Impact of wheat straw decomposition on successional patterns of soil microbial community structure

The dynamics of indigenous bacterial and fungal soil communities were followed throughout the decomposition of wheat straw residue. More precisely, such dynamics were investigated in the different soil zones under the influence of decomposing wheat straw residue (i.e. residues, soil adjacent to residue = detritusphere, and bulk soil). The genetic structures of bacterial and fungal communities were compared throughout the decomposition process long by applying B- and F-ARISA (for bacterial and fungal-automated ribosomal intergenic spacer analysis) to DNA extracts from these different zones. Residue decomposition induced significant changes in bacterial and fungal community dynamics with a magnitude of changes between the different soil zones ordered as followed: residue > detritusphere > bulk soil, confirming the spatial structuration of the sphere of residue influence to the 4-6 mm soil zone in contact with residue. Furthermore, significant differences in the structure of bacterial and fungal communities were apparent between the early (14 and 28 days) and late (from 56 to 168 days) stages of decomposition. These could be related to ecological attributes such as the succession of r- (copiotrophs) and K- (oligotrophs) strategists. Microbial diversity at the early (28 days) and late (168 days) stages of degradation was further analysed by a molecular inventory of 16S and 18S rDNA in DNA extracts from the residue zone. This confirmed the succession of different populations during residue decomposition. Fluorescent Pseudomonas spp. and Neurospora sp. were dominant in the early stage with subsequent stimulation of Actinobacteria and Deltaproteobacteria taxa, as well as Basidiomycota fungal taxa and Madurella spp. According to the ecological attributes of these populations, microbial succession on fresh organic residue incorporated in soil would be dominated by copiotrophs and r-strategists in the early stages, with oligotrophs (K-strategists) increasing in relative abundance as substrate quantity and/or quality declines over time.     

Microscale distribution and function of soil microorganisms in the interface between rhizosphere and detritusphere

The rhizosphere and the detritusphere are hot spots of microbial activity, but little is known about the interface between rhizosphere and detritusphere. We used a three-compartment pot design to study microbial community structure and enzyme activity in this interface. All three compartments were filled with soil from a long-term field trial. The two outer compartments were planted with maize (root compartment) or amended with mature wheat shoot residues from a free air CO₂ enrichment experiment (residue compartment) and were separated by a 50 μm mesh from the inner compartment. Soil, residues and maize differed in ¹³C signature (δ¹³C soil −26.5‰, maize roots −14.1‰ and wheat residues −44.1‰) which allowed tracking of root- and residue-derived C into microbial phospholipid fatty acids (PLFA). The abundance of bacterial and fungal PLFAs showed clear gradients with highest abundance in the first 1–2 mm of the root and residue compartment, and generally higher values in the vicinity of the residue compartment. The δ¹³C of the PLFAs indicated that soil microorganisms incorporated more carbon from the residues than from the rhizodeposits and that the microbial use of wheat residue carbon was restricted to 1 mm from the residue compartment. Carbon incorporation into soil microorganisms in the interface was accompanied by strong microbial N immobilisation evident from the depletion of inorganic N in the rhizosphere and detritusphere. Extracellular enzyme activities involved in the degradation of organic C, N and P compounds (β-glucosidase, xylosidase, acid phosphatase and leucin peptidase) did not show distinct gradients in rhizosphere or detritusphere. Our microscale study showed that rhizosphere and detritusphere differentially influenced microbial C cycling and that the zone of influence depended on the parameter assessed. These results are highly relevant for defining the size of different microbial hot spots and understanding microbial ecology in soils.     

