Litter type and soil minerals control temperate forest soil carbon response to climate change

Temperate forest soil organic carbon (C) represents a significant pool of terrestrial C that may be released to the atmosphere as CO₂ with predicted changes in climate. To address potential feedbacks between climate change and terrestrial C turnover, we quantified forest soil C response to litter type and temperature change as a function of soil parent material. We collected soils from three conifer forests dominated by ponderosa pine (PP; Pinus ponderosa Laws.); white fir [WF; Abies concolor (Gord. and Glend.) Lindl.]; and red fir (RF; Abies magnifica A. Murr.) from each of three parent materials, granite (GR), basalt (BS), and andesite (AN) in the Sierra Nevada of California. Field soils were incubated at their mean annual soil temperature (MAST), with addition of native ¹³C‐labeled litter to characterize soil C mineralization under native climate conditions. Further, we incubated WF soils at PP MAST with ¹³C‐labeled PP litter, and RF soils at WF MAST with ¹³C‐labeled WF litter to simulate a migration of MAST and litter type, and associated change in litter quality, up‐elevation in response to predicted climate warming. Results indicated that total CO₂ and percent of CO₂ derived from soil C varied significantly by parent material, following the pattern of GR>BS>AN. Regression analyses indicated interactive control of C mineralization by litter type and soil minerals. Soils with high short‐range‐order (SRO) mineral content exhibited little response to varying litter type, whereas PP litter enriched in acid‐soluble components promoted a substantial increase of extant soil C mineralization in soils of low SRO mineral content. Climate change conditions increased soil C mineralization greater than 200% in WF forest soils. In contrast, little to no change in soil C mineralization was noted for the RF forest soils, suggesting an ecosystem‐specific climate change response. The climate change response varied by parent material, where AN soils exhibited minimal change and GR and BS soils mineralized substantially greater soil C. This study corroborates the varied response in soil C mineralization by parent material and highlights how the soil mineral assemblage and litter type may interact to control conifer forest soil C response to climate change.     

The contribution of bryophytes to the carbon exchange for a temperate rainforest

Bryophytes blanket the floor of temperate rainforests in New Zealand and may influence a number of important ecosystem processes, including carbon cycling. Their contribution to forest floor carbon exchange was determined in a mature, undisturbed podocarp‐broadleaved forest in New Zealand, dominated by 100–400‐year‐old rimu (Dacrydium cupressimum) trees. Eight species of mosses and 13 species of liverworts contributed to the 62% cover of the diverse forest floor community. The bryophyte community developed a relatively thin (depth <30 mm), but dense, canopy that experienced elevated CO₂ partial pressures (median 46.6 Pa immediately below the bryophyte canopy) relative to the surrounding air (median 37.6 Pa at 100 mm above the canopy). Light‐saturated rates of net CO₂ exchange from 14 microcosms collected from the forest floor were highly variable; the maximum rate of net uptake (bryophyte photosynthesis – whole‐plant respiration) per unit ground area at saturating irradiance was 1.9 μmol m^?2 s^?1 and in one microcosm, the net rate of CO₂ exchange was negative (respiration). CO₂ exchange for all microcosms was strongly dependent on water content. The average water content in the microcosms ranged from 1375% when fully saturated to 250% when air‐dried. Reduction in water content across this range resulted in an average decrease of 85% in net CO₂ uptake per unit ground area. The results from the microcosms were used in a model to estimate annual carbon exchange for the forest floor. This model incorporated hourly variability in average irradiance reaching the forest floor, water content of the bryophyte layer, and air and soil temperature. The annual net carbon uptake by forest floor bryophytes was 103 g m^?2, compared to annual carbon efflux from the forest floor (bryophyte and soil respiration) of ?1010 g m^?2. To put this in perspective of the magnitude of the components of CO₂ exchange for the forest floor, the bryophyte layer reclaimed an amount of CO₂ equivalent to only about 10% of forest floor respiration (bryophyte plus soil) or ～11% of soil respiration. The contribution of forest floor bryophytes to productivity in this temperate rainforest was much smaller than in boreal forests, possibly because of differences in species composition and environmental limitations to photosynthesis. Because of their close dependence on water table depth, the contribution of the bryophyte community to ecosystem CO₂ exchange may be highly responsive to rapid changes in climate.     

