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.     

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.     

Kinetics of amino sugar formation from organic residues of different quality

Amino sugars are key compounds of microbial cell walls, which have been widely used as biomarker of microbial residues to investigate soil microbial communities and organic residue cycling processes. However, the formation dynamics of amino sugar is not well understood. In this study, two agricultural Luvisols under distinct tillage managements were amended with uniformly ¹³C-labeled wheat residues of different quality (grain, leaf and root). The isotopic composition of individual amino sugars and CO₂ emission were measured over a 21-day incubation period using liquid chromatography–isotope ratio mass spectrometry (LC–IRMS) and trace gas IRMS. Results showed that, the amount of residue derived amino sugars increased exponentially and reached a maximum within days after residue addition. Glucosamine and galactosamine followed different formation kinetics. The maxima of residue derived amino sugars formation ranged from 14 nmol g⁻¹ dry soil for galactosamine (0.8% of the original concentration) to 319 nmol g⁻¹ dry soil for glucosamine (11% of the original concentration). Mean production times of residue derived amino sugars ranged from 2.1 to 9.3 days for glucosamine and galactosamine, respectively. In general, larger amounts of amino sugars were formed at a higher rate with increasing plant residue quality. The microbial community of the no-till soil was better adapted to assimilate low quality plant residues (i.e. leaf and root). All together, the formation dynamics of microbial cell wall components was component-specific and determined by residue quality and soil microbial community.     

无机氮素加入量对玉米秸秆分解过程中棕壤氨基糖含量的影响

作物秸秆还田作为一种调控土壤养分循环、减少氮肥损失、维持和提高土壤有机质水平的有力措施,越来越受到人们的重视[1-2]。还田的秸秆是微生物的碳源和能源,常导致土壤微生物量迅速增加,相应的微生物死亡率和微生物残体积累量也提高[3-4]。根据Appuhn等[5]对微生物细胞壁组分的     

无机氮素加入量对玉米秸秆分解过程中棕壤氨基糖含量的影响

在室内恒温(25℃)培养条件下,通过气相色谱法研究高C/N比玉米秸秆降解过程中微生物来源的氨基糖含量及其占有机质比例的变化及其对无机氮素添加水平(0, 60.3, 167.2, 701.9 mg•N•kg-1土,依次标记为N0, Nlow, Nmed, Nhigh)的响应情况。结果表明：在玉米秸秆分解过程中,土壤中的氨基糖含量及其对有机质贡献的比例随着无机氮素供应水平的增加而增加,即以微生物代谢物形式截获的有机碳/氮相应增多。Nmed和Nhigh处理中氨基糖积累量显著高于Nlow和N0处理。不同微生物来源的氨基糖受外源氮素的影响情况不同,胞壁酸比氨基葡萄糖更易于受到土壤中碳氮供给的影响,具有相对较快的转化速率;而在数量上氨基葡萄糖对土壤有机质的贡献比例显著高于前者;氨基半乳糖在土壤中的积累过程较为缓慢,受外源无机氮素添加水平的影响并不明显。可见,在高C/N比作物残体分解过程中,无机氮素的供应水平是影响土壤中氨基糖积累转化的重要因素之一。但是,过多的无机氮素施入并不能被微生物完全同化利用,因此秸秆还田的土壤中必须要考虑有效氮素的水平问题。     

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.     

Separate effects of the biochemical quality and N content of crop residues on C and N dynamics in soil

This study assessed the respective roles of biochemical quality and N content of plant residues on C and N dynamics in a soil. Both ¹⁵N- and ¹³C-labeled oilseed rape residues (roots, seedpod walls) combining different biochemical characteristics and similar N content or the same biochemical characteristics and different N contents were used as amendments. These treatments were combined with two levels of soil inorganic N to ensure that decomposition was not limited by N availability. The soil was incubated under laboratory conditions for 134 days. Soil amended with residues of similar biochemical quality (i.e. the two pod walls) displayed similar C mineralization dynamics when the initial N availability (residue+soil N) ranged from 1.7 to 3.2% of residue dry matter. The roots showed poorer decomposition than the pod walls, lower cumulative C mineralization and greater accumulation of root-derived C in the >50 μm coarse fraction of the soil organic matter. The N content of the residues influenced mineral N accumulation in the soil with a lower net immobilization of residues with low C-to-N ratios. Adding an exogenous source of inorganic N had no effect on C dynamics but modified the remineralization kinetics of the previously immobilized N, suggesting changes in the microbial community involved.     

