Plant carbon inputs and environmental factors strongly affect soil respiration in a subtropical forest of southwestern China

Soil respiration is a large component of global carbon fluxes, so it is important to explore how this carbon flux varies with environmental factors and carbon inputs from plants. As part of a long-term study on the chemical and biological effects of aboveground litterfall denial, root trenching and tree-stem girdling, we measured soil respiration for three years in plots where those treatments were applied singly and in combination. Tree-stem girdling terminates the flow of carbohydrates from canopy, but allows the roots to continue water and nutrient uptake. After carbon storage below the stem girdles is depleted, the girdled trees die. Root trenching immediately terminates root exudates as well as water and nutrient uptake. Excluding aboveground litterfall removes soil carbon inputs, but allows normal root functions to continue. We found that removing aboveground litterfall and the humus layer reduced soil respiration by more than the C input from litter, a respiration priming effect. When this treatment was combined with stem girdling, root trenching or those treatments in combination, the change in soil respiration was indistinguishable from the loss of litterfall C inputs. This suggests that litterfall priming occurs only when normal root processes persist. Soil respiration was significantly related to temperature in all treatment combinations, and to soil water content in all treatments except stem girdling alone, and girdling plus trenching. Aboveground litterfall was a significant predictor of soil respiration in control, stem-girdled, trenched and stem-girdled plus trenching treatments. Stem girdling significantly reduced soil respiration as a single factor, but root trenching did not. These results suggest that in addition to temperature, aboveground carbon inputs exert strong controls on forest soil respiration.     

Response of soil nematode communities to tree girdling in a subtropical evergreen broad-leaved forest of southwest China

The impact of canopy photosynthates on soil microbial biomass and nematode trophic groups was studied in a subtropical evergreen broad-leaved forest by performing a large-scale tree girdling experiment. Total fungal biomass was unaffected by tree girdling. Bacterial biomass differed significantly between the girdled and control plots in the mineral soil, but was not affected by girdling treatment in the humus layer. Girdling reduced total nematode density in the humus layer. The reduced fungivorous nematode density in girdled plots in the humus layer suggested a modified energy flow through the fungal based pathways. There were no differences in the abundance of bacterial-feeding, herbivorous and omnivorous-predatory nematodes between the girdled and control plots in both humus and mineral soil layers. This study provides direct evidence that the termination of belowground photosynthate-C allocation achieved by tree girdling affects soil nematodes, and that different trophic groups vary in their responses to the reduction of C efflux into the soil.     

Alterations in forest detritus inputs influence soil carbon concentration and soil respiration in a Central-European deciduous forest

In a Quercetum petraeae–cerris forest in northeastern Hungary, we examined effects of litter input alterations on the quantity and quality soil carbon stocks and soil CO₂ emissions. Treatments at the Síkfőkút DIRT (Detritus Input and Removal Treatments) experimental site include adding (by doubling) of either leaf litter (DL) or wood (DW) (including branches, twigs, bark), and removing all aboveground litter (NL), all root inputs by trenching (NR), or removing all litter inputs (NI). Within 4 years we saw a significant decrease in soil carbon (C) concentrations in the upper 15 cm for root exclusion plots. Decreases in C for the litter exclusion treatments appeared later, and were smaller than declines in root exclusion plots, highlighting the role of root detritus in the formation of soil organic matter in this forest. By year 8 of the experiment, surface soil C concentrations were lower than Control plots by 32% in NI, 23% in NR and 19% in NL. Increases in soil C in litter addition treatments were less than C losses from litter exclusion treatments, with surface C increasing by 12% in DL and 6% in DW. Detritus additions and removals had significant effects on soil microclimate, with decreases in seasonal variations in soil temperature (between summer and winter) in Double Litter plots but enhanced seasonal variation in detritus exclusion plots. Carbon dioxide (CO₂) emissions were most influenced by detritus input quantity and soil organic matter concentration when soils were warm and moist. Clearly changes in detritus inputs from altered forest productivity, as well as altered litter impacts on soil microclimate, must be included in models of soil carbon fluxes and pools with expected future changes in climate.     