Global climate change and soil carbon stocks; predictions from two contrasting models for the turnover of organic carbon in soil

Enhanced release of CO₂ to the atmosphere from soil organic carbon as a result of increased temperatures may lead to a positive feedback between climate change and the carbon cycle, resulting in much higher CO₂ levels and accelerated global warming. However, the magnitude of this effect is uncertain and critically dependent on how the decomposition of soil organic C (heterotrophic respiration) responds to changes in climate. Previous studies with the Hadley Centre's coupled climate–carbon cycle general circulation model (GCM) (HadCM3LC) used a simple, single‐pool soil carbon model to simulate the response. Here we present results from numerical simulations that use the more sophisticated ‘RothC’ multipool soil carbon model, driven with the same climate data. The results show strong similarities in the behaviour of the two models, although RothC tends to simulate slightly smaller changes in global soil carbon stocks for the same forcing. RothC simulates global soil carbon stocks decreasing by 54 Gt C by 2100 in a climate change simulation compared with an 80 Gt C decrease in HadCM3LC. The multipool carbon dynamics of RothC cause it to exhibit a slower magnitude of transient response to both increased organic carbon inputs and changes in climate. We conclude that the projection of a positive feedback between climate and carbon cycle is robust, but the magnitude of the feedback is dependent on the structure of the soil carbon model.     

Light and heavy fractions of soil organic matter in response to climate warming and increased precipitation in a temperate steppe

Soil is one of the most important carbon (C) and nitrogen (N) pools and plays a crucial role in ecosystem C and N cycling. Climate change profoundly affects soil C and N storage via changing C and N inputs and outputs. However, the influences of climate warming and changing precipitation regime on labile and recalcitrant fractions of soil organic C and N remain unclear. Here, we investigated soil labile and recalcitrant C and N under 6 years' treatments of experimental warming and increased precipitation in a temperate steppe in Northern China. We measured soil light fraction C (LFC) and N (LFN), microbial biomass C (MBC) and N (MBN), dissolved organic C (DOC) and heavy fraction C (HFC) and N (HFN). The results showed that increased precipitation significantly stimulated soil LFC and LFN by 16.1% and 18.5%, respectively, and increased LFC:HFC ratio and LFN:HFN ratio, suggesting that increased precipitation transferred more soil organic carbon into the quick-decayed carbon pool. Experimental warming reduced soil labile C (LFC, MBC, and DOC). In contrast, soil heavy fraction C and N, and total C and N were not significantly impacted by increased precipitation or warming. Soil labile C significantly correlated with gross ecosystem productivity, ecosystem respiration and soil respiration, but not with soil moisture and temperature, suggesting that biotic processes rather than abiotic factors determine variations in soil labile C. Our results indicate that certain soil carbon fraction is sensitive to climate change in the temperate steppe, which may in turn impact ecosystem carbon fluxes in response and feedback to climate change.     

Short-term effects of soil moisture on soil organic carbon decomposition in a coastal wetland of the Yellow River Delta

Hydrobiologia - Soil moisture remarkably influences soil organic carbon (SOC) decomposition and is one of the key variables in ecological models influencing changes in soil carbon (C) storage....     

植物群落向土壤有机碳输入及其对气候变暖的响应研究进展

植物群落作为陆地生态系统土壤有机碳的主要来源,可通过地表凋落物分解、细根周转和根系分泌物等方式将光合作用同化的碳输入到土壤中。全球气候变暖正深刻地影响植物群落的分布、结构与功能,改变森林地上和地下凋落物产量与分解速率和根系分泌过程,从而改变植物群落向土壤输入有机碳数量。本文综述了植物群落向土壤有机碳输入过程及其对气候变暖的响应研究进展。研究表明,气候变暖可通过影响植物群落生产直接影响凋落物产量和根系分泌过程,还可通过改变凋落物分解环境条件、凋落物基质质量和分解者群落结构与活性等非生物与生物因子而间接作用于凋落物向土壤有机碳输入过程。气候变暖还可通过影响植物根系性状、根系分泌物化学组成等间接影响植物根系向土壤输入的碳量,但其具体机制还需深入探讨。未来的研究应该关注气候变暖导致植物群落结构改变进而影响土壤有机碳输入的具体机制以及粗木质残体对土壤有机碳输入的贡献,同时还应注重植物凋落物与根系分泌过程的整合研究,以期更全面地认识气候变暖背景下植物群落对土壤碳库及碳循环过程的贡献。     

