Fine Root Litter Research Articles

Defoliation and neighbouring legume plants accelerate leaf and root litter decomposition of Leymus chinensis dominating grasslands

Dominant plant species coexist with other species from the same or other trophic levels (e.g. neighbouring plants and herbivores), which may give rise to neighbourhood and defoliation effects on litter decomposition. Yet, little information exists on if and how neighbouring plant species and herbivore defoliation interactively affect the decomposition of dominant species, and further whether decomposition of leaves and roots has similar responses. Here, we simultaneously examined both the leaf and fine root litter decomposition of the dominant grass species, Leymus chinensis, in the presence and absence of a neighbouring plant species, Lathyrus quinquenervius (a functionally dissimilar legume species), and simulated herbivore defoliation by mowing. We found that both neighbouring legume plants and defoliation significantly accelerated both leaf and root litter decomposition of L. chinensis, and that neighbouring legume plants exerted stronger effects on litter decomposition than defoliation. The positive effects of neighbouring legume plants occurred across all study periods, whereas defoliation only had a positive effect during the first 15 days of litter decomposition. Neighbouring legume plants and defoliation exerted strong additive effects on root litter decomposition, and interactive effects on leaf litter decomposition. These findings highlight the significance of functionally dissimilar species coexistence to facilitate nutrient cycling in grasses - dominated grasslands.

Root Litter Mixing with That of Japanese Cedar Altered CO2 Emissions from Moso Bamboo Forest Soil

Research Highlights: This study examined the effect of mixing fine roots of Japanese cedar with moso bamboo on soil carbon dioxide (CO2) emissions with nitrogen (N) addition treatment. Background and Objectives: Moso bamboo expansion into adjacent forests and N deposition are common in subtropical China. The effects of litter input on soil CO2 emissions, especially fine root litter input, are crucial to evaluate contribution of moso bamboo expansion on greenhouse gas emissions. Materials and Methods: An in situ study over 12 months was conducted to examine mixing fine roots of Japanese cedar with moso bamboo on soil CO2 emissions with simulated N deposition. Results: Fine root litter input of Japanese cedar and moso bamboo both impacted soil CO2 emission rates, with mixed litter, positively impact soil CO2 emission rate with N addition treatment. Moso bamboo fine root litter input decreased the sensitivity of soil CO2 emission rate to soil temperature. Conclusions: The encroachment of moso bamboo into adjacent forests might benefit soil C sequestration under warming climate, which will also benefit the mitigation of global climate change.

Anthropogenic N deposition alters soil organic matter biochemistry and microbial communities on decaying fine roots.

Fine root litter is a primary source of soil organic matter (SOM), which is a globally important pool of C that is responsive to climate change. We previously established that ~20years of experimental nitrogen (N) deposition has slowed fine root decay and increased the storage of soil carbon (C; +18%) across a widespread northern hardwood forest ecosystem. However, the microbial mechanisms that have directly slowed fine root decay are unknown. Here, we show that experimental N deposition has decreased the relative abundance of Agaricales fungi (-31%) and increased that of partially ligninolytic Actinobacteria (+24%) on decaying fine roots. Moreover, experimental N deposition has increased the relative abundance of lignin-derived compounds residing in SOM (+53%), and this biochemical response is significantly related to shifts in both fungal and bacterial community composition. Specifically, the accumulation of lignin-derived compounds in SOM is negatively related to the relative abundance of ligninolytic Mycena and Kuehneromyces fungi, and positively related to Microbacteriaceae. Our findings suggest that by altering the composition of microbial communities on decaying fine roots such that their capacity for lignin degradation is reduced, experimental N deposition has slowed fine root litter decay, and increased the contribution of lignin-derived compounds from fine roots to SOM. The microbial responses we observed may explain widespread findings that anthropogenic N deposition increases soil C storage in terrestrial ecosystems. More broadly, our findings directly link composition to function in soil microbial communities, and implicate compositional shifts in mediating biogeochemical processes of global significance.

