Use of a 15N2 labelling technique to estimate exudation by white clover and transfer to companion ryegrass of symbiotically fixed N

Backgrounds and aims

N rhizodeposition by legumes leads to enrichment of N in soils and in companion plants. N rhizodeposition can be divided into two major components, root exudation and root senescence. Our aim was to quantify N root exudation in white clover (Trifolium repens L.) through an estimation of short-term N rhizodeposition and to assess its impact on N transfer to companion perennial ryegrass (Lolium perenne L.) grown in mixture with clover.

Method

¹⁵N₂ provided in the root atmosphere for 3 days was used to estimate transfer of symbiotically fixed nitrogen (SFN) to the growing medium by clover grown in pure stand and to ryegrass by clover grown in mixture for 2 months.

Results

The proportion of N rhizodeposited over the 3 days increased from 3.5 % of SFN in pure stand to 5.3 % in mixture. The ¹⁵N-enrichment of ammonium from the adhering substrate shows that a part of the rhizodeposited N was released in the form of ammonium. 4 % of the rhizodeposited N was taken up by ryegrass during the labelling period.

Conclusions

This study showed a significant contribution of root N exudation to the total N rhizodeposition of legumes and in the transfer of N to grasses.     

Decomposition of cowpea and millet amendments to a sandy Alfisol in Niger

Current inputs of organic materials to cropped lands on sandy Alfisols and Entisols in Sahelian West Africa are insufficient to arrest soil organic matter (SOM) decline. Crop residues and green manures require proper management in order to maximize their contribution to nutrient supply and SOM maintenance. The objectives of this study were to quantify the rates of C and N mineralization from cowpea (Vigna unguiculata (L.) Walp.) green manure, cowpea residue, and millet (Pennisetum glaucum (L.) R.Br.) residue under field conditions in Niger and to determine the effect of these organic amendments on pearl millet yield. Millet was grown (1) as sole crop, (2) as intercrop with cowpea, (3) as intercrop with cowpea that was incorporated as green manure during the second half of the growing season, (4) with incorporated cowpea residue (2000 kg ha^–1), (5) with millet residue mulch (3000 kg ha^–1), and (6) with N fertilizer. Carbon loss as CO₂ from soil with and without organic amendment was measured three times per week during the growing season. Nitrogen fertilizer increased millet yield only in a year with a favorable rainfall distribution. Cowpea grown in intercrop with millet during the first part of the growing season and subsequently incorporated as green manure between millet rows increased millet grain yield in a year with sufficient early rainfall, which could be attributed to the rapid rate of decomposition and nutrient release during the first 3 weeks after incorporation. In a year with limited early rainfall, however, densely planted green manure cowpeas competed for water and nutrients with the growing millet crop. Incorporated cowpea residue and millet residue mulch increased millet yield. Surface applied millet residue had high rates of decomposition only during the first 3 days after a rainfall event, with 34% of the millet residue C lost as CO₂ in one rainy season. Recovery of undecomposed millet residue at the end of the rainy season was related to presence or absence of termites, but not to seasonal C loss. Millet residue mulch increased soil organic C content of this sandy Alfisol in Niger. Cowpea and millet residues had a greater effect on SOM and millet yield than cowpea green manure due to their greater rate of application and slower rate of decomposition.     

Effect of Potassium and Nitrogen Fertilizer on Switchgrass Productivity and Nutrient Removal Rates under Two Harvest Systems on a Low Potassium Soil

Biomass demand for energy will lead to utilization of marginal, low fertility soil. Application of fertilizer to such soil may increase switchgrass (Panicum virgatum L.) biomass production. In this three-way factorial field experiment, biomass yield response to potassium (K) fertilizer (0 and 68 kg?K?ha^?1) on nitrogen (N)-sufficient and N-deficient switchgrass (0 and 135 kg?N?ha^?1) was evaluated under two harvest systems. Harvest system included harvesting once per year after frost (December) and twice per year in summer (July) at boot stage and subsequent regrowth after frost. Under the one-cut system, there was no response to N or K only (13.4 Mg?ha^?1) compared to no fertilizer (12.4 Mg?ha^?1). Switchgrass receiving both N and K (14.6 Mg?ha^?1) produced 18 % greater dry matter (DM) yield compared to no fertilizer check. Under the two-cut harvest system, N only (16.0 Mg?ha^?1) or K only (14.1 Mg?ha^?1) fertilizer produced similar DM to no fertilizer (15.1 Mg?ha^?1). Switchgrass receiving both N and K in the two-cut system (19.2 Mg?ha^?1) produced the greatest (P?<?0.05) DM yield, which was 32 % greater than switchgrass receiving both N and K in the one-cut system. Nutrient removal (biomass?×?nutrient concentration) was greatest in plots receiving both N and K, and the two-cut system had greater nutrient removal than the one-cut system. Based on these results, harvesting only once during winter months reduces nutrient removal in harvested biomass and requires less inorganic fertilizer for sustained yields from year to year compared to two-cut system.     

