Changing sources of soil respiration with time since fire in a boreal forest

Radiocarbon signatures (Δ¹⁴C) of carbon dioxide (CO₂) provide a measure of the age of C being decomposed by microbes or respired by living plants. Over a 2‐year period, we measured Δ¹⁴C of soil respiration and soil CO₂ in boreal forest sites in Canada, which varied primarily in the amount of time since the last stand‐replacing fire. Comparing bulk respiration Δ¹⁴C with Δ¹⁴C of CO₂ evolved in incubations of heterotrophic (decomposing organic horizons) and autotrophic (root and moss) components allowed us to estimate the relative contributions of O horizon decomposition vs. plant sources. Although soil respiration fluxes did not vary greatly, differences in Δ¹⁴C of respired CO₂ indicated marked variation in respiration sources in space and time. The ¹⁴C signature of respired CO₂ respired from O horizon decomposition depended on the age of C substrates. These varied with time since fire, but consistently had Δ¹⁴C greater (averaging ～120‰) than autotrophic respiration. The Δ¹⁴C of autotrophically respired CO₂ in young stands equaled those expected for recent photosynthetic products (70‰ in 2003, 64‰ in 2004). CO₂ respired by black spruce roots in stands >40 years old had Δ¹⁴C up to 30‰ higher than recent photosynthates, indicating a significant contribution of C stored at least several years in plants. Decomposition of O horizon organic matter made up 20% or less of soil respiration in the younger (<40 years since fire) stands, increasing to ～50% in mature stands. This is a minimum for total heterotrophic contribution, since mineral soil CO₂ had Δ¹⁴C close to or less than those we have assigned to autotrophic respiration. Decomposition of old organic matter in mineral soils clearly contributed to soil respiration in younger stands in 2003, a very dry year, when Δ¹⁴C of soil respiration in younger successional stands dropped below those of the atmospheric CO₂.     

Hair of grazing cattle provides an integrated measure of the effects of site conditions and interannual weather variability on δ13C of temperate humid grassland

The carbon isotope composition (δ¹³C) of C3 ecosystems is sensitive to water availability, and provides important information for the assessment of terrestrial carbon (C) sink/source activity. Here, we report the effects of plant available soil water (PAW) on community ¹³C signatures of temperate humid grassland. The 5‐year study was conducted on pastures exhibiting a large range of PAW capacity that were located on two site types: peat and mineral soils. The data set included the centennial drought year 2003, and data from wet years (2000 and 2002). Seasonal variation of PAW was modeled using PAW capacity of each pasture, precipitation inputs and evapotranspiration estimates. Community ¹³C signatures were derived from the δ¹³C of vegetation and segments of tail switch hair of cattle grown while grazing pastures. Hair ¹³C signatures provided an assimilation‐weighted ¹³C signal that integrated both spatial (paddock‐scale) and temporal (grazing season) variation of ¹³C signatures on a pasture. The δ¹³C of hair and vegetation increased with decreasing modeled PAW in the same way on mineral and peat soils. But, at a given PAW, the δ¹³C of hair was 2.6‰ less negative than that of vegetation, reflecting the diet‐hair isotopic shift. Furthermore, the δ¹³C of hair and vegetation on peat soil pastures was 0.5‰ more negative than on pastures situated on mineral soil. This may have resulted from a ～10 ppm CO₂ enrichment of canopy air derived from ongoing peat mineralization. Community‐scale season‐mean ¹³C discrimination (Δ) exhibited a saturation‐type response towards season‐mean modeled PAW (r²=0.78), and ranged between 19.8‰ on soils with low PAW capacity during the drought year of 2003, and 21.4‰ on soils with high PAW capacity in a wet year. This indicated relatively small variation in season‐mean assimilation‐weighted p_i/p_a (0.68–0.75) between contrasting sites and years. However, this range is similar to that reported in other studies, which encompass the range from subtropical arid to humid temperate grassland. Furthermore, the tight relationship between season‐mean Δ and modeled mean PAW suggests that PAW may be used as proxy for Δ.     

