Incorporation Of Plant Material Research Articles

Application of Plant Ingredients for Improving Sustainability of Fresh Pasta

Pasta is a low-cost and easy-to-prepare food product. By using fresh pasta, the drying process is omitted, which represents significant energy and financial savings, but the durability of such pasta is very limited. The addition of plant materials (parsley, oregano, thyme, cinnamon, nettle, spinach, and carob) to the dough affects the microbiological stability and thus the durability of the fresh pasta, which can then be stored longer in the refrigerator without the need to freeze the product. With significant energy savings due to the omission of the drying and freezing processes, extending the shelf life of the pasta will contribute to the reduction of food waste and thus contribute to a more sustainable production system. The aim of this work was to examine the possibility of producing fresh pasta with the addition of plant materials from the perspectives of technological and sensory quality, as well as microbiological composition. The incorporation of plant materials (10 g/100 g) decreased water absorption and had no effect on optimal cooking time. Thyme-enriched pasta had significantly lower (p < 0.05) cooking loss (3.34%) than the control sample (4.12%). In cinnamon-, spinach-, and parsley-enriched pasta, an appealing colour and pleasant smell and taste were achieved. Cinnamon and carob proved to have the most favourable effect on the microbiological quality (reduction in mesophilic, Enterobacteriaceae, and yeast counts), followed by spinach (reduction in mould count). For cinnamon-enriched pasta, the best technological (optimal cooking time of 3.23 min; cooking loss of 4.41%; firmness of 531.94 g) and sensory and microbiological quality were achieved; thus, cinnamon proved to be the preferred natural preservative for the production of enriched pasta.

Synthesis and characterization of peanut hull modified chitosan beads.

The incorporation of plant materials is an effective method to improve the stability of chitosan beads, as it further increases the adsorption of toxic dyes and metals from aqueous systems. In the present study, chitosan gels were impregnated with a novel type of powder as the groundnut hull powder in order to form composite beads by using a simple droplet-based microfluidic system. The beads were then characterised through various techniques such as SEM, TGA, FTIR, and XRD. Microscopic imaging revealed a change in the surface morphology of the composite beads, which became rough and wrinkled with more valley-like features and irregular cracks. FTIR data suggest that the impregnation of groundnut hull powder led to an increase in functional groups. The thermal analysis allowed for the assessment of composite bead hydration contents and indicated the presence of groundnut hull entrapped in the loaded beads, which was corroborated by vibrational spectroscopy. XRD analysis allows us to conclude that there is an involvement of groundnut hull in the chitosan gels, and the consequence of that is the formation of amorphous beads, which would make them suitable for the adsorption of toxic dyes and metals from water systems.

Mudanças na microbiota do solo causada pelo uso de produtos comerciais e incorporação de materiais vegetais

ABSTRACT Growers have adopted monoculture to maintain the high melon (Cucumis melo L.) production demand in the Northeastern region of Brazil. This cultivating practice culminates in up to three crop cycles per year being used in the same growing area. The main objective of this study was to evaluate if the incorporation of plant material used with polyethylene mulch and or in association with commercial soil amendment products can help to condition an environment that is beneficial to soil microbial communities. Two identical greenhouse experiments were conducted using a completely randomized design with seven treatments and seven replications. The treatments were: (C) – Control, (M) - polyethylene mulch, (C+M) - incorporation of Crotalaria juncea L. + polyethylene mulch, (P+M) - incorporation of Pennisetum glaucum L. + polyethylene mulch, (M+CS) - polyethylene mulch + (Compost-Aid® + Soil-Set®, trade names of products produced by Alltech Crop Science), (C+M+CS) - incorporation of C. juncea L. + polyethylene mulch + (Compost-Aid® + Soil-Set®), and (P+M+CS) - incorporation of P. glaucum L. + polyethylene mulch + (Compost-Aid® + Soil-Set®). To quantify the target soil microbiota (fungi, bacteria, sporulating bacteria, and actinomycetes), isolations were attempted on selective culture media specific for each group of microorganisms. The incorporation of P. glaucum together with the use of polyethylene mulch and commercial products (Compost-Aid® and Soil-Set®), (P+M+CS), increased the total fungal population by 183%, total bacteria by 55%, sporulating bacteria by 21%, and actinomycetes by 146% in relation to the control treatment.