Root production, mortality and turnover in soil profiles as affected by clipping in a temperate grassland on the Loess Plateau

Abstract Aims Clipping or mowing for hay, as a prevalent land-use practice, is considered to be an important component of global change. Root production and turnover in response to clipping have great implications for the plant survival strategy and grassland ecosystem carbon processes. However, our knowledge about the clipping effect on root dynamics is mainly based on root living biomass, and limited by the lack of spatial and temporal observations. The study aim was to investigate the effect of clipping on seasonal variations in root length production and mortality and their distribution patterns in different soil layers in semiarid grassland on the Loess Plateau. Methods Clipping was performed once a year in June to mimic the local spring livestock grazing beginning from 2014. The minirhizotron technique was used to monitor the root production, mortality and turnover rate at various soil depths (0–10, 10–20, 20–30 and 30–50 cm) in 2014 (from 30 May to 29 October) and 2015 (from 22 April to 25 October). Soil temperature and moisture in different soil layers were also measured during the study period. Important Findings Our results showed that: (i) Clipping significantly decreased the cumulative root production (P < 0.05) and increased the cumulative root mortality and turnover rates of the 0–50 cm soil profile for both years. (ii) Clipping induced an immediate and sharp decrease in root length production and an increase in root length mortality in all soil layers. However, with plant regrowth, root production increased and root mortality decreased gradually, with the root production at a depth of 30–50 cm even exceeding the control in September–October 2014 and April–May 2015. (iii) Clipping mainly reduced root length production and increased root length mortality in the upper 0–20 cm soil profile with rapid root turnover. However, roots at deeper soil layers were either little influenced by clipping or exhibited an opposite trend with slower turnover rate compared with the upper soil profile, leading to the downward transport of root production and living root biomass. These findings indicate that roots in deeper soil layers tend to favour higher root biomass and longer fine root life spans to maximize the water absorption efficiency under environmental stress, and also suggest that short-term clipping would reduce the amount of carbon through fine root litter into the soil, especially in the shallow soil profile.

Increased soil temperature stimulates changes in carbon, nitrogen, and mass loss in the fine roots of Pinus koraiensis under experimental warming and drought

The effects of warming (+3 °C) and drought (-30% precipitation) on the fine root decomposition of Pinus koraiensis seedlings were examined using a litter bag method. The study site included a full factorial design with two temperature and two precipitation levels, with three replicates. Litter bags containing fine root litter of 2-year-old P. koraiensis seedlings were retrieved after 3, 6, and 12 months of decomposition. After 12 months, the mass loss of fine roots was significantly increased in response to warming (control = 31.1%, warming = 35.9%, drought = 29.2%, and warming plus drought = 35.5%); no change was observed until 6 months. Mass loss was not influenced by drought or by the interaction between warming and drought. Warming increased the nitrogen concentration of fine root litter but decreased the carbon concentration and carbon/nitrogen ratio after 6 and 12 months. This may be because warming stimulated nitrogen immobilization, which reduced the carbon/nitrogen ratio. Therefore, the carbon/nitrogen ratio may be affected by warming prior to changes in the mass loss of fine roots because roots with a low carbon/nitrogen ratio are generally characterized by high available nitrogen for decomposers. These results suggest that climate change (especially warming) may cause rapid decomposition of organic matter.

Nitrogen addition drives convergence of leaf litter decomposition rates between Flaveria bidentis and native plant

Evidence is growing that invasive species can change decomposition rates and associated nutrient cycling within an ecosystem by changing the quality of the litter entering a system. However, the relative contribution of their distinct litter types to carbon turnover is less understood, especially in the context of enhanced N deposition. The objective of this study was to investigate the whole-plant responses of an invasive plant Flaveria bidentis in litter decay to simulated N eutrophication. A 1-year study was conducted to assess if N enhancement influenced decomposition and nutrient dynamics of litters from foliage, fine roots and twigs of F. bidentis compared to co-occurring native species Setaria viridis. N fertilization significantly decreased the decomposition rate of the foliage of the invasive F. bidentis by more than 25% relative to the water control, but had relatively minor effects on decomposition of its twigs and fine root litter or leaf litter from the native species. Collectively, decomposition rates of foliar litters of the invasive and native species become convergent over time in the presence of N addition. Moreover, net N loss was predominately influenced by litter species, followed by the litter type, while N addition had little effect on net N loss. Our study showed that the variation in litter decomposition was much greater between litter types of the invasive F. bidentis than between different plant species under the N addition and that the litter of invasive species with higher inherent decomposability did not always decompose more rapidly than the litter of native species in response to predicted N deposition enhancement.

