Microbial community structure and residue chemistry during decomposition of shoots and roots of young and mature wheat (Triticum aestivum L.) in sand

There is limited understanding of the relationship between carbon (C) chemistry and microbial community structure during decomposition of shoot and root residues and how plant age affects this. In this study, residues of young wheat shoots and roots, mature wheat shoots and roots or a 1:1 mix of mature shoot + root (MSR) were added to sand inoculated with a diverse microbial community. Respiration was measured over 60 days. On days 0, 15, 30 and 60, total C and nitrogen were measured, residue C chemistry was determined by ¹³C‐NMR (nuclear magnetic resonance) spectroscopy and microbial community structure was assessed by phospholipid fatty acid (PLFA) analyses. Cumulative respiration was least in young roots and did not differ among the other residue types. In MSR, decomposition was similar to that of shoots and roots alone; shoot material appeared to be preferentially decomposed. The decomposition rate of all residues combined was not related to C chemistry. However, mineralized C (C_min) was negatively correlated with the percentage of (aryl + O‐aryl)‐C in mature but not in young residues. Mineralized C of roots was positively correlated with the percentage of (di‐O‐alkyl + O‐alkyl)‐C, whereas this was not the case for shoots. Microbial community structure was influenced by time, plant organ and plant age. There was no general relationship between microbial community structure and C chemistry of the residues.     

A microcosm experiment to evaluate the influence of location and quality of plant residues on residue decomposition and genetic structure of soil microbial communities

The effects of location (soil surface vs. incorporated in soil) and nature of plant residues on degradation processes and indigenous microbial communities were studied by means of soil microcosms incubation in which the different soil zones influenced by decomposition i.e. residues, soil adjacent to residues (detritusphere) and distant soil unaffected by decomposition (bulk soil) were considered. Plant material decomposition, organic carbon assimilation by the soil microbial biomass and soil inorganic N dynamics were studied with ¹³C labelled wheat straw and young rye. The genetic structure of the community in each soil zone were compared between residue locations and type by applying B- and F-ARISA (for bacterial- and fungal-automated ribosomal intergenic spacer analysis) directly to DNA extracts from these different zones at 50% decomposition of each residue. Both location and biochemical quality affected residue decomposition in soil: 21% of incorporated ¹³C wheat straw and 23% left at the soil surface remained undecomposed at the end of incubation, the corresponding values for ¹³C rye being 1% and 8%. Residue decomposition induced a gradient of microbial activity with more labelled C incorporated into the microbial biomass of the detritusphere. The sphere of influence of the decomposing residues on the dynamics of soluble organic C and inorganic N in the different soil zones showed particular patterns which were influenced by both residue location and quality. Residue degradation stimulated particular genetic structure of microbial community with a gradient from residue to bulk soil, and more pronounced spatial heterogeneity for fungal than for bacterial communities. The initial residue quality strongly affected the resulting spatial heterogeneity of bacteria, with a significance between-zone discrimination for rye but weak discrimination between the detritusphere and bulk soil, for wheat straw. Comparison of the different detrituspheres and residue zones (corresponding to different residue type and location), indicated that the genetic structure of the bacterial and fungal communities were specific to a residue type for detritusphere and to its location for residue, leading to conclude that the detritusphere and residue corresponded to distinct trophic and functional niches for microorganisms.     

The microbial community composition changes rapidly in the early stages of decomposition of wheat residue

