Nitrogen deposition promotes the production of new fungal residues but retards the decomposition of old residues in forest soil fractions

Atmospheric nitrogen (N) deposition has frequently been observed to increase soil carbon (C) storage in forests, but the underlying mechanisms still remain unclear. Changes in microbial community composition and substrate use are hypothesized to be one of the key mechanisms affected by N inputs. Here, we investigated the effects of N deposition on amino sugars, which are used as biomarkers for fungal‐ and bacterial‐derived microbial residues in soil. We made use of a 4‐year combined CO₂ enrichment and N deposition experiment in model forest ecosystems, providing a distinct ¹³C signal for ‘new’ and ‘old’ C in soil organic matter and microbial residues measured in density and particle‐size fractions of soils. Our hypothesis was that N deposition decreases the amount of fungal residues in soils, with the new microbial residues being more strongly affected than old residues. The soil fractionation showed that organic matter and microbial residues are mainly stabilized by association with soil minerals in the heavy and fine fractions. Moreover, the bacterial residues are relatively enriched at mineral surfaces compared to fungal residues. The ¹³C tracing indicated a greater formation of fungal residues compared to bacterial residues after 4 years of experiment. In contradiction to our hypotheses, N deposition significantly increased the amount of new fungal residues in bulk soil and decreased the decomposition of old microbial residues associated with soil minerals. The preservation of old microbial residues could be due to decreased N limitation of microorganisms and therefore a reduced dependence on organic N sources. This mechanism might be especially important in fine heavy fractions with low C/N ratios, where microbial residues are effectively protected from decomposition by association with soil minerals.     

Pathways of mineral‐associated soil organic matter formation: Integrating the role of plant carbon source,chemistry, and point of entry

To predict the behavior of the terrestrial carbon cycle, it is critical to understand the source, formation pathway, and chemical composition of soil organic matter (SOM). There is emerging consensus that slow‐cycling SOM generally consists of relatively low molecular weight organic carbon substrates that enter the mineral soil as dissolved organic matter and associate with mineral surfaces (referred to as “mineral‐associated OM,” or MAOM). However, much debate and contradictory evidence persist around: (a) whether the organic C substrates within the MAOM pool primarily originate from aboveground vs. belowground plant sources and (b) whether C substrates directly sorb to mineral surfaces or undergo microbial transformation prior to their incorporation into MAOM. Here, we attempt to reconcile disparate views on the formation of MAOM by proposing a spatially explicit set of processes that link plant C source with MAOM formation pathway. Specifically, because belowground vs. aboveground sources of plant C enter spatially distinct regions of the mineral soil, we propose that fine‐scale differences in microbial abundance should determine the probability of substrate–microbe vs. substrate–mineral interaction. Thus, formation of MAOM in areas of high microbial density (e.g., the rhizosphere and other microbial hotspots) should primarily occur through an in vivo microbial turnover pathway and favor C substrates that are first biosynthesized with high microbial carbon‐use efficiency prior to incorporation in the MAOM pool. In contrast, in areas of low microbial density (e.g., certain regions of the bulk soil), MAOM formation should primarily occur through the direct sorption of intact or partially oxidized plant compounds to uncolonized mineral surfaces, minimizing the importance of carbon‐use efficiency, and favoring C substrates with strong “sorptive affinity.” Through this framework, we thus describe how the primacy of biotic vs. abiotic controls on MAOM dynamics is not mutually exclusive, but rather spatially dictated. Such an understanding may be integral to more accurately modeling soil organic matter dynamics across different spatial scales.     

Nitrogen limitation of decomposition and decay: How can it occur?