Temporal responses of soil microorganisms to substrate addition as indicated by amino sugar differentiation

Amino sugars are one of the important microbial residue biomarkers which are associated with soil organic matter cycling. However, little is known about their transformation kinetics in response to available substrates because living biomass only contributes a negligible portion to the total mass of amino sugars. By using ¹⁵N tracing technique, the newly synthesized (labeled) amino sugars can be differentiated from the native portions in soil matrix, making it possible to evaluate, in quantitative manner, the transformation pattern of amino sugars and to interpret the past and ongoing changes of microbial communities during the assimilation of extraneous ¹⁵N. In this study, laboratory incubations of soil samples were conducted by using ¹⁵NH₄⁺ as nitrogen source with or without glucose addition. Both the ¹⁵N enrichment (expressed as atom percentage excess, APE) and the contents of amino sugars were determined by an isotope-based gas chromatography-mass spectrometry. The significant ¹⁵N incorporation into amino sugars was only observed in glucose plus ¹⁵NH₄⁺ amendment with the APE arranged as: muramic acid (MurN) > glucosamine (GlcN) > galactosamine (GalN). The dynamics of ¹⁵N enrichment in bacterial-derived MurN and fungal-derived GlcN were fitted to the hyperbolic equations and indicative for the temporal responses of different soil microorganisms. The APE plateau of MurN and fungal-derived GlcN represented the maximal extent of bacterial and fungal populations, respectively, becoming active in response to the available substrates. The different dynamics of the ¹⁵N enrichment between MurN and GlcN indicated that bacteria reacted faster than fungi to assimilate the labile substrates initially, but fungus growth was dominant afterward, leading to integrated microbial community structure over time. Furthermore, the dynamics of labeled and unlabeled portions of amino sugars were compound-specific and substrate-dependent, suggesting their different stability in soil. GlcN tended to accumulate in soil while MurN was more likely degraded as a carbon source when nitrogen supply was excessive.     

Decomposition of mulched versus incorporated crop residues: Modelling with PASTIS clarifies interactions between residue quality and location

Crop residue management has been shown to significantly affect the decomposition process of plant debris in soil. In previous studies examining this influence, the extrapolation of laboratory data of carbon and/or nitrogen mineralization to field conditions was often limited by a number of interactions that could not be taken into account by a mere experimental approach. Therefore, we demonstrated the interactive effect between crop residue location in soil (mulch vs. incorporation) and its biochemical and physical quality, in repacked soil columns under artificial rain. Decomposition of ¹³C and ¹⁵N labelled rape and rye residues, with associated C and N fluxes, was analysed using the mechanistic model PASTIS, which turned out to be necessary to understand the interacting factors on the C and N fluxes. The influence of soil and residue water content on decomposition and nitrification was evaluated by the moisture limitation factor of PASTIS. This factor strongly depended on residue location and to a smaller extent on physical residue properties, resulting in a lower decomposition rate of about 35% for surface placed compared to incorporated residues. Irrespective of its placement, the biochemical residue quality (e.g. N availability for decomposition, amount of soluble compounds and lignin) was responsible for a faster and more advanced decomposition of about 15% in favour of rye compared to rape, suggesting only a limited interaction between residue quality and its location. Net N mineralization after nine weeks was larger for rye than for rape, equivalent to 59 and 10 kg NO₃⁻-N ha⁻¹ with incorporation, and 71 and 34 kg NO₃⁻-N ha⁻¹ with mulch, respectively. This net N mineralization in soil resulted from the interaction between soil water content, depending on residue placement, and N availability, which was determined by the biochemical residue quality. Moisture limitation appeared more important than N limitation in the decomposition of mulched residues. Modelling of gross N mineralization and immobilization also revealed that leaving crop residues at the soil surface may increase the risk of nitrate leaching compared to residue incorporation, if (i) soil water content under mulch is larger than with residue incorporation (more gross N mineralization), and (ii) availability to the applied C-source is limited (less gross N immobilization). Scenario analyses with PASTIS confirmed the importance of moisture conditions on the decomposition of mulched residues and the small interaction between biochemical crop residue quality and its location in soil.     

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.     