Leachate from fine root litter is more acidic than leaf litter leachate: A 2.5-year laboratory incubation

Some tree species increase fine root production under soil acidification, thus changing the balance of litter input from leaves and roots. Litter leaches a significant amount of acidic materials during its decomposition, which might facilitate soil acidification. In this context, we focused on dissolved organic matter (DOM) as the major component of acidic materials. We hypothesized that both the quality and quantity of DOM, which control its function (i.e., proton supply), differ between leaf and root litter. To test this hypothesis, we conducted a 2.5-year laboratory incubation experiment using fresh fine roots and fresh green leaves as litter of two coniferous species (Cryptomeria japonica and Chamaecyparis obtusa) and investigated the leachate pH and DOM composition based on the optical properties. After the early stage of decomposition when flash leaching of DOM converged, the amount of dissolved organic carbon (DOC) leached from roots increased again and leachate pH declined. In contrast, DOC concentrations continued to decrease in leaf leachates during the incubation period, and the pH decrease was not as striking as that of root leachates. Optical properties (ultraviolet visible absorption and fluorescence) of DOM revealed that humic-like substances in DOM played a central role in the acidic pH of root leachates. The total amount of protons released from roots of C. japonica and C. obtusa is about 13 and 18 times higher, respectively, than that from leaves. These results imply that the increase of fine root biomass may induce a positive plant–soil feedback in acidic soils, affecting soil biogeochemical functions of terrestrial ecosystems.

Fine root longevity and below- and aboveground litter production in a boreal Betula pendula forest

1. Fine root turnover plays a critical role in carbon and nutrient cycling in forest ecosystems. In this study, we focused on the most abundant deciduous species in Nordic countries, silver birch (Betula pendula Roth) and its fine root dynamics, including the amount of litter produced by fine roots as well as by aboveground vegetation.2. The minirhizotron method was used to quantify fine root longevity of silver birch and understory fine roots and rhizomes in northern Finland. Fine root biomass per basal area and ectomycorrhizal short root numbers per mg were also quantified. The fine root litter production was estimated by fine root biomass and longevity, and then compared with the aboveground litter collected with litter traps.3. Birch fine root biomass was 1.4-fold higher than that of understory fine roots and rhizomes (234 ± 22, 171 ± 19 g m−2 respectively). Fine root longevity of birch (372 days) was significantly (P < 0.05) shorter than that of understory vegetation (643 days). The birch fine root longevity was positively related to root diameter and soil depth. Hazard analysis showed that thicker roots, long roots, roots produced late in the growing season, and roots growing deeper in the soil had relatively lower mortality hazard compared to the reference data. The total annual soil C input, including both birch and understory, was 283 g C m−2 yr−1. The proportion of understory annual C input was 35% of the total. Total annual belowground C input was 1.4-fold greater than that of aboveground.4. Our study indicated that the total annual belowground litter production was greater than that of the aboveground litter in a boreal deciduous forest stand. Therefore, more emphasis should be put to quantify the C cycling of both above- and belowground parts of different tree species as well as understory in boreal forests.

Decomposition of Leaves and Fine Roots in Three Subtropical Plantations in China Affected by Litter Substrate Quality and Soil Microbial Community

Leaf and root litter decomposition has been a major research focus. However, the possible effects of belowground microbial community structure and diversity on this process are poorly understood. Understanding the biochemical mechanisms controlling aboveground decomposition processes is important to predict the changes of soil carbon and nutrient cycling in response to changes of forest management regimes. Here, we explore the biochemical controls of leaf and fine root decomposition in three subtropical plantations (Ford Erythrophleum (Erythrophleum fordii Oliver), Masson Pine (Pinus massoniana Lamb.)), and a mixed plantation containing both species) using the litterbag method, and soil microbial communities were determined using phospholipid fatty acid profiles. Overall, leaves decomposed more rapidly than fine roots, potentially due to the faster degradation of their cellulose component, but not lignin. In addition, leaf and fine root decomposition rates varied among plantations, being higher in E. fordii and lower in P. massoniana. Substrate quality such as N, Ca, lignin concentration, and C/N ratio were responsible for the decomposition rate changes among plantation types. Moreover, we used redundancy analysis to examine the relationships between litter decomposition and soil microbial community composition and diversity. Results revealed that actinobacteria and arbuscular mycorrhizal fungi community were the key determinants affecting leaf and fine root litter decomposition, respectively. Our work demonstrates that litter decomposition was linked to substrate quality and to the structure of soil microbial communities, and evidences the probable role of E. fordii in increasing soil nutrient availability, especially N, P and Ca. Additional data on phospholipid fatty acid (PLFA) or DNA marker groups within the litterbags over time may provide insights into litter decomposition dynamics, which represents potential objectives for future long-term decomposition studies.

