Patterns of SOC and soil 13C and their relations to climatic factors and soil characteristics on the Qinghai–Tibetan Plateau

Background and aims

SOC inventory and soil δ¹³C were widely used to access the size of soil C pool and to indicate the dynamics of C input and output. The effects of climatic factors and soil physical characteristics and plant litter input on SOC inventory and soil δ¹³C were analyzed to better understand the dynamics of carbon cycling across ecosystems on the Qinghai-Tibetan Plateau.

Methods

Field investigation was carried out along the two transects with a total of 1,875 km in length and 200 km in width. Sixty-two soil profiles, distributed in forest, meadow, steppe, and cropland, were stratified sampled every 10 cm from 0 to 40 cm.

Results

Our result showed that SOC density in forest and meadows were much higher than in steppe and highland barley. In contrast, δ¹³C in forest and meadow were lower than in steppe and highland barley. Soil δ¹³C tended to enrich with increasing soil depth but SOC decline. SOC and δ¹³C (0–40 cm) were correlated with different climatic factors in different ecosystems, such that SOC correlated negatively with MAT in meadow and positively with MAP in steppe; δ¹³C correlated positively with MAT in meadow and steppe; and δ¹³C also tended to increase with increasing MAT in forest. Of the variation of SOC, 55.15 % was explained by MAP, pH and silt content and 4.63 % was explained by the interaction between MAT and pH across all the ecosystems except for the cropland. Meanwhile, SOC density explained 27.40 % of variation of soil δ¹³C.

Conclusions

It is suggested that different climatic factors controlled the size of the soil C pool in different ecosystems on the Tibetan Plateau. SOC density is a key contributor to the variation of soil δ¹³C.     

Soil Ca alters processes contributing to C and N retention in the Oa/A horizon of a northern hardwood forest

Recent studies on the effects of calcium (Ca) additions on soil carbon (C) cycling in organic soil horizons present conflicting results, with some studies showing an increase in soil C storage and others a decrease. We tested the legacy effects of soil Ca additions on C and nitrogen (N) retention in a long-term incubation of soils from a plot-scale field experiment at the Hubbard Brook Experimental Forest, NH, USA. Two levels of Ca (850 and 4250 kg Ca/ha) were surface applied to field plots as the mineral wollastonite (CaSiO₃) in summer of 2006. Two years after field Ca additions, Oa/A horizon soils were collected from field plots and incubated in the laboratory for 343 days to test Ca effects on C mineralization, dissolved organic carbon (DOC) export, and net N transformations. To distinguish mineralization of soil organic C (SOC) from that of more recent C inputs to soil, we incubated soils with and without added ¹³C-labeled sugar maple leaf litter. High Ca additions increased exchangeable Ca and pH compared to the control. While low Ca additions had little effect on mineralization of SOC or added litter C, high Ca additions reduced mineralization of SOC and enhanced mineralization of litter C. In litter-free incubations, δ¹³C of respired C was enriched in the high Ca treatment compared to the control, indicating that Ca suppressed mineralization of ¹³C-depleted SOC sources. Leaching of DOC and NH₄ ⁺ were reduced by Ca additions in litter-free and litter-amended soils. Our results suggest that Ca availability in these organic soils influences mineralization of SOC and N primarily by stabilization processes and only secondarily through pH effects on organic matter solubility, and that SOC binding processes become important only with relatively large alterations of Ca status.     

Perennial Grass Bioenergy Cropping on Wet Marginal Land: Impacts on Soil Properties,Soil Organic Carbon,and Biomass During Initial Establishment