Mentha and Oregano Soil Amendment Induces Enhancement of Tomato Tolerance against Soilborne Diseases, Yield and Quality

Tomato is considered among the most important horticulture crops in both temperate and tropical regions, and two of the major biotic stresses include Fusarium oxysporun f.sp. lycopersici (Fol) and Verticillium dahliae (Vd). The effect of soil incorporated spearmint and oregano dried plant material on physiological, yield and quality parameters of tomato, along with their efficacy against soilborne fungal diseases, was studied in pot experiments conducted in a greenhouse environment. Tomato plants grown in soil amended with spearmint or oregano showed better agronomical characteristics (taller plants with thicker stems) and improved physiological ones (higher chlorophyll content index and photosynthetic rate). Yield was increased and the qualitative features of tomato fruits were enhanced. In addition, plants grown in soil amended with spearmint or oregano and inoculated with Fol or Vd had no visual disease symptoms 50 days from the inoculated tomato transplantation, except from plants grown in soil amended with oregano and inoculated with Fol, which showed symptoms of chlorosis and leaves loss. These enhancements on physiological parameters and on disease suppression resulted in increased fruit yields of plants–grown in soil amended with spearmint and oregano and inoculated with Fol or Vd–by 77%–95% compared with free-disease controls. GC-MS analysis of volatiles derived from soils amended of either spearmint or oregano indicated that several constituents remained in the soil environment long after incorporation of plant material, although, at lower concentrations and considerable modified. The current study reveals that direct incorporation of spearmint or oregano plant material into the soil could improve tomato tolerance against soilborne fungi, soil fertility and consequently increase yield and product quality.

Influência da adubação verde no declínio de monosporascus em solo naturalmente infestado

RESUMO A incorporação de materiais vegetais no solo pode exercer influência sobre as populações dos patógenos e/ou suas atividades patogênicas. Este trabalho objetivou avaliar o potencial da adubação verde na indução da supressividade ao declínio de monosporascus em solo naturalmente infestado por Monosporascus cannonballus. Foram realizados dois experimentos, um em casa de vegetação e outro em campo. No primeiro, foram comparados 13 tratamentos considerando diferentes combinações do melão pele de sapo e os adubos verdes (Crotalaria spectabilis, C. juncea, Canavalia ensiformis, Cajanus cajan, Dolichos lablab e Stilozobium aterrimum) em quatro ciclos de cultivo. Os maiores valores de severidade da doença foram detectados nos plantios intercalados de C. cajan ou C. spectabilis com meloeiro, e nos cultivos sucessivos de C. ensiformis com incorporação, e meloeiro sem incorporação. Os maiores níveis populacionais de M. cannonballus nos solos foram verificados nos cultivos sucessivos de meloeiro sem incorporação, D. lablab, e nos cultivos intercalados de C. ensiformis, S. aterrimum, C. cajan, C. juncea ou D. lablab com meloeiro. As menores densidades obtidas ao final do experimento de casa de vegetação foram nos solos submetidos à adubação verde sucessiva de C. spectabilis, C. cajan, C. ensiformis, C. juncea e S. aterrimum e no plantio intercalado de C. Juncea. Em campo, o tratamento mais eficiente na redução da severidade do declínio de monosporascus e que apresentou, numericamente, maior número de frutos comerciais de melão foi o tratamento com S. aterrimum.

