Long-term effects of metal-containing farmyard manure and sewage sludge on soil organic matter in a fluvisol

Our aim was to establish the long-term effects of repeated applications after 20 y of organic amendments (farmyard manure at 10 t ha⁻¹ y⁻¹, and urban sewage sludge at two different rates, 10 t ha⁻¹ y⁻¹ and 100 t ha⁻¹ every 2 y) on the quality of a sandy and poorly buffered soil (Fluvisol, pH 6). Chemical characteristics and biodegradability of the labile organic matter, which is mainly derived from microbial biomass and biodegradation products of organic residues, were chosen as indicators for soil quality. The organic C content had reached a maximal value (30.6 g C kg⁻¹ in the 100 t sludge-treated soil), i.e. about 2.5 times that in the control. Six years after the last application, the organic C content and the microbial biomass content remained higher in sludge-treated soils than in the control. In contrast, the proportion of labile organic matter was significantly lower in sludge-treated soils than in manure-treated and control soils. The labile organic matter of sludge extracts appeared less humified than that of manure-treated and control soils.     

Rewetting and litter addition influence mineralisation and microbial communities in soils from a semi-arid intermittent stream

Nitrogen (N) and carbon (C) mineralisation are triggered by pulses of water availability in arid and semi-arid systems. Intermittent streams and their associated riparian communities are obvious ‘hot spots’ for biogeochemical processes in arid landscapes where water and often C are limiting. Stream landscapes are characterized by highly heterogeneous soils that may respond variably to rewetting. We used a laboratory incubation to quantify how N and C mineralisation in rewetted soils and sediments from an intermittent stream in the semi-arid Pilbara region of north-west Australia varied with saturation level and substrate addition (as ground Eucalyptus litter). Full (100%) saturation was defined as the maximum gravimetric moisture content (%) achieved in free-draining soils and sediments after rewetting, with 50% saturation defined as half this value. We estimated rates and amounts of N mineralised from changes in inorganic N and microbial respiration as CO₂ efflux throughout the incubation. In soils and sediments subject to 50% saturation, >90% of N mineralised accumulated within the first 7 d of incubation, compared to only 48% when soils were fully saturated (100% saturation). Mineralisation rates and microbial respiration were similar in riparian and floodplain soils, and channel sediments. N mineralisation rates in litter-amended soils and sediments (0.73 mg N kg⁻¹ d⁻¹) were only one-third that of unamended samples (3.04 mg N kg⁻¹ d⁻¹), while cumulative microbial respiration was doubled in litter-amended soils, suggesting N was more rapidly immobilized. Landscape position was less important in controlling microbial activity than soil saturation when water-filled pore space (% WFPS) was greater than 40%. Our results suggest that large pulses of water availability resulting in full soil saturation cause a slower release of mineralisation products, compared to small pulse events that stimulate a rapid cycle of C and N mineralisation-immobilization.     

Electron donors and acceptors influence anaerobic soil organic matter mineralization in tidal marshes

Anaerobic decomposition in wetland soils is carried out by several interacting microbial processes that influence carbon storage and greenhouse gas emissions. To understand the role of wetlands in the global carbon cycle, it is critical to understand how differences in both electron donor (i.e., organic carbon) and terminal electron acceptor (TEA) availability influence anaerobic mineralization of soil organic matter. In this study we manipulated electron donors and acceptors to examine how these factors influence total rates of carbon mineralization and the pathways of microbial respiration (e.g., sulfate reduction versus methanogenesis). Using a field-based reciprocal transplant of soils from brackish and freshwater tidal marshes, in conjunction with laboratory amendments of TEAs, we examined how rates of organic carbon mineralization changed when soils with different carbon contents were exposed to different TEAs. Total mineralization (the sum of CO₂ + CH₄ produced) on a per gram soil basis was greater in the brackish marsh soils, which had higher soil organic matter content; however, on a per gram carbon basis, mineralization was greater in the freshwater soils, suggesting that the quality of carbon inputs from the freshwater plants was higher. Overall anaerobic metabolism was higher for both soil types incubated at the brackish site where SO₄²⁻ was the dominant TEA. When soils were amended with TEAs in the laboratory, more thermodynamically favorable respiration pathways typically resulted in greater organic matter mineralization (Fe(III) respiration > SO₄²⁻ reduction > methanogenesis). These results suggest that both electron donors and acceptors play important roles in regulating anaerobic microbial mineralization of soil organic matter.     

