Aboveground Litter Inputs Research Articles

Chemical quality of aboveground litter inputs for jack pine and black spruce stands along the Canadian Boreal Forest Transect Case Study

In the Canadian boreal forest, jack pine stands generally have a thin forest floor and occupy sites with coarsetextured soils and good drainage. Black spruce occurs more often on poorly drained sites and develops a thick mossdominated forest floor, but the common attribution of this development to poor quality of black spruce foliar litter has not been tested. We determined needle, twig, cone, and bark litter inputs during 10 y for black spruce and jack pine along the Boreal Forest Transect Case Study in northern Saskatchewan and Manitoba. Analysis of C, N, total phenolics, condensed tannins, and solid-state 13C NMR spectra from years 1–3 showed only small differences between species, notably higher tannins in black spruce cones. There was similarly little difference between area-based inputs, including classes of C structures determined by NMR. Condensed tannin input for black spruce was approximately twice that for jack pine, but both were in the very low range of reported values. Similar analyses showed that black spruce forest floor was less decomposed than that of jack pine, and for both species, aromatic litter C inputs appear to be poorly conserved, with a large influence of mosses and lichen. It is unlikely, however, that these large differences are mainly due to the small differences in aboveground litter inputs.

Combined effects of nitrogen addition and litter manipulation on nutrient resorption of Leymus chinensis in a semi‐arid grassland of northern China

Plant growth in semi-arid ecosystems is usually severely limited by soil nutrient availability. Alleviation of these resource stresses by fertiliser application and aboveground litter input may affect plant internal nutrient cycling in such regions. We conducted a 4-year field experiment to investigate the effects of nitrogen (N) addition (10 g N·m(-2) ·year(-1)) and plant litter manipulation on nutrient resorption of Leymus chinensis, the dominant native grass in a semi-arid grassland in northern China. Although N addition had no clear effects on N and phosphorus (P) resorption efficiencies in leaves and culms, N fertilisation generally decreased leaf N resorption proficiency by 54%, culm N resorption proficiency by 65%. Moreover, N fertilisation increased leaf P resorption proficiency by 13%, culm P resorption proficiency by 20%. Under ambient or enriched N conditions, litter addition reduced N and P resorption proficiencies in both leaves and culms. The response of P resorption proficiency to litter manipulation was more sensitive than N resorption proficiency: P resorption proficiency in leaves and culms decreased strongly with increasing litter amount under both ambient and enriched N conditions. In contrast, N resorption proficiency was not significantly affected by litter addition, except for leaf N resorption proficiency under ambient N conditions. Furthermore, although litter addition caused a general decrease of leaf and culm nutrient resorption efficiencies under both ambient and enriched N conditions, litter addition effects on nutrient resorption efficiency were much weaker than the effects of litter addition on nutrient resorption proficiency. Taken together, our results show that leaf and non-leaf organs of L. chinensis respond consistently to altered soil N availability. Our study confirms the strong effects of N addition on plant nutrient resorption processes and the potential role of aboveground litter, the most important natural fertiliser in terrestrial ecosystems, in influencing plant internal nutrient cycling.

Litter and Root Manipulations Provide Insights into Soil Organic Matter Dynamics and Stability

Understanding controls on C stored in soil organic matter (SOM) is of critical importance to models of biospheric C sequestration. Although ecosystem C models assume a strong relationship between plant litter inputs and soil C accumulation, there is little experimental evidence to support this assumption. The Detritus Input and Removal Treatments (DIRT) experiment at Harvard Forest was designed to assess how rates and sources of plant litter inputs control the accumulation and dynamics of organic matter in soils across decadal time scales. Carbon and SOM quantity and quality were measured in O horizon and mineral soil in five treatments: control, double litter, no litter, no roots, and no inputs. After 20 yr of manipulation, doubling litter inputs did not increase bulk soil C or N content, light or heavy fraction pools of C, or measures of labile C. However, the activities of two key enzymes (β‐glucosidase and phosphomonoesterase) increased 30% with litter additions. Exclusion of either aboveground litter or root inputs resulted in sharp declines in O‐horizon C and N but smaller decreases in total mineral soil C and N. However, decreases in light fraction C and soil respiration were significant in removal treatments. Litter exclusion resulted in an 18% decline in total profile mineral soil C, whereas root exclusion resulted in a 9% decline, indicating the importance of aboveground inputs to long‐term C pools. Soil C pools in this forest do not respond linearly or immediately to aboveground or belowground litter inputs, and thus efforts to sequester C by managing productivity and associated litter inputs will probably not result in increased C storage in short time frames.