The incorporation of plant materials in “Serra da Estrela” cheese improves antioxidant activity without changing the fatty acid profile and visual appearance

‘Serra da Estrela’ cheese is a Portuguese delicacy, which has been produced for centuries from the milk of cattle pasturing in the protected Serra da Estrela natural park. Transforming this cheese into a functional food would be a huge benefit for the market and the consumer. Decocted extracts, dried chestnut flowers and lemon balm plants were incorporated into the cheese to functionalise it, granting antioxidant activity to this foodstuff. The functionalised cheeses showed higher antioxidant activity, especially lipid peroxidation inhibition. The incorporation of dried plants appeared to be more effective than using decoctions, but the influence of the plant species was less observable. Furthermore, the fatty acids profile of the cheeses was also determined through gas chromatography. C18:1 and C16:0 were the most abundant fatty acids; saturated fatty acids prevailed over the unsaturated ones. Between the control and the incorporated samples no significant differences were found. In addition, the external colour was measured through a spectrophotometer for lightness, yellowness and redness. There were some differences recorded for each samples’ colour. In general, the results indicated that the functionalisation of this exquisite dairy product with natural plant extracts provided beneficial characteristics, both for consumers (healthier product) and producers (added‐value products).Practical applications The food industry is always searching for new products which are cheaper to produce and healthier for the consumer, withstanding longer storage periods.‘Serra da Estrela’ cheese was incorporated with chestnut flowers and lemon balm dried plants and their respective decoctions in order to achieve antioxidant properties.

Incorporation of plant materials in the control of root pathogens in muskmelon.

Avaliou-se o efeito de materiais vegetais [Crotalária (Crotalaria juncea), Mamona (Ricinus communis L.), Cassava (Manihot esculenta Crantz) e Nim (Azadirachta indica)] e do tempo de incorporação destes sobre a incidência de podridões radiculares no meloeiro, no estado do Ceará, Brasil. O experimento foi conduzido em área comercial com histórico de patógenos radiculares em cucurbitáceas. O delineamento utilizado foi de blocos casualizados, em esquema fatorial 5 x 3, com quatro repetições. Os tratamentos consistiram da combinação de quatro materiais vegetais (crotalária, mamona, mandioca brava e nim) mais uma testemunha com solo sem incorporação de material vegetal e três tempos de incorporação (28, 21, e 14 dias antes do transplantio das mudas). Menor incidência de plantas com podridão radicular foi observada em praticamente todos os tratamentos onde se incorporou materiais nos diferentes tempos, com variação entre os materiais, conforme o tempo de incorporação, em relação ao solo sem material vegetal. A crotalária proporcionou menor incidência quando a incorporação foi de 14 dias e à medida que aumentou o tempo, houve acréscimo na porcentagem de plantas com podridão. Quando se utilizou a mandioca, houve redução da podridão, proporcionalmente ao tempo de incorporação. Os patógenos isolados das plantas de melão com sintomas foram Fusarium solani, Macrophomina phaseolina, Monosporascus cannonballus e Rhizoctonia solani, sendo o F. solani o de maior frequência.

Efeitos da simulação da solarização do solo com materiais vegetais sobre o crescimento micelial de fungos fitopatogênicos habitantes do solo

A incorporação de materiais vegetais específicos associados à solarização do solo tem sido um avanço promissor no controle de fungos fitopatogênicos habitantes do solo. O objetivo do trabalho foi avaliar determinados efeitos da incorporação e decomposição de brócolis, mamona, mandioca brava e mansa, no solo, em condições de microcosmo mantido em BOD (37±2ºC), sobre o micélio de Fusarium oxysporum f. sp. lycopersici Raça 2, Macrophomina phaseolina, Rhizoctonia solani AG-4 HGI e de Sclerotium rolfsii. Assim, quatro ensaios idênticos foram instalados em conjunto de microcosmos, com cinco tratamentos e quatro períodos de tempo diferentes e independentes (7, 14, 21 e 28 dias). O parâmetro avaliado foi os efeitos inócuo, fungistático e fungicida dos tratamentos sobre o micélio dos fungos. Verificou-se efeito fungistático e fungicida no crescimento micelial de F. oxysporum f. sp. lycopersici Raça 2, R. solani AG-4 HGI e de S. rolfsii. Os fungos que apresentaram efeito fungistático apresentaram uma velocidade média de crescimento micelial inferior ao controle geral, que consistiu na incubação dos fungos em temperatura de 25±2ºC. O efeito fungicida ocorreu aos 21 dias de incubação para F. oxysporum e R. solani e aos 28 dias para S. rolfsii. Para M. phaseolina, observou-se apenas efeito inócuo. Associação da temperatura de 37±2ºC mais o período de tempo dos tratamentos foi o fator responsável pelos efeitos fungistático e fungicida no micélio dos fitopatógenos estudados. Essa associação também interferiu na velocidade do crescimento micelial dos fungos que apresentaram efeito fungistático.

