The nitrogen balance of vegetable crops irrigated with untreated effluent

In the agricultural areas near Santiago, Chile,ca. 780 kg N ha^?1 yr^?1 are added to vegetable cropsvia irrigation with untreated sewage effluent draining from the metropolitan area. Nitrate levels in surface wells in the area, from which drinking water is derived, often exceed established limits for human consumption. Of the 779 kg N ha^?1 added to crops in one year, 161–287 kg N ha^?1 yr^?1 were removed by crop harvest and much of the remainder apparently eventually leached to the 1–15 m deep water table.     

Nitrogen balance in the Trachypogon grasslands of Central Venezuela

The nitrogen balance of a Trachypogon grassland in Calabozo, Venezuela, is calculated for average conditions using biomass accumulation, nitrogen content, and turnover rates of organic matter. Burning Trachypogon grasslands results in losses of 8.5 kg N ha^?1 yr^?1, while rainfall inputs average 2.6 kg N ha^?1 yr^?1. Uptake of N by vegetation is 14.8 kg N ha^?1 yr^?1, but the total N required to build new tissue during a growing season is about 30 kg N ha^?1 yr^?1, so that about 50% of the nitrogen in the vegetation is recycled internally. Nitrogen lossesvia fire are probably balanced by biological N₂-fixation, but no data are available for N-fixation in these savannas. The calculations presented in this paper are based on few data and more measurements are needed to develop a conclusive picture of the N-balance of Trachypogon grasslands.     

Regional gains and losses of nitrogen in the Amazon basin

In order to better understand the relative importance of different ecosystems and nitrogen cycling processes within the Amazon basin to the nitrogen economy of this region, we constructed a generalized nitrogen budget for the region based on data for hydrologic losses of nitrogen and nitrogen fixation in Amazon forests. Data included information available for nitrogen in water entering and leaving both the entire basin and watersheds on oxisol and ultisol soils near Manaus, Brazil, in addition to biological nitrogen fixation in forests on ultisol, oxisol and entisol (‘varzea’) soils in Central Amazonia. Available data indicate that 4–6 kg N ha^?1 yr^?1 are lost via the River Amazonas, and that a similar amount enters in rainfall. Root-associated biological nitrogen fixation contributesca. 2 kg N ha^?1 yr^?1 to forests on oxisols, 20 kg N ha^?1 yr^?1 to forests on utisols, and 200 kg N ha^?1 yr^?1 to forests on fertile varzea soils. There is 5–10 fold more NH₄ ⁺?N than NO₃?N in rain and stream water entering and leaving the waterbasin near Manaus. Calculations based on these data plus certain assumption yield the following regional nitrogen balance estimate: inputs through bulk deposition of 36×10⁸ kg N yr^?1 and through biological nitrogen fixation of 120×10⁸ kg N yr^?1, and outputsvia the River Amazonas of 36×10⁸ kg N yr^?1 andvia denitrification and volatization (by difference) of 120×10⁸ kg N yr^?1.     

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.     

Optimizing Sweet Sorghum Production for Biofuel in the Southeastern USA Through Nitrogen Fertilization and Top Removal

Sustainable bioenergy cropping systems require not only high yields but also efficient use of inputs. Management practices optimizing production of sweet sorghum [Sorghum bicolor (L.) Moench] for bioenergy use are needed. The effects of N rate (45, 90, 135, and 180?kg N?ha^?1) and top removal (at boot stage, anthesis, and none) on biomass, brix, estimated sugar yield, and N and P recovery of sweet sorghum cv. M-81E were investigated in Florida at two sites differing in soil type. Across all data, dry biomass yields averaged 17.7 Mg?ha^?1 and were not affected by N fertilization rate at either site (P?>?0.10). Mean brix values ranged from 131 to 151?mg?g^?1 and were negatively related to N rate. Top removal, either at boot stage or anthesis, resulted in greater brix values and 13% greater sugar yields at both locations. Whole plant N recovery was positively and linearly related to N rate and ranged from 78 to 166?kg N?ha^?1, approximately two thirds of which was in leaf and grain tissues. Based on yield and nutrient recovery responses, optimal nutrient requirements were 90 to 110?kg N?ha^?1 and 15 to 20?kg P?ha^?1. Higher N fertilization led to greater N recovery, but little to modest gain in sugar yield. Thus, proper nutrient and harvest management will be needed to optimize sugar yields of sweet sorghum for production of biofuels and bio-based products. Further research is needed to refine management practices of sweet sorghum for bioenergy production, especially with regard to the use of leaf and grain tissues.     

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.     