It is generally accepted that during the early stages of residue decomposition, easily available compounds are decomposed, leading to a relative increase in more recalcitrant compounds in the later stages of decomposition and that these changes in substrate availability are associated with changes in microbial community composition. However most studies on residue decomposition are conducted over several weeks or months; little is known about the changes in microbial community composition in the first weeks of decomposition. To address this knowledge gap, we incubated wheat residues inoculated with a microbial suspension in mesh bags buried in sand for 30 days, with sampling on days 0, 2, 4, 6, 8, 10, 15, 20, 25 and 30. Of the C added with the residues, 10, 18 and 25% had been respired on days 10, 20 and 30, respectively. The sum of PLFAs (phospholipid fatty acids), as an indicator of microbial biomass, increased strongly in the first 4 days and then decreased. The concentration of bacterial fatty acids was maximal on days 2 and 4, whereas the concentration of fungal fatty acids peaked on day 15. Microbial community composition (based on PLFA patterns) changed rapidly, with significant changes in the first 8 days and from day 8 to day 20. There were no significant changes in microbial community composition after day 20. The concentration of water-soluble C decreased strongly in the first 8 days, suggesting that the rapid changes in microbial community in this period are related to the changes in water-soluble C. Residue C chemistry, assessed by ¹³C NMR spectroscopy, changed little during the incubation period. This study showed that microbial community composition in decomposing residues changes rapidly in the first 1-2 weeks, which is, at least partly, the result of competition for the easily available compounds in the water-soluble fraction. After depletion of the water-soluble compounds, the microbial community composition changes more slowly.     

Fate of nitrogen from crop residues as affected by biochemical quality and the microbial biomass

Net mineralization of N from a range of shoot and root materials was determined over a period of 6 months following incorporation into a sandy-loam soil under controlled environment conditions. Biochemical “quality” components of the materials showed better correlation with net N mineralization than did gross measures of the respiration and N content of the soil microbial community during decomposition. The quality components controlling net N mineralization changed during decomposition, with water-soluble phenolic content significantly correlated with net N mineralization at early stages, and water-soluble N, followed by cellulose at later stages. C-to-N and total N were correlated with net N mineralization towards the end of the incubation only. Cumulative microbial respiration during the early stages of decomposition was correlated with net N mineralization measured after 2 months, at which time maximum net N mineralization was recorded for most residues. However, there was no relationship between microbial-N and net N mineralization. Biochemical quality factors controlling the C and N content of the residue remaining at the end of the incubation as light fraction organic matter (LFOM) were also investigated. Both C and N content of LFOM derived from the residues were correlated with residue cellulose content, and the chemical characteristics of LFOM were highly correlated with those of the original plant material. Incorporation of low cellulose, high water-soluble N-containing shoot residues resulted in more N becoming mineralized than had been added in the residues, demonstrating that net mineralization of native soil organic matter had occurred. Large amounts of N were lost from the mineral-N pool during the incubation, which could not be accounted for by microbial immobilization.     

Use of C-labelled plant materials and ergosterol, PLFA and NLFA analyses to investigate organic matter decomposition in Antarctic soil

The relationship between organic matter decomposition and changes in microbial community structure were investigated in Antarctic soils using ¹³C-labelled plant materials. Soils with and without labelled Deschampsia antarctica (a native Antarctic grass) were incubated for 42 days and sampled at 0, 7, 14, 21, 28 and 42 days. Changes in microbial community structure were assessed using phospholipid fatty acid analysis (PLFA) and an analysis of the fatty acids associated with the neutral lipid fraction (NLFA). These studies showed that there were no significant changes in PLFA or NLFA profiles over time suggesting no change in microbial community structure during residue decomposition. There was a marked increase however, in ergosterol levels in these soils indicative of growth of the fungal biomass. Analysis of this ergosterol using gas chromatography-mass spectrometry confirmed the transformation of the plant residue by showing the incorporation of ¹³C-plant C into the ergosterol. This incorporation of ¹³C into the ergosterol increased over the incubation period. Importantly, these changes associated with fungal growth were not evident in the analysis of either the PLFA or NLFA fractions thus questioning the reliability of such approaches for studying changes in microbial communities associated with the decomposition of plant residues.     

