The role of lignin and cellulose in the carbon-cycling of degraded soils under semiarid climate and their relation to microbial biomass

A high level of biological degradation is usually observed in soils under semiarid climate where the low inputs of vegetal debris constraint the development of microbiota. Among vegetal inputs, cellulose and lignin are dominant substrates but their assimilation by the microbial community of semiarid soils is yet not understood. In the present study, ¹³C-labeled cellulose and ¹³C-labeled lignin (75 μg ¹³C g⁻¹ soil) were added to two semiarid soils with different properties and degradation level. Abanilla soil is a bare, highly degraded soil without plant cover growing on it and a total organic C content of 5.0 g kg⁻¹; Santomera soil is covered by plants (20% coverage) based on xerophytic shrubs and has a total organic C content of 12.0 g kg⁻¹. The fate of added carbon was evaluated by analysis of the carbon isotope signature of bulk soil-derived carbon and extractable carbon fractions (water and sodium-pyrophosphate extracts). At long-term (120 days), we observed that the stability of cellulose- and lignin-derived carbon was dependent on their chemical nature. The contribution of lignin-derived carbon to the pool of humic substances was higher than that of cellulose. However, at short-term (30 days), the mineralization of the added substrates was more related to the degradation level of soils (i.e. microbial biomass). Stable isotope probing (SIP) of phospholipid fatty acids (PLFA-SIP) analysis revealed that just a minor part of the microbial community assimilated the carbon derived from cellulose and lignin. Moreover, the relative contribution of each microbial group to the assimilation of lignin-derived carbon was different in each soil.     

Photodegradation of plant litter in the Sonoran Desert varies by litter type and age

Predicting litter decay rates in arid systems has proved elusive and sunlight (photodegradation) is a potentially important but poorly understood driver of litter decay in these systems. We placed three litter types (Cynodon dactylon, Larrea tridentata leaves, and L. tridentata twigs) in envelopes whose tops either transmitted all solar radiation, filtered UV-B, filtered all UV, or filtered all UV and visible solar radiation, on the soil surface of the Sonoran Desert and assessed mass loss over 14 months. Regardless of treatment, final mass loss was greatest in C. dactylon litter and least in L. tridentata twig litter, consistent with initial litter characteristics of presumed litter quality; C. dactylon had the lowest lignin concentration and lignin:N, and the highest cellulose:lignin and area:mass. Compared to litter in sunlight, excluding solar UV, or UV-B, slowed mass loss of all 3 litter types, and UV-B appeared more effective than UV-A in photodegradation. The relative contribution of UV photodegradation to mass loss increased with litter age. After 14 months, litter exposed to solar UV lost 1.2 (C. dactylon), 1.3 (L. tridentata twigs) and 1.4 (L. tridentata leaves) times as much mass as litter not exposed to UV radiation. The relative contribution of UV photodegradation to mass loss increased with the initial C:N ratio of litter, but was not related to initial lignin concentration or optical properties (i.e. UV and visible absorbance and transmittance) of litter. Within all litter type by treatment combinations, there was a strong positive correlation between litter mass loss and ash concentration. In some cases, a discontinuity in this relationship was detected, suggesting a threshold ash concentration, above which further mass loss was negligible. We expected these thresholds to be most prevalent in sunlight, because soil films could prevent sunlight from reaching litter and thereby minimize photodegradation. Contrary to expectations, thresholds were more common in shade or UV filter treatments, suggesting that reductions in photodegradation attributable to soil films were not typically responsible. The effect of shading, which likely enhanced microbial degradation via higher relative humidity due to lower temperatures, depended on litter type and time. Compared to litter in sunlight, mass loss of shaded litter was greater over the initial 3 months in all litter types, illustrating that microbial degradation in shade was greater than photodegradation in sunlight. These differences in mass loss between shaded and sunlit litter increased over the 14 month experiment in L. tridentata twigs, declined in L. tridentata leaves, and disappeared within 6 months in C. dactylon, illustrating that the timing of this shift in the dominance of photodegradation versus microbial degradation was highly dependent on litter type. In a second experiment, we reduced microclimate differences among sun and shade treatments, pre-sterilized litter to reduce microbial degradation, and examined the mass loss of young and old and L. tridentata leaf litter after 53 days outdoors. Consistent with our first experiment, mass loss attributable to photodegradation was greater in old than young litter. Unsterilized litter exposed to sunlight (UV and visible) lost 1.3 (young) and 1.5 (old) times as much mass as shaded litter. Pre-sterilized litter exposed to sunlight lost 11.4 (young litter) and 45.9 (old litter) times as much mass as shaded litter. These large differences in pre-sterilized litter were the result of the very small mass loss of shaded litter (≤0.2%), together with modest losses of sunlit litter (<5%). Taken together, our findings suggest that as litter aged, microbial degradation became a weaker driver of mass loss, while photodegradation became stronger.     