改变碳输入对南亚热带海岸沙地典型天然次生林土壤呼吸的影响

土壤呼吸是陆地生态系统碳循环的一个重要过程,开展环境因子和改变碳输入对土壤呼吸影响的研究具有重要意义.2015年3月-2016年2月,在南亚热带海岸沙地典型天然次生林中设置去除根系、去除凋落物、加倍凋落物和对照4种处理,采用LI-8100连续观测改变碳输入对土壤呼吸的影响.结果表明:改变碳输入没有显著影响l0cm土壤温度和湿度(P＞0.05);不同处理土壤呼吸速率存在明显的季节变化,表现为夏高冬低,最大值出现在5月或者6月,最小值出现在11月或12月;土壤呼吸速率的年均值为加倍凋落物＞对照＞去除根系＞去除凋落物,不同改变碳输入方式均降低了土壤呼吸的Q10值;矿质土壤呼吸、凋落物呼吸和根系呼吸对土壤总呼吸的贡献分别为41.24％、43.29％和15.45％;不同处理土壤呼吸速率分别与土壤温度和湿度呈显著的指数和线性正相关(P＜0.05),双因子模型能解释土壤呼吸变异的45％ ～ 69％;改变碳输入影响土壤可溶性有机碳和微生物生物量碳,不同处理土壤呼吸速率与可溶性有机碳和微生物生物量碳呈正相关.因此,改变碳输入引起土壤易变碳的变化进而影响土壤呼吸.     

Carbon flow into microbial and fungal biomass as a basis for the belowground food web of agroecosystems

The origin and quantity of plant inputs to soil are primary factors controlling the size and structure of the soil microbial community. The present study aimed to elucidate and quantify the carbon (C) flow from both root and shoot litter residues into soil organic, extractable, microbial and fungal C pools. Using the shift in C stable isotope values associated with replacing C3 by C4 plants we followed root- vs. shoot litter-derived C resources into different soil C pools. We established the following treatments: Corn Maize (CM), Fodder Maize (FM), Wheat + maize Litter (WL) and Wheat (W) as reference. The Corn Maize treatment provided root- as well as shoot litter-derived C (without corn cobs) whereas Fodder Maize (FM) provided only root-derived C (aboveground shoot material was removed). Maize shoot litter was applied on the Wheat + maize Litter (WL) plots to trace the incorporation of C4 litter C into soil microorganisms. Soil samples were taken three times per year (summer, autumn, winter) over two growing seasons. Maize-derived C signal was detectable after three to six months in the following pools: soil organic C (C_org), extractable organic C (EOC), microbial biomass (C_mic) and fungal biomass (ergosterol). In spite of the lower amounts of root- than of shoot litter-derived C inputs, similar amounts were incorporated into each of the C pools in the FM and WL treatments, indicating greater importance of the root- than shoot litter-derived resources for the soil microorganisms as a basis for the belowground food web. In the CM plots twice as much maize-derived C was incorporated into the pools. After two years, maize-derived C in the CM treatment contributed 14.1, 24.7, 46.6 and 76.2% to C_org, EOC, C_mic and ergosterol pools, respectively. Fungi incorporated maize-derived C to a greater extent than did total soil microbial biomass.     

