Responses of enzymatic activities within soil aggregates to 9-year nitrogen and water addition in a semi-arid grassland

Soil microorganisms secrete enzymes used to metabolize carbon (C), nitrogen (N), and phosphorus (P) from the organic materials typically found in soil. Because of the connection with the active microbial biomass, soil enzyme activities can be used to investigate microbial nutrient cycling including the microbial response to environmental changes, transformation rates and to address the location of the most active biomass. In a 9-year field study on global change scenarios related to increasing N inputs (ambient to 15 g N m⁻² yr⁻¹) and precipitation (ambient to 180 mm yr⁻¹), we tested the activities of soil β-glucosidase (BG), N-acetyl-glucosaminidase (NAG) and acid phosphomonoesterase (PME) for three soil aggregate classes: large macroaggregates (>2000 μm), small macroaggregates (250–2000 μm) and microaggregates (<250 μm). Results showed higher BG and PME activities in micro-vs. small macroaggregates whereas the highest NAG activity was found in the large macroaggregates. This distribution of enzyme activity suggests a higher contribution of fast-growing microorganisms in the micro-compared with the macroaggregates size fractions. The responses of BG and PME were different from NAG activity under N addition, as BG and PME decreased as much as 47.1% and 36.3%, respectively, while the NAG increased by as much as 80.8%, which could imply better adaption of fungi than bacteria to lower soil pH conditions developed under increased N. Significant increases in BG and PME activities by as much as 103.4 and 75.4%, respectively, were found under water addition. Lower ratio of BG:NAG and higher NAG:PME underlined enhanced microbial N limitation relative to both C and P, suggesting the repression of microbial activity and the accompanied decline in their ability to compete for N with plants and/or the accelerated proliferation of soil fungi under elevated N inputs. We conclude that changes in microbial activities under increased N input and greater water availability in arid- and semi-arid grassland ecosystems where NPP is co-limited by N and water may result in substantial redistribution of microbial activity in different-sized soil particles. This shift will influence the stability of SOM in the soil aggregates and the nutrient limitation of soil biota.     

The distribution of nematodes and soil microbial communities across soil aggregate fractions and farm management systems

We hypothesized that nematode and microbial communities vary between soil aggregate fractions due to variations in physical and/or resource constraints associated with each fraction and that this, in turn, contributes to management impacts on whole soil food webs. Nematode and microbial communities were examined within three soil fractions: large macroaggregates (LM; >1000 μm), small macroaggregates (SM; 250-1000 μm) and inter-aggregate soil and space (IS; <250 μm) isolated from soils of four agricultural management systems: conventional tomato (CON), organic tomato (ORG), a minimum till grain-legume intercrop with continuous cover (CC) and an unmanaged riparian corridor (RC). Aggregate fractions appeared to influence nematode assemblages more than did management system. In general the IS and LM fractions contained higher densities of all nematode trophic groups than did SM. Management × fraction interactions for bacterivores and fungivores, however; suggested a non uniform trend across management systems. The IS fraction exhibited stronger trophic links, per the nematode structure index (SI), while the LM and SM fractions had more active fungal decomposition channels as indicated by the channel index (CI). Higher adult to juvenile ratios in the LM and IS than the SM fraction, and a positive correlation between nematode density in the IS fraction and the proportion of macroaggregates in the soil, indicated an association between soil structure and nematode distribution. Microbial communities varied across both aggregate fractions and management systems. Phospholipid fatty acid (PLFA) analysis suggested that the LM fraction contained greater microbial biomass, gram positive bacteria, and eukaryotes than the IS fraction, while SM contained intermediate PLFA associated with these groups. Total PLFA was greater under RC and ORG than under CC or CON. Total PLFA was positively correlated with % C in soil fractions while nematode abundance exhibited no such relationship. Our findings suggest that microbial communities are more limited by resource availability than by habitable pore space or predation, while nematode communities, although clearly resource-dependent, are better associated with habitable pore space for the soil fractions studied here.     