Carbon mineralisation in soil adjacent to plant residues of contrasting biochemical quality

A fraction of the C of residues incorporated into soil diffuses into the adjacent soil where it is eventually mineralised by microorganisms. Our aim was to quantify the contribution of this adjacent soil to the overall mineralisation of residue-C. For this, we incorporated two different residues labelled with ¹³C, with contrasting biochemical characteristics, namely mature wheat straw and young rye leaves, in soil cores. When 15% mineralisation of residue-C was measured for both residues, we separated a particulate fraction (the residues), the adjacent soil (4-5 mm thick) and a distant soil fraction, and incubated them separately for 5 h. We found that 76% of the mineralised wheat straw-C came from the particulate fraction and 23% from the soil adjacent to the residues. For rye leaves, 67% of the evolved CO₂ came from the particulate fraction and 33% from the adjacent soil. It showed that the adjacent soil had a significant role in the mineralisation of carbon from the residues, even if the main source of residue-derived CO₂ was the particulate fraction itself. The functional importance of the soil adjacent to the residues increased with the amount of soluble organic compounds that had been leached from the residue into the adjacent soil, suggesting a strong interaction between the initial quality of the crop residue and the resulting spatial heterogeneity of the decomposing microorganisms and C within the soil.     

Differential and successive effects of residue quality and soil mineral N on water-stable aggregation during crop residue decomposition

Residue quality has been shown to influence soil water-stable aggregation (WSA) during crop residue decomposition, but there is still little information about its interactive effect with soil mineral N availability. The aim of this study was to determine the effect of soil mineral N on WSA during the decomposition of two high-C/N crop residues (wheat straw with C/N = 125.6 and miscanthus straw with C/N = 311.3). The two crop residues were combined with three mineral N addition rates (0, 60, and 120 mg N kg⁻¹ dry soil). Respiration, soil mineral N content, and WSA (expressed as mean-weight diameter, MWD) were measured on several dates during a 56-d incubation. The effect of decomposing crop residues on WSA followed two phases. (i) Between 0 and 7 d, the increase in WSA was related to intrinsic residue quality with higher decomposability of the wheat straw resulting in higher WSA. (ii) Thereafter, and until the end of the experiment, mineral N addition rates had a predominant but negative influence on WSA. In this second phase, the average MWD of residue-treated soils was 0.92, 0.55, and 0.44 mm for the 0, 60 and 120 mg N kg⁻¹ dry soil addition rates, respectively. Mineral N addition which did result in higher crop residue decomposition did not lead to higher WSA. WSA during crop residue decomposition is therefore not simply positively related to the induced microbial activity, and changes in microbial community composition with differential effects on WSA must be involved. The impact of high-C/N crop residues inputs on WSA, initially assumed to be low, could actually be strong and long-lasting in situations with low soil mineral N content.     

Role of soil and residue microorganisms in determining the extent of residue decomposition in soil

The effects of residue (wheat straw or sewage-sludge compost) incorporation in soil and the relative contribution of microorganisms in the residues, or in the soil to decomposition of the added residue, (CO₂ production) was evaluated in an incubation experiment. All residues and soils were adjusted to 33 kPa moisture tension and maintained at 25°C under a constant flow of CO₂-free air for 72 days. Residue decomposition was determined by monitoring CO₂ evolution from the treatments.

Mixing an aged sewage-sludge compost (10%, 224 Mg ha⁻¹) with soil stimulated decomposition of the compost 1.64-fold when compared with any of the localized placements, and indicated that the indigenous soil microorganisms were the major contributors to the transformations of this mature compost. Wheat straw was populated with organisms capable of decomposing readily-available substrates in the straw during the first stage of the decomposition, whereas it appeared that soil organisms contributed to an acceleration of straw decomposition during the final stages. After 65 days approx. 30% of the added wheat straw C had been evolved as CO₂. Soil basidiomycetes doubled the extent of decomposition when the indigenous decomposers in wheat were inactivated by γ-irradiation. Model equations are presented for residue decomposition relative to time.     

