A microcosm experiment to evaluate the influence of location and quality of plant residues on residue decomposition and genetic structure of soil microbial communities

The effects of location (soil surface vs. incorporated in soil) and nature of plant residues on degradation processes and indigenous microbial communities were studied by means of soil microcosms incubation in which the different soil zones influenced by decomposition i.e. residues, soil adjacent to residues (detritusphere) and distant soil unaffected by decomposition (bulk soil) were considered. Plant material decomposition, organic carbon assimilation by the soil microbial biomass and soil inorganic N dynamics were studied with ¹³C labelled wheat straw and young rye. The genetic structure of the community in each soil zone were compared between residue locations and type by applying B- and F-ARISA (for bacterial- and fungal-automated ribosomal intergenic spacer analysis) directly to DNA extracts from these different zones at 50% decomposition of each residue. Both location and biochemical quality affected residue decomposition in soil: 21% of incorporated ¹³C wheat straw and 23% left at the soil surface remained undecomposed at the end of incubation, the corresponding values for ¹³C rye being 1% and 8%. Residue decomposition induced a gradient of microbial activity with more labelled C incorporated into the microbial biomass of the detritusphere. The sphere of influence of the decomposing residues on the dynamics of soluble organic C and inorganic N in the different soil zones showed particular patterns which were influenced by both residue location and quality. Residue degradation stimulated particular genetic structure of microbial community with a gradient from residue to bulk soil, and more pronounced spatial heterogeneity for fungal than for bacterial communities. The initial residue quality strongly affected the resulting spatial heterogeneity of bacteria, with a significance between-zone discrimination for rye but weak discrimination between the detritusphere and bulk soil, for wheat straw. Comparison of the different detrituspheres and residue zones (corresponding to different residue type and location), indicated that the genetic structure of the bacterial and fungal communities were specific to a residue type for detritusphere and to its location for residue, leading to conclude that the detritusphere and residue corresponded to distinct trophic and functional niches for microorganisms.     

Degradability of black carbon and its impact on trace gas fluxes and carbon turnover in paddy soils

Rice paddy soils are characterized by anoxic conditions, anaerobic carbon turnover, and significant emissions of the greenhouse gas methane. A main source for soil organic matter in paddy fields is the rice crop residue that is returned to fields if not burned. We investigated as an alternative treatment the amendment of rice paddies with rice residues that have been charred to black carbon. This treatment might avoid various negative side effects of traditional rice residue treatments. Although charred biomass is seen as almost recalcitrant, its impact on trace gas (CO₂, CH₄) production and emissions in paddy fields has not been studied. We quantified the degradation of black carbon produced from rice husks in four wetland soils in laboratory incubations. In two of the studied soils the addition of carbonised rice husks resulted in a transient increase in carbon mineralisation rates in comparison to control soils without organic matter addition. After almost three years, between 4.4% and 8.5% of the black carbon added was mineralised to CO₂ under aerobic and anaerobic conditions, respectively. The addition of untreated rice husks resulted in a strong increase in carbon mineralisation rates and in the same time period 77%-100% of the added rice husks were mineralised aerobically and 31%-54% anaerobically. The ¹³C-signatures of respired CO₂ gave a direct indication of black carbon mineralisation to CO_2. In field trials we quantified the impact of rice husk black carbon or untreated rice husks on soil respiration and methane emissions. The application of black carbon had no significant effect on soil respiration but significantly enhanced methane emissions in the first rice crop season. The additional methane released accounted for only 0.14% of black carbon added. If the same amount of organic carbon was added as untreated rice husks, 34% of the applied carbon was released as CO₂ and methane in the first season. Furthermore, the addition of fresh harvest residues to paddy fields resulted in a disproportionally high increase in methane emissions. Estimating the carbon budget of the different rice crop residue treatments indicated that charring of rice residues and adding the obtained black carbon to paddy fields instead of incorporating untreated harvest residues may reduce field methane emissions by as much as 80%. Hence, the production of black carbon from rice harvest residues could be a powerful strategy for mitigating greenhouse gas emissions from rice fields.     