Drivers of soil respiration and nitrogen mineralization change after litter management at a subtropical Chinese sweetgum tree plantation

Leaf litter decomposition transfers elements from litter to soils that are essential for regulating nutrient cycles in plantation ecosystems, especially carbon and nitrogen. However, soil carbon and nitrogen dynamics in response to tree litter management remains insufficiently researched. We conducted a one-year field experiment at a fast-growing sweetgum tree plantation to evaluate the effects of leaf litter management on soil available nutrients, respiration rate and nitrogen mineralization rate. Three leaf litter treatments were applied, which were: (1) natural input (control); (2) double input and (3) non-input. It was found that the double input treatment increased soil inorganic nitrogen and microbial biomass nitrogen, but had little effect on microbial biomass carbon, dissolved organic carbon or dissolved organic nitrogen compared with natural input. The non-input treatment caused dissolved organic carbon to decrease compared with natural input. The respiration rate increased in the double input treatment, with a positive priming effect observed. Soil net ammonification, nitrification and mineralization rates also increased in the double input treatment in specific seasons. Meanwhile, positive linear relationships between respiration rate and all nitrogen transformation rates were observed for all treatments. Soil temperature was found to be an important prediction factor for predicting the respiration rate and mineralization as seasonal variations, but not for litter-induced fluctuations. Soil water content and mineral nitrogen were the primary drivers of litter-induced change to the respiration rate, whereas mineral nitrogen and microbial biomass were primary drivers of mineralization change. These results suggest that changes in soil nitrogen mineralization rate are strongly associated with the soil respiratory process, resulting in a potentially strong plant–soil feedback mechanism.     

Grasses, litter, and their interaction affect microbial biomass and soil enzyme activity

Plant effects on ecosystem processes are mediated through plant-microbial interactions belowground and soil enzyme assays are commonly used to directly relate microbial activity to ecosystem processes. Live plants influence microbial biomass and activity via differences in rhizosphere processes and detrital inputs. I utilized six grass species of varying litter chemistry in a factorial greenhouse experiment to evaluate the relative effect of live plants and detrital inputs on substrate-induced respiration (SIR, a measure of active microbial biomass), basal respiration, dissolved organic carbon (DOC), and the activities of β-glucosidase, β-glucosaminidase, and acid phosphatase. To minimize confounding variables, I used organic-free potting media, held soil moisture constant, and fertilized weekly. SIR and enzyme activities were 2-15 times greater in litter-addition than plant-addition treatments. Combining live plants with litter did not stimulate microbial biomass or activity above that in litter-only treatments, and β-glucosidase activity was significantly lower. Species-specific differences in litter N (%) and plant biomass were related to differences in β-glucosaminidase and acid phosphatase activity, respectively, but had no apparent effect on β-glucosidase, SIR, or basal respiration. DOC was negatively related to litter C:N, and positively related to plant biomass. Species identity and living plants were not as important as litter additions in stimulating microbial activity, suggesting that plant effects on soil enzymatic activity were driven primarily by detrital inputs, although the strength of litter effects may be moderated by the effect of growing plants.     

Feedback on plant productivity can be constrained by SOM in N-limited grasslands

In many terrestrial ecosystems plant productivity is limited by the availability of mineral nitrogen, which is produced by soil microbial transformations of organic N in soil organic matter (SOM-N). Mineral N availability results from two opposing processes, 1) gross mineral N production (gross ammonification/gross nitrification) and 2) microbial N immobilization. These processes can be influenced by the availability of plant-derived C (PDC) inputs to the microbes, SOM-N pool size, and the size of the microbial community (microbial biomass). We considered how changes in PDC inputs and SOM-N pool size together may alter microbial biomass, mineral N availability, and feedbacks on plant productivity. In areas dominated by one of six tallgrass prairie species along a natural gradient of PDC inputs and SOM-N pool size, we conducted a field survey of microbial biomass and gross ammonification. We also performed greenhouse manipulations of SOM-N pool size and PDC inputs on two species in our study area (Poa pratensis and Schizachyrium scoparium). Structural equation modeling of the field data showed that gross ammonification was both positively and directly related to microbial biomass and SOM-N pool size. Gross ammonification was positively and indirectly related to SOM-N pool size and belowground PDC inputs, via microbial biomass. In the short-term greenhouse study, PDC inputs and SOM-N pool size positively affected gross mineral N production, although only at high SOM-N pool size. If the patterns in the greenhouse can be applied to field conditions, this suggests that SOM-N pool size may constrain plant driven feedbacks on plant productivity by limiting gross mineral N production.     