Decomposition of soil and plant carbon from pasture systems after 9 years of exposure to elevated CO2: impact on C cycling and modeling

AbstractElevated atmospheric CO2 may alter decomposition rates through changes in plant material quality and through its impact on soil microbial activity. This study examines whether plant material produced under elevated CO2 decomposes differently from plant material produced under ambient CO2. Moreover, a long‐term experiment offered a unique opportunity to evaluate assumptions about C cycling under elevated CO2 made in coupled climate–soil organic matter (SOM) models. Trifolium repens and Lolium perenne plant materials, produced under elevated (60 Pa) and ambient CO2 at two levels of N fertilizer (140 vs. 560 kg ha−1 yr−1), were incubated in soil for 90 days. Soils and plant materials used for the incubation had been exposed to ambient and elevated CO2 under free air carbon dioxide enrichment conditions and had received the N fertilizer for 9 years. The rate of decomposition of L. perenne and T. repens plant materials was unaffected by elevated atmospheric CO2 and rate of N fertilization. Increases in L. perenne plant material C : N ratio under elevated CO2 did not affect decomposition rates of the plant material. If under prolonged elevated CO2 changes in soil microbial dynamics had occurred, they were not reflected in the rate of decomposition of the plant material. Only soil respiration under L. perenne, with or without incorporation of plant material, from the low‐N fertilization treatment was enhanced after exposure to elevated CO2. This increase in soil respiration was not reflected in an increase in the microbial biomass of the L. perenne soil. The contribution of old and newly sequestered C to soil respiration, as revealed by the 13C‐CO2 signature, reflected the turnover times of SOM–C pools as described by multipool SOM models. The results do not confirm the assumption of a negative feedback induced in the C cycle following an increase in CO2, as used in coupled climate–SOM models. Moreover, this study showed no evidence for a positive feedback in the C cycle following additional N fertilization.

The Role of Invertebrates on Leaf Litter Decomposition in Streams - a Review

Leaves entering low order streams are subject to physical abrasion, microbial degradation and invertebrate fragmentation. Aquatic invertebrates feeding on leaves are known as shredders and their densities tend to be correlated with the spatial and temporal accumulation of organic matter in streams. Shredders discriminate among the variety of leaves normally found in the stream; this discrimination may be related to differences in leaf toughness, plant nutrient content of leaves and the presence of secondary compounds. Shredders also consume leaves preferentially after the establishment of a well-developed microbial community. This preference may be the result of changes in leaf matrix carried out by the microbial community or the presence of fungal hyphae with a higher nutrition value than the leaves themselves. The immediate consequence of invertebrate feeding on leaves is the incorporation of plant material into secondary production and the fragmentation of leaves. The relative importance of fungi and invertebrates in the decomposition process depends upon the density of shredders, which, in turn, may depend on litter accumulation in streams. Therefore, the type of riparian vegetation has the potential to control the diversity and abundance of shredders and changes in riparian vegetation have the potential to affect the assemblages of aquatic invertebrates.

Organic matter stabilization in a Xanthic Ferralsol of the central Amazon as affected by single trees: chemical characterization of density, aggregate, and particle size fractions