The availability of nitrogen (N) is a critical control on the cycling and storage of soil carbon (C). Yet, there are conflicting conceptual models to explain how N availability influences the decomposition of organic matter by soil microbial communities. Several lines of evidence suggest that N availability limits decomposition; the earliest stages of leaf litter decay are associated with a net import of N from the soil environment, and both observations and models show that high N organic matter decomposes more rapidly. In direct contrast to these findings, experimental additions of inorganic N to soils broadly show a suppression of microbial activity, which is inconsistent with N limitation of decomposition. Resolving this apparent contradiction is critical to representing nutrient dynamics in predictive ecosystem models under a multitude of global change factors that alter soil N availability. Here, we propose a new conceptual framework, the Carbon, Acidity, and Mineral Protection hypothesis, to understand the effects of N availability on soil C cycling and storage and explore the predictions of this framework with a mathematical model. Our model simulations demonstrate that N addition can have opposing effects on separate soil C pools (particulate and mineral‐protected carbon) because they are differentially affected by microbial biomass growth. Moreover, changes in N availability are frequently linked to shifts in soil pH or osmotic stress, which can independently affect microbial biomass dynamics and mask N stimulation of microbial activity. Thus, the net effect of N addition on soil C is dependent upon interactions among microbial physiology, soil mineralogy, and soil acidity. We believe that our synthesis provides a broadly applicable conceptual framework to understand and predict the effect of changes in soil N availability on ecosystem C cycling under global change.     

贵州山区土壤中微生物担是能源物质碳流动的源与汇

在传统的农业生态系统的研究中 ,主要精力放在营养物 (如Ｎ)上 ,认为它们是限制生产力的因素 ;而往往忽略了土壤中碳的重要性 ,认为收获不受Ｃ限制的影响。然而 ,碳循环中的有机碳的分解作用部分控制着出现在地表下和显露在地表上的农业过程[4]。土壤中所储存的有机质 ,其数量既反映土壤从植物残留物的输入所获得的有机质与微生物群落的能量和营养需求之间的平衡 ,又反映植物对营养物的需求与有机质分解作用之间的平衡。因此 ,土壤中碳的平衡能反映出有机质中能量物质的储存[5]。大部分由光合作用形成的碳 ,是通过地表下的生态系统来流动的[…     

Soil enzyme activities and organic matter composition in a turfgrass chronosequence

Highly managed turfgrass systems accumulate considerable soil organic C, which supports a diverse and robust soil microbial community. Degradation of this soil organic C is mediated by a suite of soil enzymes. The relationship between these enzyme activities and the quality of soil organic C is central to understanding the dynamics of soil organic matter. We examined the activities of several soil enzymes involved in microbial C acquisition, including β-glucosidase, N-acetyl-β-glucosaminidase, cellulase, chitinase, and phenol oxidase, and characterized the chemical composition of soil organic matter using Fourier transform infrared spectroscopy (FTIR) in a turfgrass chronosequence (1–95 years old) and adjacent native pines. Non-metric multidimensional scaling analysis showed that the chemical composition of soil organic matter varied with turf age and land use (turf versus pines). Using the polysaccharide peak (1,060 cm⁻¹) as a reference, both aliphatic (2,930 cm⁻¹) and carboxylic (1,650 and 1,380 cm⁻¹) compounds increased with turf age, indicating that soil organic matter became more recalcitrant. Soil enzyme activities per unit soil mass increased with turf age and were correlated to soil C content. Most soil enzyme activities in native pines were similar to those in young turf, but the cellulase activity was similar to or greater than the activity in old turfgrass systems. On a soil C basis, however, the activities of N-acetyl-β-glucosaminidase and cellulase decreased with turf age; this reduction was correlated to the relative changes in the chemical composition of soil organic matter. We observed that the chemical composition of soil organic matter was significantly correlated with the enzyme activity profile when expressed per unit microbial biomass C, but not per unit soil organic C. Our results suggest that chemical composition of soil organic matter changes with turf age and this change partially determines the relative abundance of C-degrading soil enzymes, likely through the influence on microbial community composition.     