Comparative effects of sulfuric and nitric acid rain on litter decomposition and soil microbial community in subtropical plantation of Yangtze River Delta region

Acid rain is mainly caused by dissolution of sulfur dioxide and nitrogen oxides in the atmosphere, and has a significant negative effect on ecosystems. The relative composition of acid rain is changing gradually from sulfuric acid rain (SAR) to nitric acid rain (NAR) with the rapidly growing amount of nitrogen deposition. In this study, we investigated the impact of simulated SAR and NAR on litter decomposition and the soil microbial community over four seasons since March 2015. Results first showed that the effects of acid rain on litter decomposition and soil microbial were positive in the early period of the experiment, except for SAR on soil microbes. Second, soil pH with NAR decreased more rapidly with the amount of acid rain increased in summer than with SAR treatments. Only strongly acid rain (both SAR and NAR) was capable of depressing litter decomposition and its inhibitory effect was stronger on leaf than on fine root litter. Meanwhile, NAR had a higher inhibitory effect on litter decomposition than SAR. Third, in summer, autumn and winter, PLFAs were negatively impacted by the increased acidity level resulting from both SAR and NAR. However, higher acidity level of NAR (pH=2.5) had the strongest inhibitory impact on soil microbial activity, especially in summer. In addition, Gram-negative bacteria (cy19:0) and fungi (18:1ω9) were more sensitive to both SAR and NAR, and actinomycetes was more sensitive to SAR intensity. Finally, soil total carbon, total nitrogen and pH were the most important soil property factors affecting soil microbial activity, and high microbial indices (fungi/bacteria) with high soil pH. Our results suggest that the ratio of SO42− to NO3− in acid rain is an important factor which could affect litter decomposition and soil microbial in subtropical forest of China.

Ecosystems carbon budgets of differently aged downy birch stands growing on well-drained peatlands

Estimation of the carbon (C) storages and fluxes in different forest ecosystems is essential for understanding their C sequestration ability. The net ecosystem production (NEP) and the net primary production (NPP) in five downy birch (Betula pubescens) stands, aged between 12 and 78years, growing on fertile well-drained Histosols, were studied. Drainage of swamp forests is a large-scale manipulation, which causes significant shifts at the ecosystems level, altering C and nutrient cycling a great deal. Young and middle-aged downy birch stands (12–30-year-old) acted as C sink ecosystems, accumulating 1.4–3.0tCha−1yr−1. In the 38-year-old stand NEP was roughly zero; annual C budget was almost in balance. The over-matured downy birch stand (78-year-old) acted as a C source emitting 0.95tCha−1yr−1. Annual woody biomass increment of the stand was the main factor which affected the forest to act as a C accumulating system. Although the highest heterotrophic respiration (Rh) values were measured in the middle-aged stands, mean soil C emission did not differ significantly between the studied stands. Annual total soil respiration (Rs) and Rh ranged from 7.4 to 8.8tCha−1 and 4.7 to 6.2tCha−1, respectively. Soil temperature appeared to be the dominant driver of the soil CO2 effluxes. Temperature sensitivity (Q10 value) of respiration rates (3.0–5.5), as well as the Rh/Rs (0.6–0.7) varied irrespective of stand age. Both the annual aboveground litter (1.5–1.9tCha−1yr−1) and fine root litter (0.9–1.5tCha−1yr−1) input fluxes were quite similar for the studied stands. However, the annual organic C input into the soil via above- and belowground litter was smaller than the annual Rh efflux, indicating that continuous mineralization of the peat layer reduces the soil organic C pool. The main share of the C stock in the drained swamp downy birch stands was soil C; the storage of C accumulated in the woody biomass of the trees accounted for only 5–20% of the total C storage of the ecosystem.