Differential and successive effects of residue quality and soil mineral N on water-stable aggregation during crop residue decomposition

Residue quality has been shown to influence soil water-stable aggregation (WSA) during crop residue decomposition, but there is still little information about its interactive effect with soil mineral N availability. The aim of this study was to determine the effect of soil mineral N on WSA during the decomposition of two high-C/N crop residues (wheat straw with C/N = 125.6 and miscanthus straw with C/N = 311.3). The two crop residues were combined with three mineral N addition rates (0, 60, and 120 mg N kg⁻¹ dry soil). Respiration, soil mineral N content, and WSA (expressed as mean-weight diameter, MWD) were measured on several dates during a 56-d incubation. The effect of decomposing crop residues on WSA followed two phases. (i) Between 0 and 7 d, the increase in WSA was related to intrinsic residue quality with higher decomposability of the wheat straw resulting in higher WSA. (ii) Thereafter, and until the end of the experiment, mineral N addition rates had a predominant but negative influence on WSA. In this second phase, the average MWD of residue-treated soils was 0.92, 0.55, and 0.44 mm for the 0, 60 and 120 mg N kg⁻¹ dry soil addition rates, respectively. Mineral N addition which did result in higher crop residue decomposition did not lead to higher WSA. WSA during crop residue decomposition is therefore not simply positively related to the induced microbial activity, and changes in microbial community composition with differential effects on WSA must be involved. The impact of high-C/N crop residues inputs on WSA, initially assumed to be low, could actually be strong and long-lasting in situations with low soil mineral N content.     

Frequent addition of wheat straw residues to soil enhances carbon mineralization rate

In many ecosystems, residues are added frequently to soil, in the form of root turnover and litter fall. However, in most studies on residue decomposition, residues are added once and there are few studies that have investigated the effect of frequent residue addition on C mineralization and N dynamics. To close this knowledge gap, we mixed mature wheat residue (C/N 122) into soil at a total rate of 2% w/w once at the start (R1×), every 16 days (R4×), every 8 days (R8×) or every 4 days (R16×). Un-amended soil served as control. All treatments were mixed every 4 days. Soil respiration was measured continuously over the 80-day incubation. Inorganic N, K₂SO₄-extractable C and N, chloroform-labile C and N (as an estimate of microbial biomass C and N), soil pH and microbial community composition were assessed every 16 days. Increasing frequency of residue addition increased C mineralization per g residue. Compared to R1×, cumulative respiration per g residue at the end of the incubation (day 80) was increased by 57, 82 and 92% in R4×, R8× and R16×, respectively. The largest differences in soil respiration per g residue occurred in the first 30 days. Despite large increases in cumulative respiration, frequent residue addition did not affect inorganic N or K₂SO₄-extractable N concentrations, chloroform-labile C and N or soil pH. Compared to the control, all residue treatments resulted in increases in chloroform-labile C and N and soil pH but decreased inorganic and K₂SO₄-extractable N. Microbial community composition was affected by residue addition, however there were no consistent differences among residue treatments. It is concluded that experiments with single residue additions may underestimate residue decomposition rates in the field. The increased C mineralization caused by frequent residue additions does not appear to be due to an increased microbial biomass or changes in microbial community composition, but rather to increased C mineralization per unit biomass.     

Short-term carbon mineralization in saline–sodic soils

Previous studies have shown that carbon (C) mineralization in saline or sodic soils is affected by various factors including organic C content, salt concentration and water content in saline soils and soil structure in sodic soils, but there is little information about which soil properties control carbon dioxide (CO₂) emission from saline-sodic soils. In this study, eight field-collected saline–sodic soils, varying in electrical conductivity (EC_e, a measure of salinity, ranging from 3 to 262 dS m⁻¹) and sodium adsorption ratio (SAR_e, a measure of sodicity, ranging from 11 to 62), were left unamended or amended with mature wheat or vetch residues (2% w/w). Carbon dioxide release was measured over 42 days at constant temperature and soil water content. Cumulative respiration expressed per gram SOC increased in the following order: unamended soil<soil amended with wheat residues (C/N ratio 122)<soil with vetch residue (C/N ratio 18). Cumulative respiration was significantly (p < 0.05) negatively correlated with EC_e but not with SAR_e. Our results show that the response to EC_e and SAR_e of the microbial community activated by addition of organic C does not differ from that of the less active microbial community in unamended soils and that salinity is the main influential factor for C mineralization in saline–sodic soils.     