An Absorbing Markov Chain approach to understanding the microbial role in soil carbon stabilization

The number of studies focused on the transformation and sequestration of soil organic carbon (C) has dramatically increased in recent years due to growing interest in understanding the global C cycle. While it is readily accepted that terrestrial C dynamics are heavily influenced by the catabolic and anabolic activities of microorganisms, the incorporation of microbial biomass components into stable soil C pools (via microbial living cells and necromass) has received less attention. Nevertheless, microbial-derived C inputs to soils are now increasingly recognized as playing a far greater role in stabilization of soil organic matter than previously believed. Our understanding, however, is limited by the difficulties associated with studying microbial turnover in soils. Here, we describe the use of an Absorbing Markov Chain (AMC) to model the dynamics of soil C transformations among three microbial states: living microbial biomass, microbial necromass, and C removed from living and dead microbial sources. We find that AMC provides a powerful quantitative approach that allows prediction of how C will be distributed among these three states, and how long it will take for the entire amount of initial C to pass through the biomass and necromass pools and be moved into atmosphere. Further, assuming constant C inputs to the model, we can predict how C is eventually distributed, along with how much C sequestrated in soil is microbial-derived. Our work represents a first step in attempting to quantify the flow of C through microbial pathways, and has the potential to increase our understanding of the microbial role in soil C dynamics.     

Estimating vital statistics and age distributions of measurable soil organic carbon fractions based on their pathway of formation and radiocarbon content

Radiocarbon measurements have been used in combination with "bomb 14C" models to estimate turnover of soil organic carbon fractions. However, the bomb 14C models assume that all SOC fractions are formed directly from external inputs of carbon, which is not always valid because some SOC fractions may receive carbon from other SOC fractions. Due to the continuous inputs of organic carbon, we argue that the most appropriate way to describe the age of SOC is by an age distribution. We developed age distributed models of SOC fractions and derived analytical solutions to them. The models all assume that SOC fraction decay can be described by first-order kinetics, but differ in their assumptions about the pathway of SOC fraction formation. The solutions can be used to estimate age distributions at steady state of different SOC fractions based on their radiocarbon content. These age distributions can be used to calculate the mean age, mean residence time, and other vital statistics of each measurable SOC fraction. Furthermore, if a sequential scheme is used to isolate the SOC fractions, an estimated age distribution of the total SOC can be obtained by adding the contributions of each soil fraction. The age distributions can be very helpful in interpretations of soil organic carbon dynamics in different soils.     

Soil Organic Matter Dynamics Along a Vertical Vegetation Gradient in the Gongga Mountain on the Tibetan Plateau

Our knowledge about soil organic matter (SOM) dynamics is limited although this is an important issue in the study of responses of ecosystems to global climate changes. Twelve sampling plots were set up every 200 m from 1 700 to 3 900 m along the vertical vegetation gradient along the east slope of Gongga Mountain. Samples were taken from all 12 plots for SOM content measurement, although only 5 of the 12 plots were subjected to radiocarbon measurements. A radiocarbon isotope method and a time-dependent model were used to quantify the SOM dynamics and SOM turnover rates along the vertical vegetation gradient. The results showed that the SOM turnover rate decreased and turnover time increased with soil depth for all vegetation types. The litter layer turnover rates presented a clear trend along the gradient. The litter layer turnover rates decreased with an increase in elevation, except that the litter layer turnover rate of mixed forest was higher than that of evergreen forest. Climatic factors, such as temperature and precipitation, were the main factors influencing the surface soil carbon dynamics. The turnover rates of the subsoil (including the A, B, and C horizons in the soil profiles) along the vertical gradient had no clear trends. The SOM of subalpine shrub and meadow turned over more slowly than that of the forest types in almost all soil horizons. The characteristic of short roots distributing in the upper part of the soil profile leads to different SOM dynamics of shrub and meadow compared with the forest types. Coniferous and mixed forests were susceptible to carbon loss from the young carbon pool, but their long and big roots resulted in high △^14C values of the deep soil profiles and increased the input of young carbon to the deep soil. In evergreen forest, the carbon cumulative ability from the B horizon to the C horizon was weak. The different vegetation types, together with their different modes of nutrient and carbon intake, may be the mechanism conditioning the subsoil organic matter dynamics.     