The control of soil moisture, vegetation type, and prior land use on soil health parameters of perennial grass cropping systems on marginal lands is not well known. A fallow wetness-prone marginal site in New York (USA) was converted to perennial grass bioenergy feedstock production. Quadruplicate treatments were fallow control, reed canarygrass (Phalaris arundinaceae L. Bellevue) with nitrogen (N) fertilizer (75 kg N ha^?1), switchgrass (Panicum virgatum L. Shawnee), and switchgrass with N fertilizer (75 kg N ha^?1). Based on periodic soil water measurements, permanent sampling locations were assigned to various wetness groups. Surface (0–15 cm) soil organic carbon (SOC), active carbon, wet aggregate stability, pH, total nitrogen (TN), root biomass, and harvested aboveground biomass were measured annually (2011–2014). Multi-year decreases in SOC, wet aggregate stability, and pH followed plowing in 2011. For all years, wettest soils had the greatest SOC and active carbon, while driest soils had the greatest wet aggregate stability and lowest pH. In 2014, wettest soils had significantly (p?<?0.0001) greater SOC and TN than drier soils, and fallow soils had 14 to 20% greater SOC than soils of reed canarygrass + N, switchgrass, and switchgrass + N. Crop type and N fertilization did not result in significant differences in SOC, active carbon, or wet aggregate stability. Cumulative 3-year aboveground biomass yields of driest switchgrass + N soils (18.8 Mg ha^?1) were 121% greater than the three wettest switchgrass (no N) treatments. Overall, soil moisture status must be accounted for when assessing soil dynamics during feedstock establishment.     

Legume and Non-legume Trees Increase Soil Carbon Sequestration in Savanna

Savanna ecosystems are increasingly pressured by climate and land-use changes, especially around populous areas such as the Mt. Kilimanjaro region. Savanna vegetation consists of grassland with isolated trees or tree groups and is therefore characterized by high spatial variation and patchiness of canopy cover and aboveground biomass. Both are major regulators for soil ecological properties and soil-atmospheric trace gas exchange (CO₂, N₂O, CH₄), especially in water-limited environments. Our objectives were to determine spatial trends in soil properties and trace gas fluxes during the dry season and to relate above- and belowground processes and attributes. We selected a Savanna plain with vertic soil properties, south east of Mt. Kilimanjaro. Three trees were chosen from each of the two most dominant species: the legume Acacia nilotica and the non-legume Balanites aegyptiaca. For each tree, we selected one transect with nine sampling points, up to a distance of 4 times the crown radius from the stem. At each sampling point, we measured carbon (C) and nitrogen (N) content, δ¹³C of soil (0–10, 10–30 cm depth) and in plant biomass, soil C and N pools, water content, available nutrients, cation exchange capacity (CEC), temperature, pH, as well as root biomass and greenhouse-gas exchange. Tree species had no effect on soil parameters and gas fluxes under the crown. CEC, C, and N pools decreased up to 50% outside the crown-covered area. Tree leaf litter had a far lower C:N ratio than litter of the C₄ grasses. δ¹³C in soil under the crown shifted about 15% in the direction of tree leaf litter δ¹³C compared to soil in open area reflecting the tree litter contribution to soil organic matter. The microbial C:N ratio and CO₂ efflux were about 30% higher in the open area and strongly dependent on mineral N availability. This indicates N limitation and low microbial C use efficiency in the soil of open grassland areas. We conclude that the spatial structure of aboveground biomass in savanna ecosystems leads to a spatial redistribution of nutrients and thus C mineralization and sequestration. Therefore, the capability of savanna ecosystems to act as C sinks is both directly and indirectly dependent on the abundance of trees, regardless of their N-fixing status.     

Threshold responses of soil organic carbon concentration and composition to multi-level nitrogen addition in a temperate needle-broadleaved forest

Responses of soil organic carbon (SOC) cycling and C budget in forest ecosystems to elevated nitrogen (N) deposition are divergent. Little is known about the N critical loads for the shift between gain and loss of SOC storage in the old-growth temperate forest of Northeast China. The objective of this study was to investigate the nonlinear responses of SOC concentration and composition to multiple rates of N addition, as well as the microbial mechanisms responsible for SOC alteration under N enrichment. Nine rates of urea addition (0, 10, 20, 40, 60, 80, 100, 120, 140 kg N ha^?1 year^?1) with 4 replicates for each treatment were conducted. Soil samples in the 0–10 cm mineral layer were taken after 3 years of N fertilization. Soil aggregate size distribution and SOC physical fractionation were performed to examine SOC dynamics. Phospholipid fatty acid (PLFA) technique was used to measure the abundance and structure of microbial community. Three years of N addition led to significant increases in the concentrations of soil particulate organic C and aggregate-associated organic C fractions only. The responses of total N and each labile SOC fraction to the rates of N addition followed Gaussian equations, with the N critical loads being estimated to be between 80 and 100 kg N ha^?1 year^?1. The change in SOC concentration (ΔSOC) was positively correlated with the changes in aggregate associated OC (r² > 0.80) and POC concentrations (r² > 0.50). Significant correlations among the concentrations of labile SOC fractions, the percentages of soil aggregates, and the abundances of microbial PLFAs were observed, which implies a close linkage between microbial community structure and SOC accumulation and stability. Our results suggest that increase in soil moisture and shift of microbial community structure could control the critical N load for the switch between C accumulation and loss. The current N deposition rate (~ 11 kg N ha^?1 year^?1) to the northeast China’s forests is favorable for soil C accumulation over the short term.     

