Seasonal dynamics of above‐ and below‐ground biomass and nitrogen partitioning in Miscanthus × giganteus and Panicum virgatum across three growing seasons

The first replicated productivity trials of the C4 perennial grass Miscanthus × giganteus in the United States showed this emerging ligno‐cellulosic bioenergy feedstock to provide remarkably high annual yields. This covered the 5 years after planting, leaving it uncertain if this high productivity could be maintained in the absence of N fertilization. An expected, but until now unsubstantiated, benefit of both species was investment in roots and perennating rhizomes. This study examines for years 5–7 yields, biomass, C and N in shoots, roots, and rhizomes. The mean peak shoot biomass for M. × giganteus in years 5–7 was 46.5 t ha^?1 in October, declining to 38.1 t ha^?1 on completion of senescence and at harvest in December, and 20.7 t ha^?1 declining to 11.3 t ha^?1 for Panicum virgatum. There was no evidence of decline in annual yield with age. Mean rhizome biomass was significantly higher in M. × giganteus at 21.5 t ha^?1 compared to 7.2 t ha^?1 for P. virgatum, whereas root biomass was similar at 5.6–5.9 t ha^?1. M. × giganteus shoots contained 339 kg ha^?1 N in August, declining to 193 kg ha^?1 in December, compared to 168 and 58 kg ha^?1 for P. virgatum. The results suggest substantial remobilization of N to roots and rhizomes, yet still a substantial loss with December harvests. The shoot and rhizome biomass increase of 33.6 t ha^?1 during the 2‐month period between June and August for M. × giganteus corresponds to a solar energy conversion of 4.4% of solar energy into biomass, one of the highest recorded and confirming the remarkable productivity potential of this plant.     

Belowground responses of <Emphasis Type="Italic">Picea asperata</Emphasis> seedlings to warming and nitrogen fertilization in the eastern Tibetan Plateau

The impacts of global climatic change on belowground ecological processes of terrestrial ecosystems are still not clear. We therefore conducted an experiment in the subalpine coniferous forest ecosystem of the eastern edges of the Tibetan Plateau to study roots of Picea asperata seedlings and rhizosphere soil responses to soil warming and nitrogen availability from April 2007 to December 2008. The seedlings were subjected to two levels of temperature (ambient; infrared heater warming) and two nitrogen levels (0 or 25 g m⁻²year⁻¹ N). We used a free air temperature increase from an overhead infrared heater to raise both air and soil temperature by 2.1 and 2.6°C, respectively. The results showed that warming alone significantly increased total biomass, coarse root biomass and fine root biomass of P. asperata seedlings. Both total biomass and fine root biomass were increased, but coarse root biomass was significantly decreased by nitrogen fertilization and warming combined with nitrogen fertilization. Warming induced a prominent increase in soil organic carbon (SOC) and NO₃ ⁻-N of rhizosphere soil, while nitrogen fertilization significantly decreased SOC and NH₄ ⁺-N of rhizosphere soil. The warming, fertilization and warming × N fertilization interaction decreased soil microbial C significantly, but substantially increased soil microbial N. These results suggest that nitrogen deposition combined with warmer temperatures under future climatic change possibly will have no effect on fine root production of P. asperata seedlings, but could enhance the nitrification process of their rhizosphere soils in subalpine coniferous forests.     

Soil fertility and fine root dynamics in response to 4 years of nutrient (N,P, K) fertilization in a lowland tropical moist forest,Panama

The question of how tropical trees cope with infertile soils has been challenging to address, in part, because fine root dynamics must be studied in situ. We used annual fertilization with nitrogen (N as urea, 12.5 g N m^?2 year^?1), phosphorus (P as superphosphate, 5 g P m^?2 year^?1) and potassium (K as KCl, 5 g K m^?2 year^?1) within 38 ha of old‐growth lowland tropical moist forest in Panama and examined fine root dynamics with minirhizotron images. We expected that added P, above all, would (i) decrease fine root biomass but, (ii) have no impact on fine root turnover. Soil in the study area was moderately acidic (pH = 5.28), had moderate concentrations of exchangeable base cations (13.4 cmol kg^?1), low concentrations of Bray‐extractable phosphate (PO₄ = 2.2 mg kg^?1), and modest concentrations of KCl‐extractable nitrate (NO₃ = 5.0 mg kg^?1) and KCl‐extractable ammonium (NH₄ = 15.5 mg kg^?1). Added N increased concentrations of KCl‐extractable NO₃ and acidified the soil by one pH unit. Added P increased concentrations of Bray‐extractable PO₄ and P in the labile fraction. Concentrations of exchangeable K were elevated in K addition plots but reduced by N additions. Fine root dynamics responded to added K rather than added P. After 2 years, added K decreased fine root biomass from 330 to 275 g m^?2. The turnover coefficient of fine roots <1 mm diameter ranged from 2.6 to 4.4 per year, and the largest values occurred in plots with added K. This study supported the view that biomass and dynamics of fine roots respond to soil nutrient availability in species‐rich, lowland tropical moist forest. However, K rather than P elicited root responses. Fine roots smaller than 1 mm have a short lifetime (<140 days), and control of fine root production by nutrient availability in tropical forests deserves more study.     