Role of proteins in soil carbon and nitrogen storage: controls on persistence

Mechanisms of soil organic carbon (C) and nitrogen (N) stabilization are of great interest, due to the potential for increased CO₂ release from soil organic matter (SOM) to the atmosphere as a result of global warming, and because of the critical role of soil organic N in controlling plant productivity. Soil proteins are recognized increasingly as playing major roles in stabilization and destabilization of soil organic C and N. Two categories of proteins are proposed: detrital proteins that are released upon cell death and functional proteins that are actively released into the soil to fulfill specific functions. The latter include microbial surface-active proteins (e.g., hydrophobins, chaplins, SC15, glomalin), many of which have structures that promote their persistence in the soil, and extracellular enzymes, responsible for many decomposition and nutrient cycling transformations. Here we review information on the nature of soil proteins, particularly those of microbial origin, and on the factors that control protein persistence and turnover in the soil. We discuss first the intrinsic properties of the protein molecule that affect its stability, next possible extrinsic stabilizing influences that arise as the proteins interact with other soil constituents, and lastly controls on accessibility of proteins at coarser spatial scales involving microbial cells, clay particles, and soil aggregates. We conclude that research at the interface between soil science and microbial physiology will yield rapid advances in our understanding of soil proteins. We suggest as research priorities determining the relative abundance and turnover time (age) of microbial versus plant proteins and of functional microbial proteins, including surface-active compounds.     

氮沉降对森林土壤有机质和凋落物分解的影响及其微生物学机制

氮沉降持续增加背景下土壤C∶N∶P化学计量比和pH环境等的改变及其可能的土壤微生物学机制已经成为陆地生态系统与全球变化研究的新生长点和科学研究前沿.以生态化学计量学和土壤微生物生态学为理论基础,综述了氮沉降对森林土壤有机质和凋落物分解的影响及其微生物学机制的基本理论、最新进展、研究热点与难点,旨在促进全球变化背景下陆地生态系统地下生态学的研究.氮沉降持续增加会导致森林生态系统磷循环加速,导致磷限制.氮沉降不但改变森林土壤有机质和凋落物的C∶N∶P化学计量比和降低土壤pH值,而且改变土壤微生物生物量碳氮磷、细菌、真菌和放线菌的组成以及影响碳氮磷分解的关键酶活性.氮沉降对森林土壤有机质和凋落物分解的影响表现为促进、抑制和无影响,其影响的差异可能来源于微生物效应的不同.叶片在凋落前有显著的氮磷养分回收,但是根无明显的养分回收,造成土壤有机质和凋落物的C∶N∶P化学计量比存在明显差异.基于DNA/RNA等分子生物学方法为土壤微生物生态学研究提供了强有力的手段,将促进氮沉降对森林土壤有机质和凋落物化学计量比改变的微生物学机制研究.     

Soil Organic Matter Dynamics Along a Vertical Vegetation Gradient in the Gongga Mountain on the Tibetan Plateau

Our knowledge about soil organic matter (SOM) dynamics is limited although this is an important issue in the study of responses of ecosystems to global climate changes. Twelve sampling plots were set up every 200 m from 1 700 to 3 900 m along the vertical vegetation gradient along the east slope of Gongga Mountain. Samples were taken from all 12 plots for SOM content measurement, although only 5 of the 12plots were subjected to radiocarbon measurements. A radiocarbon isotope method and a time-dependent model were used to quantify the SOM dynamics and SOM turnover rates along the vertical vegetation gradient. The results showed that the SOM turnover rate decreased and turnover time increased with soil depth for all vegetation types. The litter layer turnover rates presented a clear trend along the gradient. The litter layer turnover rates decreased with an increase in elevation, except that the litter layer turnover rate of mixed forest was higher than that of evergreen forest. Climatic factors, such as temperature and precipitation,were the main factors influencing the surface soil carbon dynamics. The turnover rates of the subsoil (including the A, B, and C horizons in the soil profiles) along the vertical gradient had no clear trends. The SOM of subalpine shrub and meadow turned over more slowly than that of the forest types in almost all soil horizons. The characteristic of short roots distributing in the upper part of the soil profile leads to different SOM dynamics of shrub and meadow compared with the forest types. Coniferous and mixed forests were susceptible to carbon loss from the young carbon pool, but their long and big roots resulted in high △14C values of the deep soil profiles and increased the input of young carbon to the deep soil. In evergreen forest,the carbon cumulative ability from the B horizon to the C horizon was weak. The different vegetation types,together with their different modes of nutrient and carbon intake, may be the mechanism conditioning the subsoil organic matter dynamics.     