Simulation of net N immobilisation and mineralisation in substrate-amended soils by the NCSOIL computer model

 The capability of the NCSOIL computer model to simulate the effects of residue fractions on mineralisation-immobilisation turnover was evaluated. Heterogeneous organic substrates were represented in the model by three Van Soest pools, decomposing at different rates. Dried and ground wheat straw, sunflower stalks, wheat stubble and sheep manure (5.22 g kg^–1 soil) were respectively added to a Chromic Calcixerert and aerobically incubated for 224 days at 22±2  °C and 75% field capacity. The CO₂ evolution rates peaked shortly after the C amendments were added, with the highest rate in the sunflower- stalk-amended soils. The addition of organic substrates induced rapid N immobilisation. Net mineralisation was detected earliest in the sunflower-stalk treatment (day 14), while soils with the other amendments showed no net N mineralisation until day 52. The NCSOIL model was calibrated for this soil with CO₂ and inorganic N data from the control soil, yielding a χ² value of 0.011. The overestimation by the model of the C mineralisation data in the case of C-amended soils clearly showed that the concept of three Van Soest pools, decomposing independently at a specific rate constant, is not valid. A retardation factor, that was related to the lignin content of the decomposing material, was introduced into the model. After its introduction the model satisfactorily simulated the C mineralisation rates. However, for all plant residues, N mineralisation was underestimated towards the end of the incubation period. In the case of the soil amended with sheep manure, there was a large discrepancy between simulated and experimental N mineralisation-immobilisation kinetics, suggesting a different allocation of N in animal manure to N-containing fractions compared to that of plant residues. The results indicated that a N fractionation procedure for organic residues should be tested and incorporated into the model. Received: 9 January 1998     

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.     

Interactions in microbial use of soluble plant components in soil

Glucose, glutamic acid, alanine, valine, and albumin were used as model substances to represent readily available soluble plant components. Their mineralisation in soil was monitored by making hourly measurements of total CO₂ evolution and periodic measurements of ¹⁴CO₂ during a 5-day incubation period. When glucose and the amino acids were combined in dual substrates the two components were mineralised simultaneously. In each combination the component with the shortest lag phase or highest specific growth rate () was mineralised preferentially. Glucose accelerated the mineralisation of the amino acids that had longer lag phases than glucose (i.e., alamine and valine). During the first 8 h, glucose mineralisation was reduced in the dual substrates compared with its mineralisation as a single substrate. The smaller the difference between the lag phase of glucose and the amino acid, the greater was the reduction in the glucose mineralisation rate. During the exponential phase, the effects on glucose mineralisation were dependent on the nature and concentration of the amino acid. The interactions observed between glucose and amino acid decomposition indicated that these substances were mineralised, at least partly, by the same microorganisms. In contrast, glucose and the soluble protein albumin were decomposed independently of each other.     

Biochemical characteristics of solid fractions from animal slurry separation and their effects on C and N mineralisation in soil

Solid fractions from separated animal slurry can be used as organic fertilisers on agricultural land. Solid fractions contain variable amounts of inorganic and organic N, so it is important to synchronise their application in the field with crop demand to ensure N availability in the growing season. This study quantified C and N mineralisation for a wide range of solid fractions from slurry separation applied to soil and examined potential correlations between chemical and biochemical characteristics of solid fractions and their C and N turnover. The solid fractions were mixed with soil and incubated at 14°C for 120 days, during which CO₂ evolution and inorganic N content of the solid fractions were determined. A two-parameter exponential function fitted to the individual solid fraction C mineralisation patterns explained 98% of the data variation, while a three-parameter Monod-type equation fitted to the net N mineralisation patterns explained 89% of the variation. Between 5% and 45% of initially added C was mineralised within the incubation period, with the largest proportion tending to be mineralised from simple mechanically separated solid fractions (MEC). Nitrogen was initially immobilised by the majority of solid fractions. Solid fractions from decanter centrifuged, anaerobically digested slurry (DEC) and chemically pre-treated and separated slurry (KEM) began to re-mineralise N after 20?C40 days, whereas N was continuously immobilised from MEC solid fractions. The carbon mineralisation rate constant was correlated with the C content in the neutral detergent soluble (NDS), hemicellulose and cellulose fractions and the N content in NDS. Net nitrogen mineralisation was correlated with the C/Norg ratio of solid fractions, the N content of NDS and the C content of hemicellulose and cellulose.     

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.     