Measuring rates of gross and net mineralisation of organic phosphorus in soils

The isotopic dilution method developed by Oehl et al. [2001b. Organic phosphorus mineralisation studies using isotopic dilution techniques. Soil Science Society of America Journal 65, 780-787] to measure gross mineralisation of soil organic phosphorus (P) was tested on a range of low-P sorbing soils. This isotopic dilution method relies on accurate prediction of radiolabel behaviour due to soil physicochemical processes. Based on experimental validation of the extrapolation for isotopic dilution due to physicochemical processes using autoclaved soils, a simple power function was used for extrapolation rather than the more complex equation used in the original method. For several soils, however, a potential overestimation of gross mineralisation by 0.1-2.0 mg P kg⁻¹ d⁻¹ was revealed. In addition, the detection limit of P mineralisation ranged between 0.6 and 2.6 mg P kg⁻¹ d⁻¹. The method is likely to be at the detection limit for soils that are high in available P and low in biological activity. The method was modified with respect to the extrapolation and successfully applied to a soil with relatively high microbial P (18 mg P kg⁻¹) and soil respiration rates (29 mg C kg⁻¹ d⁻¹), revealing gross mineralisation rates of organic P of 0.9-1.2 mg P kg⁻¹ d⁻¹. Measurement of uptake of ³²P by the microbial biomass allowed derivation of a net organic P mineralisation rate of 0.5-0.9 mg P kg⁻¹ d⁻¹.     

Humus accumulation and microbial activities in calcari-epigleyic fluvisols under grassland and forest diked in for 30 years

The accumulation and transformation of organic matter during soil development is rarely investigated although such processes are relevant when discussing about carbon sequestration in soil. Here, we investigated soils under grassland and forest close to the North Sea that began its genesis under terrestrial conditions 30 years ago after dikes were closed. Organic C contents of up to 99 mg g⁻¹ soil were found until 6 cm soil depth. The humus consisted mainly of the fraction lighter than 1.6 g cm⁻³ which refers to poorly degraded organic carbon. High microbial respiratory activity was determined with values between 1.57 and 1.17 μg CO₂-C g⁻¹ soil h⁻¹ at 22 °C and 40 to 70% water-holding capacity for the grassland and forest topsoils, respectively. The microbial C to organic C ratio showed values up to 20 mg C_mic g⁻¹ C_org. Although up to 2.69 kg C m⁻² were estimated to be sequestered during 30 years, the microbial indicators showed intensive colonisation and high transformation rates under both forest and grassland which were higher than those determined in agricultural and forest topsoils in Northern Germany.     

Biochemical characterization of urban soil profiles from Stuttgart, Germany

The knowledge of biochemical properties of urban soils can help to understand nutrient cycling in urban areas and provide a database for urban soil management. Soil samples were taken from 10 soil profiles in the city of Stuttgart, Germany, differing in land use—from an essentially undisturbed garden area to highly disturbed high-density and railway areas. A variety of soil biotic (microbial biomass, enzyme activities) and abiotic properties (total organic C, elemental C, total N) were measured up to 1.9 m depth. Soil organic matter was frequently enriched in the subsoil. Microbial biomass in the top horizons ranged from 0.17 to 1.64 g C kg⁻¹, and from 0.01 to 0.30 g N kg⁻¹, respectively. The deepest soil horizon at 170-190 cm, however, contained 0.12 g C kg⁻¹ and 0.05 kg N kg⁻¹ in the microbial biomass. In general, arylsulphatase and urease activity decreased with depth but in three profiles potentially mineralizable N in the deepest horizons was higher than in soil layers directly overlying. In deeply modified urban soils, subsoil beside topsoil properties have to be included in the evaluation of soil quality. This knowledge is essential because consumption of natural soils for housing and traffic has to be reduced by promoting inner city densification.     

