Drying-rewetting events reduce C and N losses from a Norway spruce forest floor

Periods of prolonged summer drought are likely to be expected for this century, with possibly strong effects on carbon (C) and nitrogen (N) mineralization in soils. Drought generally reduces mineralization rates, but the possibility of excess mineralization pulses during rewetting raises the question about the net effect of drying-rewetting events. In this experiment, we measured C and N mineralization in undisturbed soil columns that were either kept under continuously moist conditions (control) or that were subjected to drying-rewetting. We had three treatments (D1-D3) with different drying intensity (increasing from D1 to D3) but uniform rewetting intensity (4 mm d⁻¹). Soil columns were taken from a Norway spruce forest in Bavaria, Germany. The CO₂ fluxes from control and treatment groups were identical before drying. Over the 80 d drought period, total CO₂ emissions from D1, D2, and D3 were only 72, 52 and 43% of that from the control, respectively. Rewetting resulted in a fast increase of CO₂ fluxes to approx. the same level as in the control. Rewetting could not restore soil moisture of the dry soil to the level of the control, presumably because of preferential flow and water repellency of soil organic matter. No significant excess C mineralization during the 40 d rewetting period was observed. Adding up total CO₂ fluxes during drought and rewetting period, the treatments D1, D2, and D3 emitted only 88, 71 and 67% of the CO₂ emitted by the control. Measurements of dissolved organic carbon (DOC) did only show minor differences between control and treatment columns, indicating that no significant accumulation of DOC took place during the drought period. Radiocarbon signature of emitted CO₂ indicated that C mineralization was reduced with decreasing water availability and no new substrate became bioavailable. Net N mineralization over the course of the whole experiment was reduced by drought to 77, 65 or 52% of the control. Net nitrification was virtually zero during drought whereas net ammonification continued at reduced levels. In summary, we found that drying-rewetting generally reduced C and N mineralization in this soil and that the total reduction increased with drought intensity.     

Humic Acids Increase the Phytoavailability of Cd and Pb to Wheat Plants Cultivated in Freshly Spiked, Contaminated Soil (7 pp)

Background, Aim and Scope   Humic acids (HAs) are the most important humified component of dissolved organic carbon (DOC) present in sewage water used for irrigation. It is well known that HAs affect the toxicity and availability of heavy metals (HMs) in soil-plant systems, and may increase the human exposure to HMs in contaminated soil through plant uptake. This study was conducted to assess the effects of HAs on HM availability, plant growth and HM uptake. Materials and Methods: With wheat (Triticum aestivum) as a test plant, a greenhouse pot experiment was conducted to investigate the effects of HAs in irrigation water on the phytoavailability of cadmium (Cd) and lead (Pb) in soil. Cd and Pb were added to the soil at concentrations of 1.5 and 150 mg/kg, respectively. Wheat seedlings grown in Cd and Pb-contaminated soil were watered with 4 levels of HA solution (0, 140, 280 and 560 mg/kg of HAs, respectively). Results: In control and Pb treatments, both plant biomass and plant HM concentrations increased with increasing concentrations of HAs in the solution. Plant biomass was markedly decreased when metal concentrations in plants increased, particularly in Cd and Cd/Pb treatments. In the soil, extractable metals, and water soluble organic carbon (WSOC) and its fractions significantly increased with increasing HA concentrations. Discussion: The results suggested that the application of HAs in barren soils may improve plant nutrition by mobilizing soil nutrients and providing plants with carbon sources. On the other hand, HAs present in sewage water may increase both the availability and transfer of HMs in the soil-plant continuum and subsequently increase human exposure to HMs in polluted soil. Conclusions. Conclusions: HA solution as irrigation water significantly increased HM availability to plants cultivated in the HM-amended soil and may increase the environmental risk of sewage irrigation. Recommendations and Perspectives: These results suggested that, when assessing the effect of sewage irrigation on soil quality, HAs contained in sewage water should be taken into consideration.     