Nitrogen isotope composition of tomato (Lycopersicon esculentum Mill. cv. T-5) grown under ammonium or nitrate nutrition

Studies that quantify plant δ¹⁵N often assume that fractionation during nitrogen uptake and intra-plant variation in δ¹⁵N are minimal. We tested both assumptions by growing tomato (Lycopersicon esculetum Mill. cv. T-5) at NH₄⁺ or NO^?₃ concentrations typical of those found in the soil. Fractionation did not occur with uptake; whole-plant δ¹⁵N was not significantly different from source δ¹⁵ N for plants grown on either nitrogen form. No intra-plant variation in δ¹⁵N was observed for plants grown with NH⁺₄. In contrast. δ¹⁵N of leaves was as much as 5.8% greater than that of roots for plants grown with NO^?₃. The contrasting patterns of intra-plant variation are probably caused by different assimilation patterns. NH⁺₄ is assimilated immediately in the root, so organic nitrogen in the shoot and root is the product of a single assimilation event. NO^?₃ assimilation can occur in shoots and roots. Fractionation during assimilation caused the δ¹⁵N of NO^?₃ to become enriched relative to organic nitrogen; the δ¹⁵N of NO^?₃ was 11.1 and 12.9% greater than the δ¹⁵N of organic nitrogen in leaves and roots, respectively. Leaf δ¹⁵N may therefore be greater than that of roots because the NO^?₃ available for assimilation in leaves originates from a NO^?₃ pool that was previously exposed to nitrate assimilation in the root.     

Nitrate leaching and soil nitrous oxide emissions diminish with time in a hybrid poplar short‐rotation coppice in southern Germany

Hybrid poplar short‐rotation coppices (SRC) provide feedstocks for bioenergy production and can be established on lands that are suboptimal for food production. The environmental consequences of deploying this production system on marginal agricultural land need to be evaluated, including the investigation of common management practices i.e., fertilization and irrigation. In this work, we evaluated (1) the soil‐atmosphere exchange of carbon dioxide, methane, and nitrous oxide (N₂O); (2) the changes in soil organic carbon (SOC) stocks; (3) the gross ammonification and nitrification rates; and (4) the nitrate leaching as affected by the establishment of a hybrid poplar SRC on a marginal agricultural land in southern Germany. Our study covered one 3‐year rotation period and 2 years after the first coppicing. We combined field and laboratory experiments with modeling. The soil N₂O emissions decreased from 2.2 kg N₂O‐N ha^?1 a^?1 in the year of SRC establishment to 1.1–1.4 kg N₂O‐N ha^?1 a^?1 after 4 years. Likewise, nitrate leaching reduced from 13 to 1.5–8 kg N ha^?1 a^?1. Tree coppicing induced a brief pulse of soil N₂O flux and marginal effects on gross N turnover rates. Overall, the N losses diminished within 4 years by 80% without fertilization (irrespective of irrigation) and by 40% when 40–50 kg N ha^?1 a^?1 were applied. Enhanced N losses due to fertilization and the minor effect of fertilization and irrigation on tree growth discourage its use during the first rotation period after SRC establishment. A SOC accrual rate of 0.4 Mg C ha^?1 a^?1 (uppermost 25 cm, P = 0.2) was observed 5 years after the SRC establishment. Overall, our data suggest that SRC cultivation on marginal agricultural land in the region is a promising option for increasing the share of renewable energy sources due to its net positive environmental effects.     

