Jakob Magid

Soil surface CO2 flux as an index of soil respiration in situ: A comparison of two chamber methods

Predictions of global climate change have recently focused attention on soils as major sources and sinks for atmospheric CO2, and various methodologies exist for measuring soil surface CO2 flux. A static (passive CO2 absorption in an alkali trap over 24 h) and a dynamic (portable infra-red CO2 gas analyzer over 1–2 min) chamber method were compared. Both methods were used for 100 different site × treatment × time combinations in temperate arable, forest and pasture ecosystems. Soil surface CO2 flux estimates covered a wide range from 0 to ca. 300 mg CO2C m−2 h−1 by the static method and from 0 to ca. 2500 mg CO2C m−2 h−1 by the dynamic method. The relationship between results from the two methods was highly non-linear, and was best explained by an exponential equation. When compared to the dynamic method, the static method gave on average 12% higher flux rates below 100 mg CO2C m−2 h−1, but much lower flux rates above 100 mg CO2C m−2 h−1. Spatial variability was large for both methods, necessitating a large number of replicates for reliable field data, with typical coefficients of variation being in the range 10–60%, usually higher with the dynamic than the static method. Diurnal variability in soil surface CO2 flux was partly correlated with soil temperature, whereas day-to-day variability was more unpredictable. However, use of a mechanistic simulation model of CO2 transport in soil, SOILCO2, showed that very large day-to-day changes in soil surface CO2 flux can result from rainfall events causing relatively small changes in soil water content above field capacity (ca. −10 kPa), even if CO2 production rates remained relatively unaffected.

Temporal variation of C and N mineralization, microbial biomass and extractable organic pools in soil after oilseed rape straw incorporation in the field

Abstract The temporal variation of soil microbial biomass C and N, extractable organic C and N, mineral N and soil-surface CO2 flux in situ in two arable soils (a sandy loam and a coarse sandy soil) was examined periodically for a full year after field incorporation of 0, 4 or 8 t dry mass ha−1 of oilseed rape straw in late summer. Both unlabelled and 15N-labelled straw were applied. Soil-surface CO2 flux, used as an index of soil respiration, was up to 2-fold higher in the straw-amended treatments than in the unamended treatment at both sites during the first 6–8 wk, but the general temporal pattern was mainly controlled by soil temperature and soil water content. Microbial biomass C and N increased very rapidly after the straw amendments and the 31–49% difference from the unamended treatment persisted throughout the winter. Temporal variations in soil microbial biomass C and N were only within ±13–22% of the mean at both sites and in all straw treatments over the 1 y period. Microbial biomass C-to-N ratios were not significantly different between straw treatments and were relatively constant over time. Extractable organic C and N were slightly higher in the straw-amended treatments and were higher in spring and summer than in autumn and winter. More than 90% of the added straw N could be accounted for initially and there was no loss of straw N over the winter period, in spite of a winter rainfall that was twice the 25 y average. Between 52 and 80% of the initial increase in microbial biomass N was derived from the straw N, with up to 27% of the straw N being incorporated into the microbial biomass. Rapid immobilization of soil mineral N occurred simultaneously and the sum of this and the straw-derived microbial biomass N on day 7 exceeded the total increase in microbial biomass N, indicating a very rapid turnover of microbial biomass in the first few days. Significant differences in microbial biomass C and N between the straw treatments were still found after nearly 1 y and the decay constant of straw-derived microbial biomass N was estimated to be ca. 0.26 y−1.

In search of the elusive 'active' fraction of soil organic matter: three size-density fractionation methods for tracing the fate of homogeneously 14C-labelled plant materials

Abstract To improve the understanding of nutrient cycling in soil there is a need for development of methods to quantify biologically-meaningful fractions of soil organic matter which turn over in the short or medium-term. Homogeneously 14 C-labelled shoots from ryegrass grown at ambient (350 μl l −1 ) and elevated (700 μl l −1 ) CO 2 concentrations were added to a loamy sand and incubated for up to 200 days. Three size-density methods were tested in order to elucidate the breakdown of the plant material. One approach involved density separation in Ludox TM40 (a colloidal silica suspension) but only included soil materials > 150 μm. The other two approaches in which sodium polytungstate was used as density agent included all solid and soluble soil material. One of these involved a size separation (at 100 μm) prior to density separation, while the other was performed on whole soil. Density fractionation in a centrifuge (10,000 g ) without initial size-separation substantially reduced the recovery of freshly-added plant material in the light fraction. We assume that this was partly due to the loss of air entrapped in intact tissue during centrifugation, and partly due to interactions between small heavy particles and the large light plant material. Fractionation by size and density thus seems a more powerful approach for separating soil organic matter fractions than fractionation based on density alone. Separation of finer textured materials ( 14 C-enriched fraction and contained a substantial amount of 14 C throughout the incubation. The large, light fractions consisted of identifiable plant residues and were enriched in 14 C during the 200 day incubation. Subdivision of the large fraction by density resulted in fractions with considerably different initial enrichment, presumably due to greater airfilled porosity in less decomposed or frayed materials. Losses of “native” soil carbon were small, compared with the analytical uncertainties, and thus the identification of active “native” soil fractions was hampered. Differences in the decomposition patterns between ryegrass grown at ambient and elevated CO 2 concentrations, measured by CO 2 respiration after 10 days, were observed with the large (> 150 μm) light Ludox fractions. At the end of the experiment no differences between plant material grown at ambient and elevated CO 2 concentrations were detected in earlier CO 2 evolution or in the different soil organic matter fractions. Mineralization of C from previously leached plant materials was considerably enhanced by exposure to Ludox and retarded by exposure to sodium polytungstate.