Spiders and subsidies: results from the riparian zone of a coastal temperate rainforest

1. Aquatic insects emerging from streams can provide an important energy subsidy to recipient consumers such as riparian web-building spiders. This subsidy has been hypothesized to be of little importance where the primary productivity of the recipient habitat exceeds that of the donor habitat. 2. To test this hypothesis, we manipulated emerging stream insect abundance in a productive riparian rainforest in a replicated design using greenhouse-type exclosures, contrasted with unmanipulated stream reaches (four exclosures on two streams). 3. Experimental exclosures resulted in a 62.9% decrease in aquatic insect abundance in exclusion reaches compared with control reaches. The overall density of riparian spiders was significantly positively correlated with aquatic insect abundances. Horizontal orb weavers (Tetragnathidae) showed a strong response to aquatic insect reduction - abundance at exclosure sites was 57% lower than at control sites. Several spider families that have not been associated with tracking aquatic insect subsidies also showed significantly decreased abundance when aquatic insects were reduced. 4. This result is contrary to predictions of weak subsidy effects where recipient net primary productivity is high. These results suggest that predicting the importance of resource subsidies for food webs requires a focus on the relative abundance of subsidy materials in recipient and donor habitats and not simply on the total flux of energy between systems.     

Rapid development of phosphorus limitation in temperate rainforest along the Franz Josef soil chronosequence

The aim of this study was to examine how shifts in soil nutrient availability along a soil chronosequence affected temperate rainforest vegetation. Soil nutrient availability, woody plant diversity, composition and structure, and woody species leaf and litter nutrient concentrations were quantified along the sequence through ecosystem progression and retrogression. In this super-wet, high leaching environment, the chronosequence exhibited rapid soil development and decline within 120,000 years. There were strong gradients of soil pH, N, P and C, and these had a profound effect on vegetation. N:P_leaf increased along the chronosequence as vegetation shifted from being N- to P- limited. However, high N:P_leaf ratios, which indicate P-limitation, were obtained on soils with both high and low soil P availability. This was because the high N-inputs from an N-fixing shrub caused vegetation to be P-limited in spite of high soil P availability. Woody species nutrient resorption increased with site age, as availability of N and P declined. Soil P declined 8-fold along the sequence and P resorption proficiency decreased from 0.07 to 0.01%, correspondingly. N resorption proficiency decreased from 1.54 to 0.26%, corresponding to shifts in mineralisable N. Woody plant species richness, vegetation cover and tree height increased through ecosystem progression and then declined. During retrogression, the forest became shorter, more open and less diverse, and there were compositional shifts towards stress-tolerant species. Conifers (of the Podocarpaceae) were the only group to increase in richness along the sequence. Conifers maintained a lower N:P_leaf than other groups, suggesting superior acquisition of P on poor soils. In conclusion, there was evidence that P limitation and retrogressive forests developed on old soils, but N limitation on very young soils was not apparent because of inputs from an abundant N-fixing shrub.Electronic Supplementary Material Supplementary material is available in the online version of this article at .     

稳定碳同位素在滨海湿地碳生物地球化学循环中的应用

碳作为滨海湿地中重要的生命元素,其生物地球化学循环过程是滨海湿地研究的核心内容之一.稳定同位素技术越来越多地被应用到滨海湿地碳生物地球化学循环过程的研究中,提高了其研究水平,并推动了其研究的进程.本文从有机物质生产、土壤有机质来源、食物链传递、温室气体排放以及可溶性有机碳输出5个方面,综述了滨海湿地碳生物地球化学循环过程的稳定同位素研究进展.通过植物及土壤δ13C值的测定进行有机质的生产机理研究及外源追溯,通过对比各生物种群的δ13C值分析碳在生态系统中的流动过程,通过湿地排放温室气体及可溶性有机碳δ13C值的测定揭示影响碳输出的环境因子.最后,文章总结了当前研究中存在的问题,并对其研究前景进行了展望.