亚热带杉木和米老排人工林土壤呼吸对凋落物去除和交换的响应

PDF HTML阅读 XML下载 导出引用 引用提醒 亚热带杉木和米老排人工林土壤呼吸对凋落物去除和交换的响应 DOI: 10.5846/stxb201309092240 作者: 作者单位: 湿润亚热带山地生态国家重点实验室培育基地;福建师范大学地理科学学院,湿润亚热带山地生态国家重点实验室培育基地;福建师范大学地理科学学院,湿润亚热带山地生态国家重点实验室培育基地;福建师范大学地理科学学院,湿润亚热带山地生态国家重点实验室培育基地;福建师范大学地理科学学院,湿润亚热带山地生态国家重点实验室培育基地;福建师范大学地理科学学院,湿润亚热带山地生态国家重点实验室培育基地;福建师范大学地理科学学院,福建农林大学林学院,湿润亚热带山地生态国家重点实验室培育基地;福建师范大学地理科学学院 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金项目（41371269）；教育部新世纪优秀人才支持计划（DB-168）；福建省杰出青年科学基金项目（2060203） Contrasting responses of soil respiration to litter manipulation in subtropical Mytilaria laosensis and Cunninghamia lanceolata plantations Author: Affiliation: College of Geographical Science, Fujian Normal University,College of Geographical Science, Fujian Normal University,,,,,, Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:在我国亚热带地区米老排（Mytilaria laosensis）和杉木（Cunninghamia lanceolata）人工林中设置去除凋落物、交换凋落物和对照3种处理，利用LI-8100对不同处理土壤呼吸速率进行为期14个月的连续观测。结果表明：与对照相比，去除和交换凋落物导致米老排人工林CO2年排放量显著减少29.8%和14.2%，杉木人工林则分别减少6.1%和增加37.8%。两种林分交换凋落物处理产生了不同程度的激发效应，米老排凋落物相对于杉木凋落物具有更大的激发效应。不同处理土壤呼吸速率均与土壤温度呈显著指数关系，与土壤含水量呈负相关，土壤温度和含水量的回归模型可以分别解释米老排人工林中去除、交换和对照处理土壤呼吸速率的68.9%、77.0%和69.6%，杉木人工林的53.0%、36.2%和63.8%。两种林分不同处理土壤呼吸速率与土壤可溶性有机碳含量、微生物生物量碳含量以及主要微生物种群生物量显著相关。米老排人工林去除和交换凋落物处理均降低了土壤呼吸的温度敏感性Q10值，而杉木林中仅去除凋落物降低了Q10值。研究表明，土壤呼吸对凋落物输入方式改变的响应因树种而异，这种差异与凋落物输入的数量和质量及其对土壤易变性有机碳含量和微生物生物量的影响有关。 Abstract:Future climate change is expected to alter the primary productivity of forest ecosystems, and thus potentially change the quality and quantity of aboveground litter inputs which could affect soil carbon emission and sequestration. A litter manipulation experiment was carried out in 19-year-old Mytilaria laosensis and Cunninghamia lanceolata plantations replanted in C.lanceolata woodland in subtropical China between January 2012 and February 2013. Three treatments including litter exclusion, reciprocal litter transplant and control were conducted within 2 m ×2 m plots in randomized block design. Soil respiration rate (Rs) were measured monthly by Li-Cor 8100 and soil temperature and soil moisture were also monitored. The results showed that Rs subject to three treatments exhibited similar seasonal patterns in M.laosensis and C.lanceolata plots. The average Rs in litter exclusion, litter transplant and control treatments were 1.79, 2.18, 2.45μmol CO2 m-2 s-1 in M.laosensis plots and 1.77, 2.29, 1.90μmol CO2 m-2 s-1 in C.lanceolata plots, respectively. Overall, litter exclusion and transplant significantly decreased the annual fluxes under M.laosensis plots by 29.8% and 14.2%, respectively. Litter exclusion caused a slight decline in annual flux but litter transplant markedly increased the annual flux by 37.8% in C.lanceolata plots. Using the decay constants of the two litter species in a reciprocal litter transplant decomposition experiment, priming effects were calculated as the amount of litter-derived respiration minus decomposed carbon from litter. Results showed that M.laosensis litter aroused higher priming effect than C.lanceolata litter, suggesting that the quantity and quality of litter input caused by different tree species could result in different below-ground responses. Correlations showed that Rs were exponentially correlated with the soil temperature and negatively correlated with the soil moisture under both M.laosensis and C.lanceolata plots. Non-liner regressions of soil temperature and soil moisture together accounted for 68.9%-77.0% and 36.2%-63.8% of monthly variation in soil respiration in M.laosensis and C.lanceolata plots, respectively, indicating that other factors were responsible for interpreting variations in Rs. The average Rs was significantly correlated to soil dissolved organic carbon concentration (P=0.019) and microbial biomass carbon concentration (P=0.037), which suggests that litter manipulation induced changes in soil labile carbon caused significant effects on Rs. A one time-point measured soil PLFA profiles showed that the average Rs was significantly correlated with the biomass of main signature PLFAs such as fungi (18:1 w9c, 18:2 w6,9c), which may imply that the high quality of litter input could alter soil microbial biomass and thus affect Rs. The temperature sensitivity (Q10) of Rs decreased to 1.86 and 2.05 in the litter exclusion and transplant plots compared to 2.14 in the control plots under M.laosensis plantations. However, the Q10 in the litter exclusion, litter transplanting and control plots were 1.40, 1.52 and 1.52 in C.lanceolata plantations, respectively, which are lower than other studies in subtropical China. In summary, we concluded that the effects of litter manipulation on Rs were dependent on tree species, the contributions of litter toRs. can be attributed to the quantity and quality of litter input, the changes in soil labile carbon and soil microbial biomass in subtropical forest plantations. 参考文献 相似文献 引证文献