The effects of heating,rhizosphere, and depth on root litter decomposition are mediated by soil moisture

The breakdown and decomposition of plant inputs are critical for nutrient cycling, soil development, and climate-ecosystem feedbacks, but uncertainties persist in how the rates and products of litter decomposition are affected by soil temperature, rhizosphere, and depth of input. We investigated the effects of soil warming (+ 4 °C), rhizosphere, and depth of litter placement on the decomposition of Avena fatua (wild oat grass) root litter in a Mediterranean grassland ecosystem. Field lysimeters were subjected to three environmental treatments (heating, control, and plant removal) and three ¹³C-labeled root litter addition treatments (to A horizon, to B horizon, and no-addition disturbance control) for each of two harvest time points. We buried root litter in February 2014 and measured loss of ¹³C in CO₂ from the soil surface and in leachate as dissolved organic carbon (DOC) over two growing seasons. At the end of each growing season we recovered the ¹³C remaining in the soil. Loss of root litter C occurred almost entirely via heterotrophic respiration, with an estimated < 2% lost as DOC during the initial decay period. The added roots were broken down and incorporated into bulk soil material very quickly; only ~ 30% of added root was visible after 6 months. In the first growing season, decomposition occurred faster in the B than in the A horizon, the latter having greater moisture limitation. Subsequently, there was almost no further decomposition in the B horizon. After two growing seasons, less than 20% of the added root litter C remained in the A or B horizons of all environmental treatments. Heating did not stimulate decomposition, likely because it exacerbated the moisture limitation. However, while plots without plants dried down more slowly than plots with plants, their decomposition rate was not significantly greater, possibly due to the lack of rhizosphere processes such as priming. We conclude that in this Mediterranean grassland ecosystem, soil moisture, which is affected by season, depth, heating, and rhizosphere, plays a dominant role in mediating the effect of those factors on root litter decomposition, which after two seasons did not differ by depth or by treatment.     

Shrub-encroachment induced alterations in input chemistry and soil microbial community affect topsoil organic carbon in an Inner Mongolian grassland

Shrub encroachment frequently occurs in arid and semi-arid grasslands worldwide and affects the regional carbon balance. Many previous studies have revealed the effects of shrub encroachment on bulk carbon content of grasslands, but molecular evidence is surprisingly lacking. In this study, we examined the chemical composition of plant tissues and soil organic carbon (SOC), and soil microbial communities to identify the effects of shrub (Caragana microphylla) encroachment on SOC storage in the top layer (0–10 cm) along a gradient of natural shrub cover in the grasslands of Inner Mongolia. We found that SOC in the shrub patches was derived mainly from leaves, whereas SOC in the grassy matrix was composed of a mixture of fresh root- and leaf-derived compounds. Compared with pure grassland, the SOC decreased by 29% in the shrub-encroached grasslands (SEGs), and this decrease was enhanced by increasing shrub cover. We also found that free lipids and lignin-derived phenols increased while the ratios of ω-C₁₈/∑C₁₈ and suberin/cutin decreased with increasing shrub cover. In addition, the ratios of fungal to bacterial phospholipid fatty acids (PLFAs) and gram-negative to gram-positive bacterial PLFAs decreased with increasing shrub cover. These results indicate that the encroachment of nitrogen-rich legume shrubs can lead to carbon loss by altering the chemical composition of plant inputs as well as the soil microbial community in grassland ecosystems.     