Succession of soil microarthropod communities during the aboveground and belowground litter decomposition processes

The process of litter decomposition is driven by interactions among climate, litter quality, and decomposers. However, information about the soil animal community involved in fine-root litter decomposition remains limited. We compared the composition of the soil microarthropods involved in leaf and root decomposition in field experiments using litterbags. To evaluate the relative effects of litter type and initial litter position, we set up a two-factor experiment (litter type × litter position). Litterbags containing either roots or leaves were placed at two positions (either on the soil surface or buried within the soil) and were collected to follow the succession of microarthropods in the decomposing litter for three years. We found different successional patterns of soil microarthropods between the natural processes of leaf and root litter decomposition (i.e. leaves on the soil surface and roots buried within the soil), which were caused by taxonomy-specific responses to both litter quality and litter position. Prostigmata were clearly affected by the stage of litter decomposition; they were initial colonisers of decomposition in all litterbag treatments. The abundances of other taxa were more determined by litter position; however, Oribatida showed a preference for later stages of decomposition, while Collembola and Mesostigmata were strongly determined by water content related to litter position. Although litter quality was not the primary factor controlling their distribution, the ratio of Oribatida to Collembola gradually increased during the decomposition process in the belowground litterbags. Our results indicate that Oribatida are primarily involved in root litter decomposition process, particularly in the late stages of decomposition under natural conditions.

Treeline advances and associated shifts in the ground vegetation alter fine root dynamics and mycelia production in the South and Polar Urals.

Climate warming is shifting the elevational boundary between forests and tundra upwards, but the related belowground responses are poorly understood. In the pristine South and Polar Urals with shifts of the treeline ecotone documented by historical photographs, we investigated fine root dynamics and production of extramatrical mycorrhizal mycelia (EMM) along four elevational transects reaching from the closed forest to the treeless tundra. In addition, we analysed elevational differences in climate and vegetation structure, and excavated trees to estimate related changes in the partitioning between below- and aboveground biomass. Fine root biomass of trees (<2mm) increased by 13-79% with elevation, paralleled by a 35-72% increase in ground vegetation fine roots from the closed forest to the tundra. During the first year of decomposition, mass loss of fine root litter from different vegetation types was greater at lower elevations in the forest-tundra ecotone. The ratio between fine roots of trees and stem biomass largely increased with elevation in both regions, but these increases were not accompanied by a distinct production of EMM. Production of EMM, however, increased with the presence of ectomycorrhizal trees at the transition from the tundra to the forest. Our results imply that the recorded upward expansion of forest into former tundra in the Ural Mountains by 4-8m per decade is decreasing the partitioning of plant biomass to fine roots. They further suggest that climate-driven forest advances will alter EMM production rates with potential feedbacks on soil carbon and nutrient cycling in these ecosystems.

Uncertainty of upland soil carbon sink estimate for Finland

Changes in the soil carbon stock of Finnish upland soils were quantified using forest inventory data, forest statistics, biomass models, litter turnover rates, and the Yasso07 soil model. Uncertainty in the estimated stock changes was assessed by combining model and sampling errors associated with the various data sources into variance–covariance matrices that allowed computationally efficient error propagation in the context of Yasso07 simulations. In sensitivity analysis, we found that the uncertainty increased drastically as a result of adding random year-to-year variation to the litter input. Such variation is smoothed out when using periodic inventory data with constant biomass models and turnover rates. Model errors (biomass, litter, understorey vegetation) and the systematic error of total drain had a marginal effect on the uncertainty regarding soil carbon stock change. Most of the uncertainty appears to be related to uncaptured annual variation in litter amounts. This is due to fact that variation in the slopes of litter input trends dictates the uncertainty of soil carbon stock change. If we assume that there is annual variation only in foliage and fine root litter rates and that this variation is less than 10% from year to year, then we can claim that Finnish upland forest soils have accumulated carbon during the first Kyoto period (2008–2012).