Priming effect and microbial diversity in ecosystem functioning and response to global change: a modeling approach using the SYMPHONY model

Integration of the priming effect (PE) in ecosystem models is crucial to better predict the consequences of global change on ecosystem carbon (C) dynamics and its feedbacks on climate. Over the last decade, many attempts have been made to model PE in soil. However, PE has not yet been incorporated into any ecosystem models. Here, we build plant/soil models to explore how PE and microbial diversity influence soil/plant interactions and ecosystem C and nitrogen (N) dynamics in response to global change (elevated CO₂ and atmospheric N depositions). Our results show that plant persistence, soil organic matter (SOM) accumulation, and low N leaching in undisturbed ecosystems relies on a fine adjustment of microbial N mineralization to plant N uptake. This adjustment can be modeled in the SYMPHONY model by considering the destruction of SOM through PE, and the interactions between two microbial functional groups: SOM decomposers and SOM builders. After estimation of parameters, SYMPHONY provided realistic predictions on forage production, soil C storage and N leaching for a permanent grassland. Consistent with recent observations, SYMPHONY predicted a CO₂‐induced modification of soil microbial communities leading to an intensification of SOM mineralization and a decrease in the soil C stock. SYMPHONY also indicated that atmospheric N deposition may promote SOM accumulation via changes in the structure and metabolic activities of microbial communities. Collectively, these results suggest that the PE and functional role of microbial diversity may be incorporated in ecosystem models with a few additional parameters, improving accuracy of predictions.     

Nutrient Addition Prompts Rapid Destabilization of Organic Matter in an Arctic Tundra Ecosystem

Nutrient availability in the arctic is expected to increase in the next century due to accelerated decomposition associated with warming and, to a lesser extent, increased nitrogen deposition. To explore how changes in nutrient availability affect ecosystem carbon (C) cycling, we used radiocarbon to quantify changes in belowground C dynamics associated with long-term fertilization of graminoid-dominated tussock tundra at Toolik Lake, Alaska. Since 1981, yearly fertilization with nitrogen (N) and phosphorus (P) has resulted in a shift to shrub-dominated vegetation. These combined changes have altered the quantity and quality of litter inputs, the vertical distribution and dynamics of fine roots, and the decomposition rate of soil organic C. The loss of C from the deep organic and mineral soil has more than offset the C accumulation in the litter and upper organic soil horizons. In the litter and upper organic horizons, radiocarbon measurements show that increased inputs resulted in overall C accumulation, despite being offset by increased decomposition in some soil pools. To reconcile radiocarbon observations in the deeper organic and mineral soil layers, where most of the ecosystem C loss occurred, both a decrease in input of new root material and a dramatic increase of decomposition rates in centuries-old soil C pools were required. Therefore, with future increases in nutrient availability, we may expect substantial losses of C which took centuries to accumulate.     

