and 20.4% in S _Ret and 17.2%, 28.0%, and 30.1% in S _Rem treatments at the 25, 50, and 75 kg N ha⁻¹ rates, respectively. The effects of crop residue management and N fertilization on chemical properties were generally similar for both contrasting soil types. There was no effect of crop residue management on soil pH, extractable P and residual nitrate-N. Extractable P and pH in the top 0–15 cm soil decreased significantly with N application in both soil types. Residual nitrate-N (though quite low in Breton soil) increased with application of N and also indicated some downward movement in the soil profile up to 90 cm depth in Ellerslie soil. There was generally no effect of any treatment on ammonium-N in soil. In conclusion, straw retention and N application improved organic C and N in soil, and generally differences were more pronounced for light fraction than total organic C and N, and between the most extreme treatments (S _Rem0 vs. S _Ret75). Application of N fertilizer reduced extractable P and pH in the surface soil, and showed accumulation and downward leaching of nitrate-N in the soil profile.     

Decadal Nitrogen Fertilization Decreases Mineral‐Associated and Subsoil Carbon: A 32‐Year Study

Crop residues and manure are important sources of carbon (C) for soil organic matter (SOM) formation. Crop residue return increases by nitrogen (N) fertilization because of higher plant productivity, but this often results only in minor increases of SOM. In our study, we show how N fertilization and organic C additions affected SOM and its fractions within a 32‐year‐long field‐experiment at Puch, Germany. Five organic additions, no‐addition (control), manure, slurry, straw and straw + slurry, were combined with three mineral N fertilization rates (no, medium and high fertilization), which resulted in 1·17–4·86 Mg C‐input ha^‐1 y^‐1. Topsoil (0–25 cm) SOM content increased with N fertilization, mainly because of the C in free light fraction (f‐LF). In contrast, subsoil (25–60 cm) SOM decreased with N fertilization, probably because of roots' relocation in Ap horizon with N fertilization at the surface. Despite high inputs, straw contributed little to f‐LF but prevented C losses from the mineral‐associated SOM fraction (ρ > 1·6 g cm^‐3) with N fertilization, which was observed without straw addition. Above (straw) and belowground (roots) residues had opposite effects on SOM fractions. Root C retained longer in the light‐fractions and was responsible for SOM increase with N fertilization. Straw decomposed rapidly (from f‐LF) and fueled the mineral‐associated SOM fraction. We conclude that SOM content and composition depended not only on residue quantity, which can be managed by the additions and N fertilization, but also on the quality of organics. This should be considered for maintaining the SOM level, C sequestration, and soil fertility. Copyright © 2016 John Wiley & Sons, Ltd.     

A quantitative size–density separation method to recover and characterise decomposing crop residues added to soil

Plant residues placed in soil in mesh bags do not mimic realistic decomposition conditions. Alternative techniques, e.g. combined particle size and density methods, have been proposed to monitor in situ decomposition of plant residues, but are not appropriately evaluated for their ability to quantitatively recover and characterise decomposing residues of a range of sizes mixed with soil over long incubation times. For this study, we used canola residues of three different starting sizes (<1, 5–7 and 20–25 mm) mixed in a soil and incubated for 6 months. The modified method presented here relied on initial size separation of soil and organic particles into fractions of sizes <106 and >106-μm by wet sieving, and then by repeated floatation and decantation, using water as a density agent, to recover and characterise decomposing canola residues from a mixture of >106-μm mineral plus organic materials into >106-to-500 and >500-μm fractions. On day 0, across the three residue-size treatments, the >500-μm fraction recovered 93–96% of canola residue-C, with water-soluble residue-C loss during the recovery process quantified as representing further 4–5%. The rate of loss of residue-C in the >500-μm fraction was the largest, matching well the cumulative respiration loss of residue-C. The recovery of canola residue-C in the >500-μm fraction, determined as percent of cumulative CO₂–C respired of added C, decreased to 72–76% by the end of incubation, likely due to progressive generation of finer-sized residues and microbial/faunal metabolites. The increase of N in the >500-μm fraction accounted for ca. 40% of total soil N immobilised in the residue-amended soil. The extent and patterns of changes in C, N and S contents, and C-to-N and C-to-S ratios of separated fractions were similar among all the residue treatments during decomposition, except for the ground-residue treatment. The combined size and density separation procedure can be used to study decomposition in situ of soil-mixed plant residues of different sizes that are usually found in agro-ecosystems.     