Mass loss and nutrient release during litter decay in peatland: The role of microbial adaptability to litter chemistry

In peatlands the reduced decomposition rate of plant litter is the fundamental mechanism making these peat-accumulating ecosystems effective carbon sinks. A better knowledge of litter decomposition and nutrient cycling is thus crucial to improve our predictions of the effects of anthropogenic perturbation on the capacity of peatlands to continue to behave as carbon sinks. We investigated patterns of plant litter decomposition and nutrient release along a minerotrophic-ombrotrophic gradient in a bog on the south-eastern Alps of Italy. We determined mass loss as well as P, N, K, and C release of seven vascular plant species and four moss species after 1 year in both native and transplanted habitats. Hence, differences in litter decay were supposed to reflect the degree of adaptability of microbial communities to litter quality. Polyphenols/nutrient and C/nutrient quotients appeared as the main parameters accounting for decomposition rates of Sphagnum litter. In particular, litter of minerotrophic Sphagnum species decomposed always faster than litter of ombrotrophic Sphagnum species, both in native and transplanted habitats. Decomposition rates of vascular plant litter in native habitats were always higher than the corresponding mass loss rates of Sphagnum litter. Minerotrophic forbs showed the fastest decomposition both in native and transplanted habitats in accordance with low C/P and C/N litter quotients. On the other hand, C/P quotient seems to play a primary role also in controlling decomposition of graminoids. Decomposition of deciduous and evergreen shrubs was negatively related to their high lignin content. Nitrogen release from Sphagnum litter was primarily controlled by C/N quotient, so that minerotrophic Sphagnum litter released more N than ombrotrophic Sphagnum litter. Overall, we observed slower N release from litter of ombrotrophic vascular plant species compared to minerotrophic vascular plant species. No single chemical parameter could predict the variability associated with different functional groups. The release of K was very high compared to all the other nutrients and rather similar between ombrotrophic and minerotrophic litter types. In Sphagnum litter, a higher C/P quotient was associated with a slower P mineralisation, whereas a faster P release from vascular plant litter seems primarily associated with lower C/P and polyphenols/P quotients.     

Chronic N deposition does not apparently alter the biochemical composition of forest floor and soil organic matter

Future rates of atmospheric N deposition have the potential to slow litter decay and increase the accumulation of soil organic matter by repressing the activity of lignolytic soil microorganisms. We investigated the relationship between soil biochemical characteristics and enzymatic responses in a series of sugar maple (Acer saccharum)-dominated forests that have been subjected to 16 yrs of chronic N deposition (ambient + 3 g NO₃⁻–N m⁻² yr⁻¹), in which litter decay has slowed and soil organic matter has accumulated in sandy spodosols. Cupric-oxide-extractable lignin-derived phenols were quantified to determine the presence, source, and relative oxidation state of lignin-like compounds under ambient and experimental N deposition. Pools of respired C and mineralized N, along with rate constants for these processes, were used to quantify biochemically labile substrate pools during a 16-week laboratory incubation. Extracellular enzymes mediating cellulose and lignin metabolism also were measured under ambient and experimental N deposition, and these values were compared with proxies for the relative oxidation of lignin in forest floor and surface mineral soil. Chronic N deposition had no influence on the pools or rate constants for respired C and mineralized N. Moreover, neither the total amount of extractable lignin (forest floor, P = 0.260; mineral soil, P = 0.479), nor the relative degree of lignin oxidation in the forest floor or mineral soil (forest floor P = 0.680; mineral soil P = 0.934) was influenced by experimental N deposition. Given their biochemical attributes, lignin-derived molecules in forest floor and mineral soil appear to originate from fine roots, rather than leaf litter. Under none of the studied circumstances was the presence or relative oxidation of lignin correlated with the activity of cellulolytic and lignolytic extracellular enzymes. Although chronic atmospheric N deposition has slowed litter decay and increased organic matter in our experiment, it had little effect on biochemical composition of lignin-derived molecules in forest floor and surface mineral soil suggesting organic matter has accumulated by other means. Moreover, the specific dynamics of lignin phenol decay is decoupled from short-term organic matter accumulation under chronic N deposition in this ecosystem.     