Initial decomposition of post-harvest crown and root residues of poplars as affected by N availability and particle size

An incubation experiment was carried out to investigate the impacts of residue particle size and N application on the decomposition of post-harvest residues of fast-growing poplar tree plantations as well as on the microbial biomass. Crown and root residues, differing in their C/N ratios (crown 285, root 94), were ground to two particle sizes and incubated with and without application of inorganic nitrogen (N) for 42 days in a tilled soil layer from a poplar plantation after 1 year of re-conversion to arable land. Carbon and N mineralization of the residues, microbial biomass C and N, ergosterol contents, and recovery of unused substrate as particulate organic matter (POM) were determined. Carbon mineralization of the residues accounted for 26 to 29 % of added C and caused a strong N immobilization, which further increased after N addition. N immobilization in the control soil showed that even 1 year after re-conversion, fine harvest residues still remaining in the soil were a sink for mineral N. Irrespective of the particle size, C mineralization increased only for crown residues after application of N. Nevertheless, the overall decrease in amounts of POM-C and a concurrent decrease of the C/N ratio in the POM demonstrate the mineralization of easily available components of woody residues. Microbial biomass significantly decreased during incubation, but higher cumulative CO₂ respiration after N application suggests an increased microbial turnover. Higher ergosterol to microbial biomass C ratios after residue incorporation points to a higher contribution of saprotrophic fungi in the microbial community, but fungal biomass was lower after N addition.     

Nitrogen immobilization and mineralization during initial decomposition of15N-labelled pea and barley residues

The immobilization and mineralization of N following plant residue incorporation were studied in a sandy loam soil using¹⁵N-labelled field pea (Pisum sativum L.) and spring barley (Hordeum vulgare L.) straw. Both crop residues caused a net immobilization of soil-derived inorganic N during the complete incubation period of 84 days. The maximum rate of N immobilization was found to 12 and 18 mg soil-derived N g^–1 added C after incorporation of pea and barley residues, respectively. After 7 days of incubation, 21% of the pea and 17% of the barley residue N were assimilated by the soil microbial biomass. A comparison of the¹⁵N enrichments of the soil organic N and the newly formed biomass N pools indicated that either residue N may have been assimilated directly by the microbial biomass without entering the soil inorganic N pool or the biomass had a higher preference for mineralized ammonium than for soil-derived nitrate already present in the soil. In the barley residue treatment, the microbial biomass N was apparently stabilized to a higher degree than the biomass N in the pea residue treatment, which declined during the incubation period. This was probably due to N-deficiency delaying the decomposition of the barley residue. The net mineralization of residue-derived N was 2% in the barley and 22% in the pea residue treatment after 84 days of incubation. The results demonstrated that even if crop residues have a relative low C/N ratio (15), transient immobilization of soil N in the microbial biomass may contribute to improved conservation of soil N sources.     

Nitrogen immobilization and mineralization during initial decomposition of15N-labelled pea and barley residues

The immobilization and mineralization of N following plant residue incorporation were studied in a sandy loam soil using¹⁵N-labelled field pea (Pisum sativum L.) and spring barley (Hordeum vulgare L.) straw. Both crop residues caused a net immobilization of soil-derived inorganic N during the complete incubation period of 84 days. The maximum rate of N immobilization was found to 12 and 18 mg soil-derived N g^?1 added C after incorporation of pea and barley residues, respectively. After 7 days of incubation, 21% of the pea and 17% of the barley residue N were assimilated by the soil microbial biomass. A comparison of the¹⁵N enrichments of the soil organic N and the newly formed biomass N pools indicated that either residue N may have been assimilated directly by the microbial biomass without entering the soil inorganic N pool or the biomass had a higher preference for mineralized ammonium than for soil-derived nitrate already present in the soil. In the barley residue treatment, the microbial biomass N was apparently stabilized to a higher degree than the biomass N in the pea residue treatment, which declined during the incubation period. This was probably due to N-deficiency delaying the decomposition of the barley residue. The net mineralization of residue-derived N was 2% in the barley and 22% in the pea residue treatment after 84 days of incubation. The results demonstrated that even if crop residues have a relative low C/N ratio (15), transient immobilization of soil N in the microbial biomass may contribute to improved conservation of soil N sources.     