Carbon dynamics of rhizodeposits, root- and shoot-residues in a rice soil

A laboratory incubation experiment was conducted to investigate the fates of plant-derived C during the simulated fallow period in a rice soil. The ¹³C labelled soil and plant materials were used to follow the residue decomposition and its effect on soil organic C (SOC) dynamics under the conditions of either incorporation into soil or intact root systems. The soils were incubated at 15 °C for 240 d and destructive sampling was conducted at 60, 150 and 240 d. To observe the temperature effect, one batch of incubation was shifted from 15 to 25 °C during the last 45 d (between 195 and 240 d). The results showed that the decomposition of the incorporated residues could be divided into two phases: an initial rapid phase followed by a slower phase of decomposition. The decomposition of straw residues was faster than root residues: with 73% of the straw residue being decomposed, compared with 56% of the root residue over 240-d incubation at 15 °C. The water-soluble organic C and microbial biomass C significantly increased after residue incorporation. The total SOC contents, however, slightly decreased, although significant amounts of straw C (14.2%) and root C (8.7%) were found in SOC at the end of incubation, suggesting that the degradation of native SOC occurred concomitantly. Similar to decomposition of the incorporated residues, the organic substances derived from rhizodeposition of the previous season were mineralized rapidly at first and then slowly. The decomposition of the intact root system, however, was extremely slow. This result suggested that the intact root system conserved more organic C in soils compared with the incorporation of fresh residues. Increase of temperature from 15 to 25 °C during the last 45-days of incubation significantly promoted the residue decomposition.     

Decomposition of mulched versus incorporated crop residues: Modelling with PASTIS clarifies interactions between residue quality and location

Crop residue management has been shown to significantly affect the decomposition process of plant debris in soil. In previous studies examining this influence, the extrapolation of laboratory data of carbon and/or nitrogen mineralization to field conditions was often limited by a number of interactions that could not be taken into account by a mere experimental approach. Therefore, we demonstrated the interactive effect between crop residue location in soil (mulch vs. incorporation) and its biochemical and physical quality, in repacked soil columns under artificial rain. Decomposition of ¹³C and ¹⁵N labelled rape and rye residues, with associated C and N fluxes, was analysed using the mechanistic model PASTIS, which turned out to be necessary to understand the interacting factors on the C and N fluxes. The influence of soil and residue water content on decomposition and nitrification was evaluated by the moisture limitation factor of PASTIS. This factor strongly depended on residue location and to a smaller extent on physical residue properties, resulting in a lower decomposition rate of about 35% for surface placed compared to incorporated residues. Irrespective of its placement, the biochemical residue quality (e.g. N availability for decomposition, amount of soluble compounds and lignin) was responsible for a faster and more advanced decomposition of about 15% in favour of rye compared to rape, suggesting only a limited interaction between residue quality and its location. Net N mineralization after nine weeks was larger for rye than for rape, equivalent to 59 and 10 kg NO₃⁻-N ha⁻¹ with incorporation, and 71 and 34 kg NO₃⁻-N ha⁻¹ with mulch, respectively. This net N mineralization in soil resulted from the interaction between soil water content, depending on residue placement, and N availability, which was determined by the biochemical residue quality. Moisture limitation appeared more important than N limitation in the decomposition of mulched residues. Modelling of gross N mineralization and immobilization also revealed that leaving crop residues at the soil surface may increase the risk of nitrate leaching compared to residue incorporation, if (i) soil water content under mulch is larger than with residue incorporation (more gross N mineralization), and (ii) availability to the applied C-source is limited (less gross N immobilization). Scenario analyses with PASTIS confirmed the importance of moisture conditions on the decomposition of mulched residues and the small interaction between biochemical crop residue quality and its location in soil.     

Residue addition and liming history interactively enhance mineralization of native organic carbon in acid soils