Urban nutrient balance for Bangkok

To enhance agricultural sustainability, former linkages between agriculture and urban waste production should be reintroduced. Therefore, to explore the options for recycling of nutrients from mega-cities, a nutrient balance model was developed. The parameterization were established for the Bangkok Province and considers nitrogen (N) and phosphorous (P). To model the food supply, an online database (FAOSTAT) estimating supply at country levels, was employed. It is argued that desaggregation to urban level is reasonable after adjustments for different economy in Bangkok than the average in Thailand. The balance shows that only a small fraction of nutrients are recovered, currently about 7 and 12% respectively, of the amount of N and P in the total food supply. On the other hand most (about 95%), of the total loss of N can be accounted for by elevated N levels in the Chao Phraya River from where also much (about 38%) of the loss of P can be explained. That is, in- and out-flows of N is almost found in balance but a huge amount of P must be accumulated somewhere. However the balance also shows that the Bangkok Province throws out into the river (and the sea) very huge quantities of plant nutrients that could be recovered and reused. For future research is it of particular interest to explore the maximum nutrient recovery fraction in different waste management systems.

Seasonal variation in organic and inorganic phosphorus fractions of temperate-climate sandy soils

Soils from an arable plot, a grassland plot and pasture plot were sampled over an 18-month period. Inorganic (Pi) and organic (Po) soil phosphorus fractions were extracted sequentially with resin, NaHCO3, and NaOH. Soil solution was sampled on the arable plot and pasture plot during 12 months with teflon suction cups, and the contents of Pi and Po were determined.The patterns of the variation for all soil fractions were similar for the three plots. All soil Pi fractions were at minimum in the cool moist winter period. The soil Po fractions varied less systematically than Pi fractions. The sum of Po fractions had a winter maximum and a spring minimum. For all soil P fractions temporal variation was highly significant (p<0.0001). The magnitude of change in Pi and Po soil fractions was 4–40 times greater than what would be expected from the magnitude of new N mineralization.The content of P in the inorganic soil P fractions was negatively correlated with soil moisture. The variation in organic soil P could not be explained by any single factor, but it is suggested that the variation is caused by changes in solubility rather than by biological transformations. Thus, physicochemical processes masked the impact of biological transformations on the temporal variation of soil phosphorus fractions.Both soil solution Pi and Po varied significantly with time on field scale. In contrast to soil Pi fractions, solution Pi was initially low in the early autumn, increased by a factor 4 during the following 6 weeks, and thereafter decreased to a low level by the end of the sampling period. Soil solution Po had several fluctuations during the sampling period.

Recovering decomposing plant residues from the particulate soil organic matter fraction: size versus density separation

Abstract A detailed size separation of particulate organic matter (POM) from soils amended with straw from Hordeum vulgare or Vicia sativa revealed that the loss of C during the first 56 days of incubation mainly occurred from particles >2,000 μm, without a concomitant reduction in the size of these large particles. Preliminary studies of POM from non-amended soil had shown that the stable heavy (>1.4 g cm–3) POM fraction was mainly (>80%) composed of particles <400 μm, whereas the light fraction was dominated by larger particles (>80%). Therefore we decided to compare the POM <1.4 g cm3 with POM >400 μm. There was a very close relationship between POM>400 μm and POM <1.4 g cm–3 with regard to amounts of C and N, as well as the appearance of these fractions under the microscope. Similarly there was a close relationship between changes in the C content of the POM fractions and the CO2 respired, and this was also the case when comparing changes in POM-N with net N mineralization. This indicated that the biological activity during decomposition was actually localized in the POM. Due to the lighter workload and lower expenditure for reagents in connection with size separation of POM, we recommend the size separation procedure in connection with studies of residue decomposition in arable systems.

P depletion and activity of phosphatases in the rhizosphere of mycorrhizal and non-mycorrhizal cucumber (cucumis sativus L.)

Abstract An experiment was set up to test the ability of arbuscular mycorrhizal (AM) roots and hyphae to produce extracellular phosphatases and to study the relationship between phosphatase activity and soil organic P (Po). Non-mycorrhizal cucumber and cucumber in symbiosis with either of two mycorrhizal fungi were grown in a sandy loam-sand mixture in three-compartment pots. Plant roots were separated from two consecutively adjoining compartments, first by a 37 m mesh excluding roots and subsequently by a 0.45 m membrane excluding mycorrhizal hyphae. Soil from the two root-free compartments was sectioned in a freezing microtome and analyzed for extracellular acid (pH 5.2) and alkaline (pH 8.5) phosphatase activity as well as depletion of NaHCO3-extractable inorganic P (Pi) and Po. Roots and mycorrhizal hyphae depleted the soil of Pi but did not influence the concentration of Po in spite of increased phosphatase activity in soil influenced by roots. Phosphatase activity at both pH values was highest in soil influenced by uncolonized roots, but this was attributed to higher root length densities as compared to mycorrhizal roots. Mycorrhizal hyphae showed no influence on soil phosphatase activity in spite of high hyphal length densities (> 22 m cm−3). Hyphae were also able to deplete soil of Pi beyond the membrane interface.