Estimating the contribution of nitrogen from legume cover crops to the nitrogen nutrition of grapevines using a 15N dilution technique

The aim of this controlled environment experiment was to quantify the distribution of leaf-fed-¹⁵N and canopy fed-¹³C within nodulating, non-nodulating or N fertilized non-nodulating Cicer arietinum L. and in their surrounding rhizosphere soil, excluding soil?+?root respiration. Nodulating chickpea partitioned 32% of its total N and 27% of its total recoverable C below-ground, of which only 50% of N and 36% of C were in the clean root fraction. Non-nodulating chickpea allocated equal recoverable C but slightly less N (28%) below-ground but lost less C from plant induced below-ground respiration. The importance of this below-ground partitioning for crop systems C and N balances is highlighted by their large (45% and 33%, for N and C, respectively) contribution to the total plant derived residue (recyclable) fraction. Recovered ¹⁵N and ¹³C were greater (P?<?0.05) in the outer-rhizosphere (459?µg ¹⁵N and 3.2 mg ¹³C core^?1) than in the inner-rhizosphere soil (detached from roots during freeze-drying; 18?µg ¹⁵N and 67?µg ¹³C core^?1) in relation with the relative size of these compartments. This highlights the significance of the outer-rhizosphere soil when estimating C and N budgets and quantifying rhizodeposition, and the benefit of a double (¹⁵N, ¹³C) isotope approach to determine this flow against large background soil C and N pools.     

Screening Perennial Warm-Season Bioenergy Crops as an Alternative for Phytoremediation of Excess Soil P

A recent alternative strategy to reduce environmental problems associated with P transport from agricultural soils is the use of bioenergy crops to remediate excess soil P. In addition to the positive impacts associated with P mitigation, harvested biomass used as a renewable energy source can also offset the cost associated with plant-based P remediation strategies. The objective of this study was to identify potential crop species that can be used for remediation of soil P and as a cellulosic feedstock for production of renewable energy in South Florida. Fifteen crop entries were investigated for their potential to remove P from a P-enriched soil. Dry matter (DM) yield varied among crop species with greatest yield observed for elephantgrass (Pennisetum purpureum Schum.) and sugarcane (Saccharum spp.) (43 and 39 Mg?ha^?1 year^?1, respectively). Similarly, greater P removal rates were observed for elephantgrass (up to 126 kg?P?ha^?1 year^?1 in 2008) followed by sugarcane (62 kg?P?ha^?1 year^?1 in 2008). Although there was no effect (P?=?0.45) of crop species on P reduction in the soil, soil P concentrations decreased linearly during the 3-year study. Because of its relatively greater DM yield and P removal rates, elephantgrass was shown to be a good candidate for remediation of excess soil P in South Florida Spodosols.     

Yield and Nitrogen Removal of Bioenergy Grasses as Influenced by Nitrogen Rate and Harvest Management in the Coastal Plain Region of North Carolina

The agronomic performances of giant miscanthus (Miscanthus x giganteus) and switchgrass (Panicum virgatum L.) grown as bioenergy grasses are still unclear in North Carolina, due to a relatively short period of introduction. The objectives of the study were to compare the biomass yield and annual N removal of perennial bioenergy grasses and the commonly grown coastal bermudagrass [Cynodon dactylon (L.) Pers.], and to determine the optimum N rates and harvest practices for switchgrass and miscanthus. A 4-year field trial of the grasses under five annual harvest frequencies (May/Oct, June/Oct, July/Oct, Aug/Oct, and October only) and five annual N rates (0, 67,134, 202, and 268 kg N ha^?1) was established at a research farm in Eastern North Carolina in 2011. Across harvest treatments and N rates, greatest biomass was achieved in the second growth year for both miscanthus (19.0 Mg ha^?1) and switchgrass (15.9 Mg ha^?1). Grasses demonstrated no N response until the second or the third year after crop establishment. Miscanthus reached a yield plateau with a N rate of 134 kg ha^?1 since achieving plant maturity in 2013, whereas switchgrass demonstrated an increasing fertilizer N response from 134 kg N ha^?1 in the third growth year (2014) to 268 kg N ha^?1 in the fourth growth year (2015). The two-cut harvest system is not recommended for bioenergy biomass production in this region because it does not improve biomass yield and increased N removal leads to additional costs.     