Soil texture affects soil microbial and structural recovery during grassland restoration

Many biotic and abiotic factors influence recovery of soil communities following prolonged disturbance. We investigated the role of soil texture in the recovery of soil microbial community structure and changes in microbial stress, as indexed by phospholipid fatty acid (PLFA) profiles, using two chronosequences of grasslands restored from 0 to 19 years on silty clay loam and loamy fine sand soils in Nebraska, USA. All restorations were formerly cultivated fields seeded to native warm-season grasses through the USDA’s Conservation Reserve Program. Increases in many PLFA concentrations occurred across the silty clay loam chronosequence including total PLFA biomass, richness, fungi, arbuscular mycorrhizal fungi, Gram-positive bacteria, Gram-negative bacteria, and actinomycetes. Ratios of saturated:monounsaturated and iso:anteiso PLFAs decreased across the silty clay loam chronosequence indicating reduction in nutrient stress of the microbial community as grassland established. Multivariate analysis of entire PLFA profiles across the silty clay loam chronosequence showed recovery of microbial community structure on the trajectory toward native prairie. Conversely, no microbial groups exhibited a directional change across the loamy fine sand chronosequence. Changes in soil structure were also only observed across the silty clay loam chronosequence. Aggregate mean weighted diameter (MWD) exhibited an exponential rise to maximum resulting from an exponential rise to maximum in the proportion of large macroaggregates (>2000 μm) and exponential decay in microaggregates (<250 μm and >53 μm) and the silt and clay fraction (<53 μm). Across both chronosequences, MWD was highly correlated with total PLFA biomass and the biomass of many microbial groups. Strong correlations between many PLFA groups and the MWD of aggregates underscore the interdependence between the recovery of soil microbial communities and soil structure that may explain more variation than time for some soils (i.e., loamy fine sand). This study demonstrates that soil microbial responses to grassland restoration are modulated by soil texture with implications for estimating the true capacity of restoration efforts to rehabilitate ecosystem functions.     

Contributions of soil biota to C sequestration varied with aggregate fractions under different tillage systems

It is increasingly believed that substantial soil organic carbon (SOC) can be sequestered in conservation tillage system by manipulating the functional groups of soil biota. Soil aggregates of different size provide diverse microhabitats for soil biota and consequently influence C sequestration. Our objective was to evaluate the contributions of soil biota induced by tillage systems to C sequestration among different aggregate size fractions. Soil microbial and nematode communities were examined within four aggregate fractions: large macroaggregates (>2 mm), macroaggregates (2–1 mm), small macroaggregates (1–0.25 mm) and microaggregates (<0.25 mm) isolated from three tillage systems: no tillage (NT), ridge tillage (RT) and conventional tillage (CT) in Northeast China. Soil microbial and nematode communities varied across both tillage systems and aggregate fractions. The activity and abundance of microbes and nematodes were generally higher under NT and RT than under CT. Among the four aggregate fractions, soil microbial biomass and diversity were higher in microaggregates, while soil nematode abundance and diversity were higher in large macroaggregates. Structural equation modelling (SEM) revealed that the linkage between microbial and nematode communities and their contributions to soil C accumulation in >1 mm aggregate fractions were different from those in <1 mm aggregate fractions. Higher abundance of arbuscular mycorrhizal fungi (AMF) could enhance C retention within >1 mm aggregates, while more gram-positive bacteria and plant-parasitic nematodes might increase C accumulation within <1 mm aggregates. Our findings suggested that the increase in microbial biomass and nematode abundance and the alteration in their community composition at the micro-niche within aggregates could contribute to the higher C sequestration in conservation tillage systems (NT and RT).     

Effect of grassland harvesting frequency and N-fertilization on stocks and dynamics of soil organic matter in the temperate climate

ABSTRACT

Management of grassland may affect the dynamics of soil organic carbon (SOC). Objectives were to analyze the effect of different harvesting frequencies and nitrogen fertilization regimes on SOC and total N stocks in a field trial on a sandy loam to loamy sand soil of a grassland site near Kiel (Germany). Additionally, effects on microbial biomass C (C_mic) and ergosterol (as proxy for fungi) contents, water-stable aggregate size-classes and density fractions were studied. In the surface soil (0–10 cm), SOC and total N stocks, amounts of large water-stable macroaggregates (> 2000 µm) and contents of C_mic and ergosterol were significantly higher under a five cut regime. C_mic (r_Spearman = 0.61) and ergosterol contents (r_Spearman = 0.67) were correlated with amounts of large water-stable macroaggregates suggesting that fungi and microbial biomass play an important role in binding of small macroaggregates into large macroaggregates. The free light fraction of SOM showed significantly higher C concentrations under three cut compared to five cut at 30–60 cm, presumably related to the C/N ratio and the decomposability of root litter. This study indicates the importance of cutting frequency on SOC and total N stocks, amounts of large macroaggregates and contents of C_mic and ergosterol.     