Humic acids as electron acceptors in wetland decomposition

Decomposition of organic matter in inundated wetland soils requires a number of interdependent microbial processes that ultimately generate CO₂ and CH₄. Largely as the result of anaerobic decomposition, wetland soils store globally significant amounts of organic carbon and are currently net sources of CH₄ to the atmosphere. Given the importance of wetlands in the global carbon cycle, it is important to understand controls on anaerobic decomposition in order to predict feedbacks between wetland soils and global climate change. One perplexing pattern observed in many wetland soils is the high proportion of CO₂ resulting from anaerobic decomposition that cannot be explained by any measured pathway of microbial respiration. Recent studies have hypothesized that humic substances, and in particular solid-phase humic substances in wetland soils, can support anaerobic microbial respiration by acting as organic electron acceptors. Humic substances may thus account for much of the currently unexplained CO₂ measured during decomposition in wetland soils. Here we demonstrate that humic acids extracted from a variety of wetland soils act as either electron donors or electron acceptors and alter the ratio of CO₂:CH₄ produced during anaerobic laboratory incubations. Our results suggest that soil-derived humic substances may play an important, and currently unexplored, role in anaerobic decomposition in wetland soils.     

Basal organic phosphorus mineralization in soils under different farming systems

Soil organic P (P_o) mineralization plays an important role in soil P cycling. Quantitative information on the release of available inorganic P (P_i) by this process is difficult to obtain because any mineralized P_i gets rapidly sorbed. We applied a new approach to quantify basal soil P_o mineralization, based on ³³PO₄ isotopic dilution during 10 days of incubation, in soils differing in microbiological activity. The soils originated from a 20 years old field experiment, including a conventional system receiving exclusively mineral fertilizers (MIN), a bio-organic (ORG) and bio-dynamic (DYN) system. Indicators of soil microbiological activity, such as size and activity of the soil microbial biomass and phosphatase activity, were highest in DYN and lowest in MIN. In order to assess P_o hydrolysis driven by phosphatase in sterile soils, a set of soil samples was γ-irradiated. Basal P_o mineralization rates in non-irradiated samples were between 1.4 and 2.5 mg P kg⁻¹ day⁻¹ and decreased in the order DYN>ORG≥MIN. This is an amount lower, approximately equivalent to, or higher than water soluble P_i of MIN, ORG and DYN soils, respectively, but in every soil was less than 10% of the amount of P isotopically exchangeable during one day. This shows that physico-chemical processes are more important than basal mineralization in releasing plant available P_i. Organic P mineralization rates were higher, and differences between soils were more pronounced in γ-irradiated than in non-irradiated soils, with mineralization rates ranging from 2.2 to 4.6 mg P kg⁻¹ day⁻¹. These rates of hydrolysis, however, cannot be compared to those in non-sterile soils as they are affected by the release of cellular compounds, e.g. easily mineralizable P_o, derived from microbial cells killed by γ-irradiation.     

Cyanobacterial inoculation of heated soils: effect on microorganisms of C and N cycles and on chemical composition in soil surface

Physiological groups of soil microorganisms, total C and N and available nutrients were investigated in four heated (350 °C, 1 h) soils (one Ortic Podsol over sandstone and three Humic Cambisol over granite, schist or limestone) inoculated (1.5 μg chlorophyll a g⁻¹ soil or 3.0 μg chlorophyll a g⁻¹ soil) with four cyanobacterial strains of the genus Oscillatoria, Nostoc or Scytonema and a mixture of them.Cyanobacterial inoculation promoted the formation of microbiotic crusts which contained a relatively high number of NH₄⁺-producers (7.4×10⁹ g⁻¹ crust), starch-mineralizing microbes (1.7×10⁸ g⁻¹ crust), cellulose-mineralizing microbes (1.4×10⁶ g⁻¹ crust) and NO₂⁻ and NO₃⁻ producers (6.9×10⁴ and 7.3×10³ g⁻¹ crust, respectively). These crusts showed a wide range of C and N contents with an average of 293 g C kg⁻¹ crust and 50 g N kg⁻¹ crust, respectively. In general, Ca was the most abundant available nutrient (804 mg kg⁻¹ crust), followed by Mg (269 mg kg⁻¹ crust), K (173 mg kg⁻¹ crust), Na (164 mg kg⁻¹ crust) and P (129 mg kg⁻¹ crust). There were close positive correlations among all the biotic and abiotic components of the crusts.Biofertilization with cyanobacteria induced great microbial proliferation as well as high increases in organic matter and nutrients in the surface of the heated soils. In general, cellulolytics were increased by four logarithmic units, amylolytics and ammonifiers by three logarithmic units and nitrifiers by more than two logarithmic units. C and N contents rose an average of 275 g C kg⁻¹ soil and 50 g N kg⁻¹ soil while the C:N ratio decreased up to 7 units. Among the available nutrients the highest increase was for Ca (315 mg kg⁻¹ soil) followed by Mg (189 mg kg⁻¹ soil), K (111 mg kg⁻¹ soil), Na (109 mg kg⁻¹ soil) and P (89 mg kg⁻¹ soil). Fluctuations of the microbial groups as well as those of organic matter and nutrients were positively correlated.The efficacy of inoculation depended on both the type of soil and the class of inoculum. The best treatment was the mixture of the four strains and, whatever the inoculum used, the soil over lime showed the most developed crust followed by the soils over schist, granite and sandstone. In the medium term there were not significant differences between the two inocula amounts tested.These results showed that inoculation of burned soils with alien N₂-fixing cyanobacteria may be a biotechnological means of promoting microbiotic crust formation, enhancing C and N cycling microorganisms and increasing organic matter and nutrient contents in heated soils.     