Turnover of grain legume N rhizodeposits and effect of rhizodeposition on the turnover of crop residues

The turnover of N derived from rhizodeposition of faba bean (Vicia faba L.), pea (Pisum sativum L.) and white lupin (Lupinus albus L.) and the effects of the rhizodeposition on the subsequent C and N turnover of its crop residues were investigated in an incubation experiment (168 days, 15 °C). A sandy loam soil for the experiment was either stored at 6 °C or planted with the respective grain legume in pots. Legumes were in situ ¹⁵N stem labelled during growth and visible roots were removed at maturity. The remaining plant-derived N in soil was defined as N rhizodeposition. In the experiment the turnover of C and N was compared in soils with and without previous growth of three legumes and with and without incorporation of crop residues. After 168 days, 21% (lupin), 26% (faba bean) and 27% (pea) of rhizodeposition N was mineralised in the treatments without crop residues. A smaller amount of 15–17% was present as microbial biomass and between 30 and 55% of mineralised rhizodeposition N was present as microbial residue pool, which consists of microbial exoenzymes, mucous substances and dead microbial biomass. The effect of rhizodeposition on the C and N turnover of crop residues was inconsistent. Rhizodeposition increased the crop residue C mineralisation only in the lupin treatment; a similar pattern was found for microbial C, whereas the microbial N was increased by rhizodeposition in all treatments. The recovery of residual ¹⁵N in the microbial and mineral N pool was similar between the treatments containing only labelled crop residues and labelled crop residues + labelled rhizodeposits. This indicates a similar decomposability of both rhizodeposition N and crop residue N and may be attributable to an immobilisation of both N sources (rhizodeposits and crop residues) as microbial residues and a subsequent remineralisation mainly from this pool.Abbreviations C or N_dec C or N decomposed from residues - C or N_mic microbial C or N - C or N_micres microbial residue C or N - C or N_min mineralised C or N - C or N_input added C or N as crop residues and/or rhizodeposits - dfr derived from residues - dfR derived from rhizodeposition - Ndfr N derived from residues - NdfR N derived from rhizodeposition - N_loss losses of N derived from residues - SOM soil organic matter - WHC water holding capacity     

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.     

Dynamics of litter carbon turnover and microbial abundance in a rye detritusphere

Factors determining C turnover and microbial succession at the small scale are crucial for understanding C cycling in soils. We performed a microcosm experiment to study how soil moisture affects temporal patterns of C turnover in the detritusphere. Four treatments were applied to small soil cores with two different water contents (matric potential of ?0.0063 and ?0.0316 MPa) and with or without addition of ¹³C labelled rye residues (δ¹³C=299‰), which were placed on top. Microcosms were sampled after 3, 7, 14, 28, 56 and 84 days and soil cores were separated into layers with increasing distance to the litter. Gradients in soil organic carbon, dissolved organic carbon, extracellular enzyme activity and microbial biomass were detected over a distance of 3 mm from the litter layer. At the end of the incubation, 35.6% of litter C remained on the surface of soils at ?0.0063 MPa, whereas 41.7% remained on soils at ?0.0316 MPa. Most of the lost litter C was mineralised to CO₂, with 47.9% and 43.4% at ?0.0063 and ?0.0316 MPa, respectively. In both treatments about 6% were detected as newly formed soil organic carbon. During the initial phase of litter decomposition, bacteria dominated the mineralisation of easily available litter substrates. After 14 days fungi depolymerised more complex litter compounds, thereby producing new soluble substrates, which diffused into the soil. This pattern of differential substrate usage was paralleled by a lag phase of 3 days and a subsequent increase in enzyme activities. Increased soil water content accelerated the transport of soluble substrates, which influenced the temporal patterns of microbial growth and activity. Our results underline the importance of considering the interaction of soil microorganisms and physical processes at the small scale for the understanding of C cycling in soils.     

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.     

Increased microbial activity under elevated [CO2] does not enhance residue decomposition in a semi-arid cropping system in Australia

The association between the responses of microbial activity and residue decomposition to elevated atmospheric [CO₂] under field conditions in Australian cropping systems is unknown. We measured soil CO₂ emission and decomposition of wheat and field pea residues in a wheat cropping system in the field using the Australian Grains Free-Air CO₂ Enrichment (AGFACE) facility in Horsham, Victoria. Elevated [CO₂] (550 μmol mol⁻¹) increased soil CO₂ emission by 41%, but did not affect the percentage of the original mass or C remaining for either type of residue throughout the experimental period. Our findings suggest that the rates of residue decomposition and residue C mineralization in this semi-arid wheat cropping system were not affected by elevated [CO₂] despite higher microbial activity. This has major implication for the C sequestration potential of semi-arid cropping systems under future CO₂ climates.     