Germination, field establishment patterns and nitrogen fixation of indigenous legumes on nutrient-depleted soils

Integrating N₂-fixing indigenous legumes in smallholder farming systems has potential to alleviate some of the major soil fertility constraints associated with lack of nitrogen (N) inputs in many parts of Sub-SaharanAfrica. Studies were conducted under low (450–650 mm yr^?1) and high (>800 mm yr^?1) rainfall areas in Zimbabwe to investigate the establishment and nitrogen fixation patterns of fifteen indigenous legume species. The legume seeds were broadcast in mixtures at 120 seeds m^?2 species^?1 during 2004/05 and 2005/06 rainfall seasons.Eriosema ellipticum, Crotalaria ochroleuca andC. pallida had emergence rates above 15% compared with <10% forTephrosia radicans andIndigofera astragalina. Seed hardness accounted for >50% germination failure, while low viability explained 10–30%.Crotalaria ochroleuca andC. pallida attained a maximum biomass of 5–9 t ha^?1 (dry weight) over six months, while species that reached peak biomass over three months (e.g.C. cylindrostachys andC. glauca) gave lowest yields of ≈0.5 t ha^?1. Biennials,Neonotonia wightii, E. ellipticum and Tephrosia radicans, exhibited slow growth rates and only attained their maximum biomass of ≈2 t ha^?1 in the second season. The legumes derived 60–99% of their N from the atmosphere, fixing 5–120 kg N ha^?1 under low rainfall and 78–267 kg N ha^?1 under high rainfall. These findings suggest that the legumes could contribute in restoring productivity of soils continuously cultivated with little or no nutrient inputs in most of Zimbabwe and similar agro-ecologies in SubSaharan Africa.     

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).     

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.     

Nitrogen Fertilization Does Significantly Increase Yields of Stands of Miscanthus × giganteus and Panicum virgatum in Multiyear Trials in Illinois

The C4 perennial grasses Miscanthus × giganteus and Panicum virgatum (switchgrass) are emerging bioenergy crops. They are attractive because they are productive and recycle nutrients to the overwintering belowground rhizomes, before the dry shoots are harvested. They are therefore expected to require minimal fertilizer inputs, a desirable trait for any crop. Until now, Europe has had the only long-term trials of M. × giganteus, and these have either shown no or a small effect of nitrogen fertilization, while trials of P. virgatum in the USA have shown a clear positive effect of N fertilization. This study exploited the first long-term trials of M. × giganteus in the USA, and first side-by-side comparison with P. virgatum, to test the hypothesis that N fertilization would only improve yields of the latter. A split-plot N fertility treatment (0, 67, 134, and 202 kg(N)?ha^?1) was added to >5-year-old replicated stands of the two crops at seven locations on contrasting soils in the US Midwest. Averaged across all locations, M. × giganteus yields increased significantly from 23.4 Mg ha^?1 with no N fertilization to 28.9 Mg ha^?1 (+25 %) at a N application rate of 202 kg ha^?1. P. virgatum also showed significant yield increases from 10.33 Mg ha^?1 at 0 kg(N)?ha^?1 to 13.6 Mg ha^?1 (+32 %) at 202 kg(N)?ha^?1. Both species therefore responded to N fertilization and to a similar extent. The increase per unit of added N was small compared to crops such as Zea mays and unlikely to be economically worthwhile. Nitrogen fertilization arrested most of the long-term yield decline that would otherwise have occurred in P. virgatum, but eliminated only about 40 % of the decline observed in M. × giganteus, suggesting additional causal factors for long-term yield decline in this crop. While the crops responded to nitrogen addition at some locations, they did not at others. Therefore a one-case-fits-all optimum fertilization rate cannot be prescribed.     

Physiological changes in soybean (Glycine max) Wuyin9 in response to N and P nutrition