Effects of long-term mineral fertilization on microbial biomass,microbial activity,and the presence of <Emphasis Type="Italic">r</Emphasis>- and <Emphasis Type="Italic">K</Emphasis>-strategists in soil

Long-term effects of mineral fertilization on microbial biomass C (MBC), basal respiration (R _B), substrate-induced respiration (R _S), β-glucosidase activity, and the r–K-growth strategy of soil microflora were investigated using a field trial on grassland established in 1969. The experimental plots were fertilized at three rates of mineral N (0, 80, and 160 kg ha⁻¹ year⁻¹) with 32 kg P ha⁻¹ year⁻¹ and 100 kg K ha⁻¹ year⁻¹. No fertilizer was applied on the control plots (C). The application of a mineral fertilizer led to lower values of the MBC and R _B, probably as a result of fast mineralization of available substrate after an input of the mineral fertilizer. The application of mineral N decreased the content of C extracted by 0.5 M K₂SO₄ (C _ex). A positive correlation was found between pH and the proportion of active microflora (R _S/MBC). The specific growth rate (μ) of soil heterotrophs was higher in the fertilized than in unfertilized soils, suggesting the stimulation of r-strategists, probably as the result of the presence of available P and rhizodepositions. The cessation of fertilization with 320 kg N ha⁻¹ year⁻¹ (NF) in 1989 also stimulated r-strategists compared to C soil, probably as the result of the higher content of available P in the NF soil than in the C soil.     

Influence of application of rice straw, farmyard manure, and municipal biowastes on nitrogen fixation, soil microbial biomass N, and mineral N in a model paddy microcosm

Effects of application of rice straw (RS), farmyard manure (FYM), municipal biowaste compost (MBCom), and municipal biowaste charcoal (MBCha) on soil microbial biomass N, mineral N, and nitrogen-fixing activity (NFA) of a model paddy microcosm were examined in comparison with urea fertilizer. When microcosms were added with urea, NFA decreased with increasing rates of fertilization, and it was negligible (less than 4% of the control, no urea fertilization) in the soils treated with more than 60 mg kg⁻¹ urea–N. The addition of RS, with the highest C/N ratio among the organic wastes used, stimulated N₂ fixation most effectively (40% increase compared to the control). MBCom, with the lowest C/N ratio and a comparable mineral N content to 60 mg kg⁻¹ urea–N, decreased N₂ fixation (50% decrease), but it was not markedly suppressed unlike urea. In spite of the fact that FYM contained a relatively large N, expressed as low C/N ratio, its effect on N₂ fixation was small (14% decrease). FYM and MBCom did not stimulate NFA as RS did. This may be explained by the fact that N concentrations of microbial biomass N and available N were higher in the soils than in soil treated with RS. The effect of MBCha addition on N₂ fixation was small (14% decrease). The present study demonstrated that organic wastes might affect N₂ fixation depending upon the amount of available N in the waste-treated soils, but that organic-waste-treated soils generally support higher N₂ fixation than chemical-fertilizer-treated soils.     

Effects of 17-year fertilization on soil microbial biomass C and N and soluble organic C and N in loessial soil during maize growth

As labile organic pools, soluble organic matter and soil microbial biomass are sensitive to changes in soil management and therefore good indicators of soil quality. Effects of a 17-year long-term fertilization on soil microbial biomass C (SMBC) and N (SMBN), soluble organic C, and soluble organic N during the maize growing season were evaluated in a loess soil (Eum-Orthic Anthrosol) in northwest China. The fertilization treatments included no fertilizer (CK), inorganic N, P, and K fertilizer (NPK), cattle manure plus NPK fertilizer (MNPK), and straw plus NPK fertilizer (SNPK). Our results showed that C storage in the 0–20 cm soil layer was 28% to 81% higher in the fertilized treatments compared to the unfertilized treatment. In the 0–10 cm soil layer, SMBC and SMBN in the three fertilized treatments were higher than in the unfertilized treatment on all sampling dates, while microbial biomass C and N in the 0−10 cm soil layers were the highest at grain filling. In the same soil layer, soil-soluble organic C generally decreased in the order MNPK > SNPK > NPK > CK, while soluble organic N was the highest in the MNPK followed by the SNPK treatment. There was no significant difference in soluble organic N in the NPK and CK treatments throughout most of the maize growing season. Changes in soluble organic N occurred along the growing season and were more significant than those for soluble organic C. Soluble organic N was the highest at grain filling and the lowest at harvest. Overall, our results indicated that microbial biomass and soluble organic N in the surface soil were generally the highest at grain filling when maize growth was most vigorous. Significant positive relationships were found between soluble organic C and SMBC and between soluble organic N and SMBN.     

