Agricultural peatland restoration: effects of land‐use change on greenhouse gas (CO2 and CH4) fluxes in the Sacramento‐San Joaquin Delta

Agricultural drainage of organic soils has resulted in vast soil subsidence and contributed to increased atmospheric carbon dioxide (CO₂) concentrations. The Sacramento‐San Joaquin Delta in California was drained over a century ago for agriculture and human settlement and has since experienced subsidence rates that are among the highest in the world. It is recognized that drained agriculture in the Delta is unsustainable in the long‐term, and to help reverse subsidence and capture carbon (C) there is an interest in restoring drained agricultural land‐use types to flooded conditions. However, flooding may increase methane (CH₄) emissions. We conducted a full year of simultaneous eddy covariance measurements at two conventional drained agricultural peatlands (a pasture and a corn field) and three flooded land‐use types (a rice paddy and two restored wetlands) to assess the impact of drained to flooded land‐use change on CO₂ and CH₄ fluxes in the Delta. We found that the drained sites were net C and greenhouse gas (GHG) sources, releasing up to 341 g C m^?2 yr^?1 as CO₂ and 11.4 g C m^?2 yr^?1 as CH₄. Conversely, the restored wetlands were net sinks of atmospheric CO₂, sequestering up to 397 g C m^?2 yr^?1. However, they were large sources of CH₄, with emissions ranging from 39 to 53 g C m^?2 yr^?1. In terms of the full GHG budget, the restored wetlands could be either GHG sources or sinks. Although the rice paddy was a small atmospheric CO₂ sink, when considering harvest and CH₄ emissions, it acted as both a C and GHG source. Annual photosynthesis was similar between sites, but flooding at the restored sites inhibited ecosystem respiration, making them net CO₂ sinks. This study suggests that converting drained agricultural peat soils to flooded land‐use types can help reduce or reverse soil subsidence and reduce GHG emissions.     

From Flooded to Aerobic Conditions in Rice Cultivation: Consequences for Zinc Uptake

Scarcity of water causes a shift from flooded to aerobic conditions for rice production in zinc deficient areas in Northern China. This shift alters soil conditions that affect zinc availability to the crop. This paper concerns the effect of aerobic compared to flooded conditions on crop biomass production, grain yield and zinc content. A field experiment was done with six rice genotypes (Oryza sativa L.) grown on a calcareous soil, both with (23 kg Zn ha⁻¹) and without Zn fertilization. Sampling was conducted at tillering and physiological mature stage. Zn concentration in the shoots was significantly lower at both stages in plants grown in the aerobic field. At maturity, Zn uptake, biomass production, grain yield and Zn-harvest index [grain Zn/(shoot + grain Zn)] were lower under aerobic cultivation. Rice genotypes including aerobic rice and lowland rice differ in degree of response to low Zn supply. A twofold difference was found among aerobic genotypes in grain yield and Zn uptake. Also Zn-harvest index varied significantly. Zn application affected neither grain yield nor grain Zn content, although it significantly improved biomass production in both systems in most genotypes. These results demonstrate that introduction of aerobic rice systems on calcareous soils may increase Zn deficiency problems.     