Rates of decomposition of plant residues and available nitrogen in soil, related to residue composition through simulation of carbon and nitrogen turnover

The dynamics of inorganic N in soil following the application of plant residues depends on their composition. We assumed that all plant materials are composed of similar components, each decomposing at a specific rate, but differ in the proportions of the various components. The NCSOIL model that simulates C and N turnover in soil was used to link the rates of residue decomposition to their composition, defined as soluble, cellulose-like and lignin-like C and N, and thereby integrate short and long-term effects of residues on available N dynamics in soil. Five plant residues in a wide range of C:N ratios were incubated in soil for 24 weeks at 30 °C, during which C and N mineralization were measured. The materials with large C:N ratios (corn, rice hulls and wheat straw) were also incubated with NH₄⁺-N to avoid N deficiency. The residues were analyzed for total and soluble C and N. The partitioning of insoluble C and N between cellulose- and lignin-like pools was optimized by best fit of simulated C and N mineralization to measured results. The decomposition rate constants of the soluble and lignin-like pools were assumed to be 1.0 and 10⁻⁵ d⁻¹, respectively, and that of the cellulose-like pool, obtained by model optimization against mineralization of cellulose with NH₄⁺-N in soil, was 0.051 d⁻¹. The optimized, kinetically defined lignin-like pool of all residues was considerably larger than lignin contents normally found in plant residues by the Van Soest procedure. Gross N mineralization of tobacco and rape residues was similar, but N recovery from tobacco was larger, because a larger fraction of its C was in the lignin-like pool. N in rice hulls, corn and wheat residues was mostly recalcitrant, yet rice hulls did not cause N deficiency, because most of its C was recalcitrant too. The soluble components of the residues had strong short-term effects on available N in soil, but the cellulose-like pool was equally important for short and medium-term effects. Soluble and cellulose-like C were 29 and 42% of total C, respectively, in corn and 7 and 50% in wheat. Maximal net inorganic N losses, measured in both residue treatments after 2 weeks, were 42 mg g⁻¹ C applied as corn and 31 mg g⁻¹ C applied as wheat, or 84 and 110 mg g⁻¹ decomposed C of corn and wheat, respectively. Rice hulls immobilized N slowly, but by the end of 24 weeks all three residues immobilized 26-27 mg N kg⁻¹ C applied. The different dynamics of N immobilization demonstrated the need to determine the decomposability of C and N rather than their total contents in plant residues.     

Turnover of grain legume N rhizodeposits and effect of rhizodeposition on the turnover of crop residues

The turnover of N derived from rhizodeposition of faba bean (Vicia faba L.), pea (Pisum sativum L.) and white lupin (Lupinus albus L.) and the effects of the rhizodeposition on the subsequent C and N turnover of its crop residues were investigated in an incubation experiment (168 days, 15 °C). A sandy loam soil for the experiment was either stored at 6 °C or planted with the respective grain legume in pots. Legumes were in situ ¹⁵N stem labelled during growth and visible roots were removed at maturity. The remaining plant-derived N in soil was defined as N rhizodeposition. In the experiment the turnover of C and N was compared in soils with and without previous growth of three legumes and with and without incorporation of crop residues. After 168 days, 21% (lupin), 26% (faba bean) and 27% (pea) of rhizodeposition N was mineralised in the treatments without crop residues. A smaller amount of 15–17% was present as microbial biomass and between 30 and 55% of mineralised rhizodeposition N was present as microbial residue pool, which consists of microbial exoenzymes, mucous substances and dead microbial biomass. The effect of rhizodeposition on the C and N turnover of crop residues was inconsistent. Rhizodeposition increased the crop residue C mineralisation only in the lupin treatment; a similar pattern was found for microbial C, whereas the microbial N was increased by rhizodeposition in all treatments. The recovery of residual ¹⁵N in the microbial and mineral N pool was similar between the treatments containing only labelled crop residues and labelled crop residues + labelled rhizodeposits. This indicates a similar decomposability of both rhizodeposition N and crop residue N and may be attributable to an immobilisation of both N sources (rhizodeposits and crop residues) as microbial residues and a subsequent remineralisation mainly from this pool.Abbreviations C or N_dec C or N decomposed from residues - C or N_mic microbial C or N - C or N_micres microbial residue C or N - C or N_min mineralised C or N - C or N_input added C or N as crop residues and/or rhizodeposits - dfr derived from residues - dfR derived from rhizodeposition - Ndfr N derived from residues - NdfR N derived from rhizodeposition - N_loss losses of N derived from residues - SOM soil organic matter - WHC water holding capacity     