土壤团聚体氧化亚氮排放潜势与微生物学机制

氧化亚氮（N2O）是主要温室气体之一,土壤是N2O的重要排放源,其排放主要受N2O产生和还原的功能微生物影响。土壤团聚体是由原生颗粒（砂、粉、黏粒）、胶结物质和孔隙组成的土壤基本结构单元。土壤不同粒径团聚体之间因基质和孔隙差异形成特殊独立的微生境被视为N2O的生物化学反应器。在不同的微生境中,N2O产生和还原的功能微生物分布不同,因而土壤不同粒径团聚体N2O排放可能存在差异。目前在不同生态系统土壤全土N2O排放特征的报道较多,而对于不同粒径土壤团聚体N2O排放相对贡献尚不清楚、功能微生物分布还未知、N2O产生和还原热区尚未明确。本文综述了近年来国内外关于土壤团聚体对N2O产生和排放机制的研究,总结了土壤团聚体性状特征对N2O产生和还原的影响,阐述了不同粒径土壤团聚体对N2O排放影响的微生物学机制,进一步明确了今后需加强土壤团聚体N2O产生和还原的热区、环境因子阈值范围的确定、系列功能基因（酶）整体性的研究,以期为N2O模拟排放模型优化提供参考,为土壤N2O减排提供理论依据。     

Protection of soil carbon by microaggregates within earthworm casts

Earthworms are known to play a role in aggregate formation and soil organic matter (SOM) protection. However, it is still unclear at what scale and how quickly earthworms manage to protect SOM. We investigated the effects of Aporrectodea caliginosa on aggregation and aggregate-associated C pools using ¹³C-labeled sorghum (Sorghum bicolor (L.) Moench) leaf residue. Two incubations were set up. The first incubation consisted of soil samples crushed <250 μm to break up all macroaggregates with three treatments: (i) control soil; (ii) soil+¹³C-labeled residue and (iii) soil+¹³C-labeled residue+earthworms. Earthworms were added after 8 d and 12 d (days) later, aggregate size distribution was measured together with total C and ¹³C in each aggregate fraction. A second incubation was made to assay protected versus unprotected total C and ¹³C from 21-d laboratory incubations of intact and crushed large (>2000 μm) and small (250-2000 μm) macroaggregates and microaggregates (53-250 μm). Eight different pools of aggregate-associated C were quantified: (1) and (2) unprotected C pools in large and small macroaggregates, (3) unprotected C pools in microaggregates, (4) and (5) protected C pools in large and small macroaggregates, (6) protected C pool in microaggregates, and (7) and (8) protected C pools in microaggregates within large and small macroaggregates. In the presence of earthworms, a higher proportion of large macroaggregates was newly formed and these aggregates contained more C and ¹³C compared to bulk soil. There were no significant differences between the samples with or without earthworms in the C pool-sizes protected by macroaggregates, microaggregates or microaggregates within small macroaggregates. However, in the presence of earthworms, the C protected by microaggregates within large macroaggregates was a significant pool and 22% of this C pool was newly added C. In conclusion, these results clearly indicate the direct involvement of earthworms in providing protection of soil C in microaggregates within large macroaggregates leading to a possible long-term stabilization of soil C.     

Effect of litter quality and soil fungi on macroaggregate dynamics and associated partitioning of litter carbon and nitrogen