Long-term litter decay in Canadian forests and the influence of soil microbial community and soil chemistry

Long-term rates of litter decay have been shown to be primarily influenced by temperature, moisture and litter quality. However, while decomposition is a biological process, the relative importance of microbial communities and other soil chemistry factors is not well understood. Our analysis examined long-term litter decay parameters, microbial community composition via phospholipid fatty acid (PLFA) analysis, and soil organic horizon chemistry at 14 upland forested sites. Data were collected as part of the Canadian Intersite Decomposition Experiment (CIDET), a 12-year national litter decomposition experiment. Residual errors from a two-pool exponential decay model with decay rates modified by mean annual air temperature and moisture stress were compared to PLFA marker groups and chemistry variables. Residual errors were not well explained by soil PLFA marker group abundance or concentration, soil pH, nor soil C:N ratios. The best predictor of residual error was soil carbon percent (%C), with higher %C associated with slower than predicted decomposition.     

Tree species effects on microbial respiration from decomposing leaf and fine root litter

Tree species have an impact on decomposition processes of woody litter, but the effects of different tree species on microbial heterotrophic respiration derived from decomposing litter are still unclear. Here we used leaf and fine root litter of six tree species differing in chemical and morphological traits in a temperate forest and elucidated the effects of tree species on the relationships between litter-derived microbial respiration rates and decomposition rates and morphological traits, including specific leaf area (cm² g⁻¹) and specific root length (m g⁻¹) of litter at the same site. Litterbags set in forest soil were sequentially collected five times over the course of 18 months. During litter decomposition, microbial respiration from leaf and fine root litter differed among the six tree species. Temporal changes in the remaining mass and morphology (specific leaf area and specific root length) were observed, and the magnitude of these changes differed among species. Positive correlations were observed between respiration and mass loss or morphology across species. These results revealed that litter mass loss and morphological dynamics during decomposition jointly enhanced microbial respiration, and these carbon-based litter traits explained species differences in decomposition of leaves and fine roots. In conclusion, tree species influenced the magnitude and direction of microbial respiration during leaf/fine root litter decomposition. Tree species also affected the relationship between microbial respiration and litter decomposition through direct effects of litter traits and indirect effects mediated by regulation of heterotroph requirements.     

Dynamics of litter carbon turnover and microbial abundance in a rye detritusphere

Factors determining C turnover and microbial succession at the small scale are crucial for understanding C cycling in soils. We performed a microcosm experiment to study how soil moisture affects temporal patterns of C turnover in the detritusphere. Four treatments were applied to small soil cores with two different water contents (matric potential of ?0.0063 and ?0.0316 MPa) and with or without addition of ¹³C labelled rye residues (δ¹³C=299‰), which were placed on top. Microcosms were sampled after 3, 7, 14, 28, 56 and 84 days and soil cores were separated into layers with increasing distance to the litter. Gradients in soil organic carbon, dissolved organic carbon, extracellular enzyme activity and microbial biomass were detected over a distance of 3 mm from the litter layer. At the end of the incubation, 35.6% of litter C remained on the surface of soils at ?0.0063 MPa, whereas 41.7% remained on soils at ?0.0316 MPa. Most of the lost litter C was mineralised to CO₂, with 47.9% and 43.4% at ?0.0063 and ?0.0316 MPa, respectively. In both treatments about 6% were detected as newly formed soil organic carbon. During the initial phase of litter decomposition, bacteria dominated the mineralisation of easily available litter substrates. After 14 days fungi depolymerised more complex litter compounds, thereby producing new soluble substrates, which diffused into the soil. This pattern of differential substrate usage was paralleled by a lag phase of 3 days and a subsequent increase in enzyme activities. Increased soil water content accelerated the transport of soluble substrates, which influenced the temporal patterns of microbial growth and activity. Our results underline the importance of considering the interaction of soil microorganisms and physical processes at the small scale for the understanding of C cycling in soils.     