Biomass Yield and Nutrient Removal Rates of Perennial Grasses under Nitrogen Fertilization

Perennial grasses may provide a renewable source of biomass for energy production. Biomass yield, nutrient concentrations, and nutrient removal rates of switchgrass (Panicum virgatum L.), giant miscanthus (Miscanthus x giganteus), giant reed (Arundo donax L.), weeping lovegrass [Eragrostis curvula (Shrad.) Nees], kleingrass (Panicum coloratum L.), and Johnsongrass (Sorghum halepense (L.) Pers.) were evaluated at four N fertilizer rates (0, 56, 112, or 168?kg?N?ha^?1) on a Minco fine sandy loam soil in southern Oklahoma. Species were established in 2008 and harvested for biomass in winter of 2009 and 2010. Biomass yield (dry matter basis) did not show a strong relationship with N fertilizer rate (p?=?0.08), but was affected by year and species interactions (p?<?0.01). Weeping lovegrass and kleingrass produced 29.0 and 16.0?Mg?ha^?1 in 2009, but only 13.0?Mg?ha^?1 and 9.8?Mg?ha^?1 in 2010, respectively. Biomass yields of giant reed, switchgrass, and Johnsongrass averaged 23.3, 17.8, and 6.0?Mg?ha^?1, respectively. Giant miscanthus established poorly, producing only 4.7?Mg?ha^?1. Across years, giant reed had the highest biomass yield, 33.2?Mg?ha^?1 at 168?kg?N?ha^?1, and the highest nutrient concentrations and removal rates (162 to 228?kg?N?ha^?1, 23 to 25?kg?P?ha^?1, and 121 to 149?kg?K?ha^?1) among the grasses. Although giant reed demonstrated tremendous biomass production, its higher nutrient removal rates indicate a potential for increased fertilization requirements over time. Switchgrass had consistently high biomass yields and relatively low nutrient removal rates (40 to 75?kg?N?ha^?1, 5 to 12?kg?P?ha^?1, and 44 to 110?kg?K?ha^?1) across years, demonstrating its merits as a low-input bioenergy crop.     

Nitrogen dynamics following field application of biochar in a temperate North American maize-based production system

Background and aims

Biochar additions to tropical soils have been shown to reduce N leaching and increase N use efficiency. No studies exist verifying reduced N leaching in field experiments on temperate agricultural soils or identifying the mechanism for N retention.

Methods

Biochar derived from maize stover was applied to a maize cropping system in central New York State at rates of 0, 1, 3, 12, and 30 t?ha^-1 in 2007. Secondary N fertilizer was added at 100, 90, 70, and 50 % of the recommended rate (108 kg N ha^-1). Nitrogen fertilizer enriched with ¹⁵?N was applied in 2009 to the 0 and 12 t?ha^-1 of biochar at 100 and 50 % secondary N application.

Results

Maize yield and plant N uptake did not change with biochar additions (p?>?0.05; n?=?3). Less N (by 82 %; p?<?0.05) was lost after biochar application through leaching only at 100 %?N fertilization. The reason for an observed 140 % greater retention of applied ¹⁵?N in the topsoil may have been the incorporation of added ¹⁵?N into microbial biomass which increased approximately three-fold which warrants further research. The low leaching of applied fertilizer ¹⁵?N (0.42 % of applied N; p?<?0.05) and comparatively high recovery of applied ¹⁵?N in the soil (39 %) after biochar additions after one cropping season may also indicate greater overall N retention through lower gaseous or erosion N losses with biochar.

Conclusions

Addition of biochar to fertile soil in a temperate climate did not improve crop growth or N use efficiency, but increased retention of fertilizer N in the topsoil.     