Differential effects of mineral and organic N sources,and of ectomycorrhizal infection by Hebeloma cylindrosporum,on growth and N utilization in Pinus pinaster

The effect of ectomycorrhizal association of Pinus pinaster with Hebeloma cylindrosporum was investigated in relation to the nitrogen source supplied as mineral (NH₄⁺ or NO₃^?) or organic N (L ‐glutamate) and at 5 mol m^?3. Plants were grown for 14 and 16 weeks with mineral and organic N, respectively, and samples were collected during the last 6 weeks of culture. Total fungal biomass was estimated using glucosamine amount and its viability was assessed using the glucosamine to ergosterol ratio. Non‐mycorrhizal plants grew better with NH₄⁺ than with NO₃^? and grew very slowly when supplied with L ‐glutamate. The presence of the fungus decreased the growth of the host plant with mineral N whereas it increased it with L ‐glutamate. Whatever the N source, most of the living fungal biomass was associated with the roots, whereas the main part of the total biomass was assayed outside the root. The form of mineral N did not significantly affect N accumulation rates over the 42 d in control plants. In mycorrhizal plants grown on either N source, the fungal tissues developing outside of the root were always the main N sink. The ectomycorrhizal association did not change ¹⁵NH₄⁺ uptake rate by roots, suggesting that the growth decrease of the host‐plant was related to the carbon cost for fungal growth and N assimilation rather than to a direct effect on NH₄⁺ acquisition. In contrast, in NO₃^?‐grown plants, in addition to draining carbon for NO₃^? reduction the fungus competed with the root for NO₃^? uptake. With NH₄⁺ or NO₃^? feeding, although mycorrhizal association improved N accumulation in shoots, we concluded that it was unlikely that the fungus had supplied the plant with N. In L ‐glutamate‐grown plants, the presence of the fungus increased the proportion of glutamine in the xylem sap and improved both N nutrition and the growth rate of the host plant.     

Impact of nitrogen allocation on growth and photosynthesis of Miscanthus (Miscanthus × giganteus)

Nitrogen (N) addition typically increases overall plant growth, but the nature of this response depends upon patterns of plant nitrogen allocation that vary throughout the growing season and depend upon canopy position. In this study seasonal variations in leaf traits were investigated across a canopy profile in Miscanthus (Miscanthus × giganteus) under two N treatments (0 and 224 kg ha^?1) to determine whether the growth response of Miscanthus to N fertilization was related to the response of photosynthetic capacity and nitrogen allocation. Miscanthus yielded 24.1 Mg ha^?1 in fertilized plots, a 40% increase compared to control plots. Photosynthetic properties, such as net photosynthesis (A), maximum rate of rubisco carboxylation (V_cmax), stomatal conductance (g_s) and PSII efficiency (F_v'/F_m'), all decreased significantly from the top of the canopy to the bottom, but were not affected by N fertilization. N fertilization increased specific leaf area (SLA) and leaf area index (LAI). Leaf N concentration in different canopy layers was increased by N fertilization and the distribution of N concentration within canopy followed irradiance gradients. These results show that the positive effect of N fertilization on the yield of Miscanthus was unrelated to changes in photosynthetic rates but was achieved mainly by increased canopy leaf area. Vertical measurements through the canopy demonstrated that Miscanthus adapted to the light environment by adjusting leaf morphological and biochemical properties independent of nitrogen treatments. GPP estimated using big leaf and multilayer models varied considerably, suggesting a multilayer model in which V_cmax changes both through time and canopy layer could be adopted into agricultural models to more accurately predict biomass production in biomass crop ecosystems.     

Environmental impacts of bioenergy wood production from poplar short‐rotation coppice grown at a marginal agricultural site in Germany