We investigated the effect of plant residue decomposability and fungal biomass on the dynamics of macroaggregate (250–2000 μm) formation in a three months' incubation experiment and determined the distribution of residue-derived C and N in the microbial biomass and in aggregate size fractions (250–2000 μm, 53–250 μm and <53 μm) using ¹³C and ¹⁵N data. A silty loam soil (sieved <250 μm) was incubated with and without addition of ¹⁵N labelled maize leaves (C/N = 27.4) and roots (C/N = 86.4). Each treatment was carried out with and without fungicide application. The addition of maize residues enhanced soil respiration and microbial biomass C and N and resulted in increased macroaggregate formation with a higher and more rapid maximum macroaggregation in the soil amended with maize leaves than in that with addition of roots. Fungicide application led to a significant decline of microbial biomass C and mineralization of the added residues compared to untreated soils, which demonstrates a successful suppression of part of the active microbial biomass by the fungicide. However, this was not confirmed by a generally lower ergosterol concentration. Consequently, ergosterol was no reliable fungal biomarker in periods of rapid decline of the fungal biomass. A single addition of fungicide was insufficient for continued inhibition of the fungal biomass. Yet, a significant delay (28–42 days) in macroaggregation in fungicide treated compared to untreated samples highlighted the importance of the fungal biomass in macroaggregate formation. Macroaggregates were enriched in maize-derived ¹³C and ¹⁵N compared to microaggregates or the fraction < 53 μm. They turned over rapidly with decreasing substrate availability, which entailed a transfer of maize-derived C and N stored within macroaggregates during the first weeks of incubation to microaggregates with proceeding incubation time. Our results indicate that this transfer happened within macroaggregates, because no considerable amount of free particulate organic matter (POM) was released upon macroaggregate breakdown. We conclude that substrate decomposability and fungal activity are key factors determining extent and dynamics of macroaggregation during decomposition processes. Macroaggregate formation implied rapid incorporation and thereby short-term protection of maize-derived C and N. Moreover, macroaggregates allowed a transfer of maize-derived organic matter into microaggregates within macroaggregates, which prevented the release of significant amounts of free POM upon macroaggregate breakdown. Consequently, macroaggregates constitute to the transfer of recently added C into more stable soil organic matter fractions.     

Drying and rewetting effects on soil microbial community composition and nutrient leaching

The effects of a dry-rewetting event (D/RW) on soil microbial properties and nutrient release by leaching from two soils taken from adjacent grasslands with different histories of management intensity were studied. These were a low-productivity grassland, with no history of fertilizer application and a high-productivity grassland with a history of high fertilizer application, referred to as unimproved and improved grassland, respectively. The use of phospholipid fatty acid analysis (PLFA) revealed that the soil of the unimproved grassland had a significantly greater microbial biomass, and a greater abundance of fungi relative to bacteria than did the improved grassland. Soils from both grasslands were maintained at 55% water holding capacity (WHC) or dried to 10% WHC and rewetted to 55% WHC, and then sampled on days 1, 3, 9, 16, 30 and 50 after rewetting. The D/RW stress significantly reduced microbial biomass carbon (C), fungal PLFA and the ratio of fungal-to-bacterial PLFA in both soils. In contrast, D/RW increased microbial activity, but had no effect on total PLFA and bacterial PLFA in either soil. Microbial biomass nitrogen (N) was reduced significantly by D/RW in both soils, but especially in those of the improved grassland. In terms of nutrient leaching, the D/RW stress significantly increased concentrations of dissolved organic C and dissolved organic N in leachates taken from the improved soil only. This treatment increased the concentration of dissolved inorganic N in leachate of both soils, but this effect was most pronounced in the improved soil. Overall, our data show that D/RW stress leads to greater nutrient leaching from improved than from unimproved grassland soils, which have a greater microbial biomass and abundance of fungi relative to bacteria. This finding supports the notion that soils with more fungal-rich communities are better able to retain nutrients under D/RW than are their intensively managed counterparts with lower fungal to bacterial ratios, and that D/RW can enhance nutrient leaching with potential implications for water quality.     

A re‐evaluation of the enriched labile soil organic matter fraction

Identifying ‘functional' pools of soil organic matter and understanding their response to tillage remains elusive. We have studied the effect of tillage on the enriched labile fraction, thought to derive from microbes and having an intermediate turnover time. Four soils, each under three regimes, long‐term arable use without tillage (NT), long‐term arable under conventional tillage (CT), and native vegetation (NV), were separated into four aggregate size classes. Particle size fractions of macro‐ (250–2000 μm) and microaggregates (53–250 μm) were isolated by sonication and sieving. Subsequently, densiometric and chemical analyses were made on fine‐silt‐sized (2–20 μm) particles to isolate and identify the enriched labile fraction. Across soils, the amounts of C and N in the particle size fractions were highly variable and were strongly influenced by mineralogy, specifically by the contents of Fe and Al oxides. This evidence indicates that the fractionation procedure cannot be standardized across soils. In one soil, C associated with fine‐silt‐sized particles derived from macroaggregates was 567 g C m^?2 under NV, 541 g C m^?2 under NT, and 135 g C m^?2 under CT, whereas C associated with fine‐silt‐sized particles derived from microaggregates was 552, 1018, 1302 g C m^?2 in NV, NT and CT, respectively. These and other data indicate that carbon associated with fine‐silt‐sized particles is not significantly affected by tillage. Its location is simply shifted from macroaggregates to microaggregates with increasing tillage intensity. Natural abundance ¹³C analyses indicated that the enriched labile fraction was the oldest fraction isolated from both macro‐ and microaggregates. We conclude that the enriched labile fraction is a ‘passive' pool of soil organic matter in the soil and is not derived from microbes nor sensitive to cultivation.     