Phosphorus deficiency is a very common problem in the acid soil of central China. Previous research has shown that starter N and N topdressing at the flowering stage (Rl) increased soybean (Glycine max) yield and N₂ fixation (Gan et al, 1997, 2000). However, there is little information available concerning soybean response to P‐fertiliser in soybean production in central China (Gan, 1999). A field experiment was conducted to investigate the response to P (0 kg P ha^?1, 22 kg P ha^?1, 44 kg P ha^?1 before sowing) and N fertiliser application (N1: 0 kg N ha^?1, N2: 25 kg N ha^?1 before sowing, N3: N2 + 50 kg N ha^?1 at the V2 stage and N4: N2 + 50 kg N ha^?1 at the R1 stage) on growth, yield and N₂ fixation of soybean. Both N and P fertiliser increased growth and seed yield of soybean (P < 0.01). Application of basal P fertiliser at 22 kg P ha^?1 or 44 kg P ha^?1 increased total N accumulation by 11% and 10% (P < 0.01) and seed yield by 12% and 13% (P < 0.01), respectively, compared to the zero P treatment. Although application of starter N at 25 kg N ha^?1 had no positive effect on seed yield at any P level (P > 0.05), an application of a topdressing of 50 kg N ha^?1 at the V2 or R1 stage increased total N accumulation by 11% and 14% (P < 0.01) and seed yield by 16% and 21% (P < 0.01), respectively, compared to the zero N treatment. Soybean plants were grown on sterilised Perlite in the greenhouse experiment to study the physiological response to different concentrations of phosphate (P1: 0 mM; P2: 0.05 mM; P3: 0.5 mM; P4:1.0 mN) and nitrate (N1: 0 mM with inoculation, N2: 20 mM with inoculation). The result confirmed that N and P nutrients both had positive effects on growth, nodulation and yield (P < 0.01). The relative importance of growth parameters that contributed to the larger biomass with N and P fertilisation was in decreasing order: (i) total leaf area, (ii) individual leaf area, (iii) shoot/root ratio, (iv) leaf area ratio and (v) specific leaf area. The yield increase at N and P supply was mainly associated with more seeds and a larger pod number per plant, which confirmed the result from the field experiment.     

Application of zeolite improves water and nitrogen use efficiency while increasing essential oil yield and quality of Salvia officinalis under water-deficit stress

Soil moisture and nitrogen (N) are two of the most important factors affecting the production of medicinal plants. So, the management strategy of these factors is critical and to be identified. In order to study the application of zeolite (Z) (0 and 10 ton ha^?1) in S. officinalis culture medium under different irrigation regimes (30 % depletion of available soil water (ASW)) and 60 % depletion of ASW) and N (0, 75 and 150 kg N ha^?1) a split-factorial experiment was carried out with three replicates in 2018. The highest fresh and dry weight were achieved at irrigation after 30 % depletion of ASW while using 150 kg N ha^?1 and 10 ton Z ha^?1. Maximum water use efficiency (WUE) (22.10 g.L^-1) was obtained after 60 % depletion of ASW and 150 kg N ha^?1 and 10 ton Z ha^?1. Besides, the maximum nitrogen use efficiency (NUE) was obtained after 60 % depletion of ASW and 75 kg N ha^?1 and 10 ton Z ha^?1 (14.25 kg.kg^-1N). Maximum essential oil (EO) content (1.06%) and cis-Thujone were obtained from plants subjected to 60 % depletion of ASW and, application of 75 kg N ha^?1 and 10 ton Z ha^?1. Applying Z with N, in different irrigation regimes did improve soil conditions for achieving higher, WUE and NUE, increased the EO content and yield while decreasing the negative effects from water-deficit stress and has provided a direction towards a stable system.     

Nitrogen deposition in tropical forests from savanna and deforestation fires

We used satellite‐derived estimates of global fire emissions and a chemical transport model to estimate atmospheric nitrogen (N) fluxes from savanna and deforestation fires in tropical ecosystems. N emissions and reactive N deposition led to a net transport of N equatorward, from savannas and areas undergoing deforestation to tropical forests. Deposition of fire‐emitted N in savannas was only 26% of emissions – indicating a net export from this biome. On average, net N loss from fires (the sum of emissions and deposition) was equivalent to approximately 22% of biological N fixation (BNF) in savannas (4.0 kg N ha^?1 yr^?1) and 38% of BNF in ecosystems at the deforestation frontier (9.3 kg N ha^?1 yr^?1). Net N gains from fires occurred in interior tropical forests at a rate equivalent to 3% of their BNF (0.8 kg N ha^?1 yr^?1). This percentage was highest for African tropical forests in the Congo Basin (15%; 3.4 kg N ha^?1 yr^?1) owing to equatorward transport from frequently burning savannas north and south of the basin. These results provide evidence for cross‐biome atmospheric fluxes of N that may help to sustain productivity in some tropical forest ecosystems on millennial timescales. Anthropogenic fires associated with slash and burn agriculture and deforestation in the southern part of the Amazon Basin and across Southeast Asia have substantially increased N deposition in these regions in recent decades and may contribute to increased rates of carbon accumulation in secondary forests and other N‐limited ecosystems.     