Carbon input belowground is the major C flux contributing to leaf litter mass loss: Evidences from a C labelled-leaf litter experiment

Partitioning of the quantities of C lost by leaf litter through decomposition into (i) CO₂ efflux to the atmosphere and (ii) C input to soil organic matter (SOM) is essential in order to develop a deeper understanding of the litter-soil biogeochemical continuum. However, this is a challenging task due to the occurrence of many different processes contributing to litter biomass loss. With the aim of quantifying different fluxes of C lost by leaf litter decomposition, a field experiment was performed at a short rotation coppice poplar plantation in central Italy. Populus nigra leaf litter, enriched in ¹³C (δ¹³C ∼ +160‰) was placed within collars to decompose in direct contact with the soil (δ¹³C ∼ −26‰) for 11 months. CO₂ efflux from within the collars and its isotopic composition were determined at monthly intervals. After 11 months, remaining litter and soil profiles (0-20 cm) were sampled and analysed for their total C and ¹³C content. Gas chromatography (GC), GC-mass spectrometry (MS) and GC-combustion-isotope ratio (GC/C/IRMS) were used to analyse phospholipid fatty acids (PLFA) extracted from soil samples to identify the groups of soil micro-organisms that had incorporated litter-derived C and to determine the quantity of C incorporated by the soil microbial biomass (SMB). By the end of the experiment, the litter had lost about 80% of its original weight. The fraction of litter C lost as an input into the soil (67 ± 12% of the total C loss) was found to be twice as much as the fraction released as CO₂ to the atmosphere (30 ± 3%), thus demonstrating the importance of quantifying litter-derived C input to soils, in litter decomposition studies. The mean δ¹³C values of PLFAs in soil (δ¹³C = −12.5‰) showed sustained incorporation of litter-derived C after one year (7.8 ± 1.6% of total PLFA-C). Thus, through the application of stable ¹³C isotope analyses, we have quantified two major C fluxes contributing to litter decomposition, at macroscopic and microscopic levels.     

Nutrient limitation of litter decomposition with long-term secondary succession: evidence from controlled laboratory experiments

Purpose

Understanding ecosystem processes such as litter decomposition in response to dramatic land-use change is critical for modeling and predicting carbon (C) cycles. However, the patterns of litter decomposition along with long-term secondary succession (over 100 years) are not well reported, especially concerning nutrient limitations on litter decomposition.

Materials and methods

To clarify the response of litter decomposition to changes in soil nutrient availability, we conducted four incubation experiments involving soil and litter and nutrient addition from different successional stages and investigated the changes in microbial respiration and litter mass loss.

Results and discussion

Our results revealed that microbial respiration increased with succession without any litter addition (1.19~1.73 mg C g^?1 soil), and litter addition significantly promoted microbial respiration (16.5~72.9%), especially in the early successional stage (grassland and shrubland). The decomposition rate of the same litter decreased with succession. In addition, nitrogen (N) and phosphorus (P) addition showed significant effects on litter decomposition and microbial respiration; P addition promoted litter decomposition (2.4~15.3%) and microbial respiration (10.1~34.5%) in all successional stages, while N addition promoted litter decomposition (4.0~10.3%) and microbial respiration (5.4~27.2%) in all except the last stage of succession, which showed a negative effect on litter decomposition (??7.5%) and microbial respiration (??6.1%), indicating possible N saturation of litter decomposition and microbial respiration.

Conclusions

This work highlights that soil nutrient availability and successional stages need to be taken into account to predict the changes to litter decomposition in response to global changes.

     

Nitrogen addition enhances home-field advantage during litter decomposition in subtropical forest plantations