Belowground carbon input and translocation potential of fodder radish cover-crop

We compared the soil C input potential of a common catch-crop (fodder radish) established in 6-year-old direct-drilled (DD) plots with adjacent conventionally tilled (CT) plots on a Danish sandy loam soil by use of ¹⁴C-isotope labelling techniques. Intact monoliths of soil with actively growing fodder radish seedlings were extracted in Autumn of 2008 from DD and CT field plots and labelled with ¹⁴CO₂ at different time intervals during fodder radish growth. Labelled monoliths were then sampled 6 and 100 days after termination of labelling by clipping above-ground biomass at soil level and separating below-ground components into macro-roots and macro-root-free soil at 0?C10, 10?C25 and 25?C45 cm soil depth. Using fodder radish ¹⁴C data and the preceding spring barley biomass yield data we estimated C input from the spring barley-fodder radish cycle in addition to evaluating the effect of the removal of spring barley harvestable straw on soil C input. Potential soil C input under straw removal scenarios with and without an established fodder radish crop was also evaluated. Relative to other depths, over 70% of labelled below-ground C was found in the 0?C10 cm soil depth in both DD and CT treatments for each of the two samplings. For both macro-root and macro-root-free soil and in both tillage treatments, labelled C decreased significantly with depth (P?<?0.05). A decline of labeled C in macro-root but an increase of labeled C in macro-root-free soil was observed from day 6 to day 100 for both tillage treatments. Over the autumn-winter growing period, total below-ground C input by fodder radish within the 0?C45 cm soil depth was approximately 1.0 and 1.2 Mg C ha^?1 for CT and DD, respectively. We used data from 100 days after labelling, which coincided with the incorporation of the field fodder radish biomass, to estimate that the total fodder radish contribution to below-ground C after biomass incorporation would range between 1.6 and 1.7 Mg C ha^?1 for DD and CT, respectively. The figures for spring barley straw removal with fodder radish establishment would be between 4.9 and 5.1 Mg C ha^?1, while with no fodder radish establishment, C input to the soil would range between 3.2 Mg C ha^?1 and 3.4 Mg C ha^?1, which is approximately 0.6 Mg C ha^?1 lower than the 4 Mg C ha^?1 biomass C input required to maintain long-term soil organic C. In comparison, under straw retention and fodder radish catch-crop establishment the total spring barley and fodder radish C input would be approximately 6.1 and 6.5 Mg C ha^?1 for DD and CT, respectively. We conclude that fodder radish catch-crops have a potential for mitigating against soil C depletion resulting from export of cereal straw to other uses.     

Enhanced decomposition of selenium hyperaccumulator litter in a seleniferous habitat��evidence for specialist decomposers?

Grassland canopy management (spring burn, mowing and residue removal in late-summer, or no management) and native tallgrass species composition (cool season mixture, warm season mixture, or combined cool and warm mixture) effects on C and N in aboveground biomass and soil were investigated at Brookings SD on a previously-plowed Barnes clay loam (fine-loamy, superactive, frigid Calcic Hapludoll). During the last 2 yr of the 9-yr experiment, shoot biomass was affected by canopy management with the burn (2,730 kg ha^?1) and mow (3,421 kg ha^?1) treatments containing less than no management (4,655 kg ha^?1). Burn treatment biomass contained 1,189 kg ha^?1 and 25 kg ha^?1 of C and N, mow contained 1,433 kg ha^?1 and 33 kg ha^?1 of C and N, while no management contained 2,014 kg ha^?1 and 39 kg ha^?1 of C and N, respectively. Soil C accumulation was independent of grass species composition. Soil C accumulation rates, which increased in strong linear fashion (r ² of 0.89 to 0.92) after initial grass establishment, were 387 kg C ha^?1 yr^?1, 503 kg C ha^?1 yr^?1, and 711 kg C ha^?1 yr^?1 for burn, mow, and no management treatments, respectively. Thus, grassland management methods used after conversion of cropland to grassland have important effects on grass biomass and soil C accumulation.     

Soil microbial community structure in European forests in relation to forest type and atmospheric nitrogen deposition

The objective of the present study was to evaluate the combined effect of vegetation and N deposition on microbial community composition in forest soils. For this, microbial biomass and community structure were assessed by ester linked fatty acid methyl ester (EL-FAME) analyses for 12 European forest sites representing different forest types (coniferous/deciduous) and differing in annual N loads (2?C40 kg?N?ha^?1). Microbial community composition was affected by vegetation as indicated by a higher proportion of the marker for arbuscular mycorrhiza (AM) fungi??16:1 11???in deciduous forest soils (1.2%?C5.7% of total EL-FAMEs) compared to acidic coniferous forest soils (0.5%?C1.6%). The two pine forest sites investigated showed the highest proportion of fungi (up to 28% of total EL-FAMEs) and the lowest proportions of Gram-negative and Gram-positive bacteria of all study sites. Nitrogen deposition rates were highly correlated with the ratios of cyclopropyl fatty acids to their precursors (r?=?0.82; P?<?0.01) and of bacteria to fungi (r?=?0.71; P?<?0.05). The two sites with the highest N deposition (??32.3 kg?N?ha^?1a^?1) were depleted in the marker fatty acids for AM fungi and other fungi. Our findings suggest that vegetation has a pronounced effect on microbial community structure, but this effect is masked by high N inputs (>30 kg?N?ha^?1a^?1).     