Managing native and legume-fixed nitrogen in lowland rice-based cropping systems

Lowlands comprise 87% of the 145 M ha of world rice area. Lowland rice-based cropping systems are characterized by soil flooding during most of the rice growing season. Rainfall distribution, availability of irrigation water and prevailing temperatures determine when rice or other crops are grown. Nitrogen is the most required nutrient in lowland rice-based cropping systems. Reducing fertilizer N use in these cropping systems, while maintaining or enhancing crop output, is desirable from both environmental and economic perspectives. This may be possible by producing N on the land through legume biological nitrogen fixation (BNF), minimizing soil N losses, and by improved recycling of N through plant residues. At the end of a flooded rice crop, organic- and NH₄-N dominate in the soil, with negligible amounts of NO₃. Subsequent drying of the soil favors aerobic N transformations. Organic N mineralizes to NH₄, which is rapidly nitrified into NO₃. As a result, NO₃ accumulates in soil during the aerobic phase. Recent evidence indicates that large amounts of accumulated soil NO₃ may be lost from rice lowlands upon the flooding of aerobic soil for rice production. Plant uptake during the aerobic phase can conserve soil NO₃ from potential loss. Legumes grown during the aerobic phase additionally capture atmospheric N through BNF. The length of the nonflooded season, water availability, soil properties, and prevailing temperatures determine when and where legumes are, or can be, grown. The amount of N derived by legumes through BNF depends on the interaction of microbial, plant, and environmental determinants. Suitable legumes for lowland rice soils are those that can deplete soil NO₃ while deriving large amounts of N through BNF. Reducing soil N supply to the legume by suitable soil and crop management can increase BNF. Much of the N in legume biomass might be removed from the land in an economic crop produce. As biomass is removed, the likelihood of obtaining a positive soil N balance diminishes. Nonetheless, use of legumes rather than non-legumes is likely to contribute higher quantities of N to a subsequent rice crop. A whole-system approach to N management will be necessary to capture and effectively use soil and atmospheric sources of N in the lowland rice ecosystem.IRRI-NifTAL-IFDC joint contribution.     

The effects of flooding for rice culture on soil chemical properties and subsequent maize growth

Summary The effects of flooding and lowland rice culture on soil chemical properties and subsequent maize growth were investigated in two contrasting rice soils of S.E. Australia. The effects of incorporating rice straw, either during or after flooding were also studied. The experiment was conducted in a glasshouse with the use of large intact soil cores.Previous flooding markedly reduced maize growth, leaf P concentration and P uptake, despite the application of a large quantity of P fertilizer after drainage. Soil analyses showed that previous flooding increased the Langmuir sorption terms for maximum P sorption and bonding energy. The availability of P was more closely related to the bonding energy between soil and P than to the capacity of the soils to sorb P. The increases, in the P sorption parameters, were associated with decreases in the crystallinity of the free iron oxides as determined by their oxalate solubility. It was concluded that depressed P supply to maize sown in previously flooded soils was due to stronger P sorption by the drained soils, rather than to P immobilization during flooding.Rice plants grown during flooding reduced the amount of N available to the subsequent maize crop, but did not significantly affect P availability. Rice straw added during flooding did not affect subsequent maize growth, but when added after flooding caused microbial immobilization of N.Salts, Fe or Mn from previous flooding did not affect maize growth.     

Hydrologic Regime Controls Soil Phosphorus Fluxes in Restoration and Undisturbed Wetlands

Many wetland restoration projects occur on former agricultural soils that have a history of disturbance and fertilization, making them prone to phosphorus (P) release upon flooding. To study the relationship between P release and hydrologic regime, we collected soil cores from three restoration wetlands and three undisturbed wetlands around Upper Klamath Lake in southern Oregon, U.S.A. Soil cores were subjected to one of three hydrologic regimes—flooded, moist, and dry—for 7.5 weeks, and P fluxes were measured upon reflooding. Soils from restoration wetlands released P upon reflooding regardless of the hydrologic regime, with the greatest releases coming from soils that had been flooded or dried. Undisturbed wetland soils released P only after drying. Patterns in P release can be explained by a combination of physical and biological processes, including the release of iron‐bound P due to anoxia in the flooded treatment and the mineralization of organic P under aerobic conditions in the dry treatment. Higher rates of soil P release from restoration wetland soils, particularly under flooded conditions, were associated with higher total P concentrations compared with undisturbed wetland soils. We conclude that maintaining moist soil is the means to minimize P release from recently flooded wetland soils. Alternatively, prolonged flooding provides a means of liberating excess labile P from former agricultural soils while minimizing continued organic P mineralization and soil subsidence.     