Nitrogen (N) exerts strong effects on litter decomposition through altering microbial abundance and community composition. However, the effect of N addition on plant–soil interactions such as home-field advantage (HFA: enhanced decomposition at a home environment compared to a guest environment) in relation to litter decomposition remains unclear. To fill this knowledge gap, we conducted a reciprocal litter transplant plus N addition experiment in Mytilaria laosensis and Cunninghamia lanceolata plantations for two years in subtropical China where anthropogenic N input is amongst the highest in the world. We found positive HFA effects (in which the calculation incorporates litter of both species) with litter mass loss 11.2% faster at home than in the guest environment in the N addition (50 kg N ha⁻¹ yr⁻¹) treatment, but no significant HFA effects were found in the control treatment. The magnitude of the HFA effect on carbon (C) release increased with N addition, while that on N release decreased. The HFA effects on phosphorus, potassium, calcium, sodium, and magnesium release were positive overall, but varied through time and the magnitude of the effects were different among elements. The greater HFA effects in the N addition treatment were associated with greater differences in microbial biomass and community composition between home and guest environments than in the control treatment. Our results indicate that anthropogenic N enrichment could lead to enhanced HFA effects, through modification of microbial communities, and thereby affect C sequestration and N cycling in subtropical forests.     

Effects of addition of maize litter and earthworms on C mineralization and aggregate formation in single and mixed soils differing in soil organic carbon and clay content

C mineralization and aggregate stability directly depend upon organic matter and clay content, and both processes are influenced by the activity of microorganisms and soil fauna. However, quantitative data are scarce. To achieve a gradient in C and clay content, a topsoil was mixed with a subsoil. Single soils and the soil mixture were amended with 1.0 mg maize litter C g soil⁻¹ with and without endogeic earthworms (Aporrectodea caliginosa). The differently treated soils were incubated for 49 days at 15 °C and 40% water holding capacity. Cumulative C mineralization, microbial biomass, ergosterol content and aggregate fractions were investigated and litter derived C in bulk soil and aggregates were determined using isotope analyses. Results from the soil mixture were compared with the calculated mean values of the two single soils. Mixing of soil horizons differing in carbon and clay content stimulated C mineralization of added maize residues as well as of soil organic matter. Mixing also increased contents of macro-aggregate C and decreased contents of micro-aggregate C. Although A. caliginosa had a stimulating effect on C mineralization in all soils, decomposition of added litter by A. caliginosa was higher in the subsoil, whereas A. caliginosa decreased litter decomposition in the soil mixture and the topsoil. Litter derived C in macro-aggregates was higher with A. caliginosa than with litter only. In the C poor subsoil amended with litter, A. caliginosa stimulated the microbial community as indicated by the increase in microbial biomass. Furthermore, the decrease of ergosterol in the earthworm treated soils showed the influence of A. caliginosa on the microbial community, by reducing saprotrophic fungi. Overall, our data suggest both a decrease of saprotrophic fungi by selective grazing, burrowing and casting activity as well as a stimulation of the microbial community by A. caliginosa.     

Microbial community structure mediates response of soil C decomposition to litter addition and warming

Microbial activity has been highlighted as one of the main unknowns controlling the fate and turnover of soil organic matter (SOM) in response to climate change. How microbial community structure and function may (or may not) interact with increasing temperature to impact the fate and turnover of SOM, in particular when combined with changes in litter chemistry, is not well understood. The primary aim of this study was to determine if litter chemistry impacted the decomposition of soil and litter-derived carbon (C), and its interaction with temperature, and whether this response was controlled by microbial community structure and function. Fresh or pre-incubated eucalyptus leaf litter (¹³C enriched) was added to a woodland soil and incubated at 12, 22, or 32 °C. We tracked the movement of litter and soil-derived C into CO₂, water-extractable organic carbon (WEOC), and microbial phospholipids (PLFA). The litter additions produced significant changes in every parameter measured, while temperature, interacting with litter chemistry, predominately affected soil C respiration (priming and temperature sensitivity), microbial community structure, and the metabolic quotient (a proxy for microbial carbon use efficiency [CUE]). The direction of priming varied with the litter additions (negative with fresh litter, positive with pre-incubated litter) and was related to differences in the composition of microbial communities degrading soil-C, particularly gram-positive and gram-negative bacteria, resulting from litter addition. Soil-C decomposition in both litter treatments was more temperature sensitive (higher Q₁₀) than in the soil-only control, and soil-C priming became increasingly positive with temperature. However, microbes utilizing soil-C in the litter treatments had higher CUE, suggesting the longer-term stability of soil-C may be increased at higher temperature with litter addition. Our results show that in the same soil, the growth of distinct microbial communities can alter the turnover and fate of SOM and, in the context of global change, its response to temperature.     