Microbial community composition and substrate use in a highly weathered soil as affected by crop rotation and P fertilization

A better understanding of soil microbial processes is required to improve the synchrony between nutrient release from plant residues and crop demand. Phospholipid fatty acid analysis was used to investigate the effect of two crop rotations (continuous maize and maize-crotalaria rotation) and P fertilization (0 and 50 kg P ha⁻¹ yr⁻¹, applied as triple superphosphate) on microbial community composition in a highly weathered soil from western Kenya. Microbial substrate use in soils from the field experiment was compared in incubation experiments. Higher levels of soil organic matter and microbial biomass in the maize-crotalaria rotation were connected with higher total amounts of phospholipid fatty acids and an increase in the relative abundances of indicators for fungi and gram-negative bacteria. P fertilization changed the community profile only within the continuous maize treatment. The decomposition of glucose, cellulose and three plant residues (all added at 2.5 g C kg⁻¹ soil) proceeded faster in soil from the maize-crotalaria rotation, but differences were mostly transient. Microbial P and N uptake within one week increased with the water-soluble carbon content of added plant residues. More P and N were taken up by the greater microbial biomass in soil from the maize-crotalaria rotation than from continuous maize. Re-mineralization of nutrients during the decline of the microbial biomass increased also with the initial biological activity of the soil, but occurred only for a high quality plant residue within the half year incubation period. Compared to the effect of crop rotation, P fertilization had a minor effect on microbial community composition and substrate use.     

Do plant clumps constitute microbial hotspots in semiarid Mediterranean patchy landscapes?

Three semiarid Mediterranean patchy landscapes were investigated to test the existence of a microsite effect (i.e. plant canopy vs. inter-canopy) on soil microbial communities. Surface soil samples were independently taken from both microsites under naturally changing conditions of humidity and temperature through the year. In gypsiferous soils covered with a shrub steppe, improved physical and chemical soil properties were registered underneath the plant canopy, where the densest and most active microbial communities were also detected (e.g. microbial biomass C averaged 531 and 202 mg kg⁻¹ in canopy and inter-canopy areas, respectively). In calcareous perennial tussock grasslands, either growing on soils over limestones or alluvial deposits, the microsite effect was not so marked. Soil humidity, temperature and total organic C were homogeneously distributed over the landscape conditioning their uniform microbial activity under field moisture conditions (ATP content averaged 853 and 885 nmol kg⁻¹ in canopy and intercanopy areas, respectively). However, readily mineralizable C and microbial biomass C were preferentially accumulated in soils underneath the tussocks determining their larger potential microbial activity (e.g. C hydrolysis capacity under optimal conditions). In conclusion, plant clumps either functioned as microbial hotspots where enhanced microbially driven ecosystem processes took place or as microbial banks capable of undergoing a burst of activity under favourable climatic conditions. Our results provide experimental evidence of a non-patchy distribution of certain soil microbial properties in semi-arid Mediterranean patchy ecosystems.     