Agricultural use of wetlands: opportunities and limitations

Background

Wetlands are species-rich habitats performing valuable ecosystem services such as flood protection, water quality enhancement, food chain support and carbon sequestration. Worldwide, wetlands have been drained to convert them into agricultural land or industrial and urban areas. A realistic estimate is that 50 % of the world''s wetlands have been lost.

Scope

This paper reviews the relationship between wetlands and agriculture with the aim to identify the successes and failures of agricultural use in different types of wetlands, with reference to short-term and long-term benefits and issues of sustainability. It also addresses a number of recent developments which will lead to pressure to reclaim and destroy natural wetlands, i.e. the continuous need for higher production to feed an increasing world population and the increasing cultivation of energy crops. Finally, attention is paid to the development of more flood-tolerant crop cultivars.

Conclusions

Agriculture has been carried out in several types of (former) wetlands for millennia, with crop fields on river floodplain soils and rice fields as major examples. However, intensive agricultural use of drained/reclaimed peatlands has been shown to lead to major problems because of the oxidation and subsidence of the peat soil. This does not only lead to severe carbon dioxide emissions, but also results in low-lying land which needs to be protected against flooding. Developments in South-East Asia, where vast areas of tropical peatlands are being converted into oil palm plantations, are of great concern in this respect. Although more flood-tolerant cultivars of commercial crop species are being developed, these are certainly not suitable for cultivation in wetlands with prolonged flooding periods, but rather will survive relatively short periods of waterlogging in normally improved agricultural soils. From a sustainability perspective, reclamation of peatlands for agriculture should be strongly discouraged. The opportunities for agriculture in naturally functioning floodplains should be further investigated. The development and use of crop cultivars with an even stronger flood tolerance could form part of the sustainable use of such floodplain systems. Extensive use of wetlands without drastic reclamation measures and without fertilizer and pesticides might result in combinations of food production with other wetland services, with biodiversity remaining more or less intact. There is a need for research by agronomists and environmental scientists to optimize such solutions.Key words: Wetlands, sustainable agriculture, peat subsidence, floodplains, rice fields, water use, irrigation     

Influence of draining on soil phosphorus forms and distribution in a constructed wetland

Seasonal water-table fluctuations in wetlands can result in flooded and drained conditions in the surface soil. In constructed wetlands water level drawdown and soil drainage are used in management to consolidate detrital materials, accelerate soil build up, and provide easy access for other management operations. A greenhouse study was conducted using intact peat soil cores to evaluate the changes in bioavailable P and other fractions following draining and reflooding. Measurements of floodwater dissolved reactive P (DRP) indicated that draining and soil exposure could result in large P flux to the overlying water column. Phosphorus flux in soils drained for 6 weeks was 10-fold higher (334 mg P m⁻² day⁻¹) than in soils drained for 3 weeks (33 mg P m⁻² day⁻¹). Soil exposure also resulted in an increase in bioavailable inorganic P (estimated by KCl extraction) at the expense of labile organic P pool. The KCl-P pool, which was initially less than 2% of total P (TP), increased to 3% and 13% of TP after 3 and 6 weeks draining, respectively. Results suggest that various soil P fractions, particularly those in newly accreted materials, were highly unstable and could be released in a more available form when newly accreted soils undergo drying. Water level drawdown and reflooding could result in significant P release, a possible stimulation of algal blooms and other water quality problems. Therefore, soil characteristics and chemistry and their impact on water quality should be a major consideration when one adopts the flood-drain technique in wetland management.     