Changes in soil aggregation and microbial community structure control carbon sequestration after afforestation of semiarid shrublands

Changes in plant cover after afforestation induce variations in litter inputs and soil microbial community structure and activity, which may promote the accrual and physical-chemical protection of soil organic carbon (SOC) within soil aggregates. In a long-term experiment (20 years) we have studied the effects, on soil aggregation and SOC stabilization, of two afforestation techniques: a) amended terraces with organic refuse (AT), and b) terraces without organic amendment (T). We used the adjacent shrubland (S) as control. Twenty years after stand establishment, aggregate distribution (including microaggregates within larger aggregates), sensitive and slow organic carbon (OC) fractions, basal respiration in macroaggregates, and microbial community structure were measured. The main changes occurred in the top layer (0–5 cm), where: i) both the sensitive and slow OC fractions were increased in AT compared to S and T, ii) the percentage and OC content of microaggregates within macroaggregates (Mm) were higher in AT than in S and T, iii) basal respiration in macroaggregates was also higher in AT, and iv) significant changes in the fungal (rather than bacterial) community structure were observed in the afforested soils (AT and T) – compared to the shrubland soil. These results suggest that the increase in OC pools linked to the changes in microbial activity and fungal community structure, after afforestation, promoted the formation of macroaggregates – which acted as the nucleus for the formation and stabilization of OC-enriched microaggregates.     

Distribution of microbial biomass and its activity in different soil aggregate size classes as affected by cultivation

Cultivation is known to influence the organic matter status and structural stability of soil. We investigated the effects of 69 yr of cultivation on the nature, distribution and activity of microbial biomass (MB) in different aggregate size classes of an Orthic Brown Chernozemic soil. Cultivation decreased MB content, its activity and enzyme activity in soil. Microaggregate (<0.25mm) size classes in both native and cultivated soils contained lower organic-C, MB-C, fungal biomass, arylsulfatase, acid phosphatase and respiratory activities as compared to macroaggregates. However, the negative effects of cultivation were more pronounced on macroaggregate size classes. Nutrient ratios of both whole aggregates and microbial biomass were narrower in aggregates from cultivated soil as compared to native soil. In both native and cultivated soils, mineralization of C. N and S was greater in macroaggregates as compared to that in microaggregates. The greatest effect of cultivation on nutrient and microbial characteristics was observed in the 0.25 to 1.00 mm dia size classes. These results suggest that microbial biomass, especially fungal biomass, plays an important role in the formation of macroaggregates and is the labile organic matter that serves as the primary source of C and nutrients released following cultivation.     

Rapid incorporation of carbon from fresh residues into newly formed stable microaggregates within earthworm casts

Earthworms play an important role in protecting carbon in the soil, but the exact influence of their activity on the distribution and protection of C is still poorly understood. We investigated the effect of earthworms on the formation of stable microaggregates inside newly formed macroaggregates and the distribution of C in them. We crushed (< 250 µm) soil, and subjected it to three treatments: (i) soil + ¹³C‐labelled residue + earthworms (these added after 8 days' incubation), (ii) soil + ¹³C‐labelled residue, and (iii) control (no additions), and then incubated it for 20 days. At the end, we measured the aggregate size distribution, total C and ¹³C, and we isolated microaggregates (53–250 µm) from macroaggregates (> 250 µm) formed. The ¹³C in fine particulate organic matter between and within the microaggregates was determined. Earthworms helped to form large macroaggregates (> 2000 µm). These large macroaggregates contained four times more stable microaggregates than those from samples without earthworms. There was more particulate organic matter within and between microaggregates in macroaggregates in the presence of earthworms. The larger amounts of organic matter inside stable microaggregates in casts than in bulk soil after 12 days of incubation (140 mg ¹³C kg^?1 soil compared with 20 mg ¹³C kg^?1 soil) indicates that these microaggregates are formed rapidly around freshly incorporated residues within casts. In conclusion, earthworms have a direct impact on the formation of stable microaggregates and the incorporation of organic matter inside these microaggregates, and it seems likely that their activity is of great significance for the long‐term stabilization of organic matter in soils.     