Corrigendum to "Variability of above-ground litter inputs alters soil physicochemical and biological processes: a meta-analysis of litterfall-manipulation experiments" published in Biogeosciences, 10, 1–11, 2013

In the paper “Variability of above-ground litter inputs alters soil physicochemical and biological processes: a metaanalysis of litterfall-manipulation experiments” by S. Xu et al. (Biogeosciences, 10, 1–11, 2013) the following error occured: the order of the first two affiliations from the first author S. Xu were interchanged. The updated version of the right affiliation order is shown above. In this case the affiliation of the second author has been updated as well to have the right attribution as before.

Variability of above-ground litter inputs alters soil physicochemical and biological processes: a meta-analysis of litterfall-manipulation experiments

Abstract. Global change has been shown to alter the amount of above-ground litter inputs to soil greatly, which could cause substantial cascading effects on below-ground biogeochemical cycling. Despite extensive study, there is uncertainty about how changes in above-ground litter inputs affect soil carbon and nutrient turnover and transformation. Here, we conducted a meta-analysis on 70 litter-manipulation experiments in order to assess how changes in above-ground litter inputs alter soil physicochemical properties, carbon dynamics and nutrient cycles. Our results demonstrated that litter removal decreased soil respiration by 34%, microbial biomass carbon in the mineral soil by 39% and total carbon in the mineral soil by 10%, whereas litter addition increased them by 31, 26 and 10%, respectively. This suggests that greater litter inputs increase the soil carbon sink despite higher rates of carbon release and transformation. Total nitrogen and extractable inorganic nitrogen in the mineral soil decreased by 17 and 30%, respectively, under litter removal, but were not altered by litter addition. Overall, litter manipulation had a significant impact upon soil temperature and moisture, but not soil pH; litter inputs were more crucial in buffering soil temperature and moisture fluctuations in grassland than in forest. Compared to other ecosystems, tropical and subtropical forests were more sensitive to variation in litter inputs, as altered litter inputs affected the turnover and accumulation of soil carbon and nutrients more substantially over a shorter time period. Our study demonstrates that although the magnitude of responses differed greatly among ecosystems, the direction of the responses was very similar across different ecosystems. Interactions between plant productivity and below-ground biogeochemical cycling need to be taken into account to predict ecosystem responses to environmental change.

Fire interacts with season to influence soil respiration in tropical savannas

Soil respiration (Rs) is the second-largest source of CO2 to the atmosphere in terrestrial systems. In tropical savannas seasonal moisture availability and frequent fires drive ecosystem dynamics and may have a considerable impact on soil carbon (C) cycling, including Rs. In order to test the effect of fire on soil C cycling we measured Rs in annually burnt and unburnt plots in wet and dry seasons at a long-term fire experiment established in savanna woodlands of northern Australia. There was a significant interaction between season and fire, with highest rates of daily Rs (722 mmol CO2 m−2 d−1) observed in the wet season on unburnt, leaf litter patches. The three fold higher Rs rate on unburnt plots in the wet season was due to greater root-derived respiration (Rroot: 356 mmol CO2 m−2 d−1), while smaller changes to soil-derived respiration (Rsoil: 51 mmol CO2 m−2 d−1) were simply the result of C moving through decomposition rather than combustion pathways. Relationships between instantaneous Rs and soil temperature showed hysteresis with variable direction, suggesting that season and fire treatment also influence the soil depth at which CO2 is produced. We suggest that (1) changes to fire regimes, through active management or climate change, in tropical savannas could have an impact on Rs, and (2) the direct effect of fire on soil C cycling is limited to the removal of aboveground litter inputs.