覆盖旱作方式和施氮水平对稻-麦轮作体系生产力和氮素利用的影响

在成都平原通过 3a的田间试验研究了水稻覆盖 (地膜和麦秸 )旱作和施氮水平对稻麦轮作体系生产力和氮素利用的影响。结果表明 :在施氮量为水稻季 15 0 kg/hm2 ,小麦季 12 0 kg/hm2 的条件下 ,覆盖旱作和传统淹水体系均能达到较高的产量水平。再增加施氮量对产量的影响不大 ,但使氮盈余急剧增加。不施氮或低量施氮会造成作物产量的显著下降和土壤氮素亏缺。水稻覆膜旱作对稻麦轮作的系统生产力 (水稻 小麦 )没有显著影响 ;但水稻覆麦秸旱作条件下系统的生产力有降低的趋势 ,主要由于水稻覆麦秸旱作条件下 ,水稻产量下降 ,而麦秸覆盖在小麦季的后效作用不足以弥补水稻产量的下降程度。水稻、小麦的氮素吸收表现出与作物产量类似的规律。水稻季土壤很难累积无机氮 ,而且与施肥和覆盖旱作与否没有关系。小麦季土壤中积累了较多的无机氮 ,而且随施氮水平的增加而明显增加     

Land‐use legacies regulate decomposition dynamics following bioenergy crop conversion

Land‐use conversion into bioenergy crop production can alter litter decomposition processes tightly coupled to soil carbon and nutrient dynamics. Yet, litter decomposition has been poorly described in bioenergy production systems, especially following land‐use conversion. Predicting decomposition dynamics in postconversion bioenergy production systems is challenging because of the combined influence of land‐use legacies with current management and litter quality. To evaluate how land‐use legacies interact with current bioenergy crop management to influence litter decomposition in different litter types, we conducted a landscape‐scale litterbag decomposition experiment. We proposed land‐use legacies regulate decomposition, but their effects are weakened under higher quality litter and when current land use intensifies ecosystem disturbance relative to prior land use. We compared sites left in historical land uses of either agriculture (AG) or Conservation Reserve Program grassland (CRP) to those that were converted to corn or switchgrass bioenergy crop production. Enzyme activities, mass loss, microbial biomass, and changes in litter chemistry were monitored in corn stover and switchgrass litter over 485 days, accompanied by similar soil measurements. Across all measured variables, legacy had the strongest effect (P < 0.05) relative to litter type and current management, where CRP sites maintained higher soil and litter enzyme activities and microbial biomass relative to AG sites. Decomposition responses to conversion depended on legacy but also current management and litter type. Within the CRP sites, conversion into corn increased litter enzymes, microbial biomass, and litter protein and lipid abundances, especially on decomposing corn litter, relative to nonconverted CRP. However, conversion into switchgrass from CRP, a moderate disturbance, often had no effect on switchgrass litter decomposition parameters. Thus, legacies shape the direction and magnitude of decomposition responses to bioenergy crop conversion and therefore should be considered a key influence on litter and soil C cycling under bioenergy crop management.     

Nitrogen transformation processes in relation to improved cultural practices for lowland rice

Summary Inappropriate method and timing of N fertilizer application was found to result in 50–60% N losses. Recent nitrogen transformation studies indicate that NH₃ volatilization in lowland rice soils is an important loss mechanism, causing a 5–47% loss of applied fertilizer under field conditions. Estimated denitrification losses were between 28 and 33%. Ammonia volatilization losses from lowland rice can be controlled by i) placement of fertilizer in the reduced layer and proper timing of application, ii) using phenylphosphorodiamidate (PPD) to delay urease activity in flooded soils, and iii) using algicides to help stabilize changes in floodwater pH. Appropriate fertilizer placement and timing is probably the most effective technique in controlling denitrification at the farm level. The effectivity of nitrification inhibitors as another method is still being evaluated. With 60–80% of N absorbed by the crop derived from the native N pool, substantial yield gains in lowland rice are highly possible with resources already in the land. Extensive studies on soil N and its management, and an understanding of soil N dynamics will greatly facilitate the decrease in immobilization and ammonium fixation in the soil and the increase in N availability to the rice crop. Critical research needs include greater emphasis on N transformation processes in rainfed lowland rice which is grown under more harsh and variable environmental regimes than irrigated lowland rice.     