Decomposition of N-labelled maize leaves in soil affected by endogeic geophagous Aporrectodea caliginosa

A microcosm experiment was carried out for 56 days at 12 °C to evaluate the feeding effects of the endogeic geophagous earthworm species Aporrectodea caliginosa on the microbial use of ¹⁵N-labelled maize leaves (Zea mays) added as 5 mm particles equivalent to 1 mg C and 57 μg N g⁻¹ soil. The dry weight of A. caliginosa biomass decreased in the no-maize treatment by 10% during the incubation and increased in the maize leaf treatments by 18%. Roughly 5% and 10% of the added maize leaf-C and leaf-N, respectively, were incorporated into the biomass of A. caliginosa. About 29% and 33% of the added maize leaf-C were mineralised to CO₂ in the no-earthworm and earthworm treatments, respectively. The presence of A. caliginosa significantly increased soil-derived CO₂ production by 90 μg g⁻¹ soil in the no-maize and maize leaf treatments, but increased the maize-derived CO₂ production only by 40 μg g⁻¹ soil. About 10.5% of maize leaf-C and leaf-N was incorporated into the soil microbial biomass in the absence of earthworms, but only 6% of the maize leaf-C and 3% of the maize leaf-N in the presence of earthworms. A. caliginosa preferentially fed on N rich, maize leaf-colonizing microorganisms to meet its N demand. This led to a significantly increased C/N ratio of the unconsumed microbial biomass in soil. The ergosterol-to-microbial biomass C ratio was not significantly decreased by the presence of earthworms. A. caliginosa did not directly contribute to comminution of plant residues, as indicated by the absence of any effects on the contents of the different particulate organic matter fractions, but mainly to grazing of residue-colonizing microorganisms, increasing their turnover considerably.     

Enhancing Soil Quality through Residue Management in a Rice-Wheat System in Fukuoka,Japan

A long-term experiment (LTE) on a rice-wheat system was initiated in 1963 at the Kyushu National Agricultural Experiment Station, in Fukuoka, Japan, to determine the effects of continuous application of rye grass/wheat straw, rice straw and rice straw compost, alone or in combination with inorganic N on crop yields. Increase in rice yields and enhancement of total soil C and N contents with the application of organic residues in this LTE have been reported earlier. However, evaluation of the changes in the soil microbiological properties and the decomposable C fraction of soil organic matter that is needed for soil quality assessment is still lacking. Soil samples were collected after rice harvest in 2003 from the organic residue treatments and unfertilized control, air-dried and incubated for 1 month under aerobic [50% water-filled pore space (WFPS)] and flooded conditions prior to the analysis of the amount of microbial biomass C (MBC), soil respiration and the amount of potential mineralizable N (PMN). The contents of total C (TC), total N (TN), organic C (OC), hot water-extractable C (HWEC) and permanganate-oxidizable C (POC) were determined from air-dried soils. Organic residue incorporation brought about significant increases in the contents of TC, TN, OC, POC, HWEC and PMN. The largest accumulation of total C (23%) and N (72%) in the soil was from rice straw compost, compared with that from rice straw (C, 7% and N, 33%) and rye grass/wheat straw (C, 9% and N, 29%). Incorporation of rice straw compost also increased the amount of MBC under both aerobic and flooded conditions and basal soil respiration under aerobic conditions only. An efficient utilization of C by microorganisms was indicated by a significantly lower metabolic quotient (qCO₂) in the composted and uncomposted rice straw treatments compared with the control in the “-” N treatment under aerobic conditions. Similarly, the flush of CO₂ after rewetting of dry soil per unit of HWEC was lower in the organic matter treatments, indicating a more efficient C utilization and lower C losses per unit of available C. The content of HWEC was significantly correlated with the basal soil respiration (at 50% WFPS), the amounts of MBC, PMN and with the increase in the content of soil organic C in the residuetreated soils. In the treatments without inorganic N fertilizer, grain yield was significantly correlated with the amounts of total organic C, HWEC, MBC (at 50% WFPS), basal soil respiration (at 50% WFPS) and the amount of PMN.     