Emission of CO2, CH4 and N2O from lakeshore soils in an Antarctic dry valley

We measured soil profile concentrations and emission of CO₂, CH₄ and N₂O from soils along a lakeshore in Garwood Valley, Antarctica, to assess the extent and biogeochemical significance of biogenic gas emission to C and N cycling processes. Simultaneous emission of all three gases from the same site indicated that aerobic and anaerobic processes occurred in different layers or different parts of each soil profile. The day and location of high gas concentrations in the soil profile corresponded to those having high gas emission, but the pattern of concentration with depth in the soil profile was not consistent across sites. That the highest gas concentrations were not always in the deepest soil layer suggests either limited production or gas diffusion in the deeper layers. Emission of CO₂ was as high as 47 μmol m⁻² min⁻¹ and was strongly related to soil temperature. Soil respiration differed significantly according to location on the lakeshore, suggesting that factors other than environmental variables, such as the amount and availability of O₂ and nutrients, play an important role in C mineralization processes in these soils. High surface emission (maximum: 15 μmol m⁻² min⁻¹) and profile gas concentration (maximum: 5780 μL L⁻¹) of CH₄ were at levels comparable to those in resource-rich temperate ecosystems, indicating an active indigenous population of methanogenic organisms. Emission of N₂O was low and highly variable, but the presence of this gas and NO₃ in some of the soils suggest that denitrification and nitrification occur there. No significant relationships between N₂O emission and environmental variables were found. It appears that considerable C and N turnover occurs in the lakeshore soils, and accurate accounting will require measurements of aerobic and anaerobic mineralization. The production and emission of biogenic gases confirm the importance of these soils as hotspots of biological activity in the dry valleys and probable reservoirs of biological diversity.     

Responses of soil organic matter and greenhouse gas fluxes to soil management and land use changes in a humid temperate region of southern Europe

We studied the effects of soil management and changes of land use on soils of three adjacent plots of cropland, pasture and oak (Quercus robur) forest. The pasture and the forest were established in part of the cropland, respectively, 20 and 40 yr before the study began. Soil organic matter (SOM) dynamics, water-filled pore space (WFPS), soil temperature, inorganic N and microbial C, as well as fluxes of CO₂, CH₄ and N₂O were measured in the plots over 25 months. The transformation of the cropland to mowed pasture slightly increased the soil organic and microbial C contents, whereas afforestation significantly increased these variables. The cropland and pasture soils showed low CH₄ uptake rates (<1 kg C ha⁻¹ yr⁻¹) and, coinciding with WFPS values >70%, episodes of CH₄ emission, which could be favoured by soil compaction. In the forest site, possibly because of the changes in soil structure and microbial activity, the soil always acted as a sink for CH₄ (4.7 kg C ha⁻¹ yr⁻¹). The N₂O releases at the cropland and pasture sites (2.7 and 4.8 kg N₂O-N ha⁻¹ yr⁻¹) were, respectively, 3 and 6 times higher than at the forest site (0.8 kg N₂O-N ha⁻¹ yr⁻¹). The highest N₂O emissions in the cultivated soils were related to fertilisation and slurry application, and always occurred when the WFPS >60%. These results show that the changes in soil properties as a consequence of the transformation of cropfield to intensive grassland do not imply substantial changes in SOM or in the dynamics of CH₄ and N₂O. On the contrary, afforestation resulted in increases in SOM content and CH₄ uptake, as well as decreases in N₂O emissions.     

Surface and subsurface organic carbon, microbial biomass and activity in a forest soil sequence

The distribution of organic carbon, microbial biomass and activity, from the surface down to 70 cm, was investigated through three semiarid Mediterranean soils: (1) a Typic Calcixeroll covered with a native pinewood (NP), (2) a Typic Calcixerept under a mature pine plantation (PP) on abandoned agricultural terraces and (3) a Typic Haploxerept under a grassland (GS). NP and GS had the highest and lowest soil organic carbon (SOC) pools, respectively. Both of them had decreasing SOC contents with depth. PP, which held intermediate SOC levels, showed an increase in total organic C and humic substances C with depth due to their mineralization in the anciently ploughed topsoil layer. The soils were similarly ranked as regards their microbial biomass and activity: NP>PP>GS. In general, the microbial communities were less dense and active towards the deeper horizons. More specifically, PP and GS had a very populated and active top 20-cm layer, which was attributed to the dense root system of their grass cover. NP maintained high microbial biomass and activity levels from 0 to 70 cm, progressively diminishing along with shrub root density (e.g. microbial biomass C dropped from 2342 to 394 mg kg⁻¹ soil). The latter soil presented the sharpest drop of its microbial properties with depth, what was considered an indicator of its quality. Generally decreasing patterns of microbial biomass and activity were not always coincident with previously published gradients of microbial metabolic abilities and genetic structure. This reinforces the need of combining biomass, activity and biodiversity measurements if the ecosystem's functioning is to be fully understood and a real monitoring of degradation processes and restoration strategies is to be achieved.     