Differences in soil CO2 flux between different land use types in mid-subtropical China

It is crucial to advance the understanding of the soil carbon dioxide (CO₂) flux and environmental factors for a better comprehension of carbon dynamics in subtropical ecosystems. Red soil, one of the typical agricultural soils in subtropical China, plays important roles in the global carbon budget due to their large potential to sequester C and replenish atmospheric C through soil CO₂ flux. We examined the relationship between soil CO₂ flux and environmental determinants in four different land use types of subtropical red soil-paddy (P), orchard (O), woodland (W) and upland (U) using static closed chamber method. Objectives were to evaluate the relationship of soil temperature, water-filled pore space (WFPS), and dissolved organic carbon (DOC) with the soil CO₂ flux. Soil CO₂ fluxes were measured on each site about every 14 days between 09:00 and 11:00 a.m. during 14-July 2004 to 25-April 2007 at the experimental station of Heshengqiao at Xianning, Hubei, China. Soil CO₂ fluxes revealed seasonal fluctuations, with the tendency that maximum values occurred in summer, minimum in winter and intermediate values in spring and autumn except for paddy soil when it was submerged. Further, significant differences in soil CO₂ fluxes were observed among the four soils, following the order of P > O > U  W. Average soil CO₂ fluxes were estimated as 901 ± 114, 727 ± 55, 554 ± 22 and 533 ± 27 (±S.D.) g CO₂ m⁻² year⁻¹ in paddy, orchard, upland and woodland soils, respectively. Variations in soil CO₂ flux were related to soil temperature, WFPS, and dissolved organic carbon with a combined R² of 0.49–0.75. Soil temperature was an important variable controlling 26–59% of soil CO₂ flux variability. The interaction of soil temperature and WFPS could explain 31–60% of soil CO₂ flux variations for all the land use types. We conclude that soil CO₂ flux from red soil is under environmental controls, soil temperature being the main variable, which interact with WFPS and DOC to control the supply of readily mineralizable substrates.     

How important is plant litter to the regulation of mineral‐N leaching to streams in winter? An observations‐led experimental approach

Ammonium‐N concentrations were frequently observed to exceed nitrate‐N concentrations in an intermittently flowing stream draining acid grassland in North Yorkshire. This prompted the design of a soil microcosm experiment to investigate the role of litter in the leaching of ammonium and nitrate from soil profiles during winter. Drainage water was analysed weekly for N species, pH, mineral acid anions and dissolved organic carbon (DOC) for a period of 11 weeks, while extractable mineral‐N was determined after 5 and 11 weeks. The results demonstrate that litter plays an important role in reducing mineral‐N leaching in winter months. They also suggest that DOC from the litter participates in mineral‐N retention in the soil profiles in winter. Ammonium‐N and nitrate‐N concentrations measured in the microcosm drainage water are similar to those of the stream.     

基于不同施肥模式添加生物炭后菜地土壤CO2释放特征及不同形态碳含量变化

【目的】研究4种常规施肥模式下,添加生物炭后菜地土壤（褐潮土）CO₂释放量、可溶性有机碳（DOC）和微生物生物量碳（SMBC）含量的变化,阐明添加生物炭对土壤CO₂释放及不同形态碳的影响。【方法】采用室内恒温好氧培养-气象色谱测定方法,在不施肥（CK）、施有机肥（M）、施化肥（F）、有机无机混施（M+F）4种模式下投入2%和4%（质量比：生物炭/土壤干重）生物炭,定期采集气样和土样,分析土壤CO₂的释放量及DOC、SMBC含量的动态变化,并分析DOC、SMBC含量变化与CO₂释放量变化之间的相关关系。【结果】在F和M+F基础上,添加生物炭处理的土壤CO₂释放速率在培养前期（2—8 d）显著高于未添加生物炭处理,而在10—60 d,二者CO₂释放速率无显著差异;在CK和M基础上,添加与未添加生物炭处理在整个培养期间CO₂释放速率没有显著差异。在CK基础上,添加2%和4%生物炭后CO₂累积释放量分别为2 839和3 272 mg·kg^-1,与CK（3 134 mg·kg^-1）相比均无显著差异;而在F和M+F基础上,添加2%和4%生物炭后CO₂累积释放量均显著提高,分别提高20.6%和19.8%、29.9%和40.7%。相关分析表明,未添加生物炭处理DOC、SMBC含量与CO₂释放量之间无相关关系,而添加生物炭处理DOC、SMBC含量与CO₂释放量极显著相关。【结论】将生物炭单独投入未施肥土壤中,土壤CO₂排放量未出现明显增加或降低;在有机肥基础上添加生物炭,土壤CO₂排放量随着生物炭投入量的增加而增加;在化肥、有机无机配施基础上添加生物炭后,土壤CO₂排放增加比例最高。     