Rapid fine root C and N mineralization in a northern temperate forest soil

While fine roots (≤2-mm diameter) are major suppliers of carbon (C) and nitrogen (N) to northern temperate and boreal forest soils, our understanding of how long-term plant and N inputs affect fine root decomposition rates and the amount of root-derived organic matter (OM) stabilized in forest soils is incomplete. We examined the influence of long-term aboveground and/or belowground litter and inorganic N additions on mineralization and vertical transport of fine root-derived C and N during the first 2 years of decomposition of dead fine root in the field. We used an existing long-term field manipulation experiment located in a northern Michigan forest; with (i) exclusion of above and below-ground inputs, (ii) exclusion of belowground inputs alone, or (iii) inorganic N additions, for 6 years prior to the addition of dual-labeled (13C and 15N) Acer rubrum fine roots. After 2 years in soil, labeled fine roots rapidly decomposed in all treatments, with only 20.7 % of root 13C and 35.8 % of root 15N recovered in soil (0–20 cm depth). This was likely because of the combined effects of (1) root litter chemistry, (2) processing of root litter by exotic earthworms, and (3) the low stabilization potential of the coarse-textured soil at the site. Neither the long-term exclusion of litter inputs nor increased inorganic N additions influenced root mineralization rates; and there were no detectable effects of either treatment on CO2 efflux or on dissolved organic C loss. During the 2-year study, exclusion of litter inputs did not affect root C retention in soil but lowered C:N ratios of roots recovered in that treatment. Inorganic N additions had no significant effect on root-derived C or N retention in the soil. Our results show that fine root litter turns over faster than previously thought in coarse-textured temperate forests soils that lack effective OM stabilization mechanisms.

Disentangling the Litter Quality and Soil Microbial Contribution to Leaf and Fine Root Litter Decomposition Responses to Reduced Rainfall

Climate change-induced rainfall reductions in Mediterranean forests negatively affect the decomposition of plant litter through decreased soil moisture. However, the indirect effects of reduced precipitation on litter decomposition through changes in litter quality and soil microbial communities are poorly studied. This is especially the case for fine root litter, which contributes importantly to forests plant biomass. Here we analyzed the effects of long-term (11 years) rainfall exclusion (29% reduction) on leaf and fine root litter quality, soil microbial biomass, and microbial community-level physiological profiles in a Mediterranean holm oak forest. Additionally, we reciprocally transplanted soils and litter among the control and reduced rainfall treatments in the laboratory, and analyzed litter decomposition and its responses to a simulated extreme drought event. The decreased soil microbial biomass and altered physiological profiles with reduced rainfall promoted lower fine root—but not leaf—litter decomposition. Both leaf and root litter, from the reduced rainfall treatment, decomposed faster than those from the control treatment. The impact of the extreme drought event on fine root litter decomposition was higher in soils from the control treatment compared to soils subjected to long-term rainfall exclusion. Our results suggest contrasting mechanisms driving drought indirect effects on above-(for example, changes in litter quality) and belowground (for example, shifts in soil microbial community) litter decomposition, even within a single tree species. Quantifying the contribution of these mechanisms relative to the direct soil moisture-effect is critical for an accurate integration of litter decomposition into ecosystem carbon dynamics in Mediterranean forests under climate change.

Root rather than leaf litter input drives soil carbon sequestration after afforestation on a marginal cropland

Afforestation on croplands has been proposed as a means to mitigate the increasing emission of anthropogenic CO2. However, the relative contribution of above- and belowground litter input on soil organic carbon (SOC) sequestration following afforestation is not fully understood. We used a 270-day laboratory incubation experiment to examine the impact of litter type (i.e., leaf vs. fine root litter) of a poplar tree (Populus simonii Carr., C3 plant) on soil respiration and the turnover of new vs. old soil C in surface (0–10cm) and subsurface mineral soils (40–50cm) collected from a marginal cropland planted to maize (Zea mays L., C4 plant) in a semi-arid region in northeast China. Our results showed that fine root rather than leaf litter addition helps to sequester SOC even though soil microbial respiration rates were stimulated by both leaf and fine root litter input. Neither leaf nor fine root litter addition stimulated mineralization of old soil C across the two soil layers, but more new C was incorporated into the soil with fine root addition as compared with leaf litter addition. Moreover, the subsurface soil had greater potential to sequester SOC as compared to the surface soil. Our results suggest that root rather than leaf litter input drives soil carbon sequestration on the marginal soil, especially in the subsurface soil, and planting deep-rooted trees with large belowground biomass production could be used to increase SOC sequestration in marginal croplands.