For avoiding competition with food production, marginal land is economically and environmentally highly attractive for biomass production with short‐rotation coppices (SRCs) of fast‐growing tree species such as poplars. Herein, we evaluated the environmental impacts of technological, agronomic, and environmental aspects of bioenergy production from hybrid poplar SRC cultivation on marginal land in southern Germany. For this purpose, different management regimes were considered within a 21‐year lifetime (combining measurements and modeling approaches) by means of a holistic Life Cycle Assessment (LCA). We analyzed two coppicing rotation lengths (7 × 3 and 3 × 7 years) and seven nitrogen fertilization rates and included all processes starting from site preparation, planting and coppicing, wood chipping, and heat production up to final stump removal. The 7‐year rotation cycles clearly resulted in higher biomass yields and reduced environmental impacts such as nitrate (NO₃) leaching and soil nitrous oxide (N₂O) emissions. Fertilization rates were positively related to enhanced biomass accumulation, but these benefits did not counterbalance the negative impacts on the environment due to increased nitrate leaching and N₂O emissions. Greenhouse gas (GHG) emissions associated with the heat production from poplar SRC on marginal land ranged between 8 and 46 kg CO₂‐eq. GJ^?1 (or 11–57 Mg CO₂‐eq. ha^?1). However, if the produced wood chips substitute oil heating, up to 123 Mg CO₂‐eq. ha^?1 can be saved, if produced in a 7‐year rotation without fertilization. Dissecting the entire bioenergy production chain, our study shows that environmental impacts occurred mainly during combustion and storage of wood chips, while technological aspects of establishment, harvesting, and transportation played a negligible role.     

Humic acid differentially improves nitrate kinetics under low‐ and high‐affinity systems and alters the expression of plasma membrane H+‐ATPases and nitrate transporters in rice

Humic acids (HAs) have a major effect on nutrient uptake, metabolism, growth and development in plants. Here, we evaluated the effect of HA pretreatment applied with a nutrient solution on the uptake kinetics of nitrate nitrogen (N‐NO₃^?) and the metabolism of nitrogen (N) in rice under conditions of high and low NO₃^? supply. In addition, the kinetic parameters of NO₃^? uptake, N metabolites, and nitrate transporters (NRTs) and the plasma membrane (PM) H⁺‐ATPase gene expression were examined. The plants were grown in a growth chamber with modified Hoagland and Arnon solution until 21 days after germination (DAG), and they were then transferred to a solution without N for 48 h and then to another solution without N and with and without the addition of HAs for another 48 h. After this period of N deprivation, the plants received new nutrient solutions containing 0.2 and 2.0 mM N‐NO₃^?. Treatment of rice plants with HA promoted the induction of the genes OsNRT2.1‐2.2/OsNAR2.1 and some isoforms PM H⁺‐ATPase in roots. The application of HAs differentially modified the parameters of the uptake kinetics of NO₃^? under both concentrations. When grown with 0.2 mM NO₃^?, the plants pretreated with HA had lower K_m and C_min values as well as a higher V_max/K_m ratio. When grown with 2 mM NO₃^?, the plants pretreated with HA had a higher V_max value, a greater root and shoot mass, and a lower root/shoot ratio. The N fractions were also altered by pretreatment with HA, and a greater accumulation of NO₃^? and N‐amino was observed in the roots and shoots, respectively, of plants pretreated with HA. The results suggest that pretreatment with HA modifies root morphology and gene expression of PM H⁺‐ATPases and NO₃^? transporters, resulting in a greater efficiency of NO₃^? acquisition by high‐ and low‐affinity systems.     

Nitrogen Utilization and Toxin Production by Two Diatoms of the Pseudo‐nitzschia pseudodelicatissima Complex: P. cuspidata and P. fryxelliana

The toxigenic diatom Pseudo‐nitzschia cuspidata, isolated from the U.S. Pacific Northwest, was examined in unialgal batch cultures to evaluate domoic acid (DA) toxicity and growth as a function of light, N substrate, and growth phase. Experiments conducted at saturating (120 μmol photons · m^?2 · s^?1) and subsaturating (40 μmol photons · m^?2 · s^?1) photosynthetic photon flux density (PPFD), demonstrate that P. cuspidata grows significantly faster at the higher PPFD on all three N substrates tested [nitrate (NO₃^?), ammonium (NH₄⁺), and urea], but neither cellular toxicity nor exponential growth rates were strongly associated with one N source over the other at high PPFD. However, at the lower PPFD, the exponential growth rates were approximately halved, and the cells were significantly more toxic regardless of N substrate. Urea supported significantly faster growth rates, and cellular toxicity varied as a function of N substrate with NO₃^?‐supported cells being significantly more toxic than both NH₄⁺‐ and urea‐supported cells at the low PPFD. Kinetic uptake parameters were determined for another member of the P. pseudodelicatissima complex, P. fryxelliana. After growth of these cells on NO₃^? they exhibited maximum specific uptake rates (V_max) of 22.7, 29.9, 8.98 × 10^?3 · h^?1, half‐saturation constants (K_s) of 1.34, 2.14, 0.28 μg‐at N · L^?1, and affinity values (α) of 17.0, 14.7, 32.5 × 10^?3 · h^?1/(μg‐at N · L^?1) for NO₃^?, NH₄⁺ and urea, respectively. These labo‐ratory results demonstrate the capability of P. cuspidata to grow and produce DA on both oxidized and reduced N substrates during both exponential and stationary growth phases, and the uptake kinetic results for the pseudo‐cryptic species, P. fryxelliana suggest that reduced N sources from coastal runoff could be important for maintenance of these small pennate diatoms in U.S. west coast blooms, especially during times of low ambient N concentrations.     