Impeded drainage stimulates extracellular phenol oxidase activity in riparian peat cores

Peat drainage, a common land‐use practice in Europe, has been associated with habitat degradation and increased particulate and dissolved carbon release. In the UK, peatland drain blockage has been encouraged in recent years as a management practice to preserve peatland habitats and to reduce fluvial carbon loss and municipal water discoloration. Drain blockage has, however, been found to increase drain‐water dissolved organic carbon (DOC) concentrations and coloration in the short term. In order to investigate the contribution of changes in extracellular phenol oxidase activity to the increase in water coloration following peatland drain blockage, cores collected from a riparian peatland in North Wales were incubated under impeded drainage conditions. Impeded drainage resulted in the stimulation of peat extracellular phenol oxidase activity and heightened soluble phenolic concentrations, suggesting that changes in extracellular phenol oxidase activity may be a key driver of increases in DOC and water coloration following peatland drain blockage. An increase in peat pH with impeded drainage was also observed that may have contributed to the heightened soluble phenolic concentrations – directly (through effects on solubility) and/or indirectly as a driver of the elevated extracellular phenol oxidase activity.     

Clear-cutting reduces nitrate leaching in a pine plantation of high natural N status

Following the tree harvest, the biogeochemistry of a catchment is modified by changes in soil temperature and moisture, and nutrient cycling. We monitored soil-solution and stream-water chemistry, and soil properties in a Pinus radiata D. Don plantation in New Zealand before and after clear-cutting and replanting in 1997. The annual rainfall during the study was 1440–1860 mm. The soil was a 1800-year-old pumice soil of high natural N status; the catchment had received large inputs of volcanic N in rain, probably over the 1800 years since the pumice had been deposited. The leaching loss of nitrate-N was 28 kg ha⁻¹ yr⁻¹ in 1996, and then decreased sharply after clear-cutting to 3 kg ha⁻¹ yr⁻¹ in 1998 and <1 kg ha⁻¹ yr⁻¹ in 1999. Weed growth and soil microbial biomass increased during this time, and would have removed much of the N from soil solution in the upper soil layers. Although the catchment was small (8.7 ha), there was a 2-year lag until N decreased in stream-water; the losses of dissolved organic N to stream-water were low. There was no change in soil pH over the 4 years, but spring-water pH appeared to increase, which was consistent with the increase in bicarbonate that accompanied grass/weed growth. The export of cations (mmol_c l⁻¹) in the spring-water was Na>Ca>Mg=K as expected for rhyolitic pumice, and the total concentration was probably controlled by the accompanying anions. The export of anions was NO₃=Cl>SO₄=HCO₃ before harvest and HCO₃=Cl>SO₄=NO₃ after harvest.     