Tree girdling increases soil N mineralisation in two spruce stands

Tree girdling is a common practice in forestry whenever trees are to be killed without felling. The effect of tree girdling on soil nitrogen (N) mineralisation was estimated in both an old and a young spruce forest. The dynamics of mineral N (NO₃⁻–N and NH₄⁺–N) and soil microbial biomass carbon (MBC) and N (MBN) were determined for different seasons. The in situ net N mineralisation was measured by incubating soil samples in stainless steel cylinders and the gross N mineralisation rates were measured by ¹⁵N pool dilution method. Mineral N concentrations increased significantly in the girdled plots in both old and young spruce forests and showed variations between soil horizons and between sampling times. Tree girdling significantly increased net N mineralisation in both spruce forests. Annual net N mineralisation was 64 and 39 kg N ha⁻¹ in O horizon of the girdled plots in old and young forest plots, respectively, compared to 25 and 21 kg N ha⁻¹ in the control plots. Annual N mineralisation in A horizon was similar between girdled and control plots (31 kg N ha⁻¹) in the old forest whereas in the young forest A horizon N mineralisation was about 2.5 times higher in the girdled plots. As a result, the annual carbon budget was significantly more positive in the girdled plots than in the control plots in both old and young forests. However, we found significantly higher gross N mineralisation rates in both horizons in the control plots than the girdled plots in the old forest, but no differences between the treatments in the young forest. The MBC and MBN contents only showed significant changes during the first three months of the experiment and were similar later on. They first decreased as girdling removed the root carbohydrate, amino and organic acid exudation from the C sources for microorganisms then increased two months after the treatment root dieback acted as a new source of C. Mineralising microorganisms enhanced the mineral N concentrations in girdled plots as a result of greater activity rather than larger population size.     

Greater humification of belowground than aboveground biomass carbon into particulate soil organic matter in no-till corn and soybean crops

Quantifying the amount of carbon (C) incorporated from decomposing residues into soil organic carbon (C_S) requires knowing the rate of C stabilization (humification rate) into different soil organic matter pools. However, the differential humification rates of C derived from belowground and aboveground biomass into C_S pools has been poorly quantified. We estimated the contribution of aboveground and belowground biomass to the formation of C_S in four agricultural treatments by measuring changes in δ¹³C natural abundance in particulate organic matter (C_POM) associated with manipulations of C₃ and C₄ biomass. The treatments were (1) continuous corn cropping (C₄ plant), (2) continuous soybean cropping (C₃), and two stubble exchange treatments (3 and 4) where the aboveground biomass left after the grain harvest was exchanged between corn and soybean plots, allowing the separation of aboveground and belowground C inputs to C_S based on the different δ¹³C signatures. After two growing seasons, C_POM was primarily derived from belowground C inputs, even though they represented only ∼10% of the total plant C inputs as residues. Belowground biomass contributed from 60% to almost 80% of the total new C present in the C_POM in the top 10 cm of soil. The humification rate of belowground C inputs into C_POM was 24% and 10%, while that of aboveground C inputs was only 0.5% and 1.0% for soybean and corn, respectively. Our results indicate that roots can play a disproportionately important role in the C_POM budget in soils. Keywords Particulate organic matter; root carbon inputs; carbon isotopes; humification rate; corn; soybean.     