Carbon, nitrogen and Greenhouse gases budgets over a four years crop rotation in northern France

Croplands mainly act as net sources of the greenhouse gases carbon dioxide (CO₂) and nitrous oxide (N₂O), as well as nitrogen oxide (NO), a precursor of troposheric ozone. We determined the carbon (C) and nitrogen (N) balance of a four-year crop rotation, including maize, wheat, barley and mustard, to provide a base for exploring mitigation options of net emissions. The crop rotation had a positive net ecosystem production (NEP) of 4.4?±?0.7 Mg C ha^-1 y^-1 but represented a net source of carbon with a net biome production (NBP) of -1.3?±?1.1 Mg C?ha^-1 y^-1. The nitrogen balance of the rotation was correlated with the carbon balance and resulted in net loss (?24?±?28 kg N ha^-1 y^-1). The main nitrogen losses were nitrate leaching (?11.7?±1.0 kg N ha^-1 y^-1) and ammonia volatilization (?9 kg N ha^-1 y^-1). Dry and wet depositions were 6.7?±?3.0 and 5.9?±0.1 kg N ha^-1 y^-1, respectively. Fluxes of nitrous (N₂O) and nitric (NO) oxides did not contribute significantly to the N budget (N₂O: -1.8?±?0.04; NO: -0.7?±?0.04 kg N ha^-1 y^-1) but N₂O fluxes equaled 16% of the total greenhouse gas balance. The link between the carbon and nitrogen balances are discussed. Longer term experiments would be necessary to capture the trends in the carbon and nitrogen budgets within the variability of agricultural ecosystems.     

Litter dynamics and fine root production in Schizolobium parahyba var. amazonicum plantations and regrowth forest in Eastern Amazon

Forest plantations and agroforestry systems with Schizolobium parahyba var. amazonicum have greatly expanded in the Brazilian Amazon, generally as an alternative for reforesting degraded areas. To our knowledge there are no reports of above- and below-ground production in these forest systems. We quantified litter and fine root production in 6-yr old Schizolobium-based plantation forests (monospecific: MON, mixture: MIX, and agroforestry system: AFS) and in ~25-yr old regrowth forest (REG) over 8–12 months. We used litter traps and ingrowth cores to quantify litter and fine root production, respectively. Annual litter production was significantly lower in Schizolobium-based plantations (mean ± standard error, MON?=?5.92?±?0.15, MIX?=?6.08?±?0.13, AFS?=?6.63?±?0.13 Mg ha^?1 year^?1) than in regrowth forest (8.64?±?0.08 Mg ha^?1 year^?1). Schizolobium-based plantations showed significantly higher litter stock (MON?=?7.7?±?1.0, MIX?=?7.4?±?0.1 Mg ha^?1) than REG (5.9?±?1.3 Mg ha^?1). Total fine root production over an 8-month period was significantly higher in Schizolobium-based plantations (MON?=?3.8?±?0.2, MIX?=?3.4?±?0.2, AFS?=?2.7?±?0.1 Mg ha^?1) than in REG (1.1?±?0.03 Mg ha^?1). Six-yr old Schizolobium-based plantations and ~25-yr old regrowth forests showed comparable rates of litter + fine root production, suggesting that young forest plantations may be an interesting alternative to restore degraded areas due to early reestablishment of organic matter cycling under the studied conditions.     

Niche-based assessment of contributions of legumes to the nitrogen economy of Western Kenya smallholder farms