Amino sugars as specific indices for fungal and bacterial residues in soil

Amino sugars are important indices for the contribution of soil microorganisms to soil organic matter. Consequently, the past decade has seen a great increase in the number of studies measuring amino sugars. However, some uncertainties remain in the interpretation of amino sugar data. The objective of the current opinion paper is to summarize current knowledge on amino sugars in soils, to give some advice for future research objectives, and to make a plea for the correct use of information. The study gives an overview on the origin of muramic acid (MurN), glucosamine (GlcN), galactosamine (GalN), and mannosamine (ManN). Information is also provided on measuring total amino sugars in soil but also on compound-specific δ¹³C and δ¹⁵N determination. Special attention is given to the turnover of microbial cell-wall residues, to the interpretation of the GlcN/GalN ratio, and to the reasons for converting fungal GlcN and MurN to microbial residue C. There is no evidence to suggest that the turnover of fungal residues generally differs from that of bacterial residues. On average, MurN contributes 7% to total amino sugars in soil, GlcN 60%, GalN 30%, and ManN 4%. MurN is highly specific for bacteria, GlcN for fungi if corrected for the contribution of bacterial GlcN, whereas GalN and ManN are unspecific microbial markers.     

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.     

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.     

Shifts in amino sugar and ergosterol contents after addition of sucrose and cellulose to soil

An incubation experiment with organic soil amendments was carried out with the aim to determine whether formation and use of microbial tissue (biomass and residues) could be monitored by measuring glucosamine and muramic acid. Living fungal tissue was additionally determined by the cell-membrane component ergosterol. The organic amendments were fibrous maize cellulose and sugarcane sucrose adjusted to the same C/N ratio of 15. In a subsequent step, spherical cellulose was added without N to determine whether the microbial residues formed initially were preferentially decomposed. In the non-amended control treatment, ergosterol remained constant at 0.44 μg g⁻¹ soil throughout the 67-day incubation. It increased to a highest value of 1.9 μg g⁻¹ soil at day 5 in the sucrose treatment and to 5.0 μg g⁻¹ soil at day 33 in the fibrous cellulose treatment. Then, the ergosterol content declined again. The addition of spherical cellulose had no further significant effects on the ergosterol content in these two treatments. The non-amended control treatment contained 48 μg muramic acid and 650 μg glucosamine g⁻¹ soil at day 5. During incubation, these contents decreased by 17% and 19%, respectively. A 33% increase in muramic acid and an 8% increase in glucosamine were observed after adding sucrose. Consequently, the ratio of fungal C to bacterial C based on bacterial muramic acid and fungal glucosamine was lowered in comparison with the other two treatments. No effect on the two amino sugars was observed after adding cellulose initially or subsequently during the second incubation period. This indicates that the differences in quality between sucrose and cellulose had a strong impact on the formation of microbial residues. However, the amino sugars did not indicate a preferential decomposition of microbial residues as N sources.     

Soil biodegradation of maize root residues: Interaction between chemical characteristics and the presence of colonizing micro-organisms

Due to their direct contact with the soil, roots are exposed to colonizing micro-organisms that persist after the plant has died. These micro-organisms may affect intrinsic root-chemical quality and the kinetics of root residue decomposition in soil, or interact with soil micro-organisms during the decomposition process. The aims in this work were i) to determine the interactions between the presence of root-colonizing micro-organisms and root-chemical quality and ii) to quantify the effect of these micro-organisms on root decomposition. Roots were selected from six maize genotypes cultivated in the field and harvested at physiological maturity. The roots of two genotypes (F2 and F2bm1) had a higher N content, lower neutral sugars content and higher Klason lignin content than the other genotypes (F292, F292bm3, Mexxal, Colombus). Location of the root residue micro-organisms by scanning electron microscopy and transmission electron microscopy revealed that F2 and F2bm1 roots were more colonized than roots of the other genotypes. Electron Dispersive X-Ray microanalyses of in situ N confirmed a higher N content in the colonizing micro-organisms than in the root cell walls. Residues of F2 and F2bm1 roots decomposed more slowly and to a lesser extent than those of the other genotypes during incubation in a silty loam soil under controlled conditions (15 °C, −80 kPa). After 49 days, 40.6% of the total C from F292 was mineralized but only 20.7% of from F2bm1. These results suggest that residue-colonizing micro-organisms decompose the cell-wall sugars to varying extents before soil decomposition thereby modifying the chemical quality of the residues and their mineralization pattern in soil. Due to their high N content, colonizing micro-organisms also impact on the total N content of root residues, reducing their C to N ratio. Gamma sterilized root residues were incubated under the same conditions as non-sterilized residues to see if micro-organisms colonizing root residues could modify the action of soil micro-organisms during decomposition. Similar C mineralization rates were observed for both non-sterilized and sterilized residues, indicating that the residue micro-organisms did not quantitatively affect the activity of soil micro-organisms.     