Carbon decomposition kinetics and nitrogen mineralization from corn,soybean, and wheat residues

Abstract

An incubation study was conducted for 30 days in Taloka (fine, mixed, thermic mollic Albaqualf) and Leadvale (fine, silty, siliceous, thermic typic Fragiudult) silt loam soils to evaluate carbon (C) and nitrogen (N) mineralization from soybean [Glycine max (L.) Merr.], corn (Zea mays L.), and wheat (Triticum aestivum L.) residues. Corn and soybean residues were collected at the tasseling and late vegetative stages, respectively. Wheat straw was collected after harvest. Carbon dioxide (CO₂) evolution and inorganic N accumulation were measured. Carbon mineralization was described by a sequential decomposition model with a rapid and slow phase, each described by first‐order kinetics. Rapid and slow fraction rate constants and percent rapid were determined. Decomposition ranged from 39% for wheat to 67% for soybean. Carbon dioxide evolution peaked on the third day, and 30 to 50% of residue C was decomposed during the first six days of incubation. Decomposition and N mineralization were higher in the Taloka compared to the Leadvale soil, and generally followed the sequence soybean > corn > wheat residues as did percent rapid fraction, and rapid and slow fraction rate constants. Rapid fraction rate constants ranged from 0.039±0.005 to 0.115±0.005 per day. Slow fraction rate constants ranged from 0.013±0.002 to 0.030±0.002 per day. Percent rapid fraction ranged from 13±2% to 38±2%. The half‐lives of the slow fraction ranged from 23.4±3.5 to 51.8±3.5 days. Nitrogen mineralization, as estimated by ammonium (NH₄) and nitrate (NO₃) formation occurred only with the soybean residue, whereas the corn and wheat residues were characterized by N immobilization throughout the study.     

Release of carbon and nitrogen from fodder radish (Raphanus sativus) shoots and roots incubated in soils with different management history

Plant roots are generally considered to decompose slower than shoots and contribute more to accumulation of soil organic matter, and management history is expected to shape the structure and function of decomposer communities in soil. Here we study the effect of chemical characteristics of shoots and roots from fodder radish (Raphanus sativus oleiformis L.), a widely used cover crop, on the release of their C and N after addition to soil. Shoots and roots were incubated for 180?d at 20°C using four soils with different management histories (organic versus mineral fertiliser, with and without use of cover crops), and the release of CO₂ and extractable mineral N was determined. More shoot C than root C was mineralised during the first 10?d of incubation. After 180?d, 58% of the C input was mineralised with no difference between shoots and roots. At the end of incubation, shoots had released more N (42% of shoot N) than roots (28% of root N). Moreover, management history did not affect net mineralisation of added plant C. Residues incubated in soil with a management history involving cover crops showed an enhanced net N mineralisation. Therefore, long-term decomposition of C added in radish shoots and roots is unaffected by differences in chemical characteristics or soil management history. However, the net mineralisation of N in shoots is faster than for N in roots, and net N mineralisation of added materials is higher in soil with than without a history of cover crops.

Abbreviations: CC: cover crop; IF: inorganic fertilizer; M: manure     

Carbon dioxide evolution from wheat and lentil residues as affected by grinding,added nitrogen,and the absence of soil

Summary A study was conducted to determine the effects of grinding, added N, and the absence of soil on C mineralization from agricultural plant residues with a high C:N ratio. The evolution of CO₂ from ground and unground wheat straw, lentil straw, and lentil green manure, with C:N ratios of 80, 36, and 9, respectively, was determined over a period of 98 days. Treatments with added N were included with the wheat and lentil straw. Although the CO₂ evolution was initially much faster from the lentil green manure than from the lentil or wheat straw, by 98 days similar amounts of CO₂ had evolved from all residues incubated in soil with no added N. Incubation of plant residues in the absence of soil had little effect on CO₂ evolution from the lentil green manure or lentil straw but strongly reduced CO₂ evolution from the wheat straw. Grinding did not affect CO₂ evolution from the lentil green manure but increased CO₂ evolution from the lentil straw with no added N and from the wheat straw. The addition of N increased the rate of CO₂ evolution from ground wheat straw between days 4 and 14 but not from unground wheat straw, and only slightly increased the rate of CO₂ evolution from lentil straw during the initial decomposition. Over 98 days, the added N reduced the amounts of CO₂ evolved from both lentil and wheat straw, due to reduced rates of CO₂ evolution after ca. 17 days. The lack of an N response during the early stages of decomposition may be attributed to the low C:N ratio of the soluble straw component and to microbial adaptations to an N deficiency, while the inhibitory effect of N on CO₂ evolution during the later stages of decomposition may be attributed to effects of high mineral N concentrations on lignocellulolytic microorganisms and enzymes.     