Effects of take-all (Gaeumannomyces graminis var. tritici) on crop N uptake and residual mineral N in soil at harvest of winter wheat

Background and aims

Take-all, caused by the fungus Gaeumannomyces graminis var. tritici, is the most damaging root disease of wheat. A severe attack often leads to premature ripening and death of the plant resulting in a reduction in grain yield and effects on grain quality (Gutteridge et al. in Pest Manag Sci 59:215–224, 2003). Premature death of the plant could also lead to inefficient use of applied nitrogen (Macdonald et al. in J Agric Sci 129(2):125–154, 1997). The aim of this study was to determine crop N uptake and the amount of residual mineral N in the soil after harvest where different severities of take-all had occurred.

Methods

Plant and soil samples were taken at anthesis and final harvest from areas showing good and poor growth (later confirmed to be caused by take-all disease) in three winter wheat crops grown on the same soil type on Rothamsted Farm in SE England in 1995, 2007 and 2008 (harvest sampling only). All crops received fertiliser N in spring at recomended rates (190–200?kg?N ha^?1). On each ocassion crops were assessed for severity of take-all infection (TAR) and crop N uptakes and soil nitrate plus ammonium (SMN) was determined. Grain yields were also measured.

Results

Grain yields (at 85% dry matter) of crops with moderate infection (good crops) ranged from 4.3 to 13.0?t ha^?1, compared with only 0.9–4.5?t ha^?1 for those with severe infection (poor crops). There were significant (P?<?0.05) negative relationships between crop N uptake and TAR at anthesis and final harvest. At harvest, good crops contained 129–245?kg?N ha^?1 in grain, straw and stubble, of which 85–200?kg?N ha^?1 was in the grain. In contrast, poor crops contained only 46–121?kg?N ha^?1, of which only 22–87?kg?N ha^?1 was in the grain. Positive relationships between SMN and TAR were found at anthesis and final harvest. The SMN in the 0–50?cm layer following harvest of poor crops was significantly (P?<?0.05) greater than that under good crops, and most (73–93%) was present as nitrate.

Conclusions

Localised patches of severe take-all infection decreased the efficiency with which hexaploid wheat plants recovered soil and fertiliser derived N, and increased the subsequent risk of nitrate leaching. The risk of gaseous N losses to the atmosphere from these areas may also have been enhanced.     

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.     

Nonlinear nitrous oxide (N2O) response to nitrogen fertilizer in on‐farm corn crops of the US Midwest

Row‐crop agriculture is a major source of nitrous oxide (N₂O) globally, and results from recent field experiments suggest that significant decreases in N₂O emissions may be possible by decreasing nitrogen (N) fertilizer inputs without affecting economic return from grain yield. We tested this hypothesis on five commercially farmed fields in Michigan, USA planted with corn in 2007 and 2008. Six rates of N fertilizer (0–225 kg N ha^?1) were broadcast and incorporated before planting, as per local practice. Across all sites and years, increases in N₂O flux were best described by a nonlinear, exponentially increasing response to increasing N rate. N₂O emission factors per unit of N applied ranged from 0.6% to 1.5% and increased with increasing N application across all sites and years, especially at N rates above those required for maximum crop yield. At the two N fertilizer rates above those recommended for maximum economic return (135 kg N ha^?1), average N₂O fluxes were 43% (18 g N₂O–N ha^?1 day^?1) and 115% (26 g N₂O–N ha^?1 day^?1) higher than were fluxes at the recommended rate, respectively. The maximum return to nitrogen rate of 154 kg N ha^?1 yielded an average 8.3 Mg grain ha^?1. Our study shows the potential to lower agricultural N₂O fluxes within a range of N fertilization that does not affect economic return from grain yield.     

Biomass Yield and Nutrient Responses of Switchgrass to Phosphorus Application

Increasing desire for renewable energy sources has increased research on biomass energy crops in marginal areas with low potential for food and fiber crop production. In this study, experiments were established on low phosphorus (P) soils in southern Oklahoma, USA to determine switchgrass biomass yield, nutrient concentrations, and nutrient removal responses to P and nitrogen (N) fertilizer application. Four P rates (0, 15, 30, and 45?kg?P?ha^?1) and two N fertilizer rates (0 and 135?kg?N?ha^?1) were evaluated at two locations (Ardmore and Waurika) for 3?years. While P fertilization had no effect on yield at Ardmore, application of 45?kg?P?ha^?1 increased yield at Waurika by 17% from 10.5 to 12.3?Mg?ha^?1. Across P fertilizer rates, N fertilizer application increased yields every year at both locations. In Ardmore, non-N-fertilized switchgrass produced 3.9, 6.7, and 8.8?Mg?ha^?1, and N-fertilized produced 6.6, 15.7, and 16.6?Mg?ha^?1 in 2008, 2009, and 2010, respectively. At Waurika, corresponding yields were 7.9, 8.4, and 12.2?Mg?ha^?1 and 10.0, 12.1, and 15.9?Mg?ha^?1. Applying 45?kg?P?ha^?1 increased biomass N, and P concentration and N, P, potassium, and magnesium removal at both locations. Increased removal of nutrients with N fertilization was due to both increased biomass and biomass nutrient concentrations. In soils of generally low fertility and low plant available P, application of P fertilizer at 45?kg?P?ha^?1 was beneficial for increasing biomass yields. Addition of N fertilizer improves stand establishment and biomass production on low P sites.     