Partitioning of carbon and nitrogen during decomposition of 13C15N‐labeled beech and ash leaf litter

The aim of this study was to determine the influence of leaf‐litter type (i.e., European beech—Fagus sylvatica L. and European ash—Fraxinus excelsior L.) and leaf‐litter mixture on the partitioning of leaf‐litter C and N between the O horizon, the topsoil, the soil microbial biomass, and the CO₂ emission during decomposition. In a mature beech stand of Hainich National Park, Thuringia, Germany, undisturbed soil cores (?? 24 cm) were transferred to plastic cylinders and the original leaf litter was either replaced by ¹³C¹⁵N‐labeled beech or ash leaf litter, or leaf‐litter‐mixture treatments in which only one of the two leaf‐litter types was labeled. Leaf‐litter‐derived CO₂‐C flux was measured every second week over a period of one year. Partitioning of leaf‐litter C and N to the soil and microbial biomass was measured 5 and 10 months after the start of the experiment. Ash leaf litter decomposed faster than beech leaf litter. The decomposition rate was negatively related to initial leaf‐litter lignin and positively to initial Ca concentrations. The mixture of both leaf‐litter types led to enhanced decomposition of ash leaf litter. However, it did not affect beech leaf‐litter decomposition. After 5 and 10 months of in situ incubation, recoveries of leaf‐litter‐derived C and N in the O horizon (7%–20% and 9%–35%, respectively) were higher than in the mineral soil (1%–5% and 3%–8%, respectively) showing no leaf‐litter‐type or leaf‐litter‐mixture effect. Partitioning of leaf‐litter‐derived C and N to microbial biomass in the upper mineral soil (< 1% of total leaf‐litter C and 2%–3% of total leaf‐litter N) did not differ between beech and ash. The results show that short‐term partitioning of leaf‐litter C and N to the soil after 10 months was similar for ash and beech leaf litter under standardized field conditions, even though mineralization was faster for ash leaf litter than for beech leaf litter.     

Effects of sulfuric,nitric, and mixed acid rain on litter decomposition,soil microbial biomass,and enzyme activities in subtropical forests of China

Acid rain pollution is changing gradually from sulfuric acid rain (SAR) to mixed acid rain (MAR) and then to nitric acid rain (NAR) with the rapidly growing number of motor vehicles. The influences of changed acid rain types on ecosystem functions, particularly on litter decomposition, remain unclear. Two dominant litter types from a coniferous forest and a broad-leaved forest were incubated in microcosms with original forest soils and treated by five types of acid rain with different SO₄²⁻ to NO₃⁻ ratios (1:0, 5:1, 1:1, 1:5, and 0:1). During a six-month incubation period, litter mass losses, soil microbial biomass, and enzyme activities were investigated. Results showed that various acid treatments inhibited litter decomposition, soil microbial biomass, and most enzyme activities, and the inhibitory effects of NAR were more significant than those of SAR and MAR. The resistance to external acid of microbial communities in broad-leaved forest was higher than that in coniferous forest. NAR and MAR treatments slowed down soil carbon (C), nitrogen (N), and phosphorus (P) mineralization by attenuating the correlations between litter mass losses and the enzymes involved in C, N, and P cycling. Results reveal that the ratio of SO₄²⁻ to NO₃⁻ in acid rain is an important factor which profoundly influences litter decomposition process. In the future, a decreasing ratio of SO₄²⁻ to NO₃⁻ in acid rain will be observed in subtropical forests. Thus, soil C would accumulate as a consequence of future acid precipitation, and this may seriously affect the balance of ecosystem C, N flux.     

Impacts of management on decomposition and the litter-carbon balance in irrigated and rainfed no-till agricultural systems