Soil carbon storage,respiration potential,and organic matter quality across an age and climate gradient in southwestern Greenland

Geological factors influence biological cycling of organic carbon in soils but are not well represented in our understanding of Arctic carbon dynamics. Landscape age, for instance, directly affects quantity and quality of soil carbon, which are two strong controls of the temperature sensitivity of soil organic matter. We investigated soil carbon storage, respiration potential, and organic matter quality for microbial decomposition across a climate and landscape age gradient in southwest Greenland that deglaciated during the Holocene. We measured soil respiration during a 370-day laboratory incubations of active layer soils collected from four study areas across this gradient (ages 1.8 × 10², 6.8 × 10³, and 1.0 × 10⁴, coinciding with a climate gradient from drier inland to wetter coastal terrain) and used a soil respiration model comparison approach to assess the substrate quality of stored organic matter for microbial decomposers. Soils store more than three times greater organic carbon at the 10,000-year-old, maritime climate study areas than the 180-year-old, continental climate study areas. Respiration rates were highest in the surface soils of the coastal areas. Model comparisons reveal important heterogeneity in the quality of organic matter for microbial decomposition between areas: coastal soils were best modeled by both one- and two-pooled models, and inland soils were best represented by one-pooled respiration models. Together, the measures of carbon quality (C:N, CO₂ production, and model parameters estimating initial CO₂ production rates from different organic matter pools) show that shallow soils at the southern coastal area, Kobbefjord, had the highest respiration rates from the recalcitrant carbon pool. This study reveals differences in carbon storage and turnover associated with landscape age and climate factors in western Greenland. When applied to thermodynamic theory, which predicts that temperature sensitivity increases with carbon recalcitrance, our findings suggest that carbon stored in coastal soils may be more sensitive to climate warming than inland soils.     

外源无机氮素形态对土壤氨基糖动态的影响

微生物生长对底物的可利用性存在不同的响应,外源氮素的形态可以显著影响微生物代谢过程,而土壤氨基糖作为微生物细胞壁残留物,其形成、分解和周转特征与外源碳氮供给密切相关,对土壤氨基糖的研究与同位素标记技术相结合,可以进一步反映微生物对底物的利用特征.本文以葡萄糖及15N标记的NH4+和NO3-为底物,利用气相色谱-质谱联机技术,通过测定氨基糖中同位素富集比例,跟踪新形成(标记)和原有(非标记)的土壤氨基糖的动态变化.结果表明:在培养过程中,15N标记的氨基糖含量显著增加,NH4+向氨基糖的转化显著高于NO3-,反映出微生物对NH4+的选择性利用.土壤中原有的氨基糖也发生了不同变化.其中,非标记氨基葡萄糖在N H4+为底物时,其含量有所增加,但在NO3-为底物时含量逐渐下降;非标记胞壁酸含量在2个处理中均不断下降,尤其以NO3-为底物时更为显著;非标记氨基半乳糖含量的增减幅度均小于20％.这种特异性变化表明,不同来源的微生物细胞壁残留物对土壤氮素周转和稳定的作用不同,真菌细胞壁残留物易于在土壤中积累,有利于土壤有机质的稳定,而细菌细胞壁残留物容易分解,在土壤有机质周转过程中起重要作用.     

Pathways of soil organic matter formation from above and belowground inputs in a Sorghum bicolor bioenergy crop

When aboveground materials are harvested for fuel production, such as with Sorghum bicolor, the sustainability of annual bioenergy feedstocks is influenced by the ability of root inputs to contribute to the formation and persistence of soil organic matter (SOM), and to soil fertility through nutrient recycling. Using ¹³C and ¹⁵N labeling, we traced sorghum root and leaf litter‐derived C and N for 19 months in the field as they were mineralized or formed SOM. Our in situ litter incubation experiment confirms that sorghum roots and leaves significantly differ in their inherent chemical recalcitrance. This resulted in different contributions to C and N storage and recycling. Overall root residues had higher biochemical recalcitrance which led to more C retention in soil (27%) than leaf residues (19%). However, sorghum root residues resulted in higher particulate organic matter (POM) and lower mineral associated organic matter (MAOM), deemed to be the most persistent fraction in soil, than leaf residues. Additionally, the overall higher root‐derived C retention in soil led to higher N retention, reducing the immediate recycling of fertility from root as compared to leaf decomposition. Our study, conducted in a highly aggregated clay‐loam soil, emphasized the important role of aggregates in new SOM formation, particularly the efficient formation of MAOM in microaggregate structures occluded within macroaggregates. Given the known role of roots in promoting aggregation, efficient formation of MAOM within aggregates can be a major mechanism to increase persistent SOM storage belowground when aboveground residues are removed. We conclude that promoting root inputs in S. bicolor bioenergy production systems through plant breeding efforts may be an effective means to counterbalance the aboveground residue removal. However, management strategies need to consider the quantity of inputs involved and may need to support SOM storage and fertility with additional organic matter additions.     

