J.A. Van Veen

Turnover of carbon and nitrogen through the microbial biomass in a sandy loam and a clay soil incubated with [14C(U)]glucose and [15N](NH4)2So4 under different moisture regimes

Abstract Two soils, one a sandy loam and the other of relatively high clay content, were incubated with [14C(U)]gtucose and [15N](NH4)2SO4 for 101 days, either under continuously moist conditions, or with intermittent drying of soils. Rates of evolution of 14CO2, decline in residual organic 14C, and net immobilization and mineralization of N and 15N in the sandy loam soil were more rapid than in the clay soil. First order decay rates for the decomposition of residual 14C, after 10 days, were consistently twice as fast in the sandy loam soil. By contrast, the efficiency with which glucose was utilized within the first few days, and the amounts of C, 14C, N and 15N present as soil biomass throughout the incubation, were greater in the clay soil than in the sandy loam. Biomass 14C as a percentage of residual organic 14C, was consistently 1.5 times greater in the clay soil. Compared with soils held continuously moist, soils which were intermittently dried and remoistened contained smaller amounts of isotope-labelled biomass C and N, but overall similar amounts of total residual organic 14C and 15N. Remoistening of dried soils caused a temporary (4 days) flush in C and N mineralization rates. A simulation model describes C and N behaviour in the two soils. Three features of the model are proposed to expain short-term differences between soils in the rates of C and N turnover, viz. the clay soil (a) has a greater capacity to preserve biomass C and N (b) holds a higher proportion of microbial decay products in the near vicinity of surviving cells, and, to a lesser extent, (c) utilizes glucose and metabolic products more efficiently for biosynthetic reactions.

Modelling C and N turnover through the microbial biomass in soil

Many mathematical descriptions of C and N transformations in soils have been developed in the last decade, but only a few explicitly model the activity and mass of soil organisms. Great difficulties still exist in establishing basic parameters governing the kinetics of microbial turnover. The present state of the art is discussed briefly.

Carbon Fluxes in Plant‐Soil Systems at Elevated Atmospheric CO2 Levels

The flow of carbon from photosynthesizing tissues of higher plants, through the roots and into the soil is one of the key processes in terrestrial ecosystems. An increased level of CO2 in the atmosphere will likely result in an increased input of organic carbon into the soil due to the expected increase in primary production. Whether this will lead to accumulation of greater amounts of organic carbon in soil depends on the flow of carbon through the plant into the soil and its subsequent transformation in the soil by microorganisms. In this paper the major controls of carbon translocation via roots into the soil as well as the subsequent microbial turnover of root-derived carbon are reviewed. We discuss possible consequences of an increased CO2 level in the atmosphere on these processes.

Production of root-derived material and associated microbial growth in soil at different nutrient levels

SummaryMaize plants were grown for 42 days in a sandy soil at two different mineral nutrient levels, in an atmosphere containing 14CO2. The 14C and total carbon contents of shoots, roots, soil and soil microbial biomass were measured 28, 35 and 42 days after germination. Relative growth rates of shoots and roots decreased after 35 days at the lower nutrient level, but were relatively constant at the higher nutrient level. In the former treatment, 2% of the total 14C fixed was retained as a residue in soil at all harvests while at the higher nutrient level up to 4% was retained after 42 days. Incorporation of 14C into the soil microbial biomass was close to its maximum after 35 days at the lower nutrient level, but continued to increase at the higher level. Generally a good agreement existed between microbial biomass, 14C contents and numbers of fluorescent pseudomonads in the rhizosphere. Numbers of fluorescent pseudomonads in the rhizosphere were maximal after 35 days at the lower nutrient level and continued to increase at the higher nutrient level. The proportions of the residual 14C in soil, incorporated in the soil microbial biomass, were 28% to 41% at the lower nutrient level and 20%6 – 30% at the higher nutrient level. From the lower nutrient soil 18%6 – 52%6 of the residual soil 14C could be extracted with 0.5 N K2SO4, versus 14%6 – 16% from the higher nutrient soil.Microbial growth in the rhizosphere seemed directly affected by the depletion of mineral nutrients while plant growth and the related production of root-derived materials continued.

