K.B. Zwart

Relationships between habitable pore space, soil biota and mineralization rates in grassland soils.

Abstract The hypotheses that the accessible soil pore volume determines the biomass of bacteria and their grazers, and that the activity of bacteria and the mineralization rates of C and N are affected by grazing pressure on bacteria were tested. We determined the biomass of bacteria, fungi, protozoa and nematodes, the pore-size distribution, and the potential mineralization rates of C and N in grassland soils with different textures. Bacteria constituted by far the largest biomass pool. Fungi, protozoa and nematodes together comprised only 10% of the total biomass. It was found that in loams and clays, most pores had diameters Bacterial activity [measured as the frequency of dividing-divided cells (FDDC); the number of viable cells; and the amount of CO 2 produced per cell] were not affected by grazing intensity. The amount of N mineralized bacterium −1 , however, was much higher in soils with a high grazing pressure of bacterivorous nematodes and flagellates than in soils with a low grazing pressure of these groups. This indicates that grazing of bacteria by bacterivorous nematodes and flagellates may considerably increase N mineralization. No relationship was found between the grazing pressure of amoebae and the amount of N mineralized bacterium −1 .

Relationships between soil texture, physical protection of organic matter, soil biota, and C and N mineralization in grassland soils.

Abstract The effect of soil type on carbon (C) and nitrogen (N) mineralization rates in grassland soils was investigated along with the physical and biological soil characteristics that may have caused the observed differences in mineralization rates between soil types. The percentage of mineralized organic N was higher in sandy soils than in loams and clays; this was not observed for C. In loams and clays small pores constituted a higher percentage of the total pore space than in sandy soils. Two mechanisms of physical protection of organic N were distinguished. In clay soils physical protection of organic material by its location in small pores was the main mechanism. In sandy soils, however, organic material was protected by its association with clay particles. In loams both mechanisms played a role. The protected organic material associated with clay particles consisted of amorphous undefined material that did not stain with acridine orange, indicating a high degree of decomposition, while the non-protected organic material present in the sand fraction consisted of plant debris that stained intensely with acridine orange. Physically protected organic matter had a lower C/N ratio than organic matter that was not physically protected. Grazing pressure on bacteria by bacterivorous nematodes was higher in sandy soils than in loams and clays. This coincided with a higher N mineralization rate per bacterium. The C/N ratio of the microbial biomass was higher in sandy soils than in loams and clays and was positively correlated with the N mineralization rate per unit of microbial biomass N. This is in agreement with the concepts of food webs that N mineralization is positively correlated with the C/N ratio of the consumer (bacteria) for a given sol|C/N ratio of the substrate (organic matter). It is not yet clear which of the factors investigated are the most important in determining N mineralization rates in grassland soils.

Dynamics of microorganisms, microbivores and nitrogen mineralisation in winter wheat fields under conventional and integrated management☆

Abstract To reduce environmental problems, integrated farming has been proposed, which may involve a considerable reduction of fertiliser-N input. A reduced fertiliser-N input must be compensated for by a higher N mineralisation from organic matter. To reduce losses and to facilitate optimal use of the N mineralised for crop growth, knowledge of the effects of management on soil orgaisms and on their role in N cycling is needed. Therefore, biomass and activity of bacteria, biomasses of fungi, bacterivorous amoebae, flagellates and nematodes, and in situ N mineralisation were monitored during a full year in a winter wheat field under conventional management (CONV) and integrated management (INT). Fungal biomass was about 100-fold lower than bacterial biomass. The average bacterial biomass was not significantly higher in INT than in CONV, whereas amoebae and nematodes were 64% and 22% higher, respectively. Average N mineralisation was 30% higher in INT. The differences are attributed to the approximately 30% higher organic matter content of INT. Bacterial biomass and frequency of dividing-divided cells (FDDC) were relatively low in December and January, probably owing to temperatures just above 0°C. At about 5°C in February and March, relatively high FDDC values and a doubling of bacteria occurred. During summer, FDDC values were relatively low and bacterial numbers were stable, probably owing to nutrient limitation. After harvest and skim ploughing, rapid increases in FDDC and bacteria were found. In the non-fumigated INT field, protozoan peaks coincided with the bacterial peak, whereas in CONV bacterivorous fauna were drastically reduced by soil fumigation. Nevertheless, the bacterial peaks were similar in CONV and INT, indicating that bacteria were not controlled by bacterivores. Nitrogen mineralisation was relatively low in winter. The increased bacterial growth in February and March, and in September appeared to enhance immobilisation rather than mineralisation of N. During the growing season from April to the end of August, bacterial growth was relatively low and N mineralisation was relatively high. This probably resulted from bacterivore feeding and from substrate- or nutrient-limited bacteria with a low growth efficiency. Considerable mineralisation rates after harvest confirmed the need for measures to stimulate immobilisation during periods without crop uptake.