Effects of non-native Asian earthworm invasion on temperate forest and prairie soils in the Midwestern US

Effects of invasive European earthworms in North America have been well documented, but less is known about ecological consequences of exotic Asian earthworm invasion, in particular Asian jumping worms (Amynthas) that are increasingly reported. Most earthworm invasion research has focused on forests; some Amynthas spp. are native to Asian grasslands and may thrive in prairies with unknown effects. We conducted an earthworm-addition mesocosm experiment with before–after control-impact (BACI) design and a complementary field study in southern Wisconsin, USA, in 2014 to investigate effects of a newly discovered invasion of two Asian jumping worms (Amynthas agrestis and Amynthas tokioensis) on forest and prairie litter and soil nutrient pools. In both studies, A. agrestis and A. tokioensis substantially reduced surface litter (84–95 % decline in foliage litter mass) and increased total carbon, total nitrogen, and available phosphorus in the upper 0–5 cm of soils over the 4-month period from July through October. Soil inorganic nitrogen (ammonium– and nitrate–N) concentration increased across soil depths of 0–25 cm, with greater effects on nitrate–N. Dissolved organic carbon concentration also increased, e.g., 71–108 % increase in the mesocosm experiment. Effects were observed in both forest and prairie soils, with stronger effects in forests. Effects were most pronounced late in the growing season when earthworm biomass likely peaked. Depletion of the litter layer and rapid mineralization of nutrients by non-native Asian jumping worms may make ecosystems more susceptible to nutrient losses, and effects may cascade to understory herbs and other soil biota.     

Short-term effects of crop rotation,residue management,and soil water on carbon mineralization in a tropical cropping system

The purpose of this study was to investigate the short-term effects of maize (Zea mays)-fallow rotation, residue management, and soil water on carbon mineralization in a tropical cropping system in Ghana. After 15 months of the trial, maize–legume rotation treatments had significantly (P?C ₀ (μg CO₂–C g^?1) than maize–elephant grass (Pennisetum purpureum) rotations. The C ₀ for maize–grass rotation treatments was significantly related to the biomass input (r?=?0.95; P?=?0.05), but that for the maize–legume rotation was not. The soil carbon mineralization rate constant, k (per day), was also significantly related to the rotation treatments (P?k values for maize–grass and maize–legume rotation treatments were 0.025 and 0.036 day^?1 respectively. The initial carbon mineralization rate, m ₀ (μg CO₂–C g^?1 day ^?1), was significantly (P?θ. The m ₀ ranged from 3.88 to 18.67 and from 2.30 to 15.35 μg CO₂–C g^?1 day^?1 for maize–legume and maize–grass rotation treatments, respectively, when the soil water varied from 28% to 95% field capacity (FC). A simple soil water content (θ)-based factor, f _w, formulated as: \(f_{\text{w}} = \left[ {\frac{{\theta - \theta _{\text{d}} }}{{\theta _{{\text{FC}}} - \theta _{\text{d}} }}} \right]\), where θ _d and θ _FC were the air-dry and field capacity soil water content, respectively, adequately described the variation of the m ₀ with respect to soil water (R ²?=?0.91; RMSE?=?1.6). Such a simple relationship could be useful for SOC modeling under variable soil water conditions.     

Long term decomposition: the influence of litter type and soil horizon on retention of plant carbon and nitrogen in soils

How plant inputs from above- versus below-ground affect long term carbon (C) and nitrogen (N) retention and stabilization in soils is not well known. We present results of a decade-long field study that traced the decomposition of ¹³C- and ¹⁵N-labeled Pinus ponderosa needle and fine root litter placed in O or A soil horizons of a sandy Alfisol under a coniferous forest. We measured the retention of litter-derived C and N in particulate (>2 mm) and bulk soil (<2 mm) fractions, as well as in density-separated free light and three mineral-associated fractions. After 10 years, the influence of slower initial mineralization of root litter compared to needle litter was still evident: almost twice as much root litter (44% of C) was retained than needle litter (22–28% of C). After 10 years, the O horizon retained more litter in coarse particulate matter implying the crucial comminution step was slower than in the A horizon, while the A horizon retained more litter in the finer bulk soil, where it was recovered in organo-mineral associations. Retention in these A horizon mineral-associated fractions was similar for roots and needles. Nearly 5% of the applied litter C (and almost 15% of the applied N) was in organo-mineral associations, which had centennial residence times and potential for long-term stabilization. Vertical movement of litter-derived C was minimal after a decade, but N was significantly more mobile. Overall, the legacy of initial litter quality influences total SOM retention; however, the potential for and mechanisms of long-term SOM stabilization are influenced not by litter type but by soil horizon.     

