The contribution of biogas residues to soil organic matter formation and CO2 emissions in an arable soil

The biogas production process generates as side-products biogas residues containing microbial biomass which could contribute to soil organic matter formation or induce CO₂ emissions when applied to arable soil as fertilizer. Using an isotope labelling approach, we labelled the microbial biomass in biogas residues, mainly G⁺ bacteria and methanogenic archaea via KH¹³CO₃, and traced the fate of microbial biomass carbon in soil with an incubation experiment lasting 378 days. Within the first seven days, 40% of the carbon was rapidly mineralized and after that point mineralization continued, reaching 65% by the end of the experiment. Carbon mineralization data with 93% recovery could be fitted to a two-pool degradation model which estimated proportions and degradation rate constants of readily and slowly degrading pools. About 49% of the carbon was in the slowly degrading pool with a half-life of 1.9 years, suggesting mid-term contribution to living and non-living soil organic matter formation. Biogas residues caused a priming effect at the beginning, thus their intensive application should be avoided.     

The influence of mineral and organic fertilisers on the growth of the endogeic earthworm Octolasion tyrtaeum (Savigny)

Endogeic earthworms play an important role in mobilisation and stabilisation of carbon and nitrogen in forest and arable soils. Soil organic matter is the major food resource for endogeic earthworms, but little is known about the size and origin of the organic matter pool on which the earthworms actually live. We measured changes in body mass of juvenile endogeic earthworms, Octolasion tyrtaeum (Savigny), in soils with different C and N contents resulting from different fertiliser treatments. The soil was taken from a long-term experiment (Statischer Düngungsversuch, Bad Lauchstädt, Germany). The treatments included (1) non-fertilised soil, (2) NPK fertilised soil, (3) farmyard manure fertilised soil and (4) NPK + farmyard manure fertilised soil. The soil was incubated in microcosms with and without one juvenile O. tyrtaeum for 80 days.Earthworm biomass decreased in non-fertilised soil by 48.6%, in NPK soil by 9.4%, but increased in farmyard manure soil by 19.7% and 42.8% (soil with additional NPK application). In farmyard manure treatments the biomass of bigger individuals decreased, but in smaller individuals it increased. In NPK fertilised soil without farmyard manure only small O. tyrtaeum increased in body mass, whereas in the non-fertilised soil all individuals decreased in body mass. Generally, soil respiration correlated positively with soil carbon content. Earthworms significantly increased soil respiration and nitrogen leaching and this was most pronounced in farmyard manure treatments. Microbial activity was generally higher in farmyard manure soil indicating that farmyard manure increases labile organic matter pools in soil. Also, biomass of earthworms and microorganisms was increased in farmyard manure soil. The presence of earthworms reduced microbial biomass, suggesting that earthworms feed on microorganisms or/and that earthworms and soil microorganisms competed for similar organic matter pools in soil. The results demonstrate that NPK fertilisation only is insufficient to sustain O. tyrtaeum, whereas long-term fertilisation with farmyard manure enables survival of endogeic species due to an increased pool of utilisable soil organic matter in arable soil.     

Effects of pasture improvement and intensive cultivation on microbial biomass,enzyme activities,and composition and size of earthworm populations

We investigated the quantity and distribution of organic C, microbial biomass C, protease, arylsulphatase and arylphosphatase activity, and earthworm numbers and biomass in the soil from a 37-year-old grazed pasture supplied with superphosphate at rates of 0, 188, and 376 kg ha^-1 annually. The results were compared with a non-irrigated wilderness site which had not been used for agriculture and an arable site that had been intensively cultivated for 11 consecutive years. In the 0- to 5-cm layer, organic C followed the trend arableAporrectodea caliginosa (77–89% of total numbers) although Lumbricus rubellus made an increasing contribution to the population with increasing superphosphate rates. In the unirrigated wilderness site the population consisted of 56% A. caliginosa and 44% L. rubellus. While Octolasion cyaneum and A. rosea made up a small proportion of the population in the improved pasture sites, they were not present in the wilderness or arable sites. A. caliginosa was the only species present in the arable site. The mean fresh weight of individuals followed the order arable相似文献   

Nitrogen mineralization and reorganization in casts of the geophagous tropical earthworm Pontoscolex corethrurus (Glossoscolecidae)