Biofumigation and Enhanced Biodegradation: Opportunity and Challenge in Soilborne Pest and Disease Management

Management of soilborne pests and diseases in cropping systems is often highly challenging—in implementation of acceptable methodologies and in dealing with secondary problems. The phase-out of methyl bromide brings this into particularly sharp focus. There is a need for diversified options and alternatives to fill different roles across the soilborne pest and disease management spectrum, but flexibility is limited, as practicalities demand that they fit into a prophylactic methodology. It is against such a backdrop that expectations and promotion of alternatives must be set. There is also a need to recognize potentially serious problems that may have been masked under historical management regimes, but for which the nature of the system offers little scope to avoid or manage.

Biofumigation is the beneficial use of Brassica green manures that release isothiocyanates chemically similar to methyl isothiocyanate, the active agent from the synthetic fumigant metam sodium, which is used as a substitute for methyl bromide in some systems. A systematic approach to research into biofumigation, specifically aimed at overcoming a long history of empiricism, has seen significant recent advances in both basic and applied knowledge. A key development has been achievement of maximal biofumigation potential through greatly enhanced release of appropriate isothiocyanates into soil. These advances have led to commercial adoption, demonstrating that biofumigation, when applied to appropriate production systems, can have efficacy and offer cost savings. Crucially, these systematically derived research and development findings and their adoption now provide the impetus for self-sustaining further development and market penetration of the concept. Despite this success, biofumigation is not seen as being sufficiently powerful or practical in implementation to be an alternative to methyl bromide on a broad scale and misdirection in that regard could be counterproductive to more appropriately targeted further development.

Enhanced microbial biodegradation is a cryptic phenomenon that can diminish the efficacy of soil-applied pesticides, including isothiocyanates and most other currently available methyl bromide substitutes. Because methyl bromide is not susceptible, the phenomenon has potentially serious implications in intensive production systems switching from methyl bromide to reliance on other compounds that are. It is an intractable problem once induced. Avoidance of its onset is the only feasible management strategy. This has been aided for some particularly vulnerable environments by recent clarification of key risk factors associated with soil type, soil pH, and calcium content.     

Soil organic carbon changes as influenced by agricultural land use and management: a case study in Yanhuai Basin, Beijing, China

The effects of agricultural land use and management practices on soil organic carbon (SOC) are of great concern. In this study, SOC changes were investigated in sandy loam soils (Ustochrepts, USDA Soil Taxonomy) under orchard, vegetable, corn (Zea maize L.), and soybean (Glycine max L.) cultivation in northern China. The corn fields were further classified into three categories based on their inputs, i.e. high-input, mid-input, and low-input corn fields. In April 2005, a total of 197 soil samples were collected from 42 soil sites within 100 cm soil depth in Yanhuai Basin, Beijing, China. SOC contents were determined using rapid dichromate oxidation, and ANOVA statistical analysis was used to test the significances between land use and management practices at p<0.05. The results showed that: (1) the effects of land use and management practices on SOC primarily occurred within the topsoil (0–25 cm), and the SOC contents sharply decreased with the increase in soil depth. (2) SOC content and density values of orchard, vegetable, and high-input corn fields were higher than those of soybean, mid- and low-input corn fields.     

Seasonal dynamics of fine root biomass,root length density,specific root length,and soil resource availability in a Larix gmelinii plantation