大兴安岭北部天然针叶林土壤氮矿化特征

采用顶盖埋管法对大兴安岭地区天然针叶林(樟子松林、樟子松-兴安落叶松混交林和兴安落叶松林)土壤铵态氮(NH~+_4-N)、硝态氮(NO~-_3-N)、净氮矿化速率进行研究,并探索土壤理化性质与氮矿化之间的相关性,为大兴安岭地区森林生态系统土壤养分管理及森林经营提供帮助。结果表明:观测期内(5—10月)3种林型土壤无机氮变化范围为31.51—70.42 mg/kg,以NH~+_4-N形式存在为主,占比达90%以上,且与纯林相比混交林土壤无机氮含量较高。3种林型土壤净氮矿化、净氨化、净硝化速率月变化趋势呈V型,7、8月表现为负值,其他月份为正值。净氮矿化速率变化范围樟子松林为-0.54—1.28 mg kg~(-1) d~(-1)、樟子松-兴安落叶松混交林为-0.13—0.55 mg kg~(-1) d~(-1)、兴安落叶松林为-0.80—1.05 mg kg~(-1) d~(-1)。土壤净氨化过程在土壤氮矿化中占主要地位,占比达60%以上。3种林型土壤净氮矿化、净氨化及净硝化速率垂直差异显著,0—10 cm土层矿化作用明显高于10—20 cm土层(P0.05)。土壤氮矿化速率与土壤含水量、土壤有机碳含量、土壤C/N、枯落物全氮含量和枯落物C/N均存在显著相关性。不同类型的森林土壤及枯落物的质量也存在差异,进而影响土壤氮矿化特征。     

Root decomposition of <Emphasis Type="Italic">Artemisia halodendron</Emphasis> and its effect on soil nitrogen and soil organic carbon in the Horqin Sandy Land,northeastern China

Root decomposition is a critical feedback from the plant to the soil, especially in sandy land where strong winds remove aboveground litter. As a pioneer shrub in semi-mobile dunes of the Horqin sandy land, Artemisia halodendron has multiple effects on nutrient capture and the microenvironment. However, its root decomposition has not been studied in terms of its influence on soil organic carbon (SOC) and nitrogen (N). In this study, we buried fine (≤2 mm) and coarse roots in litterbags at a depth of 15 cm below semi-mobile dunes. We measured the masses remaining and the C and N contents at intervals during 434 days of decomposition. The soils below the litterbags were then divided into layers and sampled to measure the SOC and N contents. After rapid initial decomposition, both coarse and fine roots decomposed slowly. After 53 days, 36.2 % of coarse roots and 39.8 % of fine roots had decomposed. In contrast, only 18.4 % of coarse roots and 30.5 % of fine roots decomposed in the following 381 days. Fine roots decomposed significantly faster, and their decomposition rate after the initial rapid decay was strongly related to climate (R ² = 0.716, P < 0.05). Root decomposition increased SOC and N contents below the litterbags, with larger effects for fine roots. The SOC content was more variable between soil layers than the N content. Thus, decomposition of A. halodendron roots cannot be ignored when studying SOC and N feedbacks from plants to the soil, particularly for fine roots.     

Impacts of nitrogen-fixing and non-nitrogen-fixing tree species on soil respiration and microbial community composition during forest management in subtropical China