Water salinity and inundation control soil carbon decomposition during salt marsh restoration: An incubation experiment

Coastal wetlands are a significant carbon (C) sink since they store carbon in anoxic soils. This ecosystem service is impacted by hydrologic alteration and management of these coastal habitats. Efforts to restore tidal flow to former salt marshes have increased in recent decades and are generally associated with alteration of water inundation levels and salinity. This study examined the effect of water level and salinity changes on soil organic matter decomposition during a 60‐day incubation period. Intact soil cores from impounded fresh water marsh and salt marsh were incubated after addition of either sea water or fresh water under flooded and drained water levels. Elevating fresh water marsh salinity to 6 to 9 ppt enhanced CO₂ emission by 50%?80% and most typically decreased CH₄ emissions, whereas, decreasing the salinity from 26 ppt to 19 ppt in salt marsh soils had no effect on CO₂ or CH₄ fluxes. The effect from altering water levels was more pronounced with drained soil cores emitting ~10‐fold more CO₂ than the flooded treatment in both marsh sediments. Draining soil cores also increased dissolved organic carbon (DOC) concentrations. Stable carbon isotope analysis of CO₂ generated during the incubations of fresh water marsh cores in drained soils demonstrates that relict peat OC that accumulated when the marsh was saline was preferentially oxidized when sea water was introduced. This study suggests that restoration of tidal flow that raises the water level from drained conditions would decrease aerobic decomposition and enhance C sequestration. It is also possible that the restoration would increase soil C decomposition of deeper deposits by anaerobic oxidation, however this impact would be minimal compared to lower emissions expected due to the return of flooding conditions.     

Crop performance, nitrogen and water use in flooded and aerobic rice

Irrigated aerobic rice is a new system being developed for lowland areas with water shortage and for favorable upland areas with access to supplementary irrigation. It entails the cultivation of nutrient-responsive cultivars in nonsaturated soil with sufficient external inputs to reach yields of 70–80% of high-input flooded rice. To obtain insights into crop performance, water use, and N use of aerobic rice, a field experiment was conducted in the dry seasons of 2002 and 2003 in the Philippines. Cultivar Apo was grown under flooded and aerobic conditions at 0 and at 150 kg fertilizer N ha^–1. The aerobic fields were flush irrigated when the soil water potential at 15-cm depth reached –30 kPa. A ¹⁵N isotope study was carried out in microplots within the 150-N plots to determine the fate of applied N. The yield under aerobic conditions with 150 kg N ha^–1 was 6.3 t ha^–1 in 2002 and 4.2 t ha^–1 in 2003, and the irrigation water input was 778 mm in 2002 and 826 mm in 2003. Compared with flooded conditions, the yield was 15 and 39% lower, and the irrigation water use 36 and 41% lower in aerobic plots in 2002 and 2003, respectively. N content at 150 kg N ha^–1 in leaves and total plant was nearly the same for aerobic and flooded conditions, indicating that crop growth under aerobic conditions was limited by water deficit and not by N deficit. Under aerobic conditions, average fertilizer N recovery was 22% in both the main field and the microplot, whereas under flooded conditions, it was 49% in the main field and 36% in the microplot. Under both flooded and aerobic conditions, the fraction of ¹⁵N that was determined in the soil after the growing season was 23%. Since nitrate contents in leachate water were negligible, we hypothesized that the N unaccounted for were gaseous losses. The N unaccounted for was higher under aerobic conditions than under flooded conditions. For aerobic rice, trials are suggested for optimizing dose and timing of N fertilizer. Also further improvements in water regime should be made to reduce crop water stress.     