Distribution of NO3− reduction between roots and shoots of peachtree seedlings as affected by NO3− uptake rate

The distribution of NO₃^? reduction between roots and shoots was studied in hydro-ponically-grown peach-tree seedlings (Prunus persica L.) during recovery from N starvation. Uptake, translocation and reduction of NO₃^?, together with transport through xylem and phloem of the newly reduced N were estimated, using ¹⁵N labellings, in intact plants supplied for 90 h with 0.5 mM NH₄⁺ and 0.5, 1.5 or 10 mM NO₃^?. Xylem transport of NO₃^? was further investigated by xylem sap analysis in a similar experiment. The roots were the main site of NO₃^? reduction at all 3 levels of NO₃^? nutrition. However, the contribution of the shoots to the whole plant NO₃^? reduction increased with increasing external NO₃^? availability. This contribution was estimated to be 20, 23 and 42% of the total assimilation at 0.5, 1.5 and 10 mM NO₃^?, respectively. Both ¹⁵N results and xylem sap analysis confirmed that this trend was due to an enhancement of NO₃^? translocation from roots to shoots. It is proposed that the lack of NO₃^? export to the shoots at low NO₃^? uptake rate resulted from a competition between NO₃^? reduction in the root epidermis/cortex and NO₃^? diffusion to the stele. On the other hand, net xylem transport of newly reduced N was very efficient since ca 70% of the amino acids synthesized in the roots were translocated to the shoots, regardless of the level of NO₃^? nutrition. This net xylem transport by far exceeded the net downward phloem transport of the reduced N assimilated in shoots. As a consequence, the reduced N resulting from NO₃^? assimilation, principally occurring in the roots, was mainly incorporated in the shoots.     

Effect of nitrogen addition on Miscanthus × giganteus yield,nitrogen losses,and soil organic matter across five sites

The US Department of Energy has mandated the production of 16 billion gallons (60.6 billion liters) of renewable biofuel from cellulosic feedstocks by 2022. The perennial grass, Miscanthus × giganteus, is a potential candidate for cellulosic biofuel production because of high productivity with minimal inputs. This study determined the effect of three different spring fertilizer treatments (0, 60, and 120 kg N ha^?1 yr^?1 as urea) on biomass production, soil organic matter (SOM), and inorganic N leaching in Illinois, Kentucky, Nebraska, New Jersey, and Virginia, along with N₂O and CO₂ emissions at the IL site. There were no significant yield responses to fertilizer treatments, except at the IL site in 2012 (yields in 2012, year 4, varied from 10 to 23.7 Mg ha^?1 across all sites). Potentially mineralizable N increased across all fertilizer treatments and sites in the 0–10 cm soil depth. An increase in permanganate oxidizable carbon (POX‐C, labile C) in surface soils occurred at the IL and NJ sites, which were regularly tilled before planting. Decreases in POX‐C were observed in the 0 – 10 cm soil depth at the KY and NE sites where highly managed turfgrass was grown prior to planting. Growing M. × giganteus altered SOM composition in only 4 years of production by increasing the amount of potentially mineralizable N at every site, regardless of fertilization amount. Nitrogen applications increased N leaching and N₂O emission without increasing biomass production. This suggests that for the initial period (4 years) of M. × giganteus production, N application has a detrimental environmental impact without any yield benefits and thus should not be recommended. Further research is needed to define a time when N application to M. × giganteus results in increased biomass production.     