The fate of catechol in soil as affected by earthworms and clay

The effect of endogeic earthworms (Octolasion tyrtaeum) and the availability of clay (Montmorillonite) on the mobilization and stabilization of uniformly ¹⁴C-labelled catechol mixed into arable and forest soil was investigated in a short- and a long-term microcosm experiment. By using arable and forest soil the effect of earthworms and clay in soils differing in the saturation of the mineral matrix with organic matter was investigated. In the short-term experiment microcosms were destructively sampled when the soil had been transformed into casts. In the long-term experiment earthworm casts produced during 7 days and non-processed soil were incubated for three further months. Production of CO₂ and ¹⁴CO₂ were measured at regular intervals. Accumulation of ¹⁴C in humic fractions (DOM, fulvic acids, humic acids and humin) of the casts and the non-processed soil and incorporation of ¹⁴C into earthworm tissue were determined.Incorporation of ¹⁴C into earthworm tissue was low, with 0.1 and 0.44% recovered in the short- and long-term experiment, respectively, suggesting that endogeic earthworms preferentially assimilate non-phenolic soil carbon. Cumulative production of CO₂-C was significantly increased in casts produced from the arable soil, but lower in casts produced from the forest soil; generally, the production of CO₂-C was higher in forest than in arable soil. Both soils differed in the pattern of ¹⁴CO₂-C production; initially it was higher in the forest soil than in the arable soil, whereas later the opposite was true. Octolasion tyrtaeum did not affect ¹⁴CO₂-C production in the forest soil, but increased it in the arable soil early in the experiment; clay counteracted this effect. Clay and O. tyrtaeum did not affect integration of ¹⁴C into humic fractions of the forest soil. In contrast, in the arable soil O. tyrtaeum increased the amount of ¹⁴C in the labile fractions, whereas clay increased it in the humin fraction.The results indicate that endogeic earthworms increase microbial activity and thus mineralization of phenolic compounds, whereas clay decreases it presumably by binding phenolic compounds to clay particles when passing through the earthworm gut. Endogeic earthworms and clay are only of minor importance for the fate of catechol in soils with high organic matter, clay and microbial biomass concentrations, but in contrast affect the fate of phenolic compounds in low clay soils.     

Zinc toxicity on N2O reduction declines with time in laboratory spiked soils and is undetectable in field contaminated soils

Denitrification assays in soils spiked with zinc salt have shown inhibition of the N₂O reduction resulting in increased soil N₂O fluxes with increasing soil Zn concentration. It is unclear if the same is true for environmentally contaminated soils. Net production of N₂O and N₂ was monitored during anaerobic incubations (25 °C, He atmosphere) of soils freshly spiked with ZnCl₂ and of corresponding soils that were gradually enriched with metals (mainly Zn) in the field by previous sludge amendments or by corrosion of galvanized structures. Total denitrification activity (i.e. the sum of N₂O+N₂ production rate) was not inhibited by freshly added Zn salts up to 1600 mg Zn kg⁻¹, whereas N₂O reduction decreased by 50% (EC₅₀) at total Zn concentrations of 231 mg Zn kg⁻¹ (ZEV soil) and 368 mg Zn kg⁻¹ (TM soil). In contrast, N₂O reduction was not reduced by soil Zn in any of the field contaminated soils, even at total soil Zn or soil solution Zn concentrations exceeding more than 5 times corresponding EC₅₀'s of the freshly spiked soil. The absence of adverse effects in the field contaminated soils was unrelated to soil NO₃ or organic matter concentration. Ageing (2-8 weeks) and soil leaching after spiking reduced the toxicity of Zn on N₂O reduction, either expressed as total Zn or soil solution Zn, suggesting adaptation reactions. However, no full recovery after spiking was identified at the largest incubation period in one soil. In addition, the denitrification assay performed with sewage sludge showed elevated N₂O release in Zn contaminated sludges (>6000 mg Zn kg⁻¹ dry matter) whereas this was not observed in low Zn sludge (<1000 mg Zn kg⁻¹ dry matter) suggesting limits to adaptation reactions in the sludge particles. It is concluded that the use of soils spiked with Zn salts overestimates effects on N₂O reduction. Field data on N₂O fluxes in sludge amended soils are required to identify if metals indeed promote N₂O emissions in sludge amended soils.     