Lime application is the most common method to improve crop production in acid soils and has been shown to change soil organic C content. However, the impact of liming history on the priming effect on soil organic C is not well understood. This study examined the effect of liming history on C priming in response to the addition of crop residues of different qualities. Soils with pH ranging from 4.7 to 7.4 were collected from two adjacent field experiments whereby lime was applied at different rates, 6 and 35 years ago. A 90-day incubation study was conducted by applying ¹³C-labelled wheat (C/N 42) and field-pea (C/N 29) residues at a rate of 5 g kg^?1 soil. Residue application to soils yielded the positive priming effect in all pH levels with the magnitude of C priming being the greatest at initial soil pH 6.6. In comparison, the optimal pH for residue decomposition (7.3) was higher than that for priming. The overall priming effect was about 17% greater with field-pea than wheat residue. However, cumulative decomposition of added field-pea residue was 15% lower than that of wheat residue. Furthermore, C priming was greater in soils from the 35-year-old than the 6-year-old limed plots, indicating that a longer history of liming did not enhance the protection of indigenous C from mineralization. The results suggest that increases in soil pH by liming enhanced native C priming through greater microbial biomass and activity and that the magnitude and dynamics of the priming effect largely depended on residue quality and its consequent nutrient supply to decomposer organisms. The study implies that over-liming would likely have negative impacts on the long-term C sequestration.     

Soil moisture, carbon and nitrogen dynamics following incorporation and surface application of labelled crop residues in soil columns

One way to increase the amount of carbon sequestered in agricultural land is to convert conventional tillage into no‐tillage systems. This greatly affects the location of crop residues in soil. To investigate the impact of the location of residues on soil physical and biological properties and how the interactions between those properties influence the fate of carbon and nitrogen in soil, we did a laboratory experiment with repacked soil in columns. Doubly labelled ¹³C¹⁵N oilseed rape residues were incorporated in the 0–10 cm layer or left on the soil surface. The columns were incubated for 9 weeks at 20°C and were submitted to three cycles of drying and wetting, each of them induced by a rain simulator. The location of the residues affected the water dynamics and the distribution of C and N in the soil, which in turn influenced microbial activity and the decomposition rate of the added residues. After 9 weeks of’incubation, 18.4 ± 1.5% of the surface applied residue‐C and 54.7 ± 1.3% of the incorporated residue‐C was mineralized. We observed a nitrate accumulation of 10.7 mg N kg^?1 with residues at the soil surface, 3.6 mg N kg^?1 with incorporated residues and 6.3 mg N kg^?1 without addition of fresh organic matter, which entailed net N mineralization in soil under mulch and immobilization of N with residue incorporation compared with the control soil. We concluded that application of oilseed rape residues at the soil surface increased the storage of fresh organic C in soil in the short term, compared with the incorporation treatment, but increased the risk of nitrate leaching.     

Carbon flow from C-labeled straw and root residues into the phospholipid fatty acids of a soil microbial community under field conditions

To better understand how residue quality and seasonal conditions influence the flow of C from both root and straw residues into the soil microbial community, we followed the incorporation of ¹³C-labeled crimson clover (Trifolium incarnatum) and ryegrass (Lolium multiflorum) root and straw residues into the phospholipid fatty acids (PLFA) of soil microbial biomass. After residue incorporation under field conditions in late summer (September), the ¹³C content of soil PLFA was measured in September, October, and November, 2002, and April and June, 2003. Multivariate non-metric multidimensional scaling techniques showed that the distribution of ¹³C among microbial PLFA differed among the four primary treatments (ryegrass straw and roots, clover straw and roots). Regardless of treatment, some PLFA remained poorly labeled with ¹³C throughout much of the study (16:1ω5, 10Me17:0; 0-5%), whereas other PLFA consistently contained a larger percentage of residue-derived C (16:0; 18:1ω9, 18:2ω6,9; 10-25%). The distribution of residue ¹³C among individual PLFA differed from the relative contributions of individual PLFA (mol%) to total PLFA-C, suggesting that a subset of the soil biomass was primarily responsible for assimilating residue-derived C. The distribution of ¹³C among soil PLFA differed between the sampling times, indicating that residue properties and soil conditions influenced which members of the community were assimilating residue-derived C. Our findings will provide the foundation for further studies to identify the nature of the community members responsible for residue decomposition at different times of the year, and what factors account for the dynamics of the community involved.     