Effect of nitrogen and phosphorus fertilization on the composition of rhizobacterial communities of two Chilean Andisol pastures

The effect of nitrogen (N) and phosphorus (P) fertilization on composition of rhizobacterial communities of volcanic soils (Andisols) from southern Chile at molecular level is poorly understood. This paper investigates the composition of rhizobacterial communities of two Andisols under pasture after 1- and 6-year applications of N (urea) and P (triple superphosphate). Soil samples were collected from two previously established sites and the composition of rhizobacterial communities was determined by denaturing gradient gel electrophoresis (PCR–DGGE). The difference in the composition and diversity between rhizobacterial communities was assessed by nonmetric multidimensional scaling (MDS) analysis and the Shannon–Wiener index. In Site 1 (fertilized for 1 year), PCR–DGGE targeting 16S rRNA genes and MDS analysis showed that moderate N application (270 kg N ha^?1 year^?1) without P significantly changed the composition of rhizobacterial communities. However, no significant community changes were observed with P (240 kg P ha^?1 year^?1) and N–P application (270 kg N ha^?1 year^?1 plus 240 kg P ha^?1 year^?1). In Site 2 (fertilized for 6 years with P; 400 kg P ha^?1 year^?1), PCR–DGGE targeting rpoB, nifH, amoA and alkaline phosphatase genes and MDS analysis showed changes in rhizobacterial communities only at the highest rate of N application (600 kg N ha^?1 year^?1). Quantitative PCR targeting 16S rRNA genes also showed higher abundance of bacteria at higher N application. In samples from both sites, the Shannon–Wiener index did not show significant difference in the diversity of rhizobacterial communities. The changes observed in rhizobacterial communities coincide in N fertilized pastures with lower soil pH and higher pasture yields. This study indicates that N–P application affects the soil bacterial populations at molecular level and needs to be considered when developing fertilizer practices for Chilean pastoral Andisols.     

Nutrient constraints to tropical agroecosystem productivity in long‐term degrading soils

Soil degradation is one of the most serious threats to sustainable crop production in many tropical agroecosystems where extensification rather than intensification of agriculture has occurred. In the highlands of western Kenya, we investigated soil nitrogen (N) and phosphorus (P) constraints to maize productivity across a cultivation chronosequence in which land‐use history ranged from recent conversion from primary forest to 100 years in continuous cropping. Nutrient treatments included a range of N and P fertilizer rates applied separately and in combination. Maize productivity without fertilizer was used as a proxy measure for indigenous soil fertility (ISF). Soil pools of mineral nitrogen, strongly bound P and plant‐available P decreased by 82%, 31% and 36%, and P adsorption capacity increased by 51% after 100 years of continuous cultivation. For the long rainy season (LR), grain yield without fertilizer declined rapidly as cultivation age increased from 0 to 25 years and then gradually declined to a yield of 1.6 Mg ha^?1, which was maintained as time under cultivation increased from 60 to 100 years. LR grain yield in the old conversions was only 24% of the average young conversion grain yield (6.4 Mg ha^?1). Application of either N or P alone significantly increased grain yield in both the LR and short rainy (SR) seasons, but only application of 120 kg N ha^?1 on the old conversion increased yield by >1 Mg ha^?1. In both SR and LR, there was a greater average yield increment response to N and P when applied together (ranging from 1 to 3.8 Mg ha^?1 for the LR), with the greatest responses on the old conversions. The benefit–cost ratio (BCR) for applying 120 kg N ha^?1 alone was <1 except on the old conversions, while BCRs were>1 for applying 25 kg P ha^?1 alone at all levels of conversion for both seasons. Application of both N (120 kg N ha^?1) and P (25 kg P ha^?1) on the old conversions resulted in the greatest BCRs. This study clearly indicates that maize productivity responses to N and P fertilizer are significantly affected by the age of cultivation and its influence on ISF, but that loss of productivity can be restored rapidly when these limiting nutrients are applied. Management strategies should consider ISF and economic factors to determine optimal N and P input requirements for achieving and sustaining profitable crop production on degraded soils.     

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.