Carbon dynamics of rhizodeposits, root- and shoot-residues in a rice soil

A laboratory incubation experiment was conducted to investigate the fates of plant-derived C during the simulated fallow period in a rice soil. The ¹³C labelled soil and plant materials were used to follow the residue decomposition and its effect on soil organic C (SOC) dynamics under the conditions of either incorporation into soil or intact root systems. The soils were incubated at 15 °C for 240 d and destructive sampling was conducted at 60, 150 and 240 d. To observe the temperature effect, one batch of incubation was shifted from 15 to 25 °C during the last 45 d (between 195 and 240 d). The results showed that the decomposition of the incorporated residues could be divided into two phases: an initial rapid phase followed by a slower phase of decomposition. The decomposition of straw residues was faster than root residues: with 73% of the straw residue being decomposed, compared with 56% of the root residue over 240-d incubation at 15 °C. The water-soluble organic C and microbial biomass C significantly increased after residue incorporation. The total SOC contents, however, slightly decreased, although significant amounts of straw C (14.2%) and root C (8.7%) were found in SOC at the end of incubation, suggesting that the degradation of native SOC occurred concomitantly. Similar to decomposition of the incorporated residues, the organic substances derived from rhizodeposition of the previous season were mineralized rapidly at first and then slowly. The decomposition of the intact root system, however, was extremely slow. This result suggested that the intact root system conserved more organic C in soils compared with the incorporation of fresh residues. Increase of temperature from 15 to 25 °C during the last 45-days of incubation significantly promoted the residue decomposition.     

Linking microbial community to soil water-stable aggregation during crop residue decomposition

The dynamics of soil water-stable aggregation (WSA) following organic matter (OM) addition are controlled by microbial activity, which in turn is influenced by carbon substrate quality and mineral N availability. However, the role of microbial communities in determining WSA at different stages of OM decomposition remains largely unknown. This study aimed at evaluating the role of microbial communities in WSA during OM decomposition as affected by mineral N. In a 35-day incubation experiment, we studied the decomposition of two high-C/N crop residues (miscanthus, C/N = 311.3; and wheat, C/N = 125.6) applied at 4 g C kg⁻¹ dry soil with or without mineral N addition (120 mg N kg⁻¹ dry soil). Microbial characteristics were measured at day 0, 7, and 35 of the experiment, and related to previous results of WSA. Early increase in WSA (at 7 days) was related to an overall increase of the microbial biomass (MBC) with wheat residues showing higher values in MBC and WSA than miscanthus. In the intermediate stage of decomposition (from day 7 to 35), the dynamics of WSA were more associated with the dynamics of microbial polysaccharides and greatly influenced by mineral N addition. Mineral N addition resulted in a decrease or leveling off of WSA whereas it increased in its absence. We suggest that opportunistic bacterial populations stimulated by N addition may have consumed binding agents which decreased WSA or prevented its increase. To the contrary, microbial polysaccharide production was high when no mineral N was added which led to the higher WSA in the late stage of decomposition in this treatment. The late stage of decomposition was associated with a particular fungal community also influenced by the mineral N treatment. We suggest that WSA dynamics in the late stage of decomposition can be considered as a « narrow process³ where the structure of the microbial community plays a greater role than during the initial stages.     

Dynamics of soil amino sugar pools during decomposition processes of corn residues as affected by inorganic N addition

Purpose  

Identifying the impact of inorganic-nitrogen (N) availability on soil amino sugar dynamics during corn (Zea mays L.) residue decomposition may advance our knowledge of microbial carbon (C) and N transformations and the factors controlling these processes in soils. Amino sugars are routinely used as microbial biomarkers to investigate C and N sequestration in microbial residues, and they are also involved in microbial-mediated soil organic matter (SOM) turnover. We conducted a 38-week incubation study using a Mollisol which was amended with corn residues and four levels of inorganic N (i.e., 0, 60.3, 167.2, and 701.9 mg N kg⁻¹ soil). The objective of this study was to examine the effects of inorganic-N availability on fungal and bacterial formation and stabilization of heterogeneous amino sugars during the corn residue decomposition in soil.     