Plant species richness leaves a legacy of enhanced root litter-induced decomposition in soil

Increasing plant species richness generally enhances plant biomass production, which may enhance accumulation of carbon (C) in soil. However, the net change in soil C also depends on the effect of plant diversity on C loss through decomposition of organic matter. Plant diversity can affect organic matter decomposition via changes in litter species diversity and composition, and via alteration of abiotic and/or biotic attributes of the soil (soil legacy effect). Previous studies examined the two effects on decomposition rates separately, and do therefore not elucidate the relative importance of the two effects, and their potential interaction. Here we separated the effects of litter mixing and litter identity from the soil legacy effect by conducting a factorial laboratory experiment where two fresh single root litters and their mixture were mixed with soils previously cultivated with single plant species or mixtures of two or four species. We found no evidence for litter-mixing effects. In contrast, root litter-induced CO₂ production was greater in soils from high diversity plots than in soils from monocultures, regardless of the type of root litter added. Soil microbial PLFA biomass and composition at the onset of the experiment was unaffected by plant species richness, whereas soil potential nitrogen (N) mineralization rate increased with plant species richness. Our results indicate that the soil legacy effect may be explained by changes in soil N availability. There was no effect of plant species richness on decomposition of a recalcitrant substrate (compost). This suggests that the soil legacy effect predominantly acted on the decomposition of labile organic matter. We thus demonstrated that plant species richness enhances root litter-induced soil respiration via a soil legacy effect but not via a litter-mixing effect. This implies that the positive impacts of species richness on soil C sequestration may be weakened by accelerated organic matter decomposition.     

Effects of litter addition and warming on soil carbon, nutrient pools and microbial communities in a subarctic heath ecosystem

Climatic warming leads to the expansion of deciduous shrubs and trees in the Arctic. This leads to higher leaf litter inputs, which together with warming may alter the rate of carbon and nutrient cycling in the arctic ecosystems. We assessed effects of factorial warming and additional litter on the soil ecosystem of a subarctic heath in a 7-year-long field experiment. Fine root biomass, dissolved organic carbon (DOC) and total C concentration increased in response to warming, which probably was a result of the increased vegetation cover. Litter addition increased the concentration of inorganic P in the uppermost 5 cm soil, while decreasing the pool of total P per unit area of the organic profile and having no significant effects on N concentrations or pools. Microbial biomass C and N were unaffected by the treatments, while the microbial biomass P increased significantly with litter addition. Soil ergosterol concentration was also slightly increased by the added litter in the uppermost soil, although not statistically significantly. According to a principal component analysis of the phospholipid fatty acid profiles, litter addition differed from the other treatments by increasing the relative proportion of biomarkers for Gram-positive bacteria. The combined warming plus litter addition treatment decreased the soil water content in the uppermost 5 cm soil, which was a likely reason for many interactions between the effects of warming and litter addition. The soil organic matter quality of the combined treatment was also clearly different from the control based on a near-infrared reflectance (NIR) spectroscopic analysis, implying that the treatment altered the composition of soil organic matter. However, it appears that the biological processes and the microbial community composition responded more to the soil and litter moisture conditions than to the change in the quality of the organic matter.     

Effects of long-term litter manipulation on soil carbon,nitrogen, and phosphorus in a temperate deciduous forest

Changes in above-ground litterfall can influence below-ground biogeochemical processes in forests. In order to examine how above-ground litter inputs affect soil carbon (C), nitrogen (N) and phosphorus (P) in a temperate deciduous forest, we studied a 14-year-old small-scale litter manipulation experiment that included control, litter exclusion, and doubled litter addition at a mature Fagus sylvatica L. site. Total organic C (TOC), total N (TN) and total P (TP), total organic P (TOP), bioavailable inorganic P (Pi), microbial C, N and P, soil respiration and fine root biomass were analyzed in the A and in two B horizons. Our results showed that litter manipulation had no significant effect on TOC in the mineral soil. Litter addition increased the bioavailable Pi in the A horizon but had no significant effect on N in the mineral soil. Litter exclusion decreased TN and TP in the B horizon to a depth of 10 cm. In the A horizon of the litter exclusion treatment, TP, TOP and bioavailable Pi were increased, which is most likely due to the higher root biomass in this treatment. The high fine root biomass seems to have counteracted the effects of the excluded aboveground litter. In conclusion, our study indicates that aboveground litter is not an important source for C in the mineral soil and that P recycling from root litter might be more important than from above-ground litter.     