Conversion of tropical lowland forest reduces nutrient return through litterfall, and alters nutrient use efficiency and seasonality of net primary production

Tropical landscapes are not only rapidly transformed by ongoing land-use change, but are additionally confronted by increasing seasonal climate variation. There is an increasing demand for studies analyzing the effects and feedbacks on ecosystem functioning of large-scale conversions of tropical natural forest into intensively managed cash crop agriculture. We analyzed the seasonality of aboveground litterfall, fine root litter production, and aboveground woody biomass production (ANPP(woody)) in natural lowland forests, rubber agroforests under natural tree cover ("jungle rubber"), rubber and oil palm monocultures along a forest-to-agriculture transformation gradient in Sumatra. We hypothesized that the temporal fluctuation of litter production increases with increasing land-use intensity, while the associated nutrient fluxes and nutrient use efficiency (NUE) decrease. Indeed, the seasonal variation of aboveground litter production and ANPP(woody) increased from the natural forest to the plantations, while aboveground litterfall generally decreased. Nutrient return through aboveground litter was mostly highest in the natural forest; however, it was significantly lower only in rubber plantations. NUE of N, P and K was lowest in the oil palm plantations, with natural forest and the rubber systems showing comparably high values. Root litter production was generally lower than leaf litter production in all systems, while the root-to-leaf ratio of litter C flux increased along the land-use intensity gradient. Our results suggest that nutrient and C cycles are more directly affected by climate seasonality in species-poor agricultural systems than in species-rich forests, and therefore might be more susceptible to inter-annual climate fluctuation and climate change.

Asynchronous responses of soil carbon dioxide, nitrous oxide emissions and net nitrogen mineralization to enhanced fine root input

Global environmental changes can remarkably alter the amount of fine root litter inputs to the soil; these inputs affect soil CO2 and N2O emissions and net N mineralization processes by changing C and N supply to microorganisms. However, how these C and N processes respond to the amount of fine root litter input is yet not known. In this study, a year-long incubation experiment was conducted to investigate the impacts of changes in fine root litter biomass input on soil respiration, N2O emissions, and net N mineralization. Soil samples were obtained from forests of four biomes (boreal, temperate, subtropical, and tropical), and each sample was amended with fine roots from two species (either native tree species or maize roots) with four levels of root biomass input. The cumulative CO2 emissions increased linearly with root input levels, regardless of soil and root litter types. Soil respiration responded strongly to root inputs within the first 100 days and then leveled off. Root inputs retarded soil N2O emissions and net N mineralization, and the length of delay increased with root input levels, except for temperate and subtropical soils amended with tree roots, for which N2O emission dynamics were not altered by root input. Tree roots retarded net N mineralization more intensively than maize roots except for the tropical tree roots. Cumulative N2O emissions increased linearly with root input levels in only some soil type–root species–root input level combinations. Taken together, our results suggest that increased fine root biomass production might result in a linear increase of soil C loss via heterotrophic respiration, indicating that the first-order kinetic functions that have been widely used in the soil C models are still valid for predicting C mineralization rates in response to the changes in the amount of root litter inputs. Fine roots in their initial decomposition stage could be the predominant sources of soil N2O emissions in some but not all terrestrial ecosystems. However, increased fine root input might retard net N mineralization, which might disrupt the temporal pattern of ecosystem N cycling, and thus have important consequences on plant N supply and growth.

Old carbon in young fine roots in boreal forests

A large proportion of the soil carbon (C) in boreal forests originates from roots and ectomycorrhizal fungi, and accurate quantification of fine-root litter production is needed. Methods for determination of root turnover have been under debate in recent years. Two recently used methods—radiocarbon (14C) dating and use of minirhizotrons (MR)—have yielded different results. This has been attributed to analysis of different roots by use of these methods. At Flakaliden, northern Sweden, in a long-term soil warming and irrigation experiment, we compared MR lifespan with the 14C-derived age of fine roots from soil cores of the same root diameter class. We also determined the 14C-derived age of ingrowth core roots of Norway spruce. The median lifespan of fine roots around MR tubes installed 15 years previously was shorter than 2.5 years whereas the 14C-derived age of the fine roots from soil cores varied from recently grown to 14 years. Correspondingly, the age of 14C in fine roots harvested from ingrowth cores installed in soil 3 months previously was between 1 and 20 years. Thus, cellulose in these roots contained older 14C than is possible from photosynthesis during the time of cellulose formation. By investigating whether the age of Norway spruce and Scots pine seedlings was less than their root 14C-derived age, we tested the possibility of root C originating from soil uptake. This was found to be unlikely, because fine roots of four and eight-year-old seedlings had 14C that was dated to be as old as or younger than the seedlings. We propose that further effort is required to identify the ecological conditions leading to root growth utilization of stored or recycled C.