添加黏土及植物残体混合物对沙质土壤中呼吸作用和微生物量碳、氮有效性的影响

Crop yields in sandy soils can be increased by addition of clay-rich soil, but little is known about the effect of clay addition on nutrient availability after addition of plant residues with different C/N ratios. A loamy sandy soil(7% clay) was amended with a clay-rich subsoil(73% clay) at low to high rates to achieve soil mixtures of 12%, 22%, and 30% clay, as compared to a control(sandy soil alone) with no clay addition. The sandy-clay soil mixtures were amended with finely ground plant residues at 10 g kg~(-1): mature wheat(Triticum aestivum L.) straw with a C/N ratio of 68, mature faba bean(Vicia faba L.) straw with a C/N ratio of 39, or their mixtures with different proportions(0%–100%, weight percentage) of each straw. Soil respiration was measured over days 0–45 and microbial biomass C(MBC), available N, and p H on days 0, 15, 30, and 45. Cumulative respiration was not clearly related to the C/N ratio of the residues or their mixtures, but C use efficiency(cumulative respiration per unit of MBC on day 15) was greater with faba bean than with wheat and the differences among the residue mixtures were smaller at the highest clay addition rate. The MBC concentration was lowest in sole wheat and higher in residue mixtures with 50% of wheat and faba bean in the mixture or more faba bean. Soil N availability and soil p H were lower for the soil mixtures of 22% and 30% clay compared to the sandy soil alone. It could be concluded that soil cumulative respiration and MBC concentration were mainly influenced by residue addition, whereas available N and p H were influenced by clay addition to the sandy soil studied.     

潮土中残留小麦和玉米秸秆养分含量差异及与微生物群落组成的关系

黄淮海平原典型潮土上小麦和玉米收获后的秸秆往往直接还田,但驱动它们在不同质地潮土(砂质、壤质、黏质)中分解的微生物是否与残留秸秆养分含量有关尚不清楚.本研究基于尼龙网袋法,通过10个月的田间培育试验,监测秸秆分解率、残留秸秆养分含量及微生物群落组成,评估各指标在秸秆类型和土壤质地之间的差异,探究残留秸秆养分与微生物群落...     

Impact on C and N dynamics of simultaneous application of pig slurry and wheat straw, as affected by their initial locations in soil

The joint management of animal manures and plant biomass as straw on agricultural soils may be a viable option for reducing the environmental impacts associated with livestock production and recycling nutrients efficiently. To investigate this option, an incubation in controlled conditions examined how the simultaneous addition of ¹⁵N-labeled pig slurry and ¹³C-labeled wheat straw, either on the soil surface or incorporated into the soil, affected the mineralization of C from the organic materials and the soil N dynamics. Samples from a typic hapludalf were incubated for 95 days at 25°C with eight treatments: unamended soil (S), wheat straw left on the soil surface (Ws), wheat straw incorporated in the soil (Wi), pig slurry on the soil surface (Ps), pig slurry incorporated in the soil (Pi) and three combinations of the two amendments: Pi?+?Ws, Pi?+?Wi, and Ws?+?Ps. Carbon dioxide and ¹³CO₂ emissions and soil N content were measured throughout the incubation. Pig slurry stimulated the decomposition of straw C only when wheat straw and pig slurry were left together on the soil surface. Incorporation of both wheat straw and pig slurry did not modify straw C mineralization when compared to straw incorporation alone but this promoted a higher rate of N immobilization. The results suggest that when pig slurry is used in field under no-till conditions, the best strategy to preserve environmental quality with regard to CO₂ emissions would be to apply pig slurry underneath the crop residues.     