Nitrogen (N) deficiency is a major constraint to the productivity of the African smallholder farming systems. Grain, green manure and forage legumes have the potential to improve the soil N fertility of smallholder farming systems through biological N₂-fixation. The N₂-fixation of bean (Phaseolus vulgaris), soyabean (Glycine max), groundnut (Arachis hypogaea), Lima bean (Phaseolus lunatus), lablab (Lablab purpureus), velvet bean (Mucuna pruriens), crotalaria (Crotalaria ochroleuca), jackbean (Canavalia ensiformis), desmodium (Desmodium uncinatum), stylo (Stylosanthes guianensis) and siratro (Macroptilium atropurpureum) was assessed using the ¹⁵N natural abundance method. The experiments were conducted at three sites in western Kenya, selected on an agro-ecological zone (AEZ) gradient defined by rainfall. On a relative scale, Museno represents high potential AEZ 1, Majengo medium potential AEZ 2 and Ndori low potential AEZ 3. Rainfall in the year of experimentation was highest in AEZ 2, followed by AEZ 1 and AEZ 3. Experimental fields were classified into high, medium and low fertility classes, to assess the influence of soil fertility on N₂-fixation performance. The legumes were planted with triple super phosphate (TSP) at 30 kg P ha^?1, with an extra soyabean plot planted without TSP (soyabean-P), to assess response to P, and no artificial inoculation was done. Legume grain yield, shoot N accumulation, %N derived from N₂-fixation, N₂-fixation and net N inputs differed significantly (P<0.01) with rainfall and soil fertility. Mean grain yield ranged from 0.86 Mg ha^?1, in AEZ 2, to 0.30 Mg ha^?1, in AEZ 3, and from 0.78 Mg ha^?1, in the high fertility field, to 0.48 Mg ha^?1, in the low fertility field. Shoot N accumulation ranged from a maximum of 486 kg N ha^?1 in AEZ 2, to a minimum of 10 kg N ha^?1 in AEZ 3. Based on shoot biomass estimates, the species fixed 25–90% of their N requirements in AEZ 2, 23–90% in AEZ 1, and 7–77% in AEZ 3. Mean N₂-fixation by green manure legumes ranged from 319 kg ha^?1 (velvet bean) in AEZ 2 to 29 kg ha^?1 (jackbean) in AEZ 3. For the forage legumes, mean N₂-fixation ranged from 97 kg N ha^?1 for desmodium in AEZ 2 to 39 kg N ha^?1 for siratro in AEZ 3, while for the grain legumes, the range was from 172 kg N ha^?1 for lablab in AEZ 1 to 3 kg N ha^?1 for soyabean-P in AEZ 3. Lablab and groundnut showed consistently greater N₂-fixation and net N inputs across agro-ecological and soil fertility gradients. The use of maize as reference crop resulted in lower N₂-fixation values than when broad-leaved weed plants were used. The results demonstrate differential contributions of the green manure, forage and grain legume species to soil fertility improvement in different biophysical niches in smallholder farming systems and suggest that appropriate selection is needed to match species with the niches and farmers’ needs.     

Nitrous oxide emissions and nitrate leaching from a rain-fed wheat-maize rotation in the Sichuan Basin, China

Aims

A 3-year field experiment (October 2004–October 2007) was conducted to quantify N₂O fluxes and determine the regulating factors from rain-fed, N fertilized wheat-maize rotation in the Sichuan Basin, China.

Methods

Static chamber-GC techniques were used to measure soil N₂O fluxes in three treatments (three replicates per treatment): CK (no fertilizer); N150 (300 kg N fertilizer ha^?1 yr^?1 or 150 kg N?ha^?1 per crop); N250 (500 kg N fertilizer ha^?1 yr^?1 kg or 250 kg N?ha^?1 per crop). Nitrate (NO ₃ ^? ) leaching losses were measured at nearby sites using free-drained lysimeters.

Results

The annual N₂O fluxes from the N fertilized treatments were in the range of 1.9 to 6.7 kg N?ha^?1 yr^?1 corresponding to an N₂O emission factor ranging from 0.12 % to 1.06 % (mean value: 0.61 %). The relationship between monthly soil N₂O fluxes and NO ₃ ^- leaching losses can be described by a significant exponential decaying function.

Conclusions

The N₂O emission factor obtained in our study was somewhat lower than the current IPCC default emission factor (1 %). Nitrate leaching, through removal of topsoil NO ₃ ^? , is an underrated regulating factor of soil N₂O fluxes from cropland, especially in the regions where high NO ₃ ^- leaching losses occur.     

Nitrogen management in maize cover crop rotations

Winter cover crops can affect N nutrition of the following maize crop. Although legumes have been recommend for maize rotations, in tropical areas grasses may be more interesting because they provide a longer protection of soil surface. Legumes can add N to the system and grasses can compete with maize for the available nutrient. An experiment was conducted in Botucatu, São Paulo State, Brazil, to study N dynamics in the soil surface straw-maize system as affected by N fertilization management and species included in the no-till rotation. Treatments were fallow, black oat (Avena strigosa), pearl millet (Pennisetum glaucum), white lupins (Lupinus albus), black oat fertilized with N. and pearl millet fertilized with N. Maize was grown afterwards in the same plots, receiving 0.0, 60.0 and 120.0 kg ha^?1 of N sidedressed 30 days after plant emergence. Soil, straw and maize samples were taken periodically. The highest corn yields were observed when it was cropped after pearl millet fertilized with N. Nitrogen side dressed application up to 120 kg ha^?1 was not able to avoid corn yield decrease caused by black oat. Grasses can be recommended in maize rotations in tropical areas, provided they receive nitrogen fertilizer and show no allelopathy. Due to its higher C/N ratio and dry matter yield they are better than legumes, protecting the soil surface for a longer period. Pearl millet is particularly interesting because it enhances N use efficiency by the following maize crop. For a better N availability/demand synchronism, the cover crops should be desiccated right before maize planting     

