Nitrogen fluxes during the initial stage of willows and poplars in short‐rotation coppices

Among energy crops, short‐rotation coppices (SRC) are recommended to provide renewable energy. Compared to annual crops, willows and poplars are regarded as plants with low requirements for nutrients, herbicides, pesticides, and soil maintenance. However, only little is known about N‐fertilizer effects on SRC and the few studies are even inconsistent. Therefore, we studied the effects of N on yields of willows and poplars in a field experiment. The effects of N fertilization on nitrate leaching and nitrous oxide emissions from the loamy‐sand soil were also measured. Cuttings of willows (Salix viminalis clone Inger) and poplars (Populus maximovizcii × P. nigra clone max 4) were planted on farmland in 2008. The experiment was arranged in a random block design with three levels of N fertilizer (0, 50, and 75 kg N ha^–1 y^–1). After 2 y, the trees were harvested for the first time. Fertilization did not affect the yields of willows or poplars. However, the application of 75 kg N ha^–1 y^–1 caused an average increase of N leaching in the willow and poplar plots of 25 kg N ha^–1 y^–1 and 40 kg N ha^–1 y^–1, respectively. Emissions of N₂O were increased by a maximum of only 0.2 kg N ha^–1 y^–1. Further, the N fertilizer stimulated the growth of the weed biomass in case of the willow plots by 46% and of the weed N content by 52% (r = 0.53). In conclusion, in the first 2 y, SRC could be produced in a more effective and environmentally friendly manner without mineral fertilizer.     

Residue incorporation and N fertilization affect the response of soil nematodes to the elevated CO2 in a Chinese wheat field

The interplay between the carbon and other nutrient cycles is the key to understand the responses of soil ecosystems to climatic change. Using the free-air CO₂ enrichment (FACE) techniques, we carried out a multifactorial experiment in a Chinese rice-wheat rotation system, to investigate the response of soil nematodes to elevated CO₂ under different application rates of N fertilizer (225.0 kg N ha⁻¹ (HN) and 112.5 kg N ha⁻¹(LN), respectively) and residue incorporation (0 kg C ha⁻¹ (ZR), 1000 kg C ha⁻¹ (MR) and 2000 kg C ha⁻¹ (HR), respectively). This study was conducted during the wheat growing season of 2007 after expose to the elevated CO₂ for three years. The results in our study indicated that seasonality is an important factor in determining changes in the nematode abundance and diversity. The residue addition effects were more obvious than the elevated CO₂, which significantly influenced the abundance of total nematodes and plant-parasites, and some ecological indices. The interactions between residue addition and CO₂ significantly influenced nematode dominance and structure indices. High level of N fertilization was found to decrease the nematode diversity, generic richness and maturity indices at wheat jointing stage. There are significant interactions between N fertilization and elevated CO₂ for abundance of total nematodes and different trophic groups.     

Impacts of 22-year organic and inorganic N managements on soil organic C fractions in a maize field,northeast China

Impacts of 22-year organic and inorganic N managements on total organic carbon (TOC), water-soluble organic C (WSOC), microbial biomass C (MBC), particulate organic C (POC) and KMnO₄ oxidized organic C (KMnO₄-C) concentrations, C management index (CMI), and C storage in surface soil (0–20 cm) were investigated in a maize (Zea may L.) field experiment, Northeast China. The treatments included, CK: unfertilized control, M: organic manure (135 kg N ha^− 1 year^− 1), N: inorganic N fertilizer (135 kg N ha^− 1 year^− 1) and MN: combination of organic manure (67.5 kg N ha^− 1 year^− 1) and inorganic N fertilizer (67.5 kg N ha^− 1 year^− 1). TOC concentration and C storage were significantly increased under the M and MN treatments, but not under the inorganic N treatment. The organic treatments of M and MN were more effective in increasing WSOC, MBC, POC and KMnO₄-C concentrations and CMI than the N treatment. The M treatment was most effective for sequestrating SOC (10.6 Mg ha^− 1) and showed similar increase in degree of grain yield to the N and MN treatments, therefore it could be the best option for improving soil productivity and C storage in the maize cropping system.     

Effects of two contrasting agricultural land‐use practices on nitrogen leaching in a sandy soil of Middle Germany

The objective of this study is to evaluate different agricultural land‐use practices in terms of N leaching and to give recommendations for a sustainable agriculture on sandy soils in Middle Germany. Soil mineral N (N_min) and leachate N were quantified at a sandy soil in N Saxony during 3 years. Two treatments were applied: intensive (I)—using inorganic and organic fertilizer and pesticides, and organic (O)—exclusively using organic fertilizer, legume‐based crop rotation, and no pesticides. Split application of mineral fertilizers did not result in substantial N losses at treatment I. Legumes induced a considerable increase of soil mineral N and particularly of leachate mineral N (N_{min_perc}) at treatment O. High N_{min_perc} concentrations (up to 78 mg N L^–1) were observed during as well as after the cultivation of legumes. These high N_{min_perc} concentrations are the reason why clearly higher N_{min_perc} losses were determined at treatment O (62 kg N ha^–1 y^–1) compared to treatment I (23 kg N ha^–1 y^–1). At both treatments, the quantity of N losses was strongly affected by the precipitation rates. Concentrations and losses of dissolved organic N (DON_perc) were assessed as above average at both treatments. The results suggest that the DON_perc concentration is influenced by precipitation, soil coverage, and organic fertilizers. Higher values were determined in the percolation water of treatment O. The average annual DON_perc losses amounted to 15 kg N ha^–1 at I and to 32 kg N ha^–1 at O. The average monthly percentage of DON_perc losses on the loss of the dissolved total N of percolation water (DTN_perc) ranged between <1% and 55% at O and between 2% and 56% at I. For the whole measuring period of 29 months, the relative amounts of DON_perc of DTN_perc (21% at O and 25% at I) were more or less the same for both treatments. The results show that DON_perc can contribute significantly to the total N loss, confirming the importance to consider this N fraction in N‐leaching studies. It was concluded that at sandy sites, a split application of mineral fertilizers, as applied at treatment I, seems to be more expedient for limiting the N leaching losses than legume‐based crop rotations.     