Not only the amount of organic carbon in soil is important for soil organic matter (SOM) stability, but also its physical and chemical properties. The appropriate technique for the assessment of SOM dynamics can vary between soil types, and information about this is lacking for Ferralsols of the central Amazon basin. First, this work identified SOM pools which are sensitive to land-use changes on the terra firme in the central Amazon. In a second step, the effects of single trees on SOM properties were evaluated in a mixed tree crop plantation in comparison to secondary and primary forest sites. Thus, the processes of organic matter stabilization could be studied in the highly aggregated soils. A combination of aggregate and density fractionation was found to be most suitable for physical SOM characterization. The particulate organic matter (POM, density less than 1.6 Mg m −3) varied by one order of magnitude between sites and could be used as a sensitive indicator of land-use changes. Aggregate stability was not related to SOM contents or bulk SOM properties. The incorporation of plant material into stable SOM, however, was enhanced by aggregation. Among aggregate separates, the fraction, 0.25–0.5 mm, showed single-tree effects the most. SOM replenishment was higher under tree species with low quality litter, i.e. high C-to-N and polyphenol-to-N ratios. High quality litter from a leguminous ground cover, however, showed low soil nitrogen and carbon replenishment but increased nitrogen concentrations in light fractions. Litter with a high quality may improve soil nitrogen availability but not amounts of total SOM, which could only be shown for low quality litter. The results indicate the importance of aggregation and POM dynamics for SOM stabilization in the studied Xanthic Ferralsols of the central Amazon basin.

Variations in the carbon isotope composition of late-Holocene plant macrofossils: a comparison of whole-leaf and cellulose trends

Stable carbon isotope measurements (δ13C) made on Quaternary sequences of terrestrial plant subfos sils are frequently used to infer palaeoclimatic trends. However, differential decomposition of individual con stituents during incorporation of plant material into lake sediments could influence these interpretations. There fore, we investigated down-core variations in the carbon isotope composition (δ13C) of subfossil leaves of Salix herbacea L., and their holocellulose component, from a late-Holocene ( c. 0–3300 years ago) site in northern Sweden. Locally weighted regression curves fitted to bothδ13C data sets showed similar but conserved temporal patterns of change, and there was no evidence for systematic shifts in the holocellulose yields or C:N ratios of leaves with age. Small d13C variations over the past 3300 years probably reflect natural variations in microclimate within the lake catchment area. This interpretation is quantitatively consistent with the variability determ ined from measurements made on leaves of modern S. herbacea plants growing at the same site. Overall these results imply that, under conditions of uniform decomposition,δ13C measurements made on late-Holocene sequences of whole leaves provide an interpretation of the pattern and direction of change consistent with that obtained from holocellulose isolated from the same leaves.

Measured and simulated availability and leaching of nitrogen associated with frequent use of catch crops

Abstract. Results are presented from three years (1992‐1995) of a field leaching experiment on a sandy soil in south‐west Sweden. Plots of spring cereals, either with or without an undersown perennial ryegrass catch crop, were compared for nitrogen leaching and nitrogen status in soil. Both treatments were ploughed in spring, and other tillage regimes were also identical. Measurements of nitrogen leaching from drains, nitrogen uptake in crops and mineral nitrogen in the soil were made. Two coupled, simulation models, which describe water flow and nitrogen transformations and transport in soil, were used to interpret data and to calculate the nitrogen budget and nitrogen mineralization in the soil.Nitrogen leaching was 40 50% less in the catch crop treatment compared with the control during years when the establishment of the catch crop succeeded. In the third year of the experiment nitrogen leaching was actually greater in the catch crop treatment (7 kg N/ha). This increase was caused by a poorly established catch crop coinciding with enhanced mineralization of previous catch crop residues. There was no simulated change in soil organic nitrogen in either of the treatments. Simulations showed increased nitrogen mineralization during April‐July after incorporation of plant material in spring, especially in the catch crop treatment. However, the increased nitrogen mineralization probably occurred too late for the released nitrogen to be fully available to the main crop.