Residue retention mitigated short-term adverse effect of clear-cutting on soil carbon and nitrogen dynamics in subtropical Australia

Purpose

This study aimed to investigate the benefits of retaining harvest residues on the dynamics of soil C and N pools following clear-cut harvesting of a slash pine plantation in South East Queensland of subtropical Australia.

Materials and methods

Immediately following clear-cut harvesting, macro-plots (10?×?10 m) were established on a section of the plantation in a randomised complete block design with four blocks and three treatments: (1) residue removal (RR₀), (2) single level of residue retention (RR₁) and (3) double level of residue retention (RR₂). Soils were sampled at 0, 6, 12, 18 and 24 months following clear-cutting and analysed for total C and N, microbial biomass C (MBC) and N (MBN), hot water–extractable organic C (HWEOC), hot water–extractable organic N (HWEON), NH₄⁺–N and NO_x^?–N.

Results and discussion

The study showed that although soil total C decreased in the first 12 months following clear-cutting, harvest residue retention increased soil total C and N by 45% (p?<?0.001) and 32% (p?<?0.001), respectively, over the 12–24 months. NH₄⁺–N, HWEOC, HWEON and MBC showed initial surges in the first 6 months irrespective of residue management, which declined after the 6th month. However, residue retention significantly increased HWEOC and HWEON over the 12–24 months (p?<?0.001).

Conclusions

This study demonstrated that harvest residue retention during the inter-rotation period can minimise large changes in C and nutrient pools, and can even increase soil C and nutrient pools for the next plantation rotation.

     

Effects of biochar and polyacrylamide on decomposition of soil organic matter and <Superscript>14</Superscript>C-labeled alfalfa residues

Purpose

Various soil conditioners, such as biochar (BC) and anionic polyacrylamide (PAM), improve soil fertility and susceptibility to erosion, and may alter microbial accessibility and decomposition of soil organic matter (SOM) and plant residues. To date, no attempts have been made to study the effects of BC in combination with PAM on the decomposition of soil SOM and plant residues. The objective of this study was to evaluate the effects of BC, PAM, and their combination on the decomposition of SOM and alfalfa residues.

Materials and methods

An 80-day incubation experiment was carried out to investigate the effects of oak wood biochar (BC; 10 Mg ha^?1), PAM (80 kg ha^?1), and their combination (BC?+?PAM) on decomposition of SOM and ¹⁴C-labeled alfalfa (Medicago sativa L.) residues by measuring CO₂ efflux, microbial biomass, and specific respiration activity.

Results and discussion

No conditioner exerted a significant effect on SOM decomposition over the 80 days of incubation. PAM increased cumulative CO₂ efflux at 55–80 days of incubation on average of 6.7 % compared to the soil with plant residue. This was confirmed by the increased MBN and MB¹⁴C at 80 days of incubation in PAM-treated soil with plant residue compared to the control. In contrast, BC and BC?+?PAM decreased plant residue decomposition compared to that in PAM-treated soil and the respective control soil during the 80 days. BC and BC?+?PAM decreased MBC in soil at 2 days of incubation indicated that BC suppressed soil microorganisms and, therefore, decreased the decomposition of plant residue.

Conclusions

The addition of oak wood BC alone or in combination with PAM to soil decreased the decomposition of plant residue.

     

An evaluation of C and N on fresh and aged crop residue from mixed long-term no-till cropping systems

Conservation crop residue management increases soil organic carbon (SOC) storage, nutrient cycling and availability and improves soil quality. This study was conducted to evaluate the amount of residue biomass, residue carbon to nitrogen (C:N) ratio, residue carbon (C) and nitrogen (N), and residue N fertilizer deficit (supplemental N fertilizer requirement) from crop residue decomposition in long-term no-till production. Aboveground aged and fresh residues were collected in spring 2011 and fall 2012, respectively. Results showed slightly greater residue dry matter weight in aged residue than fresh residue. C:N ratios were wider in fresh residue than the aged residue. Both aged and fresh residue also showed wider C:N ratio in the corn (Zea mays L.)-soybean (Glycine max L.) rotation (66.6 and 64.4, respectively) and narrower C:N ratio in the spring wheat (Triticum aestivum L.)-winter wheat (Triticum aestivum L.)-alfalfa (Medicago sativa L.)-alfalfa-corn (Zea mays L.)-soybean (Glycine max L.) (45.6 and 35.7, respectively). Individual fresh crop residues showed narrower C:N ratios for legume and cover crops than non-legume crops. Analysis of potential supplemental N fertilizer requirements showed greater potential N requirement for the fresh residue than the aged residue.