Predominant Bacillus spp. in agricultural soil under different management regimes detected via PCR-DGGE

A PCR system for studying the diversity of species of Bacillus and related taxa directly from soil was developed. For this purpose, a specific 24-bp forward primer located around position 110 of the 16S ribosomal RNA gene was designed and combined with a reverse bacterial primer located at the end of the gene. The specificity of this PCR system for bacilli and related taxons was confirmed on the basis of tests with diverse strains as well as with soil DNA. Analysis of a soil DNA derived clone library showed that the amplified fragments affiliated exclusively with sequences of gram-positive bacteria, with up to 95% of the sequences originating from putative Bacillus species. In particular, sequences affiliated to those of B. mycoides, B. pumilus, B. megaterium, B. thuringiensis, and B. firmus, as well as to related taxa such as Paenibacillus, were obtained. A minority, i.e., less than 6%, of the clones affiliated with other gram-positive bacteria, such as Arthrobacter spp., Frankia spp., and uncultured gram-positives. The amplified fragments were used as templates for a second PCR using bacterial 16S rDNA primers, yielding PCR products of about 410 bp, which were separated by denaturing gradient gel electrophoresis (DGGE). Amplicons indicating Bacillus spp. were found in the gel between 45% and roughly 60% denaturant, whereas those representing other, high-G+C% bacteria, were localized in gel regions with denaturant concentrations exceeding about 60%, thus allowing the distinction between these two groups of sequences. We applied this system to compare the group-specific diversity in bacterial communities in an agricultural soil under different regimes, i.e., permanent grassland, grassland recently turned to arable land, and arable land under agricultural rotation. Differences in the Bacillus-related community structures between the treatments were clearly detected. Higher diversities, as judged by Shannon–Weaver indices calculated on the basis of the molecular profiles, were consistently observed in the permanent grassland and the grassland turned into arable land, as compared to the arable land.

Plant- and soil related controls of the flow of carbon from roots through the soil microbial biomass

The flow of carbon from plant roots through the microbial biomass is one of the key processes in terrestrial ecosystems. Roots release considerable amounts of organic materials which are utilized by microbes as substrate for biosynthesis and energy supply. The fate of photosynthates and other organic material in the soil-root environment under different conditions was studied using14C-tracers. Soil structure and texture had a large effect on the turnover of the14C-labelled materials through the microbial biomas. Finer, clayey soils tended to be more ‘preservative’ than coarser, sandy soils,i.e., larger amounts of14C were incorporated in microbial biomass and soil organic matter fractions in clayey soils than in sandy soils.The soil nutrient status also appeared to affect organic matter turnover. At limiting plant-nutrient concentrations the utilization of14C-labelled photosynthates seem to be hampered. Plant roots influenced the transformation of glucose and crop residues and the effect was attributed to plant-induced changes in mineral nutrient status. The mechanisms of this process and the consequences are discussed.A number of areas for future research are identified, including the potentials for manipulating rhizodeposition.

Turnover of root-derived material and related microbial biomass formation in soils of different texture

Wheat plants were grown on two soils of different texture, a sandy soil and a silty clay loam, in an atmosphere containing 14CO2. The 14C and total C content of the shoots, roots, soil rhizosphere CO2 and soil microbial biomass were measured 21, 28, 35 and 42 days after germination. There was a pronounced effect of soil texture on the turnover of root-derived C through the microbial biomass. Turnover was relatively fast and at a constant rate in the sandy soil but slowed down in the clay soil, following an initial high assimilation of root products into the microbial biomass. Four percent of the total fixed 14C was retained in the clay loam after 6 weeks compared with a corresponding value of 1.2% for the sandy soil. The proportion of fixed 14C recovered as rhizosphere CO2 at each of the sampling times was relatively constant for the sandy soil (ca 19%) but decreased from 17% at day 28 to 11% at day 42 in the clay soil. The proportion of total fixed 14C in the soil biomass as measured by a fumigation technique increased to a maximum value of 20% after 6 weeks in the sandy soil but decreased in the clay soil from 86% at day 21 to 26% after 42 days plant growth.