Forest management with N-fixing trees can improve soil fertility and tree productivity, but have little information regarding belowground carbon processes and microbial properties. We aimed to evaluate the effects of three forest management regimes, which were Erythrophleum fordii (N-fixing tree), Pinus massoniana (non-N-fixing tree), and their mixed forest, on soil respiration and microbial community composition in subtropical China, using Barometric Process Separation and phospholipid fatty acid profiles, respectively. We found that the inclusions of N-fixing species in forests significantly increased the soil respiration, but have no effects on SOC and ecosystem total C stock. In addition, soil microbial communities were obviously different among the three forest management regimes. For instance, total and bacterial PLFAs were higher in the E. fordii and mixed forest than in the P. massoniana forest. Conversely, fungal PLFAs in the P. massoniana forest were elevated versus the other two forests. Soil total N, nitrate-N and pH were the key determinants shaping the microbial community composition. Our study suggests that variations in soil respiration in the studied forests could be primarily explained by the differences of root biomass and soil microbial biomass, but not soil organic carbon. Although soil fertility and microbial biomass were promoted, N-fixing plantings also brought on increased CO₂ emissions in laboratory assays. The future decision of tree species selection for forest management in subtropical China therefore needs to consider the potential influences of tree species on CO₂ emissions.     

Temperature Sensitivity of Soil Organic Carbon Mineralization along an Elevation Gradient in the Wuyi Mountains,China

Soil organic carbon (SOC) actively participates in the global carbon (C) cycle. Despite much research, however, our understanding of the temperature sensitivity of soil organic carbon (SOC) mineralization is still very limited. To investigate the responses of SOC mineralization to temperature, we sampled surface soils (0–10 cm) from evergreen broad-leaf forest (EBF), coniferous forest (CF), sub-alpine dwarf forest (SDF), and alpine meadow (AM) along an elevational gradient in the Wuyi Mountains, China. The soil samples were incubated at 5, 15, 25, and 35°C with constant soil moisture for 360 days. The temperature sensitivity of SOC mineralization (Q₁₀) was calculated by comparing the time needed to mineralize the same amount of C at any two adjacent incubation temperatures. Results showed that the rates of SOC mineralization and the cumulative SOC mineralized during the entire incubation significantly increased with increasing incubation temperatures across the four sites. With the increasing extent of SOC being mineralized (increasing incubation time), the Q₁₀ values increased. Moreover, we found that both the elevational gradient and incubation temperature intervals significantly impacted Q₁₀ values. Q₁₀ values of the labile and recalcitrant organic C linearly increased with elevation. For the 5–15, 15–25, and 25–35°C intervals, surprisingly, the overall Q₁₀ values for the labile C did not decrease as the recalcitrant C did. Generally, our results suggest that subtropical forest soils may release more carbon than expected in a warmer climate.     

Short-term response of CO<Subscript>2</Subscript> emissions to various leaf litters: a case study from freshwater marshes of Northeast China

Soil organic carbon (SOC) mineralization is an important process of carbon (C) cycling and budgeting associated with litter decomposition in terrestrial ecosystems. Research on altered plant-derived C input on soil C stability due to climate change is controversial and there remains considerable uncertainty in predicting soil C dynamics with the techniques currently available. In this study, we conducted a laboratory incubation experiment to test the effects of single- and mixed-Deyeuxia angustifolia (DA) and Carex lasiocarpa (CL) leaf litter addition on cumulative marshland soil CO₂ emission under waterlogged and non-waterlogged conditions in Sanjiang Plain, Northeast China. Results showed that the cumulative CO₂ emissions were significantly increased after leaf litter addition in both water conditions, and that the effect was more pronounced for DA amendment than CL regardless of water condition. The cumulative CO₂ efflux differed considerably between water conditions after DA addition, whereas no significant differences were found after CL addition. Remarkably impact of leaf litter types on cumulative CO₂ evolution was observed overall, water condition and interactions between leaf litter types and water conditions had no significant effect on CO₂ emissions, however. There were no non-additive effects of individual leaf litter type on total CO₂ efflux of the mixed-leaf litter addition treatments. The results of this study indicate that plant litter input to the C-rich marshy soil can induce rapid changes in SOC decomposition regardless of water conditions and that plant residue effects should be taken into consideration when assessing the dynamics of wetland soil system to the future climate scenarios.     