Fine root tumover is a major pathway for carbon and nutrient cycling in terrestrial ecosystems and is most likely sensitive to many global change factors.Despite the importance of fine root turnover in plant C allocation and nutrient cycling dynamics and the tremendous research efforts in the past,our understanding of it remains limited.This is because the dynamics processes associated with soil resources availability are still poorly understood.Soil moisture,temperature,and available nitrogen are the most important soil characteristics that impact fine root growth and mortality at both the individual root branch and at the ecosystem level.In temperate forest ecosystems,seasonal changes of soil resource availability will alter the pattern of carbon allocation to belowground.Therefore,fine root biomass,root length density(RLD)and specific root length(SRL)vary during the growing season.Studying seasonal changes of fine root biomass,RLD,and SRL associated with soil resource availability will help us understand the mechanistic controls of carbon to fine root longevity and turnover.The objective of this study was to understand whether seasonal variations of fine root biomass,RLD and SRL were associated with soil resource availability,such as moisture,temperature,and nitrogen,and to understand how these soil components impact fine root dynamics in Larix gmelinii plantation.We used a soil coring method to obtain fine root samples(≤2 mm in diameter)every month from Mav to October in 2002 from a 17-year-old L.gmelinii plantation in Maoershan Experiment Station,Northeast Forestry University,China.Seventy-two soil cores(inside diameter 60 mm;depth intervals:0-10 cm,10-20 cm,20-30 cm)were sampled randomly from three replicates 25 m×30 m plots to estimate fine root biomass(live and dead),and calculate RLD and SRL.Soil moisture,temperature,and nitrogen(ammonia and nitrates)at three depth intervals were also analyzed in these plots.Results showed that the average standing fine root biomass(live (32.2 g.m-2.a-1)in the middle(10-20 cm)and deep layer (20-30cm),respectively.Live and dead fine root biomass was the highest from May to July and in September,but lower in August and October.The live fine root biomass decreased and dead biomass increased during the growing soil layer.RLD and SRL in May were the highestthe other months,and RLD was the lowest in Septemberdynamics of fine root biomass,RLD,and SRL showed a close relationship with changes in soil moisture,temperature,and nitrogen availability.To a lesser extent,the temperature could be determined by regression analysis.Fine roots in the upper soil layer have a function of absorbing moisture and nutrients,while the main function of deeper soil may be moisture uptake rather than nutrient acquisition.Therefore,carbon allocation to roots in the upper soil layer and deeper soil layer was different.Multiple regression analysis showed that variation in soil resource availability could explain 71-73% of the seasonal variation of RLD and SRL and 58% of the variation in fine root biomass.These results suggested a greater metabolic activity of fine roots living in soil with higher resource availability,which resulted in an increased allocation of carbohydrate to these roots,but a lower allocation of carbohydrate to those in soil with lower resource availability.     

Characteristics of soil fauna community in the Dongjiao coconut plantation ecosystem in Hainan, China

From July 2002 to July 2004, we investigated the soil fauna in the Dongjiao coconut plantation of Hainan Island. A total of 5,378 specimens were obtained. These species represented 4 phyla, 12 classes and 27 genera. The soil animal clusters are as follows: All major animal groups presented in a tropical rain forest are present in Dongjiao coconut plantation's soil community. The Dongjiao coconut plantation demonstrated the typical characteristics of a tropical soil animal community. The number of species and the diversity index (H) increase from the high latitude areas toward the equator; The dominance index (C) decreased from the high latitude areas toward the equator; the ratio of Acarina/Collembola and the percentage occurrence of increase along the declination of latitude; the occurrence of termites, a typical member of tropical community, varies from absent to present and a dramatic increase along the declination of latitude. Compared to primary tropical rainforests, the Dongjian coconut plantation community is relatively low in species diversity, and has a high dominance index and low diversity index. This may partially due to some characters of the plantation: singular tree species, monsoon climate, seashore location, high pH and salinity of the soil, soil moisture and other environmental factors. A seasonal change occurs in the community structure but is not obvious. Soil around human residences has a higher organic material content, and has higher counts of specimens and numbers of species, compared to other three sampling sites. Coconut production at locations around human residences is higher than at any other microhabitats. The high production is positively correlated with the richness of animal community in the soil.     

A Comparison of Entomopathogenic Nematode Longevity in Soil under Laboratory Conditions