Microbial community utilization of added carbon substrates in response to long-term carbon input manipulation

The chemical composition and quantity of plant inputs to soil are primary factors controlling the size and structure of the soil microbial community. Little is known about how changes in the composition of the soil microbial community affect decomposition rates and other ecosystem functions. This study examined the degradation of universally ¹³C-labeled glucose, glutamate, oxalate, and phenol in soil from an old-growth Douglas-fir (Pseudotsuga menziesii)—western hemlock (Tsuga heterophylla) forest in the Oregon Cascades that has experienced 7 y of chronic C input manipulation. The soils used in this experiment were part of a larger Detritus Input and Removal Treatment experiment and have received normal C inputs (control), doubled wood inputs, or root and litter input exclusion (no inputs). Soil from the doubled wood treatment had a higher fungal:bacterial ratio, and soil from the no inputs treatment had a lower fungal:bacterial ratio, than the control soil. Differences in the utilization of the compounds added to the field-manipulated soils were assessed by following the ¹³C tracer into microbial biomass and respiration. In addition, ¹³C-phospholipid fatty acids (PLFA) analysis was used to examine differential microbial utilization of the added substrates. Glucose and glutamate were metabolized similarly in soils of all three litter treatments. In contrast, the microbial community in the double wood soil respired more added phenol and oxalate, whereas microbes in the no inputs soil respired less added phenol and oxalate, than the control soil. Phenol was incorporated primarily into fungal PLFA, especially in soil of the double wood treatment. The addition of all four substrates led to enhanced degradation of soil organic matter (priming) in soils of all three litter treatments, and was greater following the addition of phenol and oxalate as compared to glucose and glutamate. Priming was greater in the no inputs soil as compared to the control or doubled wood soils. These results demonstrate that altering plant inputs to soil can lead to changes in microbial utilization of C compounds. It appears that many of these changes are the result of alteration in the size and composition of the microbial community.     

Plot-scale manipulations of organic matter inputs to soils correlate with shifts in microbial community composition in a lowland tropical rain forest

Little is known about the organisms responsible for decomposition in terrestrial ecosystems, or how variations in their relative abundance may influence soil carbon (C) cycling. Here, we altered organic matter in situ by manipulating both litter and throughfall inputs to tropical rain forest soils, and then used qPCR and error-corrected bar-coded pyrosequencing to investigate how the resulting changes in soil chemical properties affected microbial community structure. The plot-scale manipulations drove significant changes in microbial community composition: Acidobacteria were present in greater relative abundance in litter removal plots than in double-litter plots, while Alphaproteobacteria were found in higher relative abundance in double-litter and throughfall reduction plots than in control or litter removal plots. In addition, the bacterial:archaeal ratio was higher in double-litter than no-litter plots. The relative abundances of Actinobacteria, Alphaproteobacteria and Gammaproteobacteria were positively correlated with microbial biomass C and nitrogen (N), and soil N and C pools, while acidobacterial relative abundance was negatively correlated with these same factors. Bacterial:archaeal ratios were positively correlated with soil moisture, total soil C and N, extractable ammonium pools, and soil C:N ratios. Additionally, bacterial:archaeal ratios were positively related to the relative abundance of Actinobacteria, Gammaproteobacteria, and Actinobacteria, and negatively correlated to the relative abundance of Nitrospira and Acidobacteria. Together, our results support the copiotrophic/oligotrophic model of soil heterotrophic microbes suggested by Fierer et al. (2007).     