Quantifying water-stable soil aggregate turnover and its implication for soil organic matter dynamics in a model study

Recent studies have concluded that the dynamics of soil structure are central to the understanding of soil organic matter (SOM) cycling and the ensuing soil‐water–nutrient relationships. Aggregate turnover directly controls the stabilization and physical protection of SOM. Therefore, quantifying aggregate dynamics will improve our ability to predict SOM behaviour as affected by ecosystem management and global change. We present an approach to directly quantify aggregate dynamics using rare‐earth oxides as tracers. A 6‐week laboratory incubation was set up to measure aggregate dynamics at different times. We made samples in which each different aggregate size‐fraction contained a different tracer. By following the redistribution of these tracers into the other aggregate size‐fractions, we could quantify all soil mass transfers between aggregate size‐fractions. A comparison with a control soil showed that the tracer did not affect soil respiration or the aggregation process itself. Tracer mixing homogeneity, recovery and immobility were tested and validated. While initially macroaggregate formation occurred rapidly, microaggregate formation occurred more slowly during the experiment. Subsequent aggregate stabilization was more pronounced for the newly formed microaggregates than for the newly formed macroaggregates. Calculated turnover times were smaller for macroaggregates than for microaggregates (i.e. 30 vs. 88 days). Further research is needed to investigate to what extent these results can be extrapolated to the field. Our results confirmed existing qualitative views and concepts on aggregate dynamics in a quantitative way and will be valuable in directly linking aggregate turnover to the stabilization and protection of SOM.     

Particulate organic matter in water stable aggregates formed after the addition of C-labeled maize residues and wetting and drying cycles in vertisols

Development of soil structure and the dynamics of water stable aggregates (WSA) in many soils are known to be closely related to the cycling of soil organic matter. In some fine and medium textured soils particulate organic matter (POM) has been found to act as a nucleus for macroaggregate formation. However, this role of POM in aggregate formation has not been demonstrated in soils dominated by smectitic clay minerals. This study explored aggregation processes in a Vertisol from a semi-arid region in Northeastern Mexico in relation to the addition of ¹⁴C-labeled maize residues and application of wetting and drying cycles during 105 days of incubation. Fractionation of the WSA formed showed that labeled residues were preferentially accumulated in large macroaggregates (>2000 μm). Treatments with addition of organic residues had three to four times more intra-aggregate particulate organic matter (iPOM) in large macroaggregates than the control after 14 days of incubation. Residue-derived carbon accounted for 53% and 41% of the total carbon stored in the iPOM fraction in amended treatments with and without wetting and drying cycles, respectively. Conversely, residue-derived carbon represented <20% of the total carbon in the iPOM fraction from small macroaggregates (250-2000 μm) and microaggregates (53-250 μm). Results also showed that the amount and concentration of carbon per large macroaggregate did not differ between the large macroaggregates formed under wetting and drying and those formed in continuous moist conditions. However, due to formation of higher number of large macroaggregates per kg of soil, more carbon could be stored in amended soils under wetting and drying than in constantly wet soil: 1.4, 1.8 and 2.7 times more ¹⁴C kg⁻¹ soil after 14, 58 and 105 incubation days, respectively. The results in this study suggest that wetting and drying enhanced protection of the added maize residues inside large macroaggregates by forming more aggregates, rather than by increasing the amount of POM entrapped per aggregate. Therefore, after the addition of organic residues, this soil could accumulate more C than continuous moist soil through the influence that wetting and drying has on soil aggregation.     

Relationships between soil macroaggregation and humic carbon in a sandy loam soil following conservation tillage

Purpose

Humic substances are recalcitrant and might act as persistent binding agents to form macroaggregates. The focus of this study is in investigating the contribution of humic carbon (HC) to soil aggregation in response to various tillage and residue managements.