Influence of long-term organic and mineral fertilization on soil nematofauna when growing Sorghum bicolor in Burkina Faso

Soil degradation has led crop yield to decline in many Sahelian countries and is a fundamental agricultural and economical threat for local populations. In Saria, Burkina Faso, long-term experiments are being performed to find efficient soil management practices that could improve soil fertility. A randomized block experiment comprising organic amendment (unamended control, straw at 8.3 t ha⁻¹, manure at 10 t ha⁻¹) coupled with mineral fertilization (no urea, urea at 60 kg ha⁻¹) was started in 1980 with a continuous sorghum (Sorghum bicolor) cropping system. Twenty-six years after the settlement of the treatments, we compared their effects on nematode populations, community structure, and ecological indices, as well as soil physical and chemical properties at three stages of sorghum’s cropping cycle.     

Carbon mineralization and microbial activity in a field site trial used for 14C turnover experiments over a period of 30 years

 Long-term experiments on different crop management systems provide essential information about turnover of soil organic matter and changes in microbial properties over a period of time. A long-term field site trial, which was established in 1967 near Vienna, Austria, to document the fate of ¹⁴C-labelled manure (straw and farmyard) under different crop management systems (crop rotation, spring wheat and bare fallow), was investigated. Soil samples were taken in 1997 and separated into size fractions (>250 μm, 250–63 μm, 63–2 μm, 2–0.1 μm and <0.1 μm) after aggregate dispersion using low-energy sonication. Organic C, total N and ¹⁴C content were measured in the bulk soil and the size fractions and microbial properties were analysed in the bulk soil. Additionally, C mineralization in bulk soil samples was monitored at 20 °C over a period of 28 days, and subsequently ¹⁴C-CO₂ content was analysed. The distribution of organic C and N within the size fractions was similar between crop rotation and spring wheat; the highest amounts of organic C and N were found in the clay-sized fraction. The amounts of C and N were significantly smaller in the bare fallow, which was depleted of organic matter in the coarse-sized fractions. ¹⁴C distribution differed significantly from unlabelled C distribution, labelled C was accumulated in the silt-sized fraction, indicating weak humification of the applied manure C. The highest rate of C mineralization was measured in the crop rotation and spring wheat, whereas the emission rate of the bare fallow was about 40% lower. The higher ¹⁴C:C ratio of the bulk soil in comparison to the emitted CO₂ indicated that labelled C compounds still remained mineralizable after a period of 30 years. Microbial properties showed a great difference between crop management systems and bare fallow, particularly regarding urease and xylanase activity. Received: 31 May 1999     

Soil microbial activity,aggregation and nutrient responses to straw pulping liquor in corn cropping

The cereal and grass seed cropping systems of the Pacific northwestern USA generate a valuable fiber source for papermaking. Pulping straw with KOH produces black liquor, an organic waste effluent with potential as a K source and soil amendment. The objectives of this study were to determine if black liquor from wheat straw pulping with KOH improves soil quality including soil aggregation, microbial biomass and microbial activity, and corn yield. A 2-year field trial with a randomized complete block design was conducted with black liquor or KCl applied at 168 and 336 kg K ha⁻¹ at two sites, respectively, planted to corn in Central Washington. Black liquor applied at rates of 168 and 336 kg K ha⁻¹ slightly increased soil pH by 0.2 to 1.4 units and electrical conductivity by 0.0 to 0.8 dS m⁻¹ at both sites, and significantly increased soil test K compared to the nonamended control by 199 to 368 mg K kg⁻¹ soil in the top 5 cm at Paterson and by 44 to 200 mg K kg⁻¹ soil in the top 5 cm at Prosser. Corn had no yield response to the application of either K sources, black liquor, or KCl applied at both rates at both sites. Generally, black liquor applied at these rates increased soil dehydrogenase, β-glucosidase, and arylsulfatase activities, microbial biomass C, and soil wet stable macroaggregates in the top 5-cm soil at both sites. The results of this study suggest that the organic-rich waste liquor from wheat straw pulping with KOH may improve soil quality and soil test K without reducing crop productivity.     

Long-term effect of straw and farmyard manure on soil organic matter in field experiment in the Czech Republic

The effect of long-term (45 years) mineral and organic fertilization on soil organic matter (SOM) quantity (organic C and N content) and quality (hot-water-soluble C content, microbial biomass C content, hydrophobic organic components of SOM, soil enzyme activities) was determined in a field experiment established in Trutnov (North Bohemia, sandy loam, Eutric Cambisol). Six treatments were chosen for investigation: unfertilized control, mineral fertilization (NPK), straw N, farmyard manure (FYM) and straw and FYM completed with mineral NPK. Soil samples were taken from the arable layer (0–20 cm) in spring over the period of 2004–2010. The positive effect of FYM on the total organic C and N content, hot-water-soluble C content and hydrophobic organic components of SOM was more than 50% higher than that of straw and mineral N fertilization. Application of straw N increased microbial biomass C content in soil and generated invertase activity above the level of FYM. Hot-water-soluble C content, hydrophobic organic components of SOM and urease activity were positively correlated with total organic C and N content (R = 0.58–0.98; p < 0.05). Addition of mineral NPK to both the straw and FYM emphasized the effect of organic fertilization in most of monitored characteristics.     