Plant-N incorporation into microbial amino sugars as affected by inorganic N addition: A microcosm study of N-labeled maize residue decomposition

Carbon (C) and/or nitrogen (N) in plant residues can be assimilated into microbial biomass during the plant residue decomposition before incorporation into SOM in the form of microbial residues. Yet, microbial transformation of plant residue-N into microbial residues and the effects of inorganic N inputs on this process have not been well documented. Here, we undertook a 38-week incubation with a silt loam soil amended with a ¹⁵N-labeled maize (Zea mays L.) residue to determine how the transformation of maize residue-N into soil amino sugars was affected by rates of inorganic N addition. The newly metabolized amino sugars derived from maize residue-N were differentiated and quantified by using an isotope-based gas chromatography-mass spectrometry technique. We found that greater amounts of maize residue-N were transformed into amino sugars with lower inorganic N addition at the early stages of the plant residue degradation. However, the trend was reversed during later stages of decay as greater percentage of maize residue-N (8.6-9.4%) were enriched in amino sugars in the N_med and N_high soils, as compared with N₀ and N_low (7.5-8.2%). This indicated that higher availability of inorganic N could delay the transformation process of plant-N into microbial residues during the mineralization of plant residues. The dynamic transformations of the plant residue-N into individual amino sugars were compound-specific, with very fast incorporation into bacterial MurA_M-new found during the initial weeks, while the dynamics of maize residue-derived GluN exhibited a delayed response to assimilate plant-N into fungal products. The findings indicated differential contributions of maize residue decomposing microorganisms over time. Moreover, we found no preferential utilization of inorganic N over plant residue-N into amino sugars during the incubation course, but inorganic N inputs altered the rate of plant-N accumulation in microbial-derived organic matters. Our results indicated that higher N availability had a positive impact on the accumulation or stabilization of newly-produced microbial residues in the long term.     

Residue addition and liming history interactively enhance mineralization of native organic carbon in acid soils

Lime application is the most common method to improve crop production in acid soils and has been shown to change soil organic C content. However, the impact of liming history on the priming effect on soil organic C is not well understood. This study examined the effect of liming history on C priming in response to the addition of crop residues of different qualities. Soils with pH ranging from 4.7 to 7.4 were collected from two adjacent field experiments whereby lime was applied at different rates, 6 and 35 years ago. A 90-day incubation study was conducted by applying ¹³C-labelled wheat (C/N 42) and field-pea (C/N 29) residues at a rate of 5 g kg^?1 soil. Residue application to soils yielded the positive priming effect in all pH levels with the magnitude of C priming being the greatest at initial soil pH 6.6. In comparison, the optimal pH for residue decomposition (7.3) was higher than that for priming. The overall priming effect was about 17% greater with field-pea than wheat residue. However, cumulative decomposition of added field-pea residue was 15% lower than that of wheat residue. Furthermore, C priming was greater in soils from the 35-year-old than the 6-year-old limed plots, indicating that a longer history of liming did not enhance the protection of indigenous C from mineralization. The results suggest that increases in soil pH by liming enhanced native C priming through greater microbial biomass and activity and that the magnitude and dynamics of the priming effect largely depended on residue quality and its consequent nutrient supply to decomposer organisms. The study implies that over-liming would likely have negative impacts on the long-term C sequestration.     

Organic nitrogen cycling microbial communities are abundant in a dry Australian agricultural soil