Summary Mineral N concentrations ranged from 133.1 to 167.8 g g^-1 dry soil in fresh casts of the endogeic earthworm Pontoscolex corethrurus fed on an Amazonian Ultisol; this was approximately five times the concentration in non-ingested soil. Most of this N was in the form of NH _inf4 ^sup+ . N also accumulated in microbial biomass, which increased from a control value of 10.5–11.3 to 67.5–74.1 g g^-1 in fresh casts. During a 16-day incubation, part of the NH _inf4 ^sup+ -N was nitrified and/or transferred to the microbial biomass. Total labile N (i.e., mineral+biomas N) decreased sharply at first (ca. 50% in the first 12 h), and then more slowly. The exact fate of this N (microbial metabolites, denitrification, or volatilization) is not known. After 16 days, the overall N content of the casts was still 28% higher than that of the control soil. Incubation of the soil before ingestion by the earthworms significantly increased the production of NH _inf4 ^sup+ in casts. We calculate that in a humid tropical pasture, 50–100 kg mineral N may be produced annually in earthworm casts. Part of this N may be conserved in the compact structure of the cast where the cast is not in close contact with plant roots.     

Changes in soil fungal:bacterial biomass ratios following reductions in the intensity of management of an upland grassland

In this study we examined the effect on soil fungal:bacterial biomass ratios of withholding fertiliser, lime, and sheep-grazing from reseeded upland grassland. The cessation of fertiliser applications on limed and grazed grassland resulted in a reduction in soil pH from 5.4 to 5.1. The cessation of fertiliser applications and liming on grazed grassland resulted in a fall in pH from 5.4 to 4.7, whereas withholding fertiliser and lime and the removal of grazing resulted in a further reduction to pH 4.5. Substrate-induced respiration was reduced in the unfertilised grazed (21%; P<0.01) and unfertilised ungrazed (36%; P<0.001) treatments. Bacterial substrate-induced respiration and bacterial fatty acids were unaffected by the treatments. The relative abundance of the fungal fatty acid 18:26 increased by 39 and 72% (P<0.05) in the limed grazed and unfertilised grazed treatments, respectively. Fungal substrate-induced respiration increased in the limed grazed (18%) and unfertilised grazed (65%; P<0.05) treatments. The ratio of 18:26: bacterial fatty acids was correlated with the ratio of fungal:bacterial substrate-induced respiration (r=0.69; P<0.001).     

Temperature and substrate controls on microbial phospholipid fatty acid composition during incubation of grassland soils contrasting in organic matter quality

Soil incubations are often used to investigate soil organic matter (SOM) decomposition and its response to increased temperature, but changes in the activity and community composition of the decomposers have rarely been included. As part of an integrated investigation into the responses of SOM components in laboratory incubations at elevated temperatures, fungal and bacterial phospholipid fatty acids (PLFAs) were measured in two grassland soils contrasting in SOM quality (i.e. SOM composition), and changes in the microbial biomass and community composition were monitored. Whilst easily-degradable SOM and necromass released from soil preparation may have fuelled microbial activity at the start of the incubation, the overall activity and biomass of soil microorganisms were relatively constant during the subsequent one-year soil incubation, as indicated by the abundance of soil PLFAs, microbial respiration rate (r), and metabolic quotient (qCO₂). PLFAs relating to fungi and Gram-negative bacteria declined relative to Gram-positive bacteria in soils incubated at higher temperatures, presumably due to their vulnerability to disturbance and substrate constraints induced by faster exhaustion of available nutrient sources at higher temperatures. A linear correlation was found between incubation temperatures and the microbial stress ratios of cyclopropane PLFA-to-monoenoic precursor (cy17:0/16:1ω7c and cy19:0/18:1ω7c) and monoenoic-to-saturated PLFAs (mono/sat), as a combined effect of temperature and temperature-induced substrate constraints. The microbial PLFA decay patterns and ratios suggest that SOM quality intimately controls microbial responses to global warming.     