Molybdenum and phosphorus interact to constrain asymbiotic nitrogen fixation in tropical forests

Biological di-nitrogen fixation (N(2)) is the dominant natural source of new nitrogen to land ecosystems. Phosphorus (P) is thought to limit N(2) fixation in many tropical soils, yet both molybdenum (Mo) and P are crucial for the nitrogenase reaction (which catalyzes N(2) conversion to ammonia) and cell growth. We have limited understanding of how and when fixation is constrained by these nutrients in nature. Here we show in tropical forests of lowland Panama that the limiting element on asymbiotic N(2) fixation shifts along a broad landscape gradient in soil P, where Mo limits fixation in P-rich soils while Mo and P co-limit in P-poor soils. In no circumstance did P alone limit fixation. We provide and experimentally test a mechanism that explains how Mo and P can interact to constrain asymbiotic N(2) fixation. Fixation is uniformly favored in surface organic soil horizons--a niche characterized by exceedingly low levels of available Mo relative to P. We show that soil organic matter acts to reduce molybdate over phosphate bioavailability, which, in turn, promotes Mo limitation in sites where P is sufficient. Our findings show that asymbiotic N(2) fixation is constrained by the relative availability and dynamics of Mo and P in soils. This conceptual framework can explain shifts in limitation status across broad landscape gradients in soil fertility and implies that fixation depends on Mo and P in ways that are more complex than previously thought.     

Alleviating soil sickness caused by aerobic monocropping: responses of aerobic rice to soil oven-heating

“Aerobic rice” system is the cultivation of nutrient-responsive cultivars in nonflooded and nonsaturated soil under supplemental irrigation. It is intended for lowland areas with water shortage and for favorable upland areas with access to supplementary irrigation. Yield decline caused by soil sickness has been reported with continuous monocropping of aerobic rice grown under nonflooded conditions. The objective of this study was to determine the growth response of rice plant to oven heating of soil with a monocropping history of aerobic rice. A series of pot experiments was conducted with soils from fields where rice has been grown continuously under aerobic or anaerobic (flooded) conditions. Soil was oven heated at different temperatures and for various durations. Plants of Apo, an upland variety that does relatively well under the aerobic conditions of lowland, were grown aerobically without fertilizer inputs in all six experiments. Plants were sampled during vegetative stage to determine stem number, plant height, leaf area, and total biomass. Heating of soil increased plant growth greatly in soils with an aerobic history but a relatively small increase was observed in soils with a flooded history as these plants nearly reached optimum growth. A growth increase with continuous aerobic soil was already observed with heating at 90°C for 12 h and at 120°C for as short as 3 h. Maximum plant growth response was observed with heating at 120°C for 12 h. Leaf area was most sensitive to soil heating, followed by total biomass and stem number. We conclude that soil heating provides a simple and quick test to determine whether a soil has any sign of sickness that is caused by continuous cropping of aerobic rice.     

Seed Banks and Species Richness Potential of Coal Slurry Ponds Reclaimed as Wetlands

Seed bank experiments are described to assess the species richness potential of coal slurry ponds reclaimed as wetlands (ranging from 6 to more than 40 years old). Experimental treatments test the drawdown and flooded conditions characteristic of the vegetation dynamics of emergent wetlands in the Upper Mississippi Valley. More seedlings, primarily annuals, emerged from exposed wet sediments (freely drained) than under continuous flooded sediments in cold ponds (339 versus 136 seedlings m^?2, respectively) and in natural ponds (163 versus 47, respectively). More seeds were produced by plants established in freely drained conditions than under flooded conditions from sediments in the coal ponds (26546 versus 1842 seeds m^?2, respectively) and the natural ponds (28430 versus 4526, respectively). Similarly, more biomass was also produced by these plants in freely drained than under flooded conditions in coal ponds (118 versus 47 g m^?2, respectively) and natural ponds (118 versus 52, respectively). Fertilization (NPK) did not affect germination for the most part, but it did affect seed set and biomass production, especially for C₄ annuals such as Echinochloa crusgalli and Panicum dichotomiflorum. I propose that lime (calcium carbonate) and fertilizer be applied during the first few scheduled drawdowns for these coal slurry ponds reclaimed as wetlands to increase the number of species and to allow their more rapid development as self-sustaining systems.     