Plant and soil responses of an alpine steppe on the Tibetan Plateau to multi-level nitrogen addition

Background

Although plant growth in alpine steppes on the Tibetan Plateau has been suggested to be sensitive to nitrogen (N) addition, the N limitation conditions of alpine steppes remain uncertain.

Methods

After 2 years of fertilization with NH₄NO₃ at six rates (0, 10, 20, 40, 80 and 160 kg N ha^?1 yr^?1), the responses of plant and soil parameters as well as N₂O fluxes were measured.

Results

At the vegetation level, N addition resulted in an increase in the aboveground N pool from 0.5?±?0.1 g m^?2 in the control plots to 1.9?±?0.2 g m^?2 in the plots at the highest N input rate. The aboveground C pool, biomass N concentration, foliar δ¹⁵N, soil NO₃ ^?-N and N₂O flux were also increased by N addition. However, as the N fertilization rate increased from 10 kg N ha^?1 yr^?1 to 160 kg N ha^?1 yr^?1, the N-use efficiency decreased from 12.3?±?4.6 kg C kg N^?1 to 1.6?±?0.2 kg C kg N^?1, and the N-uptake efficiency decreased from 43.2?±?9.7 % to 9.1?±?1.1 %. Biomass N:P ratios increased from 14.4?±?2.6 in the control plots to 20.5?±?0.8 in the plots with the highest N input rate. Biomass N:P ratios, N-uptake efficiency and N-use efficiency flattened out at 40 kg N ha^?1 yr^?1. Above this level, soil NO₃ ^?-N began to accumulate. The seasonal average N₂O flux of growing season nonlinearly increased with increased N fertilization rate and linearly increased with the weighted average foliar δ¹⁵N. At the species level, N uptake responses to relative N availability were species-specific. Biomass N concentration of seven out of the eight non-legume species increased significantly with N fertilization rates, while Kobresia macrantha and the one legume species (Oxytropics glacialis) remained stable. Both the non-legume and the legume species showed significant ¹⁵N enrichment with increasing N fertilization rate. All non-legume species showed significant increased N:P ratios with increased N fertilization rate, but not the legume species.

Conclusions

Our findings suggest that the Tibetan alpine steppes might be N-saturated above a critical N load of 40 kg N ha^?1 yr^?1. For the entire Tibetan Plateau (ca. 2.57 million km²), a low N deposition rate (10 kg N ha^?1 yr^?1) could enhance plant growth, and stimulate aboveground N and C storage by at least 1.1?±?0.3 Tg N yr^?1 and 31.5?±?11.8 Tg C yr^?1, respectively. The non-legume species was N-limited, but the legume species was not limited by N.     

Soil Greenhouse Gas Emissions and Carbon Dynamics of a No-Till,Corn-Based Cellulosic Ethanol Production System

Crop residues like corn (Zea mays L.) stover perform important functions that promote soil health and provide ecosystem services that influence agricultural sustainability and global biogeochemical cycles. We evaluated the effect of corn stover removal from a no-till, corn-soybean (Glycine max (L.) Merr) rotation on soil greenhouse gas (GHG; CO₂, N₂O, CH₄) fluxes, crop yields, and soil organic carbon (SOC) dynamics. We conducted a 4-year study using replicated field plots managed with two levels of corn stover removal (none; 55 % stover removal) for four complete crop cycles prior to initiation of ground surface gas flux measurements. Corn and soybean yields were not affected by stover removal with yields averaging 7.28 Mg ha^?1 for corn and 2.64 Mg ha^?1 for soybean. Corn stover removal treatment did not affect soil GHG fluxes from the corn phase; however, the treatment did significantly increase (107 %, P?=?0.037) N₂O fluxes during the soybean phase. The plots were a net source of CH₄ (～0.5 kg CH₄-C ha^?1 year^?1 average of all treatments and crops) during the generally wet study duration. Soil organic carbon stocks increased in both treatments during the 4-year study (initiated following 8 years of stover removal), with significantly higher SOC accumulation in the control plots compared to plots with corn stover removal (0–15 cm, P?=?0.048). Non-CO₂ greenhouse gas emissions (945 kg CO₂-eq ha^?1 year^?1) were roughly half of SOC (0–30 cm) gains with corn stover removal (1.841 Mg CO₂-eq ha^?1 year^?1) indicating that no-till practices greatly improve the viability of biennial corn stover harvesting under local soil-climatic conditions. Our results also show that repeated corn stover harvesting may increase N loss (as N₂O) from fields and thereby contribute to GHG production and loss of potential plant nutrients.     