Specific response of fungal and bacterial residues to one-season tillage and repeated slurry application in a permanent grassland soil

The dynamics of fungal and bacterial residues to a one-season tillage event in combination with manure application in a grassland soil are unknown. The objectives of this study were (1) to assess the effects of one-season tillage event in two field trials on the stocks of microbial biomass, fungal biomass, microbial residues, soil organic C (SOC) and total N in comparison with permanent grassland; (2) to determine the effects of repeated manure application to restore negative tillage effects on soil microbial biomass and residues. One trial was started 2 years before sampling and the other 5 years before sampling. Mouldboard ploughing decreased the stocks of SOC, total N, microbial biomass C, and microbial residues (muramic acid and glucosamine), but increased those of the fungal biomarker ergosterol in both trials. Slurry application increased stocks of SOC and total N only in the short-term, whereas the stocks of microbial biomass C, ergosterol and microbial residues were generally increased in both trials, especially in combination with tillage. The ergosterol to microbial biomass C ratio was increased by tillage, and decreased by slurry application in both trials. The fungal C to bacterial C ratio was generally decreased by these two treatments. The metabolic quotient qCO₂ showed a significant negative linear relationship with the microbial biomass C to SOC ratio and a significant positive relationship with the soil C/N ratio. The ergosterol to microbial biomass C ratio revealed a significant positive linear relationship with the fungal C to bacterial C ratio, but a negative one with the SOC content. Our results suggest that slurry application in grassland soil may promote SOC storage without increasing the role of saprotrophic fungi in soil organic matter dynamics relative to that of bacteria.     

The measurement of soil fungal:bacterial biomass ratios as an indicator of ecosystem self-regulation in temperate meadow grasslands

 There is much interest in the development of agricultural land management strategies aimed at enhancing reliance on ecosystem self-regulation rather than on artificial inputs such as fertilisers and pesticides. This study tested the usefulness of measures of soil microbial biomass and fungal:bacterial biomass ratios as indicators of effective conversion from an intensive grassland system, reliant mainly on fertilisers for crop nutrition, to a low-input system reliant mainly on self-regulation through soil biological pathways of nutrient turnover. Analysis of soils from a wide range of meadow grassland sites in northern England, along a gradient of long-term management intensity, showed that fungal:bacterial biomass ratios (measured by phospholipid fatty acid analysis; PLFA) were consistently and significantly higher in the unfertilised than the fertilised grasslands. There was also some evidence that microbial biomass, measured by chloroform fumigation and total PLFA, was higher in the unfertilised than in the fertilised grasslands. It was also found that levels of inorganic nitrogen (N), in particular nitrate-N, were significantly higher in the fertilised than in the unfertilised grasslands. However, microbial activity, measured as basal respiration, did not differ between the sites. A field manipulation trial was conducted to determine whether the reinstatement of traditional management on an improved mesotrophic grassland, for 6 years, resulted in similar changes in the soil microbial community. It was found that neither the cessation of fertiliser applications nor changes in cutting and grazing management significantly affected soil microbial biomass or the fungal:bacterial biomass ratio. It is suggested that the lack of effects on the soil microbial community may be related to high residual fertility caused by retention of fertiliser N in the soil. On the basis of these results it is recommended that following the reinstatement of low-input management, the measurement of a significant increase in the soil fungal:bacterial biomass ratio, and perhaps total microbial biomass, may be an indicator of successful conversion to a grassland system reliant of self-regulation. Received: 4 May 1998     

Soil organic matter,microbial properties,and aggregate stability under annual and perennial pastures

The use of annually sown pastures to provide winter forage is common in dairy farming in many regions of the world. Loss of organic matter and soil structural stability due to annual tillage under this management may be contributing to soil degradation. The comparative effects of annual ryegrass pastures (conventionally tilled and resown each year), permanent kikuyu pastures and undisturbed native vegetation on soil organic matter content, microbial size and activity, and aggregate stability were investigated on commercial dairy farms in the Tsitsikamma region of the Eastern Cape, South Africa. In comparison with soils under sparse, native grassy vegetation, those under both annual ryegrass and permanent kikuyu pasture had higher soil organic matter content on the very sandy soils of the eastern end of the region. By contrast, in the higher rainfall, western side, where the native vegetation was coastal forest, there was a loss of organic matter under both types of pasture. Nonetheless, soil organic C, K₂SO₄-extractable C, microbial biomass C, basal respiration, arginine ammonification and fluorescein diacetate hydrolysis rates and aggregate stability were less under annual than permanent pastures at all the sites. These results reflect the degrading effect of annual tillage on soil organic matter and the positive effect of grazed permanent pasture on soil microbial activity and aggregation. Soil organic C, microbial biomass C, K₂SO₄-extractable C, basal respiration and aggregate stability were significantly correlated with each other. The metabolic quotient and percentage of organic C present as microbial biomass C were generally poorly correlated with other measured properties but negatively correlated with one another. It was concluded that annual pasture involving conventional tillage results in a substantial loss of soil organic matter, soil microbial activity and soil physical condition under dairy pastures and that a system that avoids tillage needs to be developed.     