Biotic community shifts explain the contrasting responses of microbial and root respiration to experimental soil acidification

Soil respiration is comprised primarily of root and microbial respiration, and accounts for nearly half of the total CO₂ efflux from terrestrial ecosystems. Soil acidification resulting from acid deposition significantly affects soil respiration. Yet, the mechanisms that underlie the effects of acidification on soil respiration and its two components remain unclear. We collected data on sources of soil CO₂ efflux (microbial and root respiration), above- and belowground biotic communities, and soil properties in a 4-year field experiment with seven levels of acid in a semi-arid Inner Mongolian grassland. Here, we show that soil acidification has contrasting effects on root and microbial respiration in a typical steppe grassland. Soil acidification increases root respiration mainly by an increase in root biomass and a shift to plant species with greater specific root respiration rates. The shift of plant community from perennial bunchgrasses to perennial rhizome grasses was in turn regulated by the decreases in soil base cations and N status. In contrast, soil acidification suppresses microbial respiration by reducing total microbial biomass and enzymatic activities, which appear to result from increases in soil H⁺ ions and decreases in soil base cations. Our results suggest that shifts in both plant and microbial communities dominate the responses of soil respiration and its components to soil acidification. These results also indicate that carbon cycling models concerned with future climate change should consider soil acidification as well as shifts in biotic communities.     

Effects of the ecological restoration practices of prescribed burning and mechanical thinning on soil microbial enzyme activities and leaf litter decomposition

The effects of ecological restoration on belowground processes such as decomposition are generally unknown. To assess the immediate effects of prescribed fire and mechanical thinning on belowground processes, we measured the activities of five extracellular enzymes (phosphatase, β-glucosidase, β-N-acetylglucosaminidase, phenol oxidase, and lignin-peroxidase) in soils and on decomposing Quercus falcata leaf litter in unburned, burned, and burned and thinned plots in a mesic forest in northern Mississippi. Decomposition rates of Q. falcata leaf litter were also assessed at each plot. Soil phosphatase activity decreased after a prescribed burn and was related to an increase in soil organic matter in plots that had been burned. Soil β-N-acetylglucosaminidase activity increased after a burn, and was related to a decrease in leaf litter. Leaf litter enzyme activity showed no consistent patterns amongst treatments, or between individual enzymes, while decomposition rates of leaf litter were slightly accelerated in the treatment plots, but not significantly so. Decomposition rates were related to cumulative enzyme activity, with phenol oxidase and lignin-peroxidase having the highest apparent efficiencies in degrading the leaf material. Overall, the microbial degradation of Q. falcata leaf litter was more efficient in plots that were burned and thinned than in the other plots. Increases in the efficiency of litter decomposition coupled with reductions in litter inputs due to canopy thinning likely allows for increased solar penetration to the soil, and could promote the restoration of the shade-intolerant species that once dominated the understory. Post-burn increases in β-N-acetylglucosaminidase activity and decreases in phosphatase activity also suggest a potential shift in the soil community from phosphorus limitation to nitrogen limitation following a fire.     

Carbon-allocation dynamics in reed canary grass as affected by soil type and fertilization rates in northern Sweden

Abstract

A field experiment was conducted in northern Sweden between 1995 and 1997, with the objectives (1) to quantify the dynamics of carbon accumulation in above- and belowground crop components of reed canary grass (RCG) during the second and third year after sowing and (2) to examine the effect of fertilization and soil type (mineral vs. organic) on C allocation. Across all treatments, carbon accumulation in belowground organs in the top 20 cm was on average 3 and 3.4 Mg C by the end of the second and third year, respectively, with roots and rhizomes accounting for up to 80%. Roots contributed most to belowground C mass during the second growing season but during the preceding winter, root biomass C decreased by 44–67%, and, thereafter, during the third growing season, the proportion of rhizome C increased. The dynamics of root biomass was considerably high, suggesting high root turnover rates. Rhizomes support re-growth during spring and rhizome biomass seems to increase with crop age. Thus, early harvesting before October may impact on the productivity during the following season.

Among the factors studied, harvest date was the most influential and affected C allocation in all crop components considerably. Fertilization stimulated growth of shoots, rhizomes, and BSBs (belowground shoot bases) but not that of roots. However, root biomass was higher in the organic than in the mineral soil. In this study, we considered only plant components above 20 cm depth. More detailed studies are needed to calculate more complete soil C balances. However, high belowground biomass production and root turnover indicate a high C input to the soil, which may result in positive soil C balances. Therefore, RCG cropping could have considerable carbon-sequestration potential.     