Soil pH changes during legume growth and application of plant material

During cultivation of legumes soil is acidified due to proton release from roots. As a consequence of proton release, plants accumulate organic anions which may, if returned and decomposed in the soil, neutralize the soil acids. Until now the detailed processes responsible for the change in soil pH after incorporation of plant material have not been completely understood. Using a pot experiment we studied the changes in acid and base in soil during growth of field beans (Vicia faba L. cv. Alfred) and after incorporation of the plant material into the soil. Soil pH was significantly decreased by field beans from 6.00 to 5.64 in a cultivation period of 45 days. Proton release amounted to 32.7 mmol H+ pot-1, which was approximately equivalent to the accumulated alkalinity in the plant shoots (34.4 mmol). Return of field bean shoots caused a significant soil pH increase from 5.64 to 6.29. Within 7 days more than 90% of the added alkalinity was released. After 307 days incubation, soil pH decreased to 5.86 due to nitrification. In a second experiment, maize leaves (Zea mays L.), containing various concentrations of nitrogen and at various alkalinities, were incorporated into the soil. Soil pH change was positively correlated to alkalinity and malate concentration and negatively correlated to total nitrogen and water-soluble organic nitrogen of incorporated leaves. It is concluded that the soil acidification caused by legume cultivation can be partly compensated for if crop residues are returned to the soil. Addition of plant material may initially cause an increase in soil pH due to decomposition of organic anions and organic nitrogen. Soil pH may decrease if nitrification is involved. The concentrations of nitrogen and alkalinity of added plant material are decisive factors controlling soil pH change after incorporation of plant material.

Recovery of nitrogen by spring barley following incorporation of 15N-labelled straw and catch crop material

The recovery by spring barley ( Hordeum vulgare L.) of nitrogen mineralized from 15N-labelled straw and ryegrass material was followed for 3 years in the field. The effects of separate and combined applications of straw and ryegrass were studied using cross-labelling with 15N. Reference plots receiving 15NH 4 15NO 3 were included. Plant samples were taken every second week until maturity during the first growing season and at maturity in the two following years. Incorporation of plant material had no significant influence on the above-ground dry matter yield of the barley. The barley recovery of N derived from straw was not significantly different whether straw was incorporated alone or in combination with ryegrass material. The mean recovery of straw N was 4.5% in the first barley crop and 2.7% and 1.1% in the second and third crop. During the first growing season, recovery of ryegrass N in the barley was higher when the catch crop material was incorporated without straw, but the differences were only significant at one sampling date. At maturity 7.8% and 10.2% of the ryegrass N was recovered in the barley crop, when ryegrass was incorporated with or without straw, respectively. Mean recoveries of ryegrass N were 2.3% in the second year and less than 1% in the third year after incorporation. recovery of mineral fertilizer in the year of application was relatively low (29–40%), probably due to long periods of spring drought in all 3 years. The recovery of N from residual mineral fertilizer was in the second and third barley crop similar to the recovery of N from incorporated plant residues.

The fate of spring applied fertilizer N during the autumn-winter period: comparison between winter-fallow and green manure cropped soil

A field experiment was carried out at a pilot plot that was cropped with oilseed rape, and then left partly fallow and partly cropped with a green manure (mustard) during the autumn after harvest of the oilseed rape. The rape residues were incorporated in the soil. Methods used to quantify the N fluxes from harvest until sowing of the next crop were (1) 15N balance method, (2) total mineral N analysis and (3) NO emission measurements. Losses of spring applied fertilizer N were negligible in cropped plots and minimal in fallow plots during the following autumn-winter period. Most of the plant-N residues was retained by the organic N pool of the upper 30-cm soil layer. The green manure contributed slightly to soil available N at sowing of the next crop. However, the incorporation of plant material resulted in a nitrate flux that was at risk of leaching on the fallow plots, and on the green manure plots after incorporation of the green manure. This nitrate was largely derived from soil organic N, not from unused fertilizer applied in spring or from immobilized fertilizer.

Nature and properties of soil humic acid as influenced by incorporation of different plant materials

Properties of soil humic acids (HAs) due to incorporation of plant materials (berseem and wheat) were studied in black, laterite and red soil representing the soil orders vertisol, ultisol and alfisol, respectively. Incorporation of plant material was found to reduce the carbon content, but to increase the nitrogen content of soil HAs, irrespective of soil type. The total-N/available-N ratio of soil was found to be a determining factor for the variation in nitrogen content of soil HAs due to incorporation of plant materials. Cation exchange capacity (CEC) and optical density of soil HAs decreased, whereas coagulation threshold, absorption in different infrared regions, buffering beyond pH 7.0, and reduced viscosity increased due to addition of plant material. The amount of proton released and the drop in the initial pH of soil HAs after addition of different metal ions indicated formation of mono-, dihydroxy-metal-HA complexes which were unstable at pH 8.0 and above.