Evaluating a sustainability index for nutrients in a short rotation energy cropping system

In dryland environments 3–5 year rotations of tree crops and agriculture represent a major potential bioenergy feedstock and a means to restore landscape hydrologic balances and phytoremediate sites, while maintaining food production. In soils with low natural fertility, the long‐term viability of these systems will be critically affected by site nutrient status and subsequent cycling of nutrients. A nutrient assimilation index (NAI) was developed to allow comparison of species and tree component nutrient assimilation and to optimize nutrient management, by quantifying different strategies to manage site nutrients. Biomass, nutrient export and nutrient use efficiency were assessed for three short rotation tree crop species. Nutrient exports following harvest at 3 years of high density (4000 trees ha^?1) were consistently higher in Pinus radiata, with values of 85 kg ha^?1 of N, 11kg ha^?1 of P, and 62 kg ha^?1 of K, than Eucalyptus globulus and Eucalyptus occidentalis. Component NAI was generally in the order of leaf?1 for N in leaves of P. radiata to 4.7 Mg kg^?1 for P in stem‐wood of E. occidentalis, indicating higher sustainability of wood biomass compared with leaf biomass. The leaves for each species contained between 40 and 60% of the total nutrient contents while comprising around 25–30% of the total biomass. These nutrient exports via biomass removal are similar to those that follow 3 years of wheat production in the same region, indicating there is no additional drawdown of nutrient reserves during the tree cropping phase of the rotation.     

Greenhouse Gas Emissions, Nitrate Leaching, and Biomass Yields from Production of Miscanthus × giganteus in Illinois, USA

Understanding the effects of nitrogen (N) fertilization on Miscanthus × giganteus greenhouse gas emissions, nitrate leaching, and biomass production is an important consideration when using this grass as a biomass feedstock. The objective of this study was to determine the effect of three N fertilization rates (0, 60, and 120?kg?N?ha^?1 using urea as the N source) on nitrous oxide (N₂O) and carbon dioxide (CO₂) emissions, nitrogen leaching, and the biomass yields and N content of M. × giganteus planted in July 2008, and evaluated from 2009 through early 2011 in Urbana, Illinois, USA. While there was no biomass yield response to N fertilization rates in 2009 and 2010, the amount of N in the harvested biomass in 2010 was significantly greater at the 60 and 120?kg?N?ha^?1?N rates. There was no significant CO₂ emission response to N rates in 2009 or 2010. Similarly, N fertilization did not increase cumulative N₂O emissions in 2009, but cumulative N₂O emissions did increase in 2010 with N fertilization. During 2009, nitrate (NO ₃ ^? ) leaching at the 50-cm soil depth was not related to fertilization rate, but there was a significant increase in NO ₃ ^? leaching between the 0 and 120?kg?N?ha^?1 treatments in 2010 (8.9 and 28.9?kg?NO₃?CN?ha^?1?year^?1, respectively). Overall, N fertilization of M. × giganteus led to N₂O releases, increased fluxes of inorganic N (primarily NO ₃ ^? ) through the soil profile; and increased harvested N without a significant increase in biomass production.     

A meta‐analysis of fertilizer‐induced soil NO and combined NO+N2O emissions

Soils are among the important sources of atmospheric nitric oxide (NO) and nitrous oxide (N₂O), acting as a critical role in atmospheric chemistry. Updated data derived from 114 peer‐reviewed publications with 520 field measurements were synthesized using meta‐analysis procedure to examine the N fertilizer‐induced soil NO and the combined NO+N₂O emissions across global soils. Besides factors identified in earlier reviews, additional factors responsible for NO fluxes were fertilizer type, soil C/N ratio, crop residue incorporation, tillage, atmospheric carbon dioxide concentration, drought and biomass burning. When averaged across all measurements, soil NO‐N fluxes were estimated to be 4.06 kg ha^?1 yr^?1, with the greatest (9.75 kg ha^?1 yr^?1) in vegetable croplands and the lowest (0.11 kg ha^?1 yr^?1) in rice paddies. Soil NO emissions were more enhanced by synthetic N fertilizer (+38%), relative to organic (+20%) or mixed N (+18%) sources. Compared with synthetic N fertilizer alone, synthetic N fertilizer combined with nitrification inhibitors substantially reduced soil NO emissions by 81%. The global mean direct emission factors of N fertilizer for NO (EF_NO) and combined NO+N₂O (EF_c) were estimated to be 1.16% and 2.58%, with 95% confidence intervals of 0.71–1.61% and 1.81–3.35%, respectively. Forests had the greatest EF_NO (2.39%). Within the croplands, the EF_NO (1.71%) and EF_c (4.13%) were the greatest in vegetable cropping fields. Among different chemical N fertilizer varieties, ammonium nitrate had the greatest EF_NO (2.93%) and EF_c (5.97%). Some options such as organic instead of synthetic N fertilizer, decreasing N fertilizer input rate, nitrification inhibitor and low irrigation frequency could be adopted to mitigate soil NO emissions. More field measurements over multiyears are highly needed to minimize the estimate uncertainties and mitigate soil NO emissions, particularly in forests and vegetable croplands.     