Some microbial nitrogen (N) cycling processes continue under low soil moisture levels in drought-adapted ecosystems. These processes are of particular importance in winter cropping systems, where N availability during autumn sowing informs fertilizer practices and impacts crop productivity. We evaluated the organic and inorganic N-cycling communities in a key cropping soil (Vertosol), using a controlled-environment incubation study that was designed to model the autumn break in south Australian grain growing regions. Soils from wheat, lucerne, and green manure (disced-in vetch) rotations of the Sustainable Cropping Rotations in Mediterranean Environments trial (Victoria, Australia) were collected during the summer when soil moisture was low. Microbial community structure and functional capacity were measured both before and after wetting (21, 49, and 77 days post-wetting) using terminal restriction fragment length polymorphism measures of bacterial and fungal communities, and quantitative PCR of nitrogen cycling genes. Quantified genes included those associated with organic matter decomposition (laccase, cellobiohydrolase), mineralization of N from organic matter (peptidases) and nitrification (bacterial and archaeal ammonia monooxygenase and nitrite oxidoreductase). In general, the N cycling functional capacity decreased with soil wetting, and there was an apparent shift from organic-N cycling dominance to autotrophic mineral-N cycling dominance. Soil nitrate levels were best predicted by laccase and neutral peptidase genes under drought conditions, but by neutral peptidase and bacterial ammonia monooxygenase genes under moist conditions. Rotation history affected both the structural and functional resilience of the soil microbial communities to changing soil moisture. Discing in green manure (vetch) residues promoted a resilient microbial community, with a high organic-N cycling capacity in dry soils. Although this was a small-scale microcosm study, our results suggest that management strategies could be developed to control microbial organic-N processing during the summer fallow period and thus improve crop-available N levels at sowing.     

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.     

Microscale distribution and function of soil microorganisms in the interface between rhizosphere and detritusphere

The rhizosphere and the detritusphere are hot spots of microbial activity, but little is known about the interface between rhizosphere and detritusphere. We used a three-compartment pot design to study microbial community structure and enzyme activity in this interface. All three compartments were filled with soil from a long-term field trial. The two outer compartments were planted with maize (root compartment) or amended with mature wheat shoot residues from a free air CO₂ enrichment experiment (residue compartment) and were separated by a 50 μm mesh from the inner compartment. Soil, residues and maize differed in ¹³C signature (δ¹³C soil −26.5‰, maize roots −14.1‰ and wheat residues −44.1‰) which allowed tracking of root- and residue-derived C into microbial phospholipid fatty acids (PLFA). The abundance of bacterial and fungal PLFAs showed clear gradients with highest abundance in the first 1–2 mm of the root and residue compartment, and generally higher values in the vicinity of the residue compartment. The δ¹³C of the PLFAs indicated that soil microorganisms incorporated more carbon from the residues than from the rhizodeposits and that the microbial use of wheat residue carbon was restricted to 1 mm from the residue compartment. Carbon incorporation into soil microorganisms in the interface was accompanied by strong microbial N immobilisation evident from the depletion of inorganic N in the rhizosphere and detritusphere. Extracellular enzyme activities involved in the degradation of organic C, N and P compounds (β-glucosidase, xylosidase, acid phosphatase and leucin peptidase) did not show distinct gradients in rhizosphere or detritusphere. Our microscale study showed that rhizosphere and detritusphere differentially influenced microbial C cycling and that the zone of influence depended on the parameter assessed. These results are highly relevant for defining the size of different microbial hot spots and understanding microbial ecology in soils.     

Soil properties and N application effects on microbial activities in two winter wheat cropping systems

The application of mineral N fertilizers may influence biologically mediated processes that are important in nutrient transformations and availability. This study was conducted to assess the effect of N application on microbial activities in irrigated and non-irrigated winter wheat systems. Carbon decomposition and microbial biomass C in soils with three N application rates (0, 150, and 300 kg N ha^–1 as urea) were measured over 40 days in a laboratory incubation experiment. Carbon, N, and P contents in the soil under the irrigated wheat were higher than those in the soil under the non-irrigated wheat. The reverse trend was observed for soil pH and Ca and Mg contents. However, soils from the two systems had similar C/N ratios. Carbon decomposition and microbial biomass C in the soil under the irrigated wheat increased significantly (p <0.05). Increasing rates of N fertilizer resulted in higher C decomposition and microbial biomass C levels in both soil systems. Results indicate that different wheat cropping systems affect soil properties that will then have an impact on C turnover in the soil. Moreover, the irrigated wheat system favors soil conditions required for a faster C turnover. In conclusion, it is likely that due to positive effects on microbial activity, N fertilization will increase nutrient cycling and, subsequently, crop productivity will improve in N-poor soils.