Microbial activity in pig slurry-amended soils under semiarid conditions

Soil amendment with manures from intensive animal industries is nowadays a common practice that may favorably or adversely affect several soil properties, including soil microbial activity. In this work, the effect of consecutive annual additions of pig slurry (PS) at rates of 30, 60, 90, 120 and 150 m³ ha⁻¹ y⁻¹ over a 4-year period on soil chemical properties and microbial activity was investigated and compared to that of an inorganic fertilization and a control (without amendment). Field plot experiment conducted under a continuous barley monoculture and semiarid conditions were used. Eight months after the fourth yearly PS and mineral fertilizer application (i.e. soon after the fourth barley harvest), surface soil samples (Ap horizon, 0-15 cm depth) from control and amended soils were collected and analysed for pH, electrical conductivity (EC), contents of total organic C, total N, available P and K, microbial biomass C, basal respiration and different enzymatic activities. The control soil had a slightly acidic pH (6.0), a small EC (0.07 dS m⁻¹), adequate levels of total N (1.2 g kg⁻¹) and available K (483 mg kg⁻¹) for barley growth, and small contents of total organic C (13.2 g kg⁻¹) and available P (52 mg kg⁻¹). With respect to the control and mineral fertilized soils, the PS-amended soils had greater pH values (around neutrality or slightly alkaline), electrical conductivities (still low) and contents of available P and K, and slightly larger total N contents. A significant decrease of total organic C was observed in soils amended at high slurry rate (12.3 g kg⁻¹). Compared with the control and mineral treatments, which produced almost similar results, the PS-amended soils were characterized by a higher microbial biomass C content (from 311 to 442 g kg⁻¹), microbial biomass C/total organic C ratio (from 2.3 to 3.6%) and dehydrogenase (from 35 to 173 μg INTF g⁻¹), catalase (from 5 to 24 μmol O₂ g⁻¹ min⁻¹), BAA-protease (from 0.7 to 1.9 μmol  g⁻¹ h⁻¹) and β-glucosidase (from 117 to 269 μmol PNP g⁻¹ h⁻¹) activities, similar basal respirations (from 48 to 77 μg C-CO₂ g⁻¹ d⁻¹) and urease activities (from 1.5 to 2.2 μmol  g⁻¹ h⁻¹), and smaller metabolic quotients (from 6.4 to 7.7 ng C-CO₂ μg⁻¹ biomass C h⁻¹) and phosphatese activities (from 374 to 159 μmol PNP g⁻¹ h⁻¹). For example, statistical analysis of experimental data showed that, with the exception of metabolic quotient and total organic C content, these effects generally increased with increasing cumulative amount of PS. In conclusion, cumulative PS application to soil over time under semiarid conditions may produce not only beneficial effects but also adverse effects on soil properties, such us the partial mineralization of soil organic C through extended microbial oxidation. Thus, PS should not be considered as a mature organic amendment and should be treated appropriately before it is applied to soil, so as to enhance its potential as a soil organic fertilizer.     

Seasonality of organic phosphorus mineralization in the rhizosphere of the native savanna grass, Trachypogon plumosus

Savanna ecosystems have low primary productivity, strong seasonality, and acid soils with low phosphorus (P) content. Organic P (Po) comprises around 50% of the total soil P and is plant-available only after mineralization. Rhizosphere processes mediated by plants, microorganisms and arbuscular mycorrhiza (AM) are important for plant P nutrition. We studied P transformation rates, Po-fractionation, acid phosphomonoesterase activity (APA), AM status, dehydrogenase activity (DHA), and bacterial and fungal plate counts in the rhizosphere of the native dominant grass Trachypogon plumosus. We collected samples from three acid savanna soils differing in order and P content (Entisol, Vertisol and Ultisol) at Estación Experimental La Iguana (Northeastern Venezuela) during the dry, rainy and transitional seasons over a 2-year-period. Less available Po fractions (moderately labile, moderately and highly resistant) seem to be involved in short-term P-cycling transformations as they significantly varied with season. During the rainy season plant P content (576-1160 mg P kg⁻¹ dry weight) and APA (44-200 mg PNP kg⁻¹ dry soil) were higher, while microbial number and activity (DHA) were lower. The higher P availability in the Entisol (6-9 mg P kg⁻¹ dry soil) resulted in a better plant nutritional status and inhibited APA. T. plumosus seems to be highly dependent on AM symbiosis (45-71% AM colonized root length, 0.6-8 AM spores g⁻¹ dry soil), especially during the rainy season. Po mineralization processes, mediated by biological associations in the rhizosphere, are crucial for understanding seasonal P-cycling and fertility in acid savanna soils.