Population dynamics in the belowground food webs in two different agricultural systems

Abstract The biomass of 17 different groups of organisms was established every 6 weeks during 1 year in two arable fields cropped to winter wheat; one field was under conventional management (CONV) and the other under integrated management (INT). Bacteria showed the highest average biomass, followed by earthworms (INT only) and amoebae. Most of the groups of organisms had higher biomasses in INT than in CONV. The difference was statistically significant for protozoans, bacterivorous, fungivorous, and phytophagous nematodes and earthworms. Predatory Collembola, cryptostigmatic and bacterivorous mites, and enchytraeids showed a smaller biomass in INT than in CONV. The annual biomass production for each group was estimated using simulation model calculations. Bacteria showed the highest production followed by amoebae and earthworms (INT only). Most of the groups showed a higher biomass production in INT than in CONV. Exceptions were predatory and nematophagous mites, predatory and omnivorous Collembola, and enchytraeids. The total annual production was approximately 32 kg C ha−1 cm−1 depth in CONV and approximately 57 kg C in INT. The population dynamics were analysed by hierarchical cluster analysis. Four different clusters were found in CONV and INT. Bacteria, fungi, protozoans, bacterivorous nematodes and predatory mites showed the same trend in population dynamics in CONV and INT. All other groups showed different population dynamics in CONV and INT. This observation and the composition of these clusters suggested different conditions in CONV and INT.

Simulation of dynamics in nitrogen mineralisation in the belowground food webs of two arable farming systems

Food web dynamics in a conventional (high-input) and an integrated (reduced-input) arable farming system were modelled to simulate the dynamics in N mineralisation during 1 year under winter wheat. The simulated N mineralisation rates were compared with the observed in situ N mineralisation rates. In the lower depth layers (10–25 cm) the simulated rates matched the observed rates better than in the upper depth layers (0–10 cm). Declines in N mineralisation were better matched than peaks in N mineralisation. The food web model simulated net N immobilisation in the conventional practice and net N mineralisation in the integrated practice for the period following harvest, which was combined with the addition of crop residues and tillage, and in the conventional practice also with soil fumigation. These simulated rates were in agreement with the observed rates. The results indicate that in the investigated arable soils, N mineralisation depended strongly on bacteria decomposing soil organic matter and microbivores, especially protozoans, releasing N from the bacterial biomass.

Does the combination of biochar and clinoptilolite enhance nutrient recovery from the liquid fraction of biogas digestate

ABSTRACT Concentrating nutrients on biochar and clinoptilolite and subsequently using the nutrient-enriched sorbents as a fertiliser could be an alternative way to manage nutrients in digestate. In this study, we investigated the use of biochar and clinoptilolite columns in removing ammonium, potassium, orthophosphate and dissolved organic carbon (DOC) from the liquid fraction of digestate. Our objectives were to investigate the effect of the initial loading ratio between liquid and biochar on nutrient removal, and to investigate the effect of combining biochar with clinoptilolite on nutrient and DOC removal efficiency. Increasing the initial loading ratios increased nutrient concentrations on biochar to 8.61 mg NH4-N g−1, 1.95 mg PO4-P g−1 and 13.01 mg DOC g−1, but resulted in decreasing removal efficiencies. The combination of biochar and clinoptilolite resulted in improved ammonium, potassium and DOC removal efficiencies compared to biochar alone, but did not significantly change PO4-P removal efficiencies. Removal efficiencies with combined sorbents were up to 67% for ammonium, 58% for DOC and 58% for potassium. Clinoptilolite showed higher removal efficiencies compared to biochar alone, and combining clinoptilolite with biochar improved only total P removal efficiency. Concentrating nutrients with clinoptilolite and biochar may be an option when both sorbents are available at low cost.