We compared the longevity of 29 strains representing 11 entomopathogenic nematode species in soil over 42 to 56 d. A series of five laboratory experiments were conducted with six to eight nematode strains in each and one or more nematode strains in common, so that qualitative comparisons could be made across experiments. Nematodes included Heterorhabditis bacteriophora (four strains), H. indica (Homl), H. marelatus (Point Reyes), H megidis (UK211), H. mexicana (MX4), Steinernema carpocapsae (eight strains), S. diaprepesi, S. feltiae (SN), S. glaseri (NJ43), S. rarum (17C&E), and S. riobrave (nine strains). Substantial within-species variation in longevity was observed in S. carpocapsae, with the Sal strain exhibiting the greatest survival. The Sal strain was used as a standard in all inter-species comparisons. In contrast, little intra-species variation was observed in S. riobrave. Overall, we estimated S. carpocapsae (Sal) and S. diaprepesi to have the highest survival capability. A second level of longevity was observed in H. bacteriophora (Lewiston), H. megidis, S. feltiae, and S. riobrave (3–3 and 355). Lower levels of survivability were observed in other H. bacteriophora strains (Hb, HP88, and Oswego), as well as S. glaseri and S. rarum. Relative to S. glaseri and S. rarum, a lower tier of longevity was observed in H. indica and H. marelatus, and in H. mexicana, respectively. Although nematode persistence can vary under differing soil biotic and abiotic conditions, baseline data on longevity such as those reported herein may be helpful when choosing the best match for a particular target pest.     

亚热带典型森林地上和地下凋落物输入对土壤新老有机碳动态平衡的影响

凋落物是森林土壤有机碳(SOC)形成、稳定和周转的重要影响因子。目前针对亚热带不同类型森林地上和地下凋落物对新SOC累积和老SOC输出动态平衡的影响仍不清楚。本研究以中亚热带常绿阔叶天然林、马尾松人工林和杉木人工林为对象,基于C3/C4植物-土壤置换试验,利用稳定同位素¹³C示踪方法开展3年野外定位试验,分析了森林地上、地下凋落物输入对SOC周转的影响。结果表明: 森林类型、凋落物处理和时间均能显著影响SOC含量、土壤δ¹³C值、新SOC和老SOC含量,且存在显著的森林类型×凋落物处理交互效应。地上和地下凋落物输入均能显著提高SOC含量和净增量,与杉木人工林相比,天然林SOC对凋落物输入的响应更敏感。凋落物输入显著降低了土壤δ¹³C值,且天然林、马尾松人工林土壤δ¹³C显著低于杉木人工林。在马尾松人工林,地下凋落物处理的新SOC含量显著高于地上凋落物;在天然林和马尾松人工林,地下凋落物输入处理的老SOC含量显著低于地上凋落物处理。此外,地上凋落物归还量和地下根生物量与SOC含量和净增量呈显著正相关,而地下根凋落物量和C/N与新SOC含量呈显著正相关。森林地下凋落物比地上凋落物输入对SOC周转的影响更重要,且不同森林凋落物输入对SOC的影响存在差异性。本研究可为揭示亚热带典型森林土壤有机碳库的形成和可持续管理提供依据。     

南亚热带马尾松-红椎混交林及其纯林土壤细菌群落结构与功能

营建乡土阔叶树种人工纯林和针阔混交林是我国亚热带地区森林经营的发展趋势,但对人工纯林和针阔混交林土壤细菌群落结构及功能知之甚少。本研究以南亚热带乡土针叶树种马尾松、阔叶树种红椎人工纯林及二者的混交林为对象,运用细菌16S rRNA基因高通量测序技术和PICRUSt基因功能预测,分析了3种人工林不同土层(0～20、20～40和40～60 cm)土壤细菌群落的结构与功能。结果表明: 混交林和马尾松林土壤细菌群落结构相似,但与红椎林差异显著,红椎林土壤细菌群落多样性、生物通路代谢功能和氮循环功能低于马尾松林和混交林;土壤全氮、硝态氮和C/N是导致红椎林与马尾松林和混交林土壤细菌群落结构及功能差异的主要土壤理化因子。就土壤细菌群落结构与功能而言,在该地区营造红椎和马尾松针阔混交林要优于红椎纯林。     

季节性干旱地区典型树种长期水分利用特征与模式

在季节性干旱地区,水分是影响植物生长发育的关键核心因子.基于长期连续观测数据探究植物水分利用模式,对于季节性干旱地区植被建设具有重要意义.本研究以北京山区侧柏人工林为对象,利用稳定氢氧同位素技术测定了2012-2017年间土壤、植物枝条和降水同位素组成,通过MixSIAR模型定量分析侧柏对不同土层土壤水分的贡献率.结果...