Yield gaps, nutrient use efficiencies and response to fertilisers by maize across heterogeneous smallholder farms of western Kenya

The need to promote fertiliser use by African smallholder farmers to counteract the current decline in per capita food production is widely recognised. But soil heterogeneity results in variable responses of crops to fertilisers within single farms. We used existing databases on maize production under farmer (F-M) and researcher management (R-M) to analyse the effect of soil heterogeneity on the different components of nutrient use efficiency by maize growing on smallholder farms in western Kenya: nutrient availability, capture and conversion efficiencies and crop biomass partitioning. Subsequently, we used the simple model QUEFTS to calculate nutrient recovery efficiencies from the R-M plots and to calculate attainable yields with and without fertilisers based on measured soil properties across heterogeneous farms. The yield gap of maize between F-M and R-M varied from 0.5 to 3 t grain ha^?1 season^?1 across field types and localities. Poor fields under R-M yielded better than F-M, even without fertilisers. Such differences, of up to 1.1 t ha^?1 greater yields under R-M conditions are attributable to improved agronomic management and germplasm. The relative response of maize to N–P–K fertilisers tended to decrease with increasing soil quality (soil C and extractable P), from a maximum of 4.4-fold to ?0.5-fold relative to the control. Soil heterogeneity affected resource use efficiencies mainly through effects on the efficiency of resource capture. Apparent recovery efficiencies varied between 0 and 70% for N, 0 and 15% for P, and 0 to 52% for K. Resource conversion efficiencies were less variable across fields and localities, with average values of 97 kg DM kg^?1 N, 558 kg DM kg^?1 P and 111 kg DM kg^?1 K taken up. Using measured soil chemical properties QUEFTS over-estimated observed yields under F-M, indicating that variable crop performance within and across farms cannot be ascribed solely to soil nutrient availability. For the R-M plots QUEFTS predicted positive crop responses to application of 30 kg P ha^?1 and 30 kg P ha^?1 + 90 kg N ha^?1 for a wide range of soil qualities, indicating that there is room to improve current crop productivity through fertiliser use. To ensure their efficient use in sub-Saharan Africa mineral fertilisers should be: (1) targeted to specific niches of soil fertility within heterogeneous farms; and (2) go hand-in-hand with the implementation of agronomic measures to improve their capture and utilisation.     

Nitrogen biogeochemistry of a mature Scots pine forest subjected to high nitrogen loads

Nitrogen (N) biogeochemistry of a mature Scots pine (Pinus sylvestris L.) stand subjected to an average total atmospheric N deposition of 48 kg ha^?1 year^?1 was studied during the period 1992–2007. The annual amount of dissolved inorganic nitrogen (DIN) in throughfall (TF) averaged 34 kg ha^?1 year^?1 over the 16-year monitoring period. The throughfall fluxes contained also considerable amounts of dissolved organic nitrogen (DON) (5–8.5 kg N ha^?1 year^?1), which should be incorporated in the estimate of N flux using throughfall collectors. Throughfall DIN fluxes declined at a rate of ?0.9 kg N ha^?1 year^?1, mainly due to the decreasing TF fluxes of ammonium (NH₄), which accounted for 70% to TF DIN. The decrease in TF DIN was accompanied by a decrease in DIN leaching in the seepage water (?1.6 kg N ha^?1 year^?1), which occurred exclusively as nitrate (NO₃ ^?). Nitrate losses in the leachate of the forest floor (LFH) equalled the TF NO₃ ^? delivered to the LFH-layer. On the contrary, about half of the TF NH₄ ⁺ was retained within the LFH-layer. Approximately 60% of the TF DIN fluxes were leached indicating that N inputs were far in excess of the N requirements of the forest. For DON, losses were only substantial from the LFH-layer, but no DON was leached in the seepage water. Despite the high N losses through nitrate leaching and NO _x emission, the forest was still accumulating N, especially in the aggrading LFH-layer. The forest stand, on the contrary, was found to be a poor N sink.