The litter carbon (C) pool of a single litter cohort in an agroecosystem is the difference between net primary productivity and decomposition and comprises 11–13% of the total C pool (litter and soil 0–15 cm depth) post-harvest. This litter-C pool is highly dynamic and up to 50% can be decomposed in the first 12 months of decomposition. Thus, understanding litter-C dynamics is key in understanding monthly and annual total ecosystem carbon dynamics. While the effects of management practices such as irrigation and fertilization on productivity are well understood, the effects on decomposition are less studied. While irrigation and fertilization increase productivity, this will only lead to increased litter-C residence time and litter-C pool accretion if these techniques do not also result in equivalent or greater increases in decomposition. Management could potentially have impacts on litter-C accretion by increasing litter inputs, changing plant-C allocation, plant tissue quality, or decomposition rates. We examined carbon loss of one annual cohort of maize litter using in situ nylon litter bags for 3 years in three no-till fields with differing management regimes: irrigated continuous maize with a pre-planting fertilization application and two fertigation events, irrigated maize–soybean rotation with the same fertilization regime as the irrigated continuous maize management regime, and rainfed maize–soybean rotation with a single pre-planting fertilization event. We addressed the effects of these different management regimes on net primary productivity and litter inputs, litter nitrogen (N) concentrations and carbon quality measures, plant C allocation, decomposition rates and the potential changes in the overall litter-C balance. We found that irrigation/fertigation management increased litter inputs, led to changes in plant tissue quality, had no effect on carbon allocation, and increased decomposition rates. This balance of both greater litter inputs and outputs of C from the irrigated management regimes led to a similar litter-C balance for this litter cohort in the irrigated and rainfed management regimes after 3 years of decomposition. Our data clearly show that merely increasing litter-C inputs through irrigation/fertigation practices is not sufficient to increase litter-C residence time because decomposition rates also increase. Therefore, close monitoring of decomposition rates is essential for understanding litter-C pool dynamics.     

Major mechanisms contributing to the macrofauna-mediated slow down of litter decomposition

To understand why excrements of soil macrofauna often decompose more slowly than leaf litter, we fed Bibio marci larvae the litter of tree species differing in litter quality (Alnus glutinosa, Salix caprea, and Quercus robur) and then measured respiration induced by litter and excrements. We also measured respiration induced by the same litter artificially modified to mimic faunal effects; the litter was modified by grinding, grinding with alkalinization to pH = 11, grinding with coating by kaolinite, and grinding with both alkalinization and coating. Decomposition of excrements tended to be slower for willow and was significantly slower for oak and alder than for the corresponding litter. With oak, decomposition was slower for all artificially modified litter than for non-modified litter. The reduction in the decomposition was similar for excrements and for alder and willow litter that was ground, coated, and alkalinized. In alder, a similar reduction was found in ground and alkalinized litter. ¹³C NMR indicated that gut passage increases aliphatic components and decreases polysaccharides. Pyrolysis indicated that gut passage increases the ratio of guaiacyl to hydroxymethyl derivatives in lignin. Our findings indicate that the decreased decomposition rate of excrements might result from the removal of easily available polysaccharides, the increase in aliphatic components, an increase in the resistant components of lignin, the accumulation of microbial cell walls, and the binding of nitrogen into complexes with aromatic components. Several of these mechanisms are supported or determined by litter alkalinization during gut passage.     

Fate of Chinese-fir litter during decomposition as a result of inorganic N additions

We conducted a controlled experiment to evaluate Chinese-fir litter decomposition and its response to the addition of inorganic N. Litter-derived CO₂, microbial biomass carbon (MBC), and dissolved organic carbon (DOC) were monitored during an 87-d incubation of a mixed soil–litter substrate using the ¹³C tracer technique. Litter C was mostly converted to CO₂ (47.4% of original mass), followed by MBC (3.6%), and DOC (1.0%), with 48% remaining unaltered in the soil. The litter decomposition rate significantly increased with the addition of inorganic N, although the effect depended on whether N was added as NH₄⁺ or NO₃⁻. Soil-derived CO₂, MBC, and DOC also increased following the combined addition of litter and N. The results showed that only a small percentage of litter C was retained as MBC or DOC and that the conversion rate depended, in part, on the form of inorganic N added to the Chinese-fir plantation soil.     

Impact of litter quality on mineralization processes in managed and abandoned pasture soils in Southern Ecuador