Abundance, production and stabilization of microbial biomass under conventional and reduced tillage

Soil tillage practices affect the soil microbial community in various ways, with possible consequences for nitrogen (N) losses, plant growth and soil organic carbon (C) sequestration. As microbes affect soil organic matter (SOM) dynamics largely through their activity, their impact may not be deduced from biomass measurements alone. Moreover, residual microbial tissue is thought to facilitate SOM stabilization, and to provide a long term integrated measure of effects on the microorganisms. In this study, we therefore compared the effect of reduced (RT) and conventional tillage (CT) on the biomass, growth rate and residues of the major microbial decomposer groups fungi and bacteria. Soil samples were collected at two depths (0-5 cm and 5-20 cm) from plots in an Irish winter wheat field that were exposed to either conventional or shallow non-inversion tillage for 7 growing seasons. Total soil fungal and bacterial biomasses were estimated using epifluorescence microscopy. To separate between biomass of saprophytic fungi and arbuscular mycorrhizae, samples were analyzed for ergosterol and phospholipid fatty acid (PLFA) biomarkers. Growth rates of saprophytic fungi were determined by [¹⁴C]acetate-in-ergosterol incorporation, whereas bacterial growth rates were determined by the incorporation of ³H-leucine in bacterial proteins. Finally, soil contents of fungal and bacterial residues were estimated by quantifying microbial derived amino sugars. Reduced tillage increased the total biomass of both bacteria and fungi in the 0-5 cm soil layer to a similar extent. Both ergosterol and PLFA analyses indicated that RT increased biomass of saprophytic fungi in the 0-5 cm soil layer. In contrast, RT increased the biomass of arbuscular mycorrhizae as well as its contribution to the total fungal biomass across the whole plough layer. Growth rates of both saprotrophic fungi and bacteria on the other hand were not affected by soil tillage, possibly indicating a decreased turnover rate of soil microbial biomass under RT. Moreover, RT did not affect the proportion of microbial residues that were derived from fungi. In summary, our results suggest that RT can promote soil C storage without increasing the role of saprophytic fungi in SOM dynamics relative to that of bacteria.     

Earthworm activities and the soil system

Summary Earthworms find in soil the energy, nutrient resources, water and buffered climatic conditions that they need. According to the food resource they exploit and the general environmental conditions, earthworms can be grouped into different functional categories which differ essentially in morphology, size, pigmentation, distribution in the soil profile, ability to dig galleries and produce surface casts, demographic profiles and relationships with the soil microflora. Soil characteristics are both the determinant and the consequence of earthworm activities, since these animals greatly influence the functioning of the soil system. When present, they build and maintain the soil structure and take an active part in energy and nutrient cycling through the selective activation of both mineralization and humification processes. By their physical activities and resultant chemical effects, earthworms promote short and rapid cycles of nutrients and assimilable carbohydrates. Thus earthworms represent a key component in the biological strategies of nutrient cycling in soils and the structure of their communities gives a clear indication of the type of soil system that they inhabit.     

Changes in microbial biomass and microbial activity of pig slurry after the transit through the gut of the earthworm Eudrilus eugeniae (Kinberg, 1867)

Here we studied the effects of gut transit through the earthworm Eudrilus eugeniae, on the physicochemical, biochemical, and microbial characteristics of pig slurry, by analyzing fresh casts. The reduction in the dissolved organic C contents in casts we recorded suggests that during digestion, earthworms assimilated labile organic C preferentially, which is a limiting growth factor for them. Furthermore, both microbial biomass and activity in pig slurry were significantly decreased by earthworm gut transit. It appears that E. eugeniae is able to digest microorganisms, although the addition of glucose to the food increased respiration, indicating that growth of microorganisms in casts could be limited by depletion of labile C. Despite reduced microbial biomass and activity, the metabolic diversity of microbial communities was greater in casts than in original pig slurry. Community level physiological profiles obtained from Biolog Ecoplate data revealed that, after earthworm gut transit, the microbial communities in casts and pig slurry were clearly differentiated by their physiological profiles. The results indicate that first stage in vermicomposting of pig slurry by E. eugeniae, i.e., casting, produced changes that will influence the dynamics of the organic matter degradation by reducing forms of N and C available to microorganisms, hence restricting their growth and multiplication. Nevertheless, the reduced microflora of casts was characterized by an increased catabolic potential that might lead to thorough degradation of pig slurry.     