Leaf Litter Consumption by Macroarthropods and Burial of their Faeces Enhance Decomposition in a Mediterranean Ecosystem

In many terrestrial ecosystems, large amounts of leaf litter are consumed by macroarthropods. Most of it is deposited as faeces that are easily transferred into deeper soil layers. However, the decomposition of this large pool of organic matter remains poorly studied. We addressed the question of how leaf litter transformation into macroarthropod faeces, and their burial in the soil, affect organic matter decomposition in a Mediterranean dry shrubland. We compared mass loss of intact leaf litter of two dominant shrub species (Quercus coccifera, Cistus albidus) with that of leaf litter-specific faeces from the abundant millipede Ommatoiulus sabulosus. Leaf litter and faeces were exposed in the field for 1 year, either on the soil surface or buried at 5 cm soil depth. Chemical and physical quality of faeces differed strongly from that of leaf litter, but distinctively between the two shrub species. On the soil surface, faeces decomposed faster than intact leaf litter in Quercus, but at similar rates in Cistus. When buried in the soil, faeces and leaf litter decomposed at similar rates in either species, but significantly faster compared to the soil surface, most likely because of higher moisture within the soil enhancing microbial activity. The combined effects of leaf litter transformation into faeces and their subsequent burial in the topsoil led to a 1.5-fold increase in the annual mass loss. These direct and indirect macroarthropod effects on ecosystem-scale decomposition are likely more widespread than currently acknowledged, and may play a particularly important role in drought-influenced ecosystems.     

Opposite effects of litter and hemiparasites on a dominant grass under different water regimes and competition levels

Direct and indirect biotic interactions may affect plant growth and development, but the magnitude of these effects may vary depending on environmental conditions. In grassland ecosystems, competition is a strong structuring force. Nonetheless, if hemiparasitic plant species are introduced the competition intensity caused by the dominant species may be affected. However, the outcome of these interactions may change between wet or dry periods. In order to study this, we performed a pot experiment with different densities of the dominant species Schedonorus arundinaceus (1, 2 or 4 individuals) under constantly moist or intermittently dry conditions. The different Schenodorus densities were crossed with presence or absence of hemiparasites (either Rhinanthus minor or R. alectorolophus). Additionally, pots remained with bare ground or received a grass litter layer (400 g m^?2). We expected that indirect litter effects on vegetation (here Schedonorus or Rhinanthus) vary depending on soil moisture. We measured Schedonorus and Rhinanthus aboveground biomass and C stable isotope signature (δ¹³C) as response variables. Overall, Schedonorus attained similar biomass under moist conditions with Rhinanthus as in pots under dry conditions without Rhinanthus. Presence of Rhinanthus also increased δ¹³C in moist pots, indicating hemiparasite-induced water stress. Litter presence increased Schedonorus biomass and reduced δ¹³C, indicating improved water availability. Plants under dry conditions with litter showed similar biomass as under wet conditions without litter. Hemiparasites and litter had opposite effects: hemiparasites reduced Schedonorus biomass while litter presence facilitated grass growth. Contrary to our expectations, litter did not compensate Schedonorus biomass when Rhinanthus was present.     

Experimental litterfall manipulation drives large and rapid changes in soil carbon cycling in a wet tropical forest

Global changes such as variations in plant net primary production are likely to drive shifts in leaf litterfall inputs to forest soils, but the effects of such changes on soil carbon (C) cycling and storage remain largely unknown, especially in C‐rich tropical forest ecosystems. We initiated a leaf litterfall manipulation experiment in a tropical rain forest in Costa Rica to test the sensitivity of surface soil C pools and fluxes to different litter inputs. After only 2 years of treatment, doubling litterfall inputs increased surface soil C concentrations by 31%, removing litter from the forest floor drove a 26% reduction over the same time period, and these changes in soil C concentrations were associated with variations in dissolved organic matter fluxes, fine root biomass, microbial biomass, soil moisture, and nutrient fluxes. However, the litter manipulations had only small effects on soil organic C (SOC) chemistry, suggesting that changes in C cycling, nutrient cycling, and microbial processes in response to litter manipulation reflect shifts in the quantity rather than quality of SOC. The manipulation also affected soil CO ₂ fluxes; the relative decline in CO ₂ production was greater in the litter removal plots (?22%) than the increase in the litter addition plots (+15%). Our analysis showed that variations in CO ₂ fluxes were strongly correlated with microbial biomass pools, soil C and nitrogen (N) pools, soil inorganic P fluxes, dissolved organic C fluxes, and fine root biomass. Together, our data suggest that shifts in leaf litter inputs in response to localized human disturbances and global environmental change could have rapid and important consequences for belowground C storage and fluxes in tropical rain forests, and highlight differences between tropical and temperate ecosystems, where belowground C cycling responses to changes in litterfall are generally slower and more subtle.     