Carbon flow from C-labeled clover and ryegrass residues into a residue-associated microbial community under field conditions

Microbial colonization of soil-incorporated, ¹³C-labeled, crimson clover and ryegrass straw residues was followed under western Oregon field conditions from late summer (September) to the following early summer (mid-June) by measuring the ¹³C content of phospholipid fatty acid (PLFA) extracted from residues recovered from soil. Residue type influenced the rate of appearance of specific PLFA during early decomposition, with branch chain bacterial PLFA (i15:0, a15:0, i16:0) appearing on clover and ryegrass residues in October and November, respectively. By April, additional PLFA (16:1ω5, 16:1ω7, cy17:0, 18:0, 18:1ω9) had appeared on both residues. Between April and June, microbial community structure shifted again with significant increases (cy17:0, 18:0, 18:1ω9), and decreases (18:1ω7+10Me18:0) detected in the quantities of specific PLFA on both residue types. In the case of clover, the PLFA-C was derived primarily from residue C (85-100%), whereas in the case of ryegrass, both residue C (57-66%), and soil C contributed substantially to the PLFA-C.     

Available N and P,Microbial Activity,and Biomass in Saline Sandy and Clayey Soils Amended with Residues of Wheat and Alfalfa

Salt accumulation has a negative effect on microorganisms, but plant residues may enhance the microbial activity and biomass. An experiment was conducted over 50 days to evaluate the effect of wheat and alfalfa residues on microbial activity and biomass and nitrogen (N) and phosphorus (P) availability in sandy and clayey soils at different salinity levels. Equivalent amounts of calcium (Ca⁺²) and sodium (Na⁺) salts were added to both soils. Values of electrical conductivity (EC_1:5), denoted S1, S2, and S3 in each soil, were 0.16, 1.10 and 1.98 dS m^?1 in the sand and 0.19, 0.82 and 1.75 dS m^?1 in the clay. Residues of wheat and alfalfa were added at 2% (w/w). Cumulative respiration and microbial biomass decreased with increasing salinity, but with residue addition they increased with a greater rate in amended sandy soil than in clay soil, with a more pronounced effect for alfalfa than for wheat residue. After 10 days, with wheat residue available N values were 113, 86, and 71 μg in the clay and 144, 114, and 94 μg g^?1 soil in the sand in S1, S2, and S3, respectively. Relative to wheat residue, alfalfa residue increased N availability by 9, 13, and 19% and 22, 24, and 24% in the clay and in the sand in S1, S2, and S3, respectively. Compared to the control, in the clay P availability increased by 33, 57, and 100% with wheat residue and by 58, 128, and 175% with alfalfa residue, whereas in the sand it increased by 92, 45, and 40% with wheat residue and by 130, 145, and 280% with alfalfa residue in S1, S2, and S3, respectively. Availability of N and P increased from day 10 to day 50 in both soils, but with different magnitudes. Residue addition can increase microbial activity and nutrient availability in saline soils, particularly in coarser textured soils.     

Influence of straw size on activity and biomass of soil microorganisms during decomposition

This study relates to the pattern of activity and biomass of soil microorganisms due to varying residue particle sizes during incubation. Wheat straw (8 t ha^–1) of different sizes (powdered, 0.9 cm, 1.8 cm, 2.9 cm and 4.4 cm) was incubated for 90 days at 50% water holding capacity in a loamy sand soil of Typic Camborthid. Dehydrogenase activity, an indicator of the total microbial activity, and microbial biomass were influenced by straw sizes during incubation. The peak dehydrogenase activity was recorded 21 days after incorporation of residue and it was highest in the powdered straw and decreased with increase in the straw length. The maximum biomass C build up was observed between 15 (< 1 cm) and 45 (> 1 cm) days after incorporation. The C:N ratio in the soil after 90 days of residue incorporation varied, with increase in straw size, between 12.1:1 and 20.8:1. The results reveal that for faster decomposition the length of the wheat straw should not exceed 1 cm.     

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.     