红壤侵蚀退化地土壤对不同来源可溶性有机碳的吸附特征

选取南方典型红壤侵蚀退化地恢复后的马尾松林为研究对象,通过在室内模拟芒萁(Dicranopteris dichotoma)和马尾松(Pinus massoniana)的鲜叶与凋落叶的浸提液在侵蚀退化地原状土柱的淋溶过程,分析了植被恢复过程中马尾松林土壤对不同来源可溶性有机碳(DOC)吸附特征及影响因素。结果表明:(1)红壤侵蚀退化地对不同来源DOC的吸附作用具有明显差异,来自马尾松鲜叶的DOC平均截留量最大为2.39mg/kg,来自芒萁鲜叶的DOC平均截留量最小为1.67mg/kg,说明马尾松鲜叶的DOC更易被表层土壤吸附,芒萁鲜叶的DOC更易进入深层土壤,不同来源DOC组成和性质的差异是其主要原因。(2)随着退化地的植被恢复,土壤渗滤液的DOC浓度增加,土壤截留DOC能力下降。土壤DOC截留量与粉粒和土壤pH呈正相关,与土壤DOC含量、土壤有机碳含量和砂粒呈负相关,其中土壤有机碳含量可以解释DOC截留量变化的51.4%,是影响土壤DOC截留能力的关键因素。(3)光谱特征表明芳香类化合物、腐殖类物质易被土壤吸附,吸附能力更强的物质可以解吸土壤中亲水性腐殖类物质。淋溶后DOC光谱特征的变化由不同来源DOC的化学组成和土壤有机碳的性质共同决定。红壤侵蚀退化地对不同植物来源的DOC吸附作用特征主要受DOC和土壤SOC性质的共同调控,对进一步认识退化红壤的固碳机制具有重要参考价值。     

杉木林土壤渗滤水溶解有机碳含量与迁移

森林是陆地生态系统中最重要的碳库,森林土壤中的碳占全球土壤有机碳的73%,在全球碳循环中起着重要作用(David et al.,1988).溶解有机碳(dissolved organic carbon,简称DOC)虽然仅占有机碳的很小部分,但对调节阳离子淋洗、金属溶解、矿物风化、土壤微生物活动以及其他土壤化学、物理和生物学过程具有重要意义.     

Changes in dissolved organic matter with depth suggest the potential for postharvest organic matter retention to increase subsurface soil carbon pools

Research into postharvest management of forests often focuses on balancing the need for increased biomass yield against factors that may directly impact the productivity of the subsequent stand (e.g. nutrient and water availability, soil microclimate, etc.). Postharvest organic matter management, however, also exerts a strong influence over the translocation of carbon (C) into and through the soil profile and may provide a mechanism to increase soil C content. The effects of contrasting postharvest organic matter retention treatments (bole-only removal, BO; whole-tree removal, WT) on soil solution C concentration and quality were quantified at the Fall River and Matlock Long-term Soil Productivity (LTSP) studies in Washington state. Solutions were collected monthly at depths of 20 and 100 cm and analyzed for dissolved organic C (DOC), dissolved organic nitrogen (DON) and DOC:DON ratio. Comparisons of DOC concentrations with depth illustrate divergent trends between the two treatments, with an overall decrease in DOC with depth in the BO treatment and either an increase or no change with depth in the WT treatment. Trends in DON concentrations with depth were less clear, partly due to the very low concentrations observed, although the relationship of DOC:DON with depth shows a decrease in the BO treatment and little to no change in DOC quality in the WT treatment. This illustrates that more recalcitrant organic matter (higher DOC:DON) is being removed from solution as it moves through the soil profile. Only 35–40% of the DOC moving past 20 cm in the BO treatment is present at 100 cm. Conversely, 98–117% of the DOC at 20 cm in the WT treatment is present at 100 cm. Thus, 11 and 30 kg C ha⁻¹ yr⁻¹ are removed from solution between 20 and 100 cm in the BO treatment at the Matlock and Fall River LTSP studies, respectively. Although much of this C is often assumed to be utilized for microbial respiration, DOC:DON ratios of the potential organic substrates and the unique mineralogy of the soils of this region suggest that a significant portion may in fact be incorporated into a more recalcitrant soil C pool. Thus, postharvest organic matter retention may provide a mechanism to increase soil C sequestration on these soils.