Assimilate translocation to the rhizosphere of two wheat lines and subsequent utilization by rhizosphere microorganisms at two soil nitrogen concentrations.

Abstract The transport of photosynthates from the leaves to the rhizosphere of wheat and incorporation in the soil microbial biomass was investigated by growing plants in a continuously 14C-labelled atmosphere. Two different varieties of wheat (C-R5B and C-R5D) were used and two nitrogen concentrations in soil were applied. At the high N relatively more 14C was released by the roots as indicated by higher percentages of the translocated 14C found in root-soil respiration and soil residue. The amount of 14C-labelled microbial biomass in the soil, as welt as rhizosphere bacterial numbers, was significantly higher at the high N. It would appear that most of the increase in microbial biomass 14C resulted from a higher exudation rate at the higher N. However, in response to a higher N concentration the relative increase in 14C-labelled microbial biomass was larger than the relative increase in release of 14C-labelled C from the roots. This indicated that the root exudates were more efficiently used at the high N. No differences in soil microbial biomass or rhizosphere bacterial numbers were found between the two wheat lines. While the higher N treatment stimulated the decomposition and microbial utilization of root-released materials it appeared to have a negative effect on the decomposition of native soil organic matter since the rate of respiration of unlabelled C from the soil decreased.

Rhizosphere microbial community and its response to plant species and soil history

The plant rhizosphere is a dynamic environment in which many parameters may influence the population structure, diversity and activity of the microbial community. Two important factors determining the structure of microbial community present in the vicinity of plant roots are plant species and soil type. In the present study we assessed the structure of microbial communities in response to four plant species (i.e. maize (Zea mays L.), oat (Avena sativa L.), barley (Hordeum vulgare L.) and commercial grass mix) planted in soil with different land use history (i.e. arable land under crop rotation, maize monoculture and permanent grassland). Both factors, plant species and land use history, showed clear effects on microbial community and diversity as determined by PCR-DGGE fingerprinting with universal and group-specific bacterial primers. Moreover, we explored the rhizosphere effect of these plant species on the abundance of bacterial antagonists of the potato pathogen Rhizoctonia solani AG3. The data showed that the abundance and taxonomic composition of antagonists differed clearly between the different plants. The highest percentages of antagonists were found in maize and grass rhizosphere. When antagonistic Pseudomonas populations were compared, the highest, abundance and diversity of antagonists were detected in barley and oat rhizospheres, as compared to maize and grass rhizosphere. The results obtained in our study demonstrate clearly that plant species and soil type are two important factors affecting the structure of total bacterial, Pseudomonas and Bacillus community.

14C pulse-labelling of field-grown spring wheat: An evaluation of its use in rhizosphere carbon budget estimations

Abstract Quantitative data on rhizodeposition under ecologically realistic conditions are scarce. Yet they are necessary to understand various aspects of soil organic matter dynamics. To evaluate the use of 14C pulse-labelling for rhizosphere carbon budget estimations and to develop a standard labelling procedure, the dynamics of 14C partitioning and factors affecting the representativity of the assimilated 14C for the average daily assimilation were investigated. Field-grown spring wheat plants were pulse-labelled with 14C at five different development stages between elongation and dough ripening. Allocation of 14C in shoot tissue and soil-root respiration was complete by day 19 after labelling. The distribution of net fixed 14C was not affected by the time of day when labelling was performed. Therefore, net assimilated 14C was representative for the average daily net assimilation. The proportion of net fixed 14C recovered in the shoot increased from 61% at elongation to 85% at dough ripening. In the roots this proportion decreased from 15 to 2% and in soil-root respiration from 14 to 7%, while in the soil organic C the percentage did not change with the development stage. 14C in roots and soil organic C decreased exponentially with depth. We can conclude that 14C pulse-labelling of wheat plants with an allocation period of about 3 weeks is a satisfactory method to estimate assimilate distribution at different development stages.