The effects of afforestation on soil organic and inorganic carbon: A case study of the Loess Plateau of China

The determination of the changes in soil organic carbon (SOC) and inorganic carbon (SIC) in the subsoil following afforestation is meaningful and necessary for assessing carbon sequestration, but such knowledge is limited. In this case study, a paired-site approach was used to determine the differences in the SOC and SIC stock in the topsoil and subsoil, respectively, between a black locust (Robinia pseudoacacia L.) forest site (converted from cropland 30 years ago) and a cropland site in the middle of the Loess Plateau of China. Compared with the cropland, the SOC stock was significantly greater under the forest in both the top 20 cm and the subsoil (30–60 cm layer). The annual litter input under the forest was more than twice that of the cropland, and the fine root biomass was significantly higher in the forest. We conclude that the higher litter input and fine root biomass may partly contribute to the greater SOC in the forest. In addition, the soil nitrogen (N) content changed in synchronicity with SOC during afforestation, which indicates that SOC accumulation in the top/subsoil may be determined by the increase in soil N in these layers. In contrast, the SIC stock in the top 20 cm of the forest was significantly lower than that of the cropland. However, this decrease in the SIC level in the topsoil of the forest was offset by an increase in SIC in the subsoil (60–100 cm). The change in SIC along the soil profile following afforestation could be explained by the dissolution and leaching of SIC from the topsoil and subsequent precipitation in the subsoil. The dissolution and leaching of SIC in the forest topsoil were due to the high biological activity associated with the high aboveground litter input, fine root biomass and SOC stock as well as the high soil water content, whereas the precipitation of the leached SIC in the subsoil was a result of the dramatic decrease in the soil water content and fine root biomass in the subsoil. These findings suggest that soil can accumulate organic carbon in the topsoil and subsoil following black locust plantation establishment on cropland (as in the Loess Plateau study area) and that this type of cropland to forest plantation conversion in this area can redistribute SIC along the soil profile without affecting the net SIC accumulation.

Early-stage impacts of sika deer on structure and function of the soil microbial food webs in a temperate forest: A large-scale experiment

Modification of forest vegetation caused by an overabundance of mammalian herbivores has been reported in temperate and subarctic regions all over the world. However, the indirect effects of these herbivores on the structure and functions of soil decomposer systems are not fully understood, especially in temperate forests. We investigated the early effects of sika deer invasion on soil decomposer systems in a Japanese temperate forest using two large-scale experimental enclosures with low and high densities of deer (LD: 25ha, 4deerkm−2; HD: 6.25ha, 16deerkm−2) including control plots without deer (WD). Three years after deer introduction the understory cover of dwarf bamboo (Sasa nipponica) declined due to deer browsing in both enclosures. At the same time, measurements were made of the soil microbial community, soil nematode community, soil nitrogen (N) mineralization rate, and carbon (C) and N content in dwarf bamboo leaves. In LD, soil microbial biomass was lower from WD, probably due to the decrease of fresh aboveground litter from dwarf bamboo. Surprisingly, there were no cascading effects on total abundance of soil nematodes and soil total N mineralization potential which were unaffected by deer in the LD treatment, while soil NH4+-N content was lower and soil nematode community structure was different (abundance of 4 families was higher and that of 3 families was lower, but the functional structure was not different) from WD. Specifically, the responses to deer introduction varied between microbes and nematodes, and the change of balance in the microbial food webs may have altered N mineralization processes. In contrast, in the HD treatment, all the variables measured were not significantly different from those of WD treatment. Intensive browsing by deer may have cancelled out the effects of the decrease in aboveground litter input on the soil decomposer systems through other pathways, such as a transitory increase in belowground litter input caused by induced changes in allocation patterns of bamboo. No changes in total N mineralization potential, leaf N, and composition of understory vegetation in both enclosures indicated that deer introduction did not facilitate nor retard N cycling regardless of deer density. This study showed that sika deer browsing can affect soil decomposer systems at an early stage of invasion even at low density, which contrasts with previous studies on the subject. Linking our findings of early-stage effects of deer on soil decomposer systems to longer-term dynamics of understory vegetation and tree regeneration will be needed to evaluate the adequacy of deer management practices with respect to the sustainability of soil nutrient supplies.