河西走廊中段绿洲退化土地退耕种植苜蓿的固碳效应

土地利用变化和耕作管理是人类影响陆地生态系统碳过程一个重要方面.对河西走廊中段张掖绿洲退化土地退耕种植苜蓿5a后土壤性状的分析表明, 49个退耕苜蓿地土壤与相邻未退耕农田土壤配对样本的比较,退耕苜蓿地0～15cm土层土壤粒级组成和容重并未发生显著变化,但土壤pH平均提高了0.11个单位,电导率降低34.8%,土壤有机碳(SOC)和全氮(全N)含量较对照农田土壤平均提高18.5%和9.3%,活性有机碳(labile C)增加53.3%.SOC含量受海拔高度和土壤粒粉粒含量的影响,退耕后SOC和全N的增加幅度沙壤土高于粉壤土,而labile C的增加幅度沙壤土低于粉壤土.退耕苜蓿地0～15cm土层SOC和全N储量较农田土壤分别增加2.84Mg hm~(-2)和0.21Mg hm~(-2),土壤C、N的固存率平均为0.57Mg hm~(-2)a~(-1)和0.04 Mg hm~(-2)a~(-1),表明退化土地由1年生作物向多年生牧草的转变有显著的固碳效应和潜力.活性有机碳的变化较总有机碳的变化更为显著,表明活性有机碳对土地利用变化的响应更为敏感.     

The rate of permafrost carbon release under aerobic and anaerobic conditions and its potential effects on climate

Recent observations suggest that permafrost thaw may create two completely different soil environments: aerobic in relatively well‐drained uplands and anaerobic in poorly drained wetlands. The soil oxygen availability will dictate the rate of permafrost carbon release as carbon dioxide (CO₂) and as methane (CH₄), and the overall effects of these emitted greenhouse gases on climate. The objective of this study was to quantify CO₂ and CH₄ release over a 500‐day period from permafrost soil under aerobic and anaerobic conditions in the laboratory and to compare the potential effects of these emissions on future climate by estimating their relative climate forcing. We used permafrost soils collected from Alaska and Siberia with varying organic matter characteristics and simultaneously incubated them under aerobic and anaerobic conditions to determine rates of CO₂ and CH₄ production. Over 500 days of soil incubation at 15 °C, we observed that carbon released under aerobic conditions was 3.9–10.0 times greater than anaerobic conditions. When scaled by greenhouse warming potential to account for differences between CO₂ and CH₄, relative climate forcing ranged between 1.5 and 7.1. Carbon release in organic soils was nearly 20 times greater than mineral soils on a per gram soil basis, but when compared on a per gram carbon basis, deep permafrost mineral soils showed carbon release rates similar to organic soils for some soil types. This suggests that permafrost carbon may be very labile, but that there are significant differences across soil types depending on the processes that controlled initial permafrost carbon accumulation within a particular landscape. Overall, our study showed that, independent of soil type, permafrost carbon in a relatively aerobic upland ecosystems may have a greater effect on climate when compared with a similar amount of permafrost carbon thawing in an anaerobic environment, despite the release of CH₄ that occurs in anaerobic conditions.     

Pyrosequencing Reveals Changes in Soil Bacterial Communities after Conversion of Yungas Forests to Agriculture

The Southern Andean Yungas in Northwest Argentina constitute one of the main biodiversity hotspots in the world. Considerable changes in land use have taken place in this ecoregion, predominantly related to forest conversion to croplands, inducing losses in above-ground biodiversity and with potential impact on soil microbial communities. In this study, we used high-throughput pyrosequencing of the 16S ribosomal RNA gene to assess whether land-use change and time under agriculture affect the composition and diversity of soil bacterial communities. We selected two areas dedicated to sugarcane and soybean production, comprising both short- and long-term agricultural sites, and used the adjacent native forest soils as a reference. Land-use change altered the composition of bacterial communities, with differences between productive areas despite the similarities between both forests. At the phylum level, only Verrucomicrobia and Firmicutes changed in abundance after deforestation for sugarcane and soybean cropping, respectively. In cultivated soils, Verrucomicrobia decreased sharply (~80%), while Firmicutes were more abundant. Despite the fact that local diversity was increased in sugarcane systems and was not altered by soybean cropping, phylogenetic beta diversity declined along both chronosequences, evidencing a homogenization of soil bacterial communities over time. In spite of the detected alteration in composition and diversity, we found a core microbiome resistant to the disturbances caused by the conversion of forests to cultivated lands and few or none exclusive OTUs for each land-use type. The overall changes in the relative abundance of copiotrophic and oligotrophic taxa may have an impact in soil ecosystem functionality. However, communities with many taxa in common may also share many functional attributes, allowing to maintain at least some soil ecosystem services after forest conversion to croplands.     