Organic matter status and the size,activity and metabolic diversity of the soil microflora as indicators of the success of rehabilitation of mined sand dunes

The effectiveness of the rehabilitation of mined sand dunes on the northern coast of KwaZulu–Natal, South Africa, was assessed based on measurements of the total and labile organic matter content and the size, activity and metabolic diversity of the soil microflora. Soil was sampled (0–10 cm) after 0, 5, 10, 20 and 25 years of rehabilitation and compared with soil under undisturbed native forest and under long-term commercial pine forest. Following topsoil removal, stockpiling and respreading on reformed dunes, there was a massive loss of organic C such that, at time zero, organic C content was only 24% of that present under native forest. Soil organic C content increased progressively during rehabilitation until, after 25 years, it represented 93% of that present under native forest. The pattern of change in light-fraction C, KMnO₄-extractable C, water-soluble C, microbial biomass C, basal respiration and arginine ammonification rate was broadly similar to that for organic C, but the extent of the initial loss and the magnitude of the subsequent increase differed. Microbial biomass C, water-soluble C and KMnO₄-extractable C, expressed as a percentage of organic C, declined during rehabilitation as humic substances progressively accumulated. Principal component (PC) analysis of catabolic response profiles to 36 substrates revealed that the catabolic diversity of microbial communities differed greatly between native forest, commercial pine forest, 0 years and 10 years of rehabilitation. On the PC1 axis, values for soils under native forest and after 25 years rehabilitation were similar, but there was still separation on the PC2 axis. The main factor explaining variation in response profiles on the PC1 axis was organic C content; and the greatest catabolic diversity occurred in soils under native forest and after 25 years of rehabilitation.     

Spring dynamics of soil carbon, nitrogen, and microbial activity in earthworm middens in a no-till cornfield

Earthworm activity may be an important cause of spatial and temporal heterogeneity of soil properties in agroecosystems. Structures known as “earthworm middens,” formed at the soil surface by the feeding and casting activities of some earthworms, may contribute significantly to this heterogeneity. We compared the temporal dynamics of carbon (C), nitrogen (N), and microbial acitivity in Lumbricus terrestris middens and in surrounding non-midden (bulk) soil during the spring, when seasonal earthworm activity was high. We sampled soil from middens and bulk soil in a no-till cornfield on four dates during May and June 1995. Soil water content and the weight of coarse organic litter (>2mm) were consistently higher in middens than in bulk soil. Total C and N concentrations, C:N ratios, and microbial activity also were greatest in midden soil. Concentrations of ammonium-nitrogen and dissolved organic N were greater in middens than in bulk soil on most dates, suggesting accelerated decomposition and mineralization in middens. However, concentrations of nitrate were usually lower in middens, indicating reduced nitrification or increased leaching and denitrification losses from middens, relative to bulk soil. Fungal activity, as well as total microbial activity, was consistently greater in middens. The contribution of fungae to overall microbial activity differed significantly between middens and bulk soil only on one date when both soils were very dry; the contribution of fungae to microbial activity was lower in the middens on this date. We conclude that the midden-forming activity of L. terrestris can be a major determinant of spatial heterogeneity in some agricultural soils, and that this can potentially affect overall rates of soil processes such as organic matter decomposition, N mineralization, denitrification, and leaching. Received: 4 April 1997     

Organic matter status and the size, activity and metabolic diversity of the soil microbial community in the row and inter-row of sugarcane under burning and trash retention