Reponse de la biomasse microbienne a l'adjonction au sol de materiel vegetal marque au 14C: Role des racines vivantes

14C and 15N-labelled immature wheat straw was incubated in the laboratory for 450 days in either a sandy soil or a clay soil, under controlled conditions of temperature and humidity. One-half of the treatments were cropped 4 times in succession with spring wheat. After each harvest, the roots and shoots were removed from the soil. The remaining treatments were kept bare, without plants. After 277 days, 1% unlabelled wheat straw was again mixed with the soils. Microbial biomass was measured after 0, 25, 53, 80, 185, 318 and 430 days, using the fumigation technique. This paper presents the 14C-data. The half-life of the labelled compounds in soil was from 60 to 70 days. After 430 days about 10% more labelled C remained in bare soil than in cropped soil. Labelled biomass carbon reached its maximum before day 25. By then 50% of the biomass-C was labelled and the biomass represented 20% of the total labelled C remaining in the soils. This percentage decreased slowly to 15% after 430 days in bare sandy soil and to 17% in bare clay soil. A second incorporation of plant material, this time unlabelled, did not appreciably alter the shape of the curve representing the decrease of labelled C in biomass, expressed as % of the total remaining labelled C. Total biomass-C (labelled + unlabelled) in cropped soil was sometimes higher and sometimes lower than in bare soil. However, the labelled C/total C ratio in biomass was always lower; in cropped soils than in soils without plants, clearly showing the effect of rhizodeposition. From days 25 to 430 an increasing difference appeared between the ratio labelled C/total and C in CO 2 and the corresponding ratio labelled C/total C in biomass. In CO 2-C the ratio diminished rapidly, in biomass-C it remained at a high level, most probably indicating a lower turnover of C in resting but living microorganisms. Other explanations are also discussed. The amount of CO 2-C released mg −1 of biomass-C was higher in cropped than in bare soil, presumably because the microorganisms were activated by the living (or dying) root system.

Microbial biomass formed from 14C, 15N-labelled plant material decomposing in soils in the field

The decomposition of 14C, 14N-labelled medic ( Medicago littoralis) material and the net formation and decay of isotope-labelled biomass have been measured in four South Australian soils in the field over 4 yr. The field sites were in similar climatic zones but two sites received about twice as much rainfall as the others. The soils were calcareous and of similar pH, but differed in texture and organic matter content. The decomposition of the organic- 14C and organic- 15N residues were, for a given site, similar. Initially, the concentrations of labelled residues decreased rapidly, then very slowly. Decomposition rates in a heavy clay soil were significantly less than in the other soils during the first 16 weeks after incorporation of plant material, but thereafter, rates of decomposition in all soils were similar, despite differences in soil texture and climate. More than 50% of the medic- 14C had disappeared from all soils after 4 weeks of decomposition and only 15–20% of the medic- 14C remained as organic residues after 4 yr. Of the medic- 15N 60–65% remained as organic residues after 32 weeks decomposition; the percentage decreased to 45–50% after 4 yr. The amounts of 14C, 14N-labelled biomass, formed from decomposing plant material, were maximal 4–8 weeks after incorporation of plant material into the soils. In samples taken at 8 weeks from the sandy Roseworthy soil, biomass- 14C and - 15N accounted for 14 and 22% respectively of the total organic- 14C and - 15N residues present. Thereafter in this soil, the concentrations of biomass- 14C and - 15N decreased, rapidly at first then more slowly. Nevertheless, throughout most of the decomposition the rates of decrease in the concentrations of biomass- 14C and - 15N exceeded those of the non-biomass, labelled organic residues. The proportions of 14C, 15N-labelled materials accounted for in the labelled biomass varied between soils. Soils of higher clay content generally retained higher proportions of residual organic- 14C and - 14N in the biomass, even though the net rates of decomposition of total labelled residues did not differ significantly between soils during most of the decomposition.