Materials and methods

Arable soils following 8-year contrasting managements were collected to determine aggregate size distribution and stability and HC fractions including humic acid (HA) and fulvic acid (FA). The contribution of HC to aggregation was divided into three special effects including positive effect (PE), negative effect (NE), and combined effect (CE), and these effects were measured using aggregate fractionation techniques.

Results and discussion

As well as to promote structural stability, HC bounds predominantly with the silt + clay fraction and secondarily with microaggregates to form larger aggregates. The PE increased with increasing aggregate size, whereas the NE followed the opposite pattern. A positive CE was observed for large and small macroaggregates, whereas the CE for microaggregates and the silt + clay fraction was negative. Compared to continuous tillage, reduced- and no-tillage decreased the PE for large and small macroaggregates by 1.58–30.98% at the 0–20 cm depth, and straw returning also slightly decreased the corresponding PE relative to straw removing. By contrast, a significantly higher NE for small macroaggregates at the 0–10 cm depth while 6.33–81.11% decreases in CE for large and small macroaggregates at the 0–10 cm depth as well as for large macroaggregates at the 10–20 cm depth, were observed under reduced- and no-tillage. The extraction of HC significantly reduced the aggregate stability and reduced- and no-tillage effectively limited its decrease magnitude. Small macroaggregates and microaggregates made larger contributions to soil HC accumulation than did other fractions. An averagely increased contribution from large or small macroaggregates was observed under both reduced-/no-tillage and straw returning at the 0–20 cm depth. A significant and positive relationship was found between the mass proportion of macroaggregates and the HC accumulation in 0–20 cm soil. Large macroaggregates had significantly higher HA/FA ratios than small macroaggregates, and reduced- and no-tillage significantly increased these ratios both in large and in small macroaggregates. The CE for large or small macroaggregates was also significantly negatively correlated with their HA/FA ratios.

Conclusions

Overall, the HC accumulation in soil is likely to play a key role in macroaggregation, but conservation tillage might decrease the contribution magnitude of HC to large or small macroaggregation through increasing the corresponding HA/FA ratios.

     

Reciprocal transfer of carbon and nitrogen by decomposer fungi at the soil-litter interface

We have investigated whether decomposer fungi translocate litter-derived C into the underlying soil while simultaneously translocating soil-derived inorganic N up into the litter layer. We also located and quantified where the translocated C is deposited within the soil aggregate structure. When ¹³C-labeled wheat straw was decomposed on the surface of soil amended with ¹⁵N-labeled inorganic N, we found that C and N were reciprocally transferred by fungi, with a significant quantity (121-151 μg C g⁻¹ whole soil) of litter-derived C being deposited into newly formed macroaggregates (>250 μm sized aggregates). Fungal inhibition reduced fungal biomass and the bidirectional C and N flux by approximately 50%. The amount of litter-derived C found in macroaggregates was positively correlated with litter-associated fungal biomass. This fungal-mediated litter-to-soil C transfer, which to our knowledge has not been demonstrated before for saprophytic fungi, may represent an important mechanism by which litter C enters the soil and becomes stabilized as soil organic matter within the macroaggregate structure.     

Microbial community composition and carbon cycling within soil microenvironments of conventional, low-input, and organic cropping systems

This study coupled stable isotope probing with phospholipid fatty acid analysis (¹³C-PLFA) to describe the role of microbial community composition in the short-term processing (i.e., C incorporation into microbial biomass and/or deposition or respiration of C) of root- versus residue-C and, ultimately, in long-term C sequestration in conventional (annual synthetic fertilizer applications), low-input (synthetic fertilizer and cover crop applied in alternating years), and organic (annual composted manure and cover crop additions) maize-tomato (Zea mays - Lycopersicum esculentum) cropping systems. During the maize growing season, we traced ¹³C-labeled hairy vetch (Vicia dasycarpa) roots and residues into PLFAs extracted from soil microaggregates (53-250 μm) and silt-and-clay (<53 μm) particles. Total PLFA biomass was greatest in the organic (41.4 nmol g⁻¹ soil) and similar between the conventional and low-input systems (31.0 and 30.1 nmol g⁻¹ soil, respectively), with Gram-positive bacterial PLFA dominating the microbial communities in all systems. Although total PLFA-C derived from roots was over four times greater than from residues, relative distributions (mol%) of root- and residue-derived C into the microbial communities were not different among the three cropping systems. Additionally, neither the PLFA profiles nor the amount of root- and residue-C incorporation into the PLFAs of the microaggregates were consistently different when compared with the silt-and-clay particles. More fungal PLFA-C was measured, however, in microaggregates compared with silt-and-clay. The lack of differences between the mol% within the microbial communities of the cropping systems and between the PLFA-C in the microaggregates and the silt-and-clay may have been due to (i) insufficient differences in quality between roots and residues and/or (ii) the high N availability in these N-fertilized cropping systems that augmented the abilities of the microbial communities to process a wide range of substrate qualities. The main implications of this study are that (i) the greater short-term microbial processing of root- than residue-C can be a mechanistic explanation for the higher relative retention of root- over residue-C, but microbial community composition did not influence long-term C sequestration trends in the three cropping systems and (ii) in spite of the similarity between the microbial community profiles of the microaggregates and the silt-and-clay, more C was processed in the microaggregates by fungi, suggesting that the microaggregate is a relatively unique microenvironment for fungal activity.     