Dissolved Organic Nitrogen Concentrations and Ratios of Dissolved Organic Carbon to Dissolved Organic Nitrogen in Throughfall and Soil Waters in Norway Spruce and Scots Pine Forest Stands Throughout Norway

Dissolved organic nitrogen (DON) plays an important ecological role in forest ecosystems, and its concentration is related to that of dissolved organic carbon (DOC). We investigated DON concentrations and ratios of DOC to DON in throughfall and soil waters in 16 Norway spruce and two Scots pine forest stands sampled at weekly intervals between 1996 and 2006. The stands are all included in the ICP Forests Level II monitoring program and are located throughout Norway. DON concentrations were significantly and positively related to DOC concentrations in throughfall (r2 = 0.72, p < 0.0001) and soil water at 5, 15, and 40 cm (r2 = 0.86, 0.32, and 0.84 and p < 0.0001, 0.04, and <0.0001, respectively). At most sites, the annual median DOC/DON ratio in throughfall ranged from 20.3 to 55.5, which is lower than values in soil water, which ranged from 24.5 to 81.3, gradually decreasing with soil depth. DON concentrations varied seasonally in throughfall at many plots and in soil water at 5-cm depth at one plot only, with higher values in the growing season, but there was no noticeable seasonality at greater depth. The ratios of DOC/DON in soil water were significantly positively related to the C/N ratio in soil at the same depth. Above-ground litter input was the main factor having a significant, negative relationship to DOC/DON in soil water at all depths studied. This might reflect the effect of site conditions on both DOC/DON ratios and litter quantity.

Carbon storage in post-mining forest soil, the role of tree biomass and soil bioturbation

Carbon storage in aboveground tree biomass and soil organic matter (in depth of A layer development i.e., up to 20 cm) was studied in 22–32 year-old post-mining sites in the northwest of the Czech Republic. Four replicated sites afforested with different tree species (spruce, pine, larch, oak, lime or alder) were compared with sites left to natural regeneration which were dominated by aspen, birch and willow. No topsoil was applied at the sites; hence carbon accumulation resulted from in situ soil development on alkaline tertiary clays that were dumped on the heaps. In aboveground tree biomass, carbon storage ranged from 17.0 ± 5.9 (mean ± SEM) to 67.6 ± 5.9 t ha−1 and the rate of C accumulation increased from 0.60 ± 0.09 to 2.31 ± 0.23 t ha−1 year−1 (natural regeneration < pine < spruce < oak < lime < alder < larch). Carbon storage in soil organic matter varied from 4.5 ± 3.7 to 38.0 ± 7.1 t ha−1 and the rate of C accumulation in soil organic matter increased from 0.15 ± 0.05 to 1.28 ± 0.34 t ha−1 year−1 at sites in the order: natural regeneration < spruce < pine, oak < larch < alder < lime. Carbon storage in the soil was positively correlated with aboveground tree biomass. Soil carbon was equivalent to 98.1% of the carbon found in aboveground tree biomass at lime dominated sites, but only 21.8% at sites with natural regeneration. No significant correlation was found between C storage in soil and aboveground litter input. Total soil carbon storage was correlated positively and significantly with earthworm density, and occurrence of earthworm cast in topsoil, which indicated that bioturbation could play an important role in soil carbon storage. Hence, not only restoring of wood production, but also restoring of soil community is critical for C storage in soil and whole ecosystem.

Increased coniferous needle inputs accelerate decomposition of soil carbon in an old-growth forest

Changes in temperature, precipitation, and atmospheric carbon dioxide (CO 2) concentration that are expected in the coming decades will have profound impacts on terrestrial ecosystem net primary production (NPP). Nearly all models linking forest NPP with soil carbon (C) predict that increased NPP will result in either unchanged or increased soil C storage, and that decreased NPP will result in decreased soil C storage. However, linkages between forest productivity and soil C storage may not be so simple and direct. In an old-growth coniferous forest located in the H.J. Andrews Experimental Forest, OR, USA, we experimentally doubled needle litter inputs, and found that actual soil respiration rates exceeded those expected due to the C added by the extra needles. Here, we estimated that this ‘priming effect’ accounted for 11.5–21.6% of annual CO 2 efflux from litter-amended plots, or an additional 137–256 g C m −2 yr −1 loss of stored C to the atmosphere. Soil priming was seasonal, with greatest amounts occurring in June–August coincident with peaks in temperature and dry summer conditions. As a result of priming, mineral soil was more resistant to further mineralization during laboratory incubations. Soil lignin-derived phenols in the Double Litter plots were more oxidized than in the control, suggesting that the soil residue was more degraded. Our hypothesis that excess dissolved organic C produced from the added litter provided the link between the forest floor and mineral soil and a substrate for soil priming was not supported. Instead, the rhizosphere, and associated mycorrhizal fungi, likely responded directly to the added aboveground litter inputs. Our results revealed that enhanced NPP may lead to accelerated processing of some stored soil C, but that the effects of increased NPP on ecosystem C storage will be based on a net balance among all ecosystem C pools and are likely to be ecosystem-dependant. Forest C models need to include these complex linkages between forest productivity and soil C storage.