Switchgrass nitrogen response and estimated production costs on diverse sites

Switchgrass (Panicum virgatum L.) has been the principal perennial herbaceous crop investigated for bioenergy production in North America given its high production potential, relatively low input requirements, and potential suitability for use on marginal lands. Few large trials have determined switchgrass yields at field scale on marginal lands, including analysis of production costs. Thus, a field‐scale study was conducted to develop realistic yield and cost estimates for diverse regions of the USA. Objectives included measuring switchgrass response to fertility treatments (0, 56, and 112 kg N ha^?1) and generating corresponding estimates of production costs for sites with diverse soil and climatic conditions. Trials occurred in Iowa, New York, Oklahoma, South Dakota, and Virginia, USA. Cultivars and management practices were site specific, and field‐scale equipment was used for all management practices. Input costs were estimated using final harvest‐year (2015) prices, and equipment operation costs were estimated with the MachData model ($2015). Switchgrass yields generally were below those reported elsewhere, averaging 6.3 Mg ha^?1 across sites and treatments. Establishment stand percent ranged from 28% to 76% and was linked to initial year production. No response to N was observed at any site in the first production year. In subsequent seasons, N generally increased yields on well‐drained soils; however, responses to N were nil or negative on less well‐drained soils. Greatest percent increases in response to 112 kg N ha^?1 were 57% and 76% on well‐drained South Dakota and Virginia sites, where breakeven prices to justify N applications were over $70 and $63 Mg^?1, respectively. For some sites, typically promoted N application rates may be economically unjustified; it remains unknown whether a bioenergy industry can support the breakeven prices estimated for sites where N inputs had positive effects on switchgrass yield.     

A rapid assessment of anthropogenic disturbances in East African wetlands

The use of East African freshwater wetlands for agriculture has increased in recent decades, raising concerns about potential impacts on wetlands and the long-term sustainability of such land use trends. WET-health is an indicator-based rapid wetland assessment approach developed in South Africa. It allows determining the conditions of wetlands in four assessment modules (hydrology, geomorphology, vegetation, and water quality) by observing the degree of deviation of a wetland from its anticipated natural reference state. We tested the transferability of the WET-health concept for East African inland valley swamps and floodplain wetlands based on 114 assessment units at four study sites. Due to large wetland areas and different environmental settings in East Africa, we modified the original approach using a random selection of assessment units and an assessment scheme based on disturbance types (Appendices A and B). Estimated WET-health impact scores were matched with biophysical and socioeconomic variables using a generalized linear mixed model. Land use included largely undisturbed wetland units occurring side by side with seasonally cropped or grazed units, and drained, permanently cultivated units. A strong differentiation of impact scores between the four assessment modules was apparent with highest scores for vegetation and lowest scores for geomorphology. Vegetation and water quality responded most sensitively to land use changes. The magnitude of wetland disturbance is predominantly determined by management factors such as land use intensity, soil tillage, drainage intensity, and the application of agrochemicals and influences vegetation attributes and the provision of ecosystem services. The proposed modification of WET-health enables users to assess large wetland areas during relatively short periods of time. While further studies will be required, WET-health appears to be a promising concept to be applied to wetlands in East Africa and possibly beyond.