Abundance, production and stabilization of microbial biomass under conventional and reduced tillage

Soil tillage practices affect the soil microbial community in various ways, with possible consequences for nitrogen (N) losses, plant growth and soil organic carbon (C) sequestration. As microbes affect soil organic matter (SOM) dynamics largely through their activity, their impact may not be deduced from biomass measurements alone. Moreover, residual microbial tissue is thought to facilitate SOM stabilization, and to provide a long term integrated measure of effects on the microorganisms. In this study, we therefore compared the effect of reduced (RT) and conventional tillage (CT) on the biomass, growth rate and residues of the major microbial decomposer groups fungi and bacteria. Soil samples were collected at two depths (0-5 cm and 5-20 cm) from plots in an Irish winter wheat field that were exposed to either conventional or shallow non-inversion tillage for 7 growing seasons. Total soil fungal and bacterial biomasses were estimated using epifluorescence microscopy. To separate between biomass of saprophytic fungi and arbuscular mycorrhizae, samples were analyzed for ergosterol and phospholipid fatty acid (PLFA) biomarkers. Growth rates of saprophytic fungi were determined by [¹⁴C]acetate-in-ergosterol incorporation, whereas bacterial growth rates were determined by the incorporation of ³H-leucine in bacterial proteins. Finally, soil contents of fungal and bacterial residues were estimated by quantifying microbial derived amino sugars. Reduced tillage increased the total biomass of both bacteria and fungi in the 0-5 cm soil layer to a similar extent. Both ergosterol and PLFA analyses indicated that RT increased biomass of saprophytic fungi in the 0-5 cm soil layer. In contrast, RT increased the biomass of arbuscular mycorrhizae as well as its contribution to the total fungal biomass across the whole plough layer. Growth rates of both saprotrophic fungi and bacteria on the other hand were not affected by soil tillage, possibly indicating a decreased turnover rate of soil microbial biomass under RT. Moreover, RT did not affect the proportion of microbial residues that were derived from fungi. In summary, our results suggest that RT can promote soil C storage without increasing the role of saprophytic fungi in SOM dynamics relative to that of bacteria.     

Influence of ciprofloxacin on microbial community structure and function in soils

Microcosm experiments were designed to investigate the effects of the widely used antibiotic ciprofloxacin (CIP) on soil microbial communities by using four different concentrations (0, 1, 5, and 50 mg kg^?1 of soil) and five sampling times (1, 3, 9, 22, and 40 days). Untreated controls only received water. The addition of CIP significantly decreased microbial biomass (p?<?0.05) but did not affect soil respiration at high doses. Potential nitrification rates were stimulated at low CIP concentrations (1 mg kg^?1) and inhibited at high CIP concentrations (50 mg kg^?1) after 9 days of incubation. The nitrate and ammonium contents of soil were not altered after CIP addition at any time. The structure of soil microbial communities was assessed by phospholipid fatty acid (PLFA) analysis. The addition of CIP decreased the ratio of bacteria to fungi and increased the ratio of Gram-positive to Gram-negative bacteria. Principal component analysis of the PLFA data clearly distinguished among the different CIP concentrations. Redundancy analysis indicated that the CIP concentration and incubation time explained 33.5 % of the total variance in the PLFA data. These results confirmed that a single addition of CIP can influence structure and function of microbial communities in soil.