GRASSLAND ESTABLISHMENT UNDER VARYING RESOURCE AVAILABILITY: A TEST OF POSITIVE AND NEGATIVE FEEDBACK

The traditional logic of carbon (C) and nitrogen (N) interactions in ecosystems predicts further increases or decreases in productivity (positive feedback) in response to high and low fertility in the soil, respectively; but the potential for development of feedback in ecosystems recovering from disturbance is less well understood. Furthermore, this logic has been challenged in grassland ecosystems where frequent fires or grazing may reduce the contribution of aboveground litter inputs to soil organic matter pools and nutrient supply for plant growth, relative to forest ecosystems. Further, if increases in plant productivity increase soil C content more than soil N content, negative feedback may result from increased microbial demand for N making less available for plant growth. We used a field experiment to test for feedback in an establishing grassland by comparing aboveground net primary productivity (ANPP) and belowground pools and fluxes of C and N in soil with enriched, ambient, and reduced N availability. For eight years annual N enrichment increased ANPP, root N, and root tissue quality, but root C:N ratios remained well above the threshold for net mineralization of N. There was no evidence that N enrichment increased root biomass, soil C or N accrual rates, or storage of C in total, microbial, or mineralizable pools within this time frame. However, the net nitrogen mineralization potential (NMP) rate was greater following eight years of N enrichment, and we attributed this to N saturation of the microbial biomass. Grassland developing under experimentally imposed N limitation through C addition to the soil exhibited ANPP, root biomass and quality, and net NMP rate similar to the ambient soil. Similarity in productivity and roots in the reduced and ambient N treatments was attributed to the potentially high nitrogen-use efficiency (NUE) of the dominant C4 grasses, and increasing cover of legumes over time in the C-amended soil. Thus, in a developing ecosystem, positive feedback between soil N supply and plant productivity may promote enhanced long-term N availability and override progressive N limitation as C accrues in plant and soil pools. However, experimentally imposed reduction in N availability did not feed back to reduce ANPP, possibly due to shifts in NUE and functional group composition.

Response of dissolved organic matter in the forest floor to long-term manipulation of litter and throughfall inputs

Dissolved organic matter (DOM) contributes to organic carbon either stored in mineral soil horizons or exported to the hydrosphere. However, the main controls of DOM dynamics are still under debate. We studied fresh leaf litter and more decomposed organic material as the main sources of DOM exported from the forest floor of a mixed beech/oak forest in Germany. In the field we doubled and excluded aboveground litter input and doubled the input of throughfall. From 1999 to 2005 we measured concentrations and fluxes of dissolved organic C and N (DOC, DON) beneath the Oi and Oe/Oa horizon. DOM composition was traced by UV and fluorescence spectroscopy. In selected DOM samples we analyzed the concentrations of phenols, pentoses and hexoses, and lignin-derived phenols by CuO oxidation. DOC and DON concentrations and fluxes almost doubled instantaneously in both horizons of the forest floor by doubling the litter input and DOC concentrations averaged 82 mg C l−1 in the Oe/Oa horizon. Properties of DOM did not suggest a change of the main DOM source towards fresh litter. In turn, increasing ratios of hexoses to pentoses and a larger content of lignin-derived phenols in the Oe/Oa horizon of the Double litter plots in comparison to the Control plots indicated a priming effect: Addition of fresh litter stimulated microbial activity resulting in increased microbial production of DOM from organic material already stored in Oe/Oa horizons. Exclusion of litter input resulted in an immediate decrease in DOC concentrations and fluxes in the thin Oi horizon. In the Oe/Oa horizon DOC concentrations started to decline in the third year and were significantly smaller than those in the Control after 5 years. Properties of DOM indicated an increased proportion of microbially and throughfall derived compounds after exclusion of litter inputs. Dissolved organic N did not decrease upon litter exclusion. We assume a microbial transformation of mineral N from throughfall and N mineralization to DON. Increased amounts of throughfall resulted in almost equivalently increased DOC fluxes in the Oe/Oa horizon. However, long-term additional throughfall inputs resulted in significantly declining DOC concentrations over time. We conclude that DOM leaving the forest floor derives mainly from decomposed organic material stored in Oe/Oa horizons. Leaching of organic matter from fresh litter is of less importance. Observed effects of litter manipulations strongly depend on time and the stocks of organic matter in forest floor horizons. Long-term experiments are particularly necessary in soils/horizons with large stocks of organic matter and in studies focusing on effects of declined substrate availability. The expected increased primary production upon climate change with subsequently enhanced litter input may result in an increased production of DOM from organic soil horizons.