Tropical regions are currently undergoing remarkable rates of land use change accompanied by altered litter inputs to soil. In vast areas of Southern Ecuador forests are clear cut and converted for use as cattle pastures. Frequently these pasture sites are invaded by bracken fern, when bracken becomes dominant pasture productivity decreases and the sites are abandoned. In the present study implications of invasive bracken on soil biogeochemical properties were investigated. Soil samples (0-5 cm) were taken from an active pasture with Setaria sphacelata as predominant grass and from an abandoned pasture overgrown by bracken. Grass (C₄ plant) and bracken (C₃ plant) litter, differing in C:N ratio (33 and 77, respectively) and lignin content (Klason-lignin: 18% and 45%, respectively), were incubated in soils of their corresponding sites and vice versa for 28 days at 22 °C. Unamended microcosms containing only the respective soil or litter were taken as controls. During incubation the amount of CO₂ and its δ¹³C-signature were determined at different time intervals. Additionally, the soil microbial community structure (PLFA-analysis) as well as the concentrations of KCl-extractable C and N were monitored. The comparison between the control soils of active and abandoned pasture sites showed that the massive displacement of Setaria-grass by bracken after pasture abandonment was characterized by decreased pH values accompanied by decreased amounts of readily available organic carbon and nitrogen, a lower microbial biomass and decreased activity as well as a higher relative abundance of actinomycetes. The δ¹³C-signature of CO₂ indicated a preferential mineralization of grass-derived organic carbon in pasture control soils. In soils amended with grass litter the mineralization of soil organic matter was retarded (negative priming effect) and also a preferential utilization of easily available organic substances derived from the grass litter was evident. Compared to the other treatments, the pasture soil amended with grass litter showed an opposite shift in the microbial community structure towards a lower relative abundance of fungi. After addition of bracken litter to the abandoned pasture soil a positive priming effect seemed to be supported by an N limitation at the end of incubation. This was accompanied by an increase in the ratio of Gram-positive to Gram-negative bacterial PLFA marker. The differences in litter quality between grass and bracken are important triggers of changes in soil biogeochemical and soil microbial properties after land use conversion.     

Temporal dynamics of microbial communities on decomposing leaf litter of 10 plant species in relation to decomposition rate

Few empirical studies have examined how microbial communities on decomposing litters change in relation to litter chemistry or how microbial community composition is related to the rate of decomposition. We examined the relationships among microbial community composition, litter chemistry, and decomposition rates in a common garden experiment of the decomposition of leaf litters of 10 plant species. Microbial community composition, as measured by phospholipid fatty acids (PLFA), and 7 litter chemistry variables (%N, C:N, four carbon fractions, and lignin:N) were examined at 1, 2, and 8 months into decomposition. Both microbial and litter chemistry variables were reduced to a single axis each through nonmetric multidimensional scaling (NMS) to examine the relationship between microbes, litter chemistry, and decomposition rates. Although microbial communities were separated according initial litter chemistry and lability, individual measures of litter chemistry had limited ability to predict microbial community composition during decomposition. Decomposition rate constants were explained by litter chemistry of initial, 1-, 2- and, 8-month old litters (60–72% of the variance), and by microbial community composition at the 8-month collection date (67%). The results suggest that initial litter chemistry determines the rate of decomposition and microbial community composition early in decomposition while the composition of the microbial community plays a more important role in determining decomposition rate later in decomposition.     

C-NMR analysis of decomposing litter and fine roots in the semi-arid Mulga Lands of southern Queensland

Plant litter and fine roots are important carbon (C) inputs to soil and a direct source of CO₂ to the atmosphere. Solid-state carbon-13 nuclear magnetic resonance (¹³C-NMR) spectroscopy was used to investigate the nature of C changes during decomposition of plant litter and fine roots of mulga (Acacia aneura F. Muell. Ex. Benth.), wheat (Triticum aestivum L.), lucerne (Medicago sativa) and buffel grass (Cenchrus ciliaris) over an 18-month period. Alkyl C was closely associated with total N concentrations in all litter materials during decay and as alkyl C increased so did total N, indicating an increase in refractory biomacromolecules. Mulga phyllodes had the greatest alkyl C concentration of all litter and fine root materials, and also exhibited the NMR peaks assigned to tannins that may slow or hinder decomposition rates and nitrification. Mulga litter and fine roots decomposed slower than all other litter materials and the soil under mulga had the highest soil C concentration, indicating slower CO₂ release. The alkyl C-to-O-alkyl C ratio is generally used as an index of the extent of decomposition, but is not useful for the decay of woody components. Of all the NMR ratios studied that may indicate the extent of decomposition, the carbohydrate C-to-methoxyl C ratio proved to have the strongest and most consistent relationship with decay time, fraction of mass remaining and total C, even though increases in alkyl C were observed with decreases in carbohydrate C.