The concentrations of organic C, labile organic fractions and the size and activity of the microbial community were measured to a depth of 30 cm below the plant row and at distances of 30 and 60 cm into the inter-row area under sugarcane under pre-harvest burning or green cane harvesting with retention of a crop residue (trash) mulch. Total root mass was similar under burning and trashing but under trashing there was a redistribution of roots towards the surface 0-10 cm in the inter-row space as roots proliferated beneath the trash mulch. Soil organic C content decreased in response to both increasing distance from the plant row (to a depth of 20 cm) and burning rather than trashing (to a depth of 10 cm). Declines in K₂SO₄-extractable C, light fraction C, microbial biomass C, basal respiration and aggregate stability in response to distance and burning were much more marked than those for organic C and occurred to a depth of 30 cm. Bulk density was greater under burnt than trashed sugarcane and was greater in the inter-row than row, particularly under burning. Heterotrophic functional diversity (measured by analysis of catabolic response profiles to 36 substrates) was also investigated. Principal component analysis of response profiles demonstrated that soils below the row and those under trashing at 30 cm out from this row were separated from the other soils on PC1 and the sample from the inter-row centre (60 cm out) under burning was separated from the others on PC2. Catabolic evenness was least for the latter soil. It was concluded that soil in the inter-row of burnt sugarcane receives few inputs of organic matter and that conversion to green cane harvesting with retention of a trash mulch greatly improves the organic matter, microbial and physical status of the inter-row soil.     

添加生物质炭对红壤水稻土有机碳矿化和微生物生物量的影响

通过室内培育试验,研究了添加生物质炭对江西红壤水稻土有机碳矿化和微生物生物量碳、氮含量的影响。结果表明:红壤有机碳矿化速率在培育第2天达最大值后迅速降低,培养7天后下降缓慢并趋于平稳;添加生物质炭降低了土壤有机碳的矿化速率和累积矿化量,培养结束时,不加生物质炭的对照处理中有机碳的累积矿化量分别比添加0.5%和1.0%生物质炭的处理高10.0%和10.8%。此外,生物质炭的加入显著提高了土壤微生物生物量,添加0.5%生物质炭处理的土壤微生物生物量碳、氮含量分别比对照高111.5%～250.6%和11.6%～97.6%,添加1.0%生物质炭处理的土壤微生物生物量碳、氮含量分别比对照高58.9%～243.6%和55.9%～110.4%。相同处理中,干旱的水分条件下(40%田间持水量)微生物生物量要高于湿润的水分条件(70%田间持水量)。同时,添加0.5%和1.0%的生物质炭使土壤代谢熵分别降低2.4%和26.8%,微生物商减少了43.7%和31.7%。     

Microbial community structure and characteristics of the organic matter in soils under Pinus sylvestris, Picea abies and Betula pendula at two forest sites

 Microbial biomass C (C_mic), C mineralization rate, phospholipid fatty acid (PLFA) profiles and community level physiological profiles (CLPPs) using Biolog were determined from the humus and mineral soil layers in adjacent stands of Scots pine (Pinus sylvestris L.), Norway spruce [Picea abies (L.) Karst.] and silver birch (Betula pendula Roth) at two forest sites of different fertility. In addition, the Fourier-transformed infrared (FTIR) spectra were run on the samples for characterization of the organic matter. C_mic and C mineralization rate tended to be lowest under spruce and highest under birch, at the fertile site in all soil layers and at the less fertile site in the humus layer. There were also differences in microbial community structure in soils under different tree species. In the humus layer the PLFAs separated all tree species and in the mineral soil spruce was distinct from pine and birch. CLPPs did not distinguish microbial communities from the different tree species. The FTIR spectra did not separate the tree species, but clearly separated the two sites. Received: 3 December 1999     

Impact of agricultural practices on the size and activity of the microbial biomass in a long-term field experiment

The Dehérain long-term field experiment was initiated in 1875 to study the impact of fertilization on a wheat-sugarbeet rotation. In 1987, the rotation was stopped to be replaced by continuous maize. Crop residues were soil-incorporated and the mineral fertilization was doubled in some plots. The impact of those changes on the microbial biomass and activity are presented. In spring 1987, the soil was still in a steady-state condition corresponding to the rotation. The microbial biomass was correlated with total organic C and decreased in the order farmyard manure>mineral NPK>unfertilized control. Microbial specific respiratory activity was higher in the unfertilized treatments. The soil biomass was closely related to soil N plant uptake. In 1989, after 2 years of maize and crop residue incorporation, the steady-state condition corresponding to the previous agricultural practices disappeared. So did the relationship between the biomass and total organic C, and the soil N plant uptake. Biomass specific respiratory activity increased because of low efficiency in the use of maize residues by microbes under N stress.     