Decomposition and distribution of residual activity of some 13C-microbial polysaccharides and cells,glucose, cellulose and wheat straw in soil

Studies were made to determine the rate of decomposition of some ¹⁴C-labeled microbial polysaccharides, microbial cells, glucose, cellulose and wheat straw in soil, the distribution of the residual ¹⁴C in various humic fractions and the influence of the microbial products on the decomposition of plant residues in soil. During 16 weeks from 32 to 86 per cent of the C of added bacterial polysaccharides had evolved as ¹⁴CO₂. Chromobacterium violaceum polysaccharide was most resistant and Leuconostoc dextranicus polysaccharide least resistant. In general the polysaccharides, microbial cells, and glucose exerted little effect on the decomposition of the plant products. Upon incubation the ¹⁴C-activity was quickly distributed in the humic. fulvic and extracted soil fractions. The pattern of distribution depended upon the amendment and the degree of decomposition. The distribution was most uniform in the highly decomposed amendments. After 16 weeks the bulk of the residual activity from Azotobacter indicus polysaccharide remained in the NaOH extracted soil. From C. violaceum polysaccharide both the extracted soil and the humic acid fraction contained high activity. About 50–80 per cent of the residual activity from the ¹⁴C-glucose, cellulose and wheat straw amended soils could be removed by hydrolysis with 6 n HCl. The greater part of this activity in the humic acid fraction was associated with the amino acids and that from the fulvic acids and residual soils after NaOH extraction with the carbohydrates. About 8 16 per cent of the activity of the humic acid fraction was present in substances (probably aromatic) extracted by ether after reductive or oxidative degradation.     

Chemical composition controls residue decomposition in soils differing in initial pH

Laboratory incubation studies were conducted to determine the dynamics of low-molecular-weight aliphatic carboxylic acids and their anions (LACAs) and respiration in three soils incorporated with five types of plant residues differing in chemical composition. Concentrations of total and individual LACAs in soils decreased sharply with initial 3 days after addition of plant residues, and varied with types of plant residue and soil. Irrespective of soil types, the amount of total extractable LACAs was highest in the soils treated with chickpea, followed by lucerne and high-N wheat straw, and lowest with low-N wheat straw. Lancelin soil [initial pH(CaCl₂) 5.06] contained higher concentration of extractable LACAs compared with Bodallin (pH 4.54) and Wodjil soil (pH 3.87). Soil respiration rate was rapidly increased by the addition of plant residues, and reached the peak by Day 3. Respiration rate was the greatest in Lancelin soil and the smallest in Wodjil soil. The amounts of CO₂ respired over 7 days were equivalent to 6-25% of the added C for Wodjil soil, 6-39% for Bodallin soil and 6-47% for Lancelin soil, depending on type of plant residue. Those amounts over 105 days increased further. Irrespective of plant residue and soil type, the cumulative respiration correlated positively with concentrations of N, excess cations and soluble C in plant residues and initial soil pH, and negatively with C:N of plant residues. The results suggest that chemical composition of plant residues plays an important role at the initial stage of residue decomposition.     

Differences in CO2 and N2O emission rates following crop residue incorporation with or without field burning: A case study of adzuki bean residue and wheat straw

In the context of sustainable soil-quality management and mitigating global warming, the impacts of incorporating raw or field-burned adzuki bean (Vigna angularis (Willd.) Ohwi & Ohashi) and wheat (Triticum aestivum L.) straw residues on carbon dioxide (CO₂) and nitrous oxide (N₂O) emission rates from soil were assessed in an Andosol field in northern Japan. Losses of carbon (C) and nitrogen (N) in residue biomass during field burning were much greater from adzuki bean residue (98.6% of C and 98.1% of N) than from wheat straw (85.3% and 75.3%, respectively). Although we noted considerable inputs of carbon (499 ± 119 kg C ha^–1) and nitrogen (5.97 ± 0.76 kg N ha^–1) from burned wheat straw into the soil, neither CO₂ nor N₂O emission rates from soil (over 210 d) increased significantly after the incorporation of field-burned wheat straw. Thus, the field-burned wheat straw contained organic carbon fractions that were more resistant to decomposition in soil in comparison with the unburned wheat straw. Our results and previously reported rates of CO₂, methane (CH₄) and N₂O emission during wheat straw burning showed that CO₂-equivalent greenhouse gas emissions under raw residue incorporation were similar to or slightly higher than those under burned residue incorporation when emission rates were assessed during residue burning and after subsequent soil incorporation.