The effects of biocidal treatments on metabolism in soil—VI. Fumigation with carbon disulphide

Used in high concentration as a soil fumigant, CS₂ was broadly similar to CHCl₃ in its effects on metabolism in soil; the amount of N mineralised in 10 days increased roughly 10-fold. the O₂ consumption almost tripled and the evolution of CO₂ more than doubled. However, the effects of CS₂ were consistently slightly less than those of CHCl₃.Used at low concentration (10 μg.g^?1 soil) on a soil rich in organic matter (2.93% organic C), CS₂ stopped nitrification completely, almost without other effect on soil respiration and mineralisation of N. In contrast, when used on a poorer soil (1.07% organic C) even 10 μgCS₂.g_?1 soil was sufficient to cause a detectable increase in both respiration and mineralisation of N, in addition to stopping nitrification.     

Turnover of organic matter in a Miscanthus field: effect of time in Miscanthus cultivation and inorganic nitrogen supply

To accurately predict the potential environmental benefits of energy crops, the sequestration of carbon in soil needs to be quantified. The aim of this study was to investigate the mineralisation rate of the perennial C₄ grass Miscanthus giganteus and Miscanthus-derived soil organic matter under contrasting nitrogen supply. Soils were collected from sites where Miscanthus had been grown for 11 and 18 years, respectively, and where a C₃-grass (Lolium spp.) had been grown for 7 years. The soils were incubated for 4 months at two levels of soil inorganic nitrogen with or without dead root material of Miscanthus.Addition of root material (residues) increased carbon mineralisation of indigenous organic matter when no nitrogen was added. Added inorganic nitrogen decreased carbon mineralisation in all soils. Nitrogen addition did not affect carbon mineralisation of the residues. Using the ¹³C fraction to calculate the proportion of respiratory CO₂ derived from Miscanthus showed that nitrogen addition decreased carbon mineralisation in soils, but it did not affect carbon mineralisation of the residues. Nitrogen mineralisation was highest in the C₃ grass soil without added residues. Nitrification decreased pH, especially in the treatments where nitrogen was added. The Miscanthus-derived organic matter is at least as stable as C₃ grassland-derived organic matter. Furthermore, the turnover time of the organic matter increases with time under Miscanthus cultivation.The CENTURY soil organic matter sub-model was used to simulate the organic matter decomposition in the experiment. Carbon mineralisation was accurately simulated but there were unexplained discrepancies in the simulation of the δ¹³C in the respiration from the treatment with residues. The δ¹³C in respiration did not decrease with time as predicted, indicating that lignin accumulation did not influence the measurements.     

Decomposition of carbon-14-labelled wheat straw in repeatedly fumigated and non-fumigated soils with different levels of heavy metal contamination

We analysed the decomposition of ¹⁴C-labelled straw at five different levels of heavy metal contamination (100-20,000 µg total Zn g^-1 soil) in non-fumigated and repeatedly fumigated soils. The soils were not spiked with Zn, but were taken from sites containing different heavy metal concentrations. Zn was only used as a reference and the effects observed are most likely due to this metal. Microbial biomass decreased with increasing heavy metal content of soils, paralleled generally by the decreasing amount of wheat straw ¹⁴C incorporated into the microbial biomass. In addition, the newly synthesised microbial biomass declined more rapidly as the incubation proceeded. In the repeatedly fumigated soils, microbial biomass ¹⁴C corresponded to roughly 50% of the maximum ¹⁴C incorporation of the non-fumigated soil. The relative decline during incubation was similar to that of the non-fumigated soil at the respective contamination level. These results reveal clearly that heavy metal effects on straw decomposition do not depend on the ratio of substrate C to microbial biomass C. In contrast to microbial biomass C, the mineralisation of the wheat straw was not seriously affected by heavy metal contamination. The same was true for all of the repeatedly fumigated treatments, where a much smaller microbial biomass mineralised nearly the same amount of straw as in the non-fumigated soils. However, repeated fumigation caused a strong reduction in the decomposition of soil organic matter. The ratio of CO₂-¹⁴C to microbial biomass ¹⁴C after 60 days was linearly related to the Zn concentration in both non-fumigated and repeatedly fumigated samples, clearly indicating that an additional energy cost is required by soil microorganisms with increasing heavy metal concentrations.