Rhizosphere priming of soil organic matter by bacterial groups in a grassland soil

Plants often impact the rate of native soil organic matter turnover through root interactions with soil organisms; however the role of root-microbial interactions in mediation of the “priming effect” is not well understood. We examined the effects of living plant roots and N fertilization on belowground C dynamics in a California annual grassland soil (Haploxeralf) during a two-year greenhouse study. The fate of ¹³C-labeled belowground C (roots and organic matter) was followed under planted (Avena barbata) and unplanted conditions, and with and without supplemental N (20 kg N ha⁻¹ season⁻¹) over two periods of plant growth, each followed by a dry, fallow period of 120 d. Turnover of belowground ¹³C SOM was followed using ¹³C-phospholipid fatty acid (PLFA) biomarkers. Living roots increased the turnover and loss of belowground ¹³C compared with unplanted soils. Planted soils had 20% less belowground ¹³C present than in unplanted soils after 2 cycles of planting and fallow. After 2 treatment cycles, unlabeled soil C was 4.8% higher in planted soils than unplanted. The addition of N to soils decreased the turnover of enriched belowground ¹³C during the first treatment season in both planted and unplanted soils, however no effect of N was observed thereafter. Our findings suggest that A. barbata may increase soil C levels over time because root and exudate C inputs are significant, but that increase will be moderated by an overall faster C mineralization rate of belowground C. N addition may slow soil C losses; however, the effect was minor and transient in this system. The labeled root-derived ¹³C was initially recovered in gram negative (highest enrichment), gram positive, and fungal biomarkers. With successive growing seasons, the labeled C in the gram negative and fungal markers declined, while gram positive markers continued to accumulate labeled belowground C. The rhizosphere of A. barbata shifted the microbial community composition, resulting in greater abundances of gram negative markers and lower abundances of gram positive, actinobacteria and cyclopropyl PLFA markers compared to unplanted soil. However, the longer-term utilization of labeled belowground C by gram positive bacteria was enhanced in the rhizosphere microbial community compared with unplanted soils. We suggest that the activities of gram positive bacteria may be major controllers of multi-year rhizosphere-related priming of SOM decomposition.     

Transformations and recovery of residue and fertilizer nitrogen-15 in a sandy Lixisol of West Africa

 The fate of ¹⁵N-labeled plant residues from different cover-cropping systems and labeled inorganic N fertilizer in the organic, soil mineral, microbial biomass and soil organic matter (SOM) particle-size fractions was investigated in a sandy Lixisol. Plant residues were from mucuna (legume), lablab (legume), imperata (grass), maize (cereal) and mixtures of mucuna or lablab with imperata or maize, applied as a surface mulch. Inorganic N fertilizer was applied as ¹⁵N-(NH₄)₂SO₄ at two rates (21 and 42 mg N kg^–1 soil). Total N release from mucuna or lablab residues was 2–3 times higher than from the other residues, whereas imperata immobilized N throughout the study period. In contrast, ¹⁵N was mineralized from all the plant residues irrespective of the mineralization–immobilization pattern observed for total N. After 168 days, 69% of soil mineral N in mucuna- or lablab-mulched soils was derived from the added residues, representing 4–8% of residue N, whereas 9–30% of inorganic N was derived from imperata, maize and the mixed residues. At the end of the study, 4–19% of microbial biomass N was derived from the added residue/fertilizer-N, accounting for 1–3% of added residue-N. Averaged across treatments, particulate SOM fractions accounted for less than 1% of the total soil by weight but contained 20% of total soil C and 8% of soil N. Soils amended with mucuna or lablab incorporated more N in the 250–2000 μm SOM pool, whereas soil amended with imperata or the mixed residues incorporated similar proportions of labeled N in the 250–2000 μm and 53–250 μm fractions. In contrast, in soils receiving the maize or inorganic fertilizer-N treatments, higher proportions of labeled N were incorporated into the 53–250 μm than the 250–2000 μm fractions. The relationship between these differences in residue/fertilizer-N partitioning into different SOM particle-size fractions and soil productivity is discussed. Received: 12 March 1999