Disproportionately high N-mineralisation rates from green manures at low temperatures – implications for modeling and management in cool temperate agro-ecosystems

We examined the decomposition of Medicago lupulina, Melilotus alba and Poa pratensis at 3, 9, and 25 °C during 4 weeks. There was a strong temperature effect on the rate of CO2 evolution, and thus the extent of energy exhaustion from the added substrates. However, there was no concomitant retardation of N mineralisation at low temperatures. In the analysis of variance of mineralized N the residue type gave a 10 times larger contribution to the regression than the temperature (T), whereas for CO2 evolution residue type and temperature were equally important contributors. This indicates that although the temperature has a statistically significant effect on N-mineralisation it is substantially less than compared with the effect on carbon mineralisation in the materials examined. The retardation of carbon mineralisation was least strong in Melilotus alba that had a relatively low cellulose content, and a higher content of low molecular compounds. Though more research will be necessary to consolidate and explain this phenomena, it is likely that an important factor is a decrease in the bioavailability of C-rich polymers at low temperatures, and thus a preferential utilization of N-rich low molecular substances. Nitrification was not effectively deterred at 3 °C. Thus, in terms of management, it is pertinent to reconsider the timing of green manure and catch crop incorporation in cool temperate climate regions, since the rapid release of nitrogen, coupled with the relatively undeterred nitrification may result in a high N leaching risk by early incorporation, but a low risk for N immobilization at late incorporation, if N rich residues are used.

Decomposition of white clover (Trifolium repens) and ryegrass (Lolium perenne) components: C and N dynamics simulated with the DAISY soil organic matter submodel

Using data from a decomposition study, we aimed to test the parameterisation of the soil organic matter module of the DAISY model, and link measurable plant litter fractions (lignin, water-soluble) with the model defined plant litter pools. Shoot and root material from perennial ryegrass and white clover was incubated in a sandy loam soil at 9 °C for 94 days. Accumulated CO2 evolution, soil mineral nitrogen (N) and soil microbial biomass-N were measured during the incubation. Marked differences in decomposition rates between above- and below-ground material as well as between the two plant species were observed. The DAISY model was used to interpret the incubation results. Decomposition rates and utilisation efficiencies were modified, under the constraint that rates of specific pools were independent of the type of material, to obtain good agreement between observed and simulated values. Measurable quality parameters were evaluated against the sizes of pools in the model and measured fluxes. The size of the slowest decomposing fraction of the DAISY model was proportional to the lignin content of the plant material, but twice as large. The easily decomposable fraction in the model was well correlated with the water-soluble fraction of the plant material (r2=0.84). The size of this pool in the model was larger than the water-soluble fraction of the plant material in three of the five plant materials. The initial carbon mineralisation was correlated with water-solubility of the plant material and total mineralisation with the lignin:N ratio. Net N mineralisation was well correlated with the C:N ratio and the N content of the added material. At the end of the experiment, the mineral N content was overestimated by the DAISY model for all treatments, except one. A soil microbial residual pool, consisting of undecomposed microbial tissue is suggested as a possible N-sink during the incubation. The study demonstrated a correlation between the model-defined pools and chemical plant fractions, but also that the pools in the model were larger than their measured counterparts.

Temporal variation of C and N turnover in soil after oilseed rape straw incorporation in the field: simulations with the soil-plant-atmosphere model DAISY

Abstract The Soil Organic Matter submodel of the soil-plant-atmosphere model DAISY was evaluated using data from one year field trials with and without incorporation of 8 t ha −1 rape straw into the soil. Periodic measurements of soil microbial biomass (C and N), mineral N and light particulate soil organic matter in the top 15 cm of the soil, and of soil CO 2 -evolution were made. The simulation of the temporary pattern of soil microbial biomass and mineral N was improved markedly by systematic modification of the default turnover rate coefficients, the substrate utilization efficiencies and the initial levels of soil microbial biomass. Metabolic quotients ( q CO 2 ) turnover rates of microbial biomass and substrate utilization efficiencies derived from the parametrization of the model were evaluated against literature data. The soil microbial biomass seems to be associated with the production of temporarily protected microbial residual products. The production of these residuals might be responsible for the nitrogen immobilization observed after incorporation of rape straw. In the early stage of rape straw decomposition, measured carbon in light particulate soil organic matter seemed to be represented by one of the added organic matter pools simulated in the DAISY model. We propose that turnover rate coefficients of microbial biomass and added organic matter obtained by fitting a model to measured values may be used as a tool to characterize the physiological state of microbial populations in their natural environment.