Soil characteristics determine soil carbon and nitrogen availability during leaf litter decomposition regardless of litter quality

Climate and litter quality have been identified as major drivers of litter decomposition, but our knowledge of how soil characteristics (e.g. microbial community and chemical properties) determine carbon (C) and nitrogen (N) availability derived from the decomposition of litter of different qualities is still scarce. We conducted a microcosm experiment to evaluate how soils with contrasting microbial communities and soil properties (denoted Soils A and B hereafter, where Soil B has higher bacterial and fungal abundance, fungal:bacterial ratio, and organic C than Soil A) determine the availability of soil C (carbohydrates, proteins, amino acids and phenols) and N (dissolved organic and inorganic N, microbial biomass N and available N) during the decomposition of litter of contrasting quality (C:N ratios ranging from 20 to 102). We also evaluated the relative importance of soil characteristics and litter quality as drivers of C and N inputs to the soil during this process. Overall, higher soil C and N availability after litter decomposition was found in Soil B than in Soil A. Soil characteristics had a higher positive effect on soil C and N contents than litter quality during litter decomposition. We also found that changes in N availability and organic matter quality registered after litter decomposition, linked to different soil characteristics, were able to promote dissimilarities in the potential mineralization rates. In conclusion, our study provides evidence that soil characteristics (e.g. microbial communities and chemical properties) can be more important than litter quality in determining soil C and equally important for N availability during the decomposition of leaf litter.     

Eucalyptus grandis and Acacia mangium in monoculture and intercropped plantations: Evolution of soil and litter microbial and chemical attributes during early stages of plant development

Soil microorganisms and microbial processes are influenced by the quality and quantity of plant waste entering the soil, by its seasonal and spatial distribution, by the ratio of above- to below-ground inputs, and by changes in nutrient inputs. Soil management strategies sometimes promote mixed-species plantations to mitigate the loss of soil nutrients and improve biogeochemical cycling. The objective of this study was to explore changes in microbiological and chemical attributes of soils and litter in the early stages of the second rotation of mixed and pure plantations of Eucalyptus grandis and Acacia mangium, and to look for correlations between attributes. Soil samples at 0–10 cm depth were collected two, seven, 14, and 20 months after planting in the following treatments: monocultures of A. mangium and E. grandis, a monoculture of E. grandis with N-fertilizer, and an intercropped plantation with E. grandis and A. mangium. Microbial soil attributes varied dramatically between treatments 20 months after planting. Total C, N and P contents in litter showed the strongest correlations with microbial biomass C and N (C_mic and N_mic), microbial respiration, and dehydrogenase activity in all sampling periods. Lower C/N and C/P ratios in litter and lower C/N and C_mic/tC ratios in soils after 20 months in the intercropped plantation illustrated the system's capacity for supplying inputs of high-quality organic matter rich in N and P, but this did not result in higher contents of these elements or greater microbial activity in soils. An implication of this finding is that, at least in the initial growth phase of these plantations, chemical attributes of the litter and variation in those attributes govern microbial processes and, consequently, are mostly responsible for plant development. Canonical discriminant analysis revealed changes in the microbiological and chemical attributes of soil in the intercropped plantation due to the plants growth and the leaf litter accumulation. Twenty months after planting, the different plantations could be discriminated by differences in litter chemistry (C, N, and P), total soil C, N_mic, and dehydrogenase activity, which were very similar in intercropped plantations and E. grandis with N-fertilizer. These results from the early stages of plantation development are important for understanding the dynamics of soil attributes in these systems, and especially in intercropped plantations. In intercropped areas the cumulative effect of microbial attributes reflects a more sustainable system. Long-term studies are needed to identify patterns that develop after 20 months, during the growth period of these plantations.