Roots and fungi accelerate carbon and nitrogen cycling in forests exposed to elevated CO2

A common finding in multiple CO(2) enrichment experiments in forests is the lack of soil carbon (C) accumulation owing to microbial priming of 'old' soil organic matter (SOM). However, soil C losses may also result from the accelerated turnover of 'young' microbial tissues that are rich in nitrogen (N) relative to bulk SOM. We measured root-induced changes in soil C dynamics in a pine forest exposed to elevated CO(2) and N enrichment by combining stable isotope analyses, molecular characterisations of SOM and microbial assays. We find strong evidence that the accelerated turnover of root-derived C under elevated CO(2) is sufficient in magnitude to offset increased belowground inputs. In addition, the C losses were associated with accelerated N cycling, suggesting that trees exposed to elevated CO(2) not only enhance N availability by stimulating microbial decomposition of SOM via priming but also increase the rate at which N cycles through microbial pools.     

不同施肥管理措施对农田土壤中植物和微生物残留组分的影响

在农田生态系统中,施肥是维持和提高土壤有机碳(SOC)水平的重要管理措施。微生物代谢和植物组分存留共同控制着有机碳的截获过程。本研究利用肥料与肥力长期(30年)定位试验,以氨基糖和木质素分别作为微生物和植物残留组分标识物,探讨长期不同施肥处理对黑土农田中微生物和植物残体组分积累及有机碳库的影响。结果表明: 与未施肥处理相比,施用无机肥(单施氮肥或有机无机肥配施)可增加作物生物量和土壤氨基糖的积累,但对木质素和SOC含量无显著影响,说明无机肥施入刺激了微生物底物同化,加速了有机碳和木质素在耕层的周转。与无机肥相比,长期施用有机肥促进了SOC的累积(增幅38.3%),但是氨基糖在土壤有机碳中所占的比例并未发生显著变化,说明微生物残留物对SOC积累的贡献具有饱和性;而有机肥施入增加了木质素在SOC中的比例,即增加了植物残体对SOC长期积累的贡献。与单施有机肥相比,有机无机肥配施增加了微生物残留物对SOC的积累。因此,长期施肥可以调节微生物残留物和植物残留组分的不同积累过程,从而影响SOC的积累和稳定机制。     

Dynamics of fine root carbon in Amazonian tropical ecosystems and the contribution of roots to soil respiration

Radiocarbon (¹⁴C) provides a measure of the mean age of carbon (C) in roots, or the time elapsed since the C making up root tissues was fixed from the atmosphere. Radiocarbon signatures of live and dead fine (<2 mm diameter) roots in two mature Amazon tropical forests are consistent with average ages of 4–11 years (ranging from <1 to >40 years). Measurements of ¹⁴C in the structural tissues of roots known to have grown during 2002 demonstrate that new roots are constructed from recent (<2‐year‐old) photosynthetic products. High Δ¹⁴C values in live roots most likely indicate the mean lifetime of the root rather than the isotopic signature of inherited C or C taken up from the soil. Estimates of the mean residence time of C in forest fine roots (inventory divided by loss rate) are substantially shorter (1–3 years) than the age of standing fine root C stocks obtained from radiocarbon (4–11 years). By assuming positively skewed distributions for root ages, we can effectively decouple the mean age of C in live fine roots (measured using ¹⁴C) from the rate of C flow through the live root pool, and resolve these apparently disparate estimates of root C dynamics. Explaining the ¹⁴C values in soil pore space CO₂, in addition, requires that a portion of the decomposing roots be cycled through soil organic matter pools with decadal turnover time.