Non-native liana, <Emphasis Type="Italic">Euonymus fortunei</Emphasis>, associated with increased soil nutrients,unique bacterial communities,and faster decomposition rate

Invasive plants have wide-ranging impacts on native systems including reducing native plant richness and altering soil chemistry, microbes, and nutrient cycling. Increasingly, these effects are found to linger long after removal of the invader. We examined how soil chemistry, bacterial communities, and litter decomposition varied with cover of Euonymus fortunei, an invasive evergreen liana, in two central Kentucky deciduous forests. In one forest, E. fortunei invaded in the late 1990s but invasion remained patchy and we paired invaded and uninvaded plots to examine the associations between E. fortunei cover and our response variables. In the second forest, E. fortunei had completely invaded the forest by 2005; areas where it had been selectively removed by 2010 were paired with an adjacent invaded plot. Where E. fortunei had patchily invaded, E. fortunei patches had up to 3.5× nitrogen, 2.7× carbon, and 1.9× more labile glomalin in soils than uninvaded plots, whereas there were no differences in soil characteristics between invaded and removal plots. In the patchily invaded forest, bacterial community composition varied among invaded and non-invaded plots, whereas bacterial communities did not vary among invaded and removal plots. Finally, E. fortunei leaf litter decomposed faster (k = 4.91 year^?1) than the native liana (k = 3.77 year^?1), Vitis vulpina; decomposition of both E. fortunei and V. vulpina was faster in invaded (k = 7.10 year^?1) than removal plots (k = 4.77 year^?1). Our findings suggest that E. fortunei invasion increases the rate of leaf litter decomposition via high-quality litter, alters the decomposition environment, and shifts in the soil biotic communities associated with a dense mat of wintercreeper. Land managers with limited resources should target the densest mats for the greatest restoration potential and remove wintercreeper patches before they establish dense mats.     

Selenium fertilization strategies for bio-fortification of food: an agro-ecosystem approach

Background and aims

Although numerous studies have quantified the effects of land-use changes on soil organic carbon (SOC) stocks, few have examined simultaneously the weight of carbon (C) inputs vs. outputs in shaping these changes. We quantified the relative importance of soil C inputs and outputs in determining SOC changes following the conversion of natural ecosystems to pastures or tree plantations, and evaluated them in light of variations in biomass production, its quality (C:N) and above/belowground allocation patterns.

Methods

We sampled soils up to one-meter depth under native grasslands or forests and compared them to adjacent sites with pastures or plantations to estimate the proportion of new SOC (SOC_new) retained in the soil and the decomposition rates of old SOC (k _SOC-old ) based on δ ¹³C shifts. We also analyzed these changes in the particulate organic matter fraction (POM) and estimated above and belowground net primary production (ANPP and BNPP) from satellite images, as well as changes in vegetation and soil’s C:N ratios.

Results

The conversion of grasslands to tree plantations decreased total SOC contents while the conversion of forests to pastures increased SOC contents in the topsoil but decreased them in deep layers, maintaining similar soil stocks up to 1 m. Changes in POM were less important and occurred only in the topsoil after cultivating pastures, following SOC changes. Surprisingly, both land-use trajectories showed similar decomposition rates in the topsoil and therefore overall SOC changes were not correlated with C outputs (k _SOC-old ) but were significantly correlated with C inputs and their stabilization as SOC_new (similar results were obtained for the POM fraction). Pastures although decreased ANPP (as compared to forest) they increased belowground allocation and C:N ratios of their inputs to the soil, probably favoring the retention and stabilization of their new C inputs. In contrast, tree plantations increased ANPP but decreased BNPP (as compared to grasslands) and scarcely accumulated SOC_new probably as a result of the high C retention in standing biomass.

Conclusions

Our results suggest that SOC changes are mainly controlled by the quantity and quality of C inputs and their retention in the soil, rather than by C outputs in these perennial subtropical ecosystems.