Differential recovery of above‐ and below‐ground rich fen vegetation following fertilization

Abstract. For seven years we studied the recovery of vegetation in a Belgian P limited rich fen (Caricion davallianae), which had been fertilized with nitrogen (200 g.m^?2) and phosphorus (50 g.m^?2) in 1992. The vegetation in this fen has low above‐ground biomass production (< 100 g m^?2) due to the strong P limitation. Above‐ground biomass was harvested from 1992 to 1998 and P and N concentrations measured. In 1998, below‐ground biomass was also harvested. The response to fertilization differed markedly between below‐ and above‐ground compartments. Above‐ground, P was the single most important factor that continued to stimulate growth 7 yr after fertilization. Below‐ground, N tended to decrease live root biomass and increase dead root biomass and seemed to have a toxic effect on the roots. In the combined NP treatment the stimulating effect of P (an increase of live root biomass) was countered by N. The 1998 soil analysis showed no difference in soil P in the plots. Thus, Fe hydroxides are not capable of retaining P in competition with fen vegetation uptake. However, higher capture of P in root Fe coatings from N plots may partially explain this negative N effect. The results suggest that N root toxicity will be amplified in strongly P limited habitats but that its persistence will be less influenced by P availability. This mechanism may be a competitive advantage for N₂ fixing species that grow in strongly P limited wetlands.     

Lignocellulosic‐based bioenergy and water quality parameters: a review

High rates of crop residue removal as biofuel feedstocks could increase losses of nonpoint source pollutants, negatively affecting water quality. An alternative to residue removal can be growing dedicated bioenergy crops such as warm season grasses (WSGs) and short‐rotation woody crops (SRWCs). Yet, our understanding of the implications of growing dedicated bioenergy crops on water quality is limited. Thus, we (i) synthesized and compared the impacts of crop residue removal, WSGs, and SRWCs on water quality parameters (i.e., sediment and nutrient runoff, and nutrient leaching) and (ii) identified research gaps for growing dedicated energy crops. Literature indicates that residue removal at rates >50% (residue retention up to 4.71 Mg ha^?1) can increase runoff by 5–15 mm, sediment loss by 0.2–7 Mg ha^?1, NO₃–N by 0.58–1 kg ha^?1, and sediment‐associated C by 0.3–57 kg ha^?1 per rainstorm event compared to no residue removal. Crop residue removal may also increase nutrient leaching. Studies on the impacts of growing WSGs as dedicated bioenergy crops at field scale on water quality parameters are few. However, WSGs when used as conservation buffers reduce losses of sediment by 66–97%, nutrients by 21–94%, and contaminants by 9–98%. This suggests that if WSGs were grown as dedicated bioenergy crops at larger scales, they could reduce losses of nonpoint source pollutants. Literature indicates that SRWCs can consistently reduce NO₃–N leaching. More modeled than field data are available, warranting further field research on (i) field data collection from WSGs and SRWCs from marginal lands, (ii) growing monoculture or polyculture of WSGs, and (iii) large‐scale production of energy crops. Overall, dedicated bioenergy crops, particularly WSGs, can reduce losses of nonpoint source pollutants compared to residue removal and be an important strategy to improve water quality if grown at larger scales.     

Implications of productivity and nutrient requirements on greenhouse gas balance of annual and perennial bioenergy crops