The influence of crop residues with different C: N ratios on the N<Subscript>2</Subscript>O emission from a loamy sand soddy-podzolic soil

The influence of crop residues with different C: N ratios on the N₂O emission from differently managed loamy sand soddy-podzolic soils was studied in a 50-day laboratory experiment. The application of crop residues into the soil increased the N₂O emission from the soil. The N₂O emission was lower from the poorly managed soil as compared to the soil with the high degree of cultivation. The crop residues form the following decreasing sequence in terms of their effect on the cumulative N₂O flow: cabbage > red clover > perennial grasses > straw of spring wheat. The composting of crop residues with a wide C: N ratio for 50 days did not exceed the critical value of the emission factor (1.25%), whereas, in the composting of crop residues with a narrow C: N ratio, the critical value of the emission factor was 1.3–2.0 times higher.     

Is burial of wheat straw in ditches a way to reduce CH4 emissions from rice cultivation?

Abstract

Burial of wheat straw in ditches and incorporation of wheat straw are the two main ways of returning wheat straw prior to rice cultivation in China. To examine the effect of burying wheat straw in ditches on CH₄ emissions from rice cultivation, a field experiment was conducted at Yixing, Jiangsu, China in 2004. CH₄ flux was measured using a closed-chamber technique in three treatments (CK, no wheat straw application; WI, evenly incorporating 3.75 t ha^?1 wheat straw into the 0.1 m topsoil; WD, burying 3.75 t ha^?1 wheat straw in 0.14-m deep by 0.25-m wide ditches). Seasonal CH₄ emissions ranged from 49.7 to 218.4 kg CH₄ ha^?1. The application of wheat straw in these two ways significantly increased CH₄ emissions by 4.0-fold and 4.4-fold, respectively (P < 0.05). Although CH₄ flux from the non-ditch area in the WD treatment was as low as that in the CK treatment, it was counter-balanced by extremely high CH₄ flux from the ditch, which was approximately 6.0-fold as much as that from WI, leading to comparability between treatments WI and WD in total CH₄ emissions (P > 0.05). No significant difference was observed between the three treatments in grain yield (P > 0.05). The results indicated that burial of wheat straw in ditches is not a way to reduce CH₄ emission from rice cultivation.     

Changes in microbial community structure in soil as a result of different amounts of nitrogen fertilization

The changes in size, activity and structure of soil microbial community caused by N fertilization were studied in a laboratory incubation experiment. The rates of N fertiliser applied (KNO₃) were 0 (control), 100 and 2,000 μg N g⁻¹ soil. Despite no extra C sources added, a high percentage of N was immobilized. Whereas no significant increase of microbial C was revealed during incubation period, microbial growth kinetics as determined by the substrate-induced growth-response method demonstrated a significant decrease in the specific growth rate of microbial community in soil treated with 2,000 μg N g⁻¹ soil. Additionally, a shift in microbial community structure resulting in an increase in fungal biomarkers, mainly in the treatment with 2,000 μg N g⁻¹ soil was visible.     

Impact of coastal wetland cultivation on microbial biomass, ammonia-oxidizing bacteria, gross N transformation and N2O and NO potential production

A ¹⁵N dilution experiment was carried out to investigate effects of cultivation on the gross N transformation rate in coastal wetland zone. Microbial community composition was estimated by phospholipid fatty acid (PLFA) analysis and abundance of soil ammonia-oxidizing bacteria (AOB) was quantified by real-time polymerase chain reaction (PCR). Soil salinity decreased significantly, while total N increased after coastal wetland was cultivated. Microbial biomass (total PLFA), bacterial biomass, fungal biomass, and actinomycete biomass of the native coastal wetland soils were significantly (p < 0.05) lower than those of the cultivated soils whereas AOB population size also significantly increased after coastal wetland cultivation. Multiple regression analysis showed that total PLFA biomass and soil total N (TN) explained 97% of the variation of gross N mineralization rate in the studied soils (gross mineralization rate = 0.179 total PLFA biomass + 5.828TN − 2.505, n = 16, p < 0.01). Gross nitrification rate increased by increasing the soil AOB population size and gross mineralization rate (M) (gross nitrification rate = 3.39AOB + 0.18 M − 0.075, R ² = 0.98, n = 16, p < 0.01). Management of salt discharge and mineral N fertilization during the cultivation of wetland soils might have changed composition of soil microflora and AOB population size, thus influencing mineralization and nitrification. Probably, the cultivation of coastal wetland soils increased the risk of N losses from soil through nitrate leaching and gas emission (e.g., N₂O and NO).