Effects of throughfall and litterfall manipulation on concentrations of methylmercury and mercury in forest‐floor percolates

AbstractThe forest floor was shown to be an effective sink of atmospherically deposited methylmercury (MeHg) but less for total mercury (Hgtotal). We studied factors controlling the difference in dynamics of MeHg and Hgtotal in the forest floor by doubling the throughfall input and manipulating aboveground litter inputs (litter removal and doubling litter addition) in the snow‐free period in a Norway spruce forest in NE Bavaria, Germany, for 14 weeks. The MeHg concentrations in the forest‐floor percolates were not affected by any of the manipulation and ranged between 0.03 (Oa horizon) and 0.11 (Oi horizon) ng Hg L–1. The Hgtotal concentrations were largest in the Oa horizon (24 ng Hg L–1) and increased under double litterfall (statistically significant in the Oi horizon). Similarly, concentrations of dissolved organic C (DOC) increased after doubling of litterfall. The concentrations of Hgtotal and DOC correlated significantly in forest‐floor percolates from all plots. However, we did not find any effect of DOC on MeHg concentrations. The difference in the coupling of Hgtotal and MeHg to DOC might be one reason for the differences in the mobility of Hg species in forest floors with a lower mobility of MeHg not controlled by DOC.

Stand age‐related effects on soil respiration in a first rotation Sitka spruce chronosequence in central Ireland

AbstractThe effect of stand age on soil respiration and its components was studied in a first rotation Sitka spruce chronosequence composed of 10‐, 15‐, 31‐, and 47‐year‐old stands established on wet mineral gley in central Ireland. For each stand age, three forest stands with similar characteristics of soil type and site preparation were used. There were no significant differences in total soil respiration among sites of the same age, except for the case of a 15‐year‐old stand that had lower soil respiration rates due to its higher productivity. Soil respiration initially decreased with stand age, but levelled out in the older stands. The youngest stands had significantly higher respiration rates than more mature sites. Annual soil respiration rates were modelled by means of temperature‐derived functions. The average Q10 value obtained treating all the stands together was 3.8. Annual soil respiration rates were 991, 686, 556, and 564 g C m−2 for the 10‐, 15‐, 31‐, and 47‐year‐old stands, respectively. We used the trenching approach to separate soil respiration components. Heterotrophic respiration paralleled soil organic carbon dynamics over the chronosequence, decreasing with stand age to slightly increase in the oldest stand as a result of accumulated aboveground litter and root inputs. Root respiration showed a decreasing trend with stand age, which was explained by a decrease in fine root biomass over the chronosequence, but not by nitrogen concentration of fine roots. The decrease in the relative contribution of autotrophic respiration to total soil CO2 efflux from 59.3% in the youngest stand to 49.7% in the oldest stand was explained by the higher activity of the root system in younger stands. Our results show that stand age should be considered if simple temperature‐based models to predict annual soil respiration in afforestation sites are to be used.

Dynamics of soil organic matter in primary and secondary forest succession on sandy soils in The Netherlands: An application of the ROMUL model

We applied the simulation model ROMUL of soil organic matter dynamics in order to analyse and predict forest soil organic matter (SOM) changes following stand growth and also to identify gaps of data and modelling problems. SOM build-up was analysed (a) from bare sand to forest soil during a primary succession in Scots pine forest and (b) on mature forest soil under Douglas fir plantations as an example of secondary succession in The Netherlands. As some of the experimental data were unreliable we compiled a set of various scenarios with different soil moisture regime, initial SOM pools and amount and quality of above and below ground litter input. This allowed us to find the scenarios that reflect the SOM dynamics more realistically. In the Scots pine forest, total litter input was estimated as 0.50 kg m −2 year −1. Two scenarios were defined for the test runs: (a) forest floor moisture regimes—‘dry, mesic and hydric’ and (b) augmenting a root litter pool with three ratios of needles and branches to roots: 1:1, 1:1.5 and 1:2.0. The scenario finally compiled had the following characteristics: (a) climate for dry site with summer drought and high winter moisture of forest floor; (b) a litter input of 0.25 kg m −2 year −1 above ground and 0.50 kg m −2 year −1 below ground; (c) a low nitrogen and ash content in all litter fall fractions. The test runs for the estimation of the initial SOM pools and the amount and proportion of above and below ground litter fall were also performed in the Douglas fir plantation. The inputs of above ground litter tested in various combinations were 0.30 and 0.60 kg m −2 year −1, and below ground litter 0.30, 0.60 and 0.90 kg m −2 year −1. The scenario that fitted the experimental data had an SOM pool of 20–25 kg m −2, an aboveground litter input of 0.6 kg m −2 year −1and a below ground litter input of 0.9 kg m −2 year −1. The long-term simulation corresponded well with the observed patterns of soil organic matter accumulation associated with the forest soil development in primary and secondary succession. During primary succession in Scots pine forest on dry sand there is a consistent accumulation of a raw humus forest floor. The soil dynamics in the Douglas fir plantation also coincide with the observed patterns of SOM changes during the secondary succession, with SOM decreasing significantly under young forest, and SOM being restored in the older stands.