Biomass from dedicated crops is expected to contribute significantly to the replacement of fossil resources. However, sustainable bioenergy cropping systems must provide high biomass production and low environmental impacts. This study aimed at quantifying biomass production, nutrient removal, expected ethanol production, and greenhouse gas (GHG) balance of six bioenergy crops: Miscanthus × giganteus, switchgrass, fescue, alfalfa, triticale, and fiber sorghum. Biomass production and N, P, K balances (input‐output) were measured during 4 years in a long‐term experiment, which included two nitrogen fertilization treatments. These results were used to calculate a posteriori ‘optimized’ fertilization practices, which would ensure a sustainable production with a nil balance of nutrients. A modified version of the cost/benefit approach proposed by Crutzen et al. (2008), comparing the GHG emissions resulting from N‐P‐K fertilization of bioenergy crops and the GHG emissions saved by replacing fossil fuel, was applied to these ‘optimized’ situations. Biomass production varied among crops between 10.0 (fescue) and 26.9 t DM ha^?1 yr^?1 (miscanthus harvested early) and the expected ethanol production between 1.3 (alfalfa) and 6.1 t ha^?1 yr^?1 (miscanthus harvested early). The cost/benefit ratio ranged from 0.10 (miscanthus harvested late) to 0.71 (fescue); it was closely correlated with the N/C ratio of the harvested biomass, except for alfalfa. The amount of saved CO₂ emissions varied from 1.0 (fescue) to 8.6 t CO₂eq ha^?1 yr^?1 (miscanthus harvested early or late). Due to its high biomass production, miscanthus was able to combine a high production of ethanol and a large saving of CO₂ emissions. Miscanthus and switchgrass harvested late gave the best compromise between low N‐P‐K requirements, high GHG saving per unit of biomass, and high productivity per hectare.     

Miscanthus × giganteus leaf senescence,decomposition and C and N inputs to soil

Energy crops are currently promoted as potential sources of alternative energy that can help mitigate the climate change caused by greenhouse gases (GHGs). The perennial crop Miscanthus × giganteus is considered promising due to its high potential for biomass production under conditions of low input. However, to assess its potential for GHG mitigation, a better quantification of the crop's contribution to soil organic matter recycling under various management systems is needed. The aim of this work was to study the effect of abscised leaves on carbon (C) and nitrogen (N) recycling in a Miscanthus plantation. The dynamics of senescent leaf fall, the rate of leaf decomposition (using a litter bag approach) and the leaf accumulation at the soil surface were tracked over two 1‐year periods under field conditions in Northern France. The fallen leaves represented an average yearly input of 1.40 Mg C ha^?1 and 16 kg N ha^?1. The abscised leaves lost approximately 54% of their initial mass in 1 year due to decomposition; the remaining mass, accumulated as a mulch layer at the soil surface, was equivalent to 7 Mg dry matter (DM) ha^?1 5 years after planting. Based on the estimated annual leaf‐C recycling rate and a stabilization rate of 35% of the added C, the annual contribution of the senescent leaves to the soil C was estimated to be approximately 0.50 Mg C ha^?1yr^?1 or 10 Mg C ha^?1 total over the 20‐year lifespan of a Miscanthus crop. This finding suggested that for Miscanthus, the abscised leaves contribute more to the soil C accumulation than do the rhizomes or roots. In contrast, the recycling of the leaf N to the soil was less than for the other N fluxes, particularly for those involving the transfer of N from the tops of the plant to the rhizome.     

Elevated CO2 plus chronic warming reduce nitrogen uptake and levels or activities of nitrogen‐uptake and ‐assimilatory proteins in tomato roots

Atmospheric CO₂ enrichment is expected to often benefit plant growth, despite causing global warming and nitrogen (N) dilution in plants. Most plants primarily procure N as inorganic nitrate (NO₃^?) or ammonium (NH₄⁺), using membrane‐localized transport proteins in roots, which are key targets for improving N use. Although interactive effects of elevated CO₂, chronic warming and N form on N relations are expected, these have not been studied. In this study, tomato (Solanum lycopersicum) plants were grown at two levels of CO₂ (400 or 700 ppm) and two temperature regimes (30 or 37°C), with NO₃^? or NH₄⁺ as the N source. Elevated CO₂ plus chronic warming severely inhibited plant growth, regardless of N form, while individually they had smaller effects on growth. Although %N in roots was similar among all treatments, elevated CO₂ plus warming decreased (1) N‐uptake rate by roots, (2) total protein concentration in roots, indicating an inhibition of N assimilation and (3) shoot %N, indicating a potential inhibition of N translocation from roots to shoots. Under elevated CO₂ plus warming, reduced NO₃^?‐uptake rate per g root was correlated with a decrease in the concentration of NO₃^?‐uptake proteins per g root, reduced NH₄⁺ uptake was correlated with decreased activity of NH₄⁺‐uptake proteins and reduced N assimilation was correlated with decreased concentration of N‐assimilatory proteins. These results indicate that elevated CO₂ and chronic warming can act synergistically to decrease plant N uptake and assimilation; hence, future global warming may decrease both plant growth and food quality (%N).