Contribution of aboveground litter, belowground litter, and rhizosphere respiration to total soil CO2 efflux in an old growth coniferous forest

In an old growth coniferous forest located in the central Cascade Mountains, Oregon, we added or removed aboveground litter and terminated live root activity by trenching to determine sources of soil respiration. Annual soil efflux from control plots ranged from 727 g C m−2 year−1 in 2002 to 841 g C m−2 year−1 in 2003. We used aboveground litter inputs (149.6 g C m−2 year−1) and differences in soil CO2 effluxes among treatment plots to calculate contributions to total soil efflux by roots and associated rhizosphere organisms and by heterotrophic decomposition of organic matter derived from aboveground and belowground litter. On average, root and rhizospheric respiration (Rr) contributed 23%, aboveground litter decomposition contributed 19%, and belowground litter decomposition contributed 58% to total soil CO2 efflux, respectively. These values fall within the range of values reported elsewhere, although our estimate of belowground litter contribution is higher than many published estimates, which we argue is a reflection of the high degree of mycorrhizal association and low nutrient status of this ecosystem. Additionally, we found that measured fluxes from plots with doubled needle litter led to an additional 186 g C m−2 year−1 beyond that expected based on the amount of additional carbon added; this represents a priming effect of 187%, or a 34% increase in the total carbon flux from the plots. This finding has strong implications for soil C storage, showing that it is inaccurate to assume that increases in net primary productivity will translate simply and directly into additional belowground storage.

Soil C Accumulation in a White Oak CO2-Enrichment Experiment via Enhanced Root Production

Abstract After four growing seasons, soil below white oak trees exposed to elevated atmospheric carbon dioxide levels (ambient + 300 ppm) had an average of 14% more soil carbon than soil below trees exposed to ambient levels of carbon dioxide. The soil carbon inventories in five soil cores collected from ambient chambers and six soil cores collected from elevated chambers at the Global Change Field Research Site, Oak Ridge, Tennessee, were measured. The authors conclude that the increase in soil carbon was due to an increase in belowground soil carbon input, because aboveground litter inputs were excluded by experimental design. These findings are consistent with the hypothesis that elevated carbon dioxide levels are increasing the amount of carbon stored in soil.

Conversion of grassy cerrado into riparian forest and its impact on soil organic matter dynamics in an Oxisol from southeast Brazil

The purpose of this study was to evaluate possible changes in soil organic matter (SOM) dynamics after establishing riparian forests on soils previously under Brazilian savannah (“cerrado”). We selected a site with a homogeneous Typic Acric Red–Yellow Latosol (Anionic Acrustox). Part of this site was maintained under native vegetation (grassy cerrado C 4-dominated), and part was planted with riparian species (C 3) in 1992. Litter and soil samples were collected and analysed (total organic carbon, total nitrogen, δ 13C isotopic analysis, and SOM density fractionation). Due to the predominance of grasses, carbon input was mainly below ground in cerrado. In such a soil, the decomposition process was more efficient, and much C and N were transferred to the heavy fraction. When forest was planted, there was a change from belowground to aboveground litter input (largely superficial), leading to higher C and N stocks in the light and lower stocks in the heavy fraction (resulting in lower stocks for bulk soil). The introduction of the C 3 vegetation decreased the soil δ 13C signature. It has occurred particularly in the topsoil (0–5 cm) due to the deposition of C 3 litter on the soil surface. At the same time, the presence of cerrado-remaining C below 5 cm maintained higher δ 13C values in this layer. During the 8 years after forest plantation, the input mode influenced both the δ 13C distribution with depth, and the C replacement: between 0 and 2.5 cm, nearly 50% of cerrado-derived C was replaced by forest-derived C, while below 5 cm, replacement was around 20%. The relatively rapid C dynamics in this Oxisol (27% replacement in the top 20 cm after 8 years of forest plantation) shows that, under tropical conditions, significant changes may occur in a short period of time.