Transformations of mineral nitrogen applied to peat soil during sequential oxic/anoxic cycling

Shifts in oxic and anoxic conditions in soil are most frequently caused by water table fluctuations, heavy rain, snowmelt or flooding, with potentially significant impacts on microbial processes and the ability of soils to convert mineral nitrogen to nitrogen gases efficiently. The impact of oxic/anoxic cycles on nitrogen transformation rates was therefore explored in the upper layer (0-30 cm) of partially degraded peat soil. We hypothesized that high denitrification potential would be conserved due to the high organic matter content of this soil. Mineral nitrogen was applied to approximately 1-cm deep layers of homogenized soil in microcosms, with no external source of readily degradable carbon. Microcosms were subjected to three cycles, each consisting of an oxic phase of 8-11 days and an anoxic phase of 21-28 days. Approximately 2% of the ammonium load was lost through ammonia volatilization during oxic phases and the remainder was nitrified. The accumulated nitrate decreased soil pH from 8.0 to 6.8 before its transformation through denitrification. Nitrification and denitrification rates during the three oxic/anoxic cycles (approximately three months) were 2.9-3.2 kg N ha⁻¹ d⁻¹ and 1.0-2.3 kg N ha⁻¹ d⁻¹, respectively. Extrapolation of these values to 30-cm deep soil layers gave rates that were sufficient for complete transformation of at least 1700 kg N ha⁻¹ of ammonium to nitrogen gases, which is ten-fold greater than the annual nitrogen application of 170 kg N ha⁻¹ permitted by the European directive. Denitrification rates decreased linearly during the three cycles (from 36 ± 2 to 16 ± 1 μg N g⁻¹ d⁻¹ dry soil), projecting cessation of denitrification activity and CO₂ production during the fifth cycle. Storage of peat soil at 4 °C most probably allowed slow degradation of organic matter that was completely oxidized to CO₂ after the soil was exposed to higher temperature (28 °C). Storage of soil for one year did not affect nitrification rate, but reduced denitrification rate, unless soil was amended with a readily degradable carbon source. The data suggest that, despite the high carbon content of this soil, it cannot sustain transformations of high N loads to nitrogen gases for prolonged periods without amendment with readily available carbon.     

Priming effect of biuret addition on native soil N mineralisation under laboratory conditions

This study was carried out to quantify the priming effect of biuret on native soil nitrogen (N) mineralisation during a 112-day incubation. Addition of biuret (100 mg ¹⁵N-labelled biuret kg⁻¹ soil) increased the turnover rate constant of soil organic matter and had a positive priming effect on native soil N mineralisation in two soils. The additional mineralisation was 0.65% of the total soil N (equivalent to 47.1 kg N ha⁻¹) in a sandy loam soil and 0.62% of the soil N (equivalent to 46.5 kg N ha⁻¹) in a silt loam soil.     

Tillage and cropping sequence impacts on nitrogen cycling in dryland farming in eastern Montana, USA

Information on N cycling in dryland crops and soils as influenced by long-term tillage and cropping sequence is needed to quantify soil N sequestration, mineralization, and N balance to reduce N fertilization rate and N losses through soil processes. The 21-yr effects of the combinations of tillage and cropping sequences was evaluated on dryland crop grain and biomass (stems + leaves) N, soil surface residue N, soil N fractions, and N balance at the 0–20 cm depth in Dooley sandy loam (fine-loamy, mixed, frigid, Typic Argiboroll) in eastern Montana, USA. Treatments were no-tilled continuous spring wheat (Triticum aestivum L.) (NTCW), spring-tilled continuous spring wheat (STCW), fall- and spring-tilled continuous spring wheat (FSTCW), fall- and spring-tilled spring wheat–barley (Hordeum vulgare L.) (1984–1999) followed by spring wheat–pea (Pisum sativum L.) (2000–2004) (FSTW-B/P), and spring-tilled spring wheat–fallow (STW-F). Nitrogen fractions were soil total N (STN), particulate organic N (PON), microbial biomass N (MBN), potential N mineralization (PNM), NH₄-N, and NO₃-N. Annualized crop grain and biomass N varied with treatments and years and mean grain and biomass N from 1984 to 2004 were 14.3–21.2 kg N ha⁻¹ greater in NTCW, STCW, FSTCW, and FSTW-B/P than in STW-F. Soil surface residue N was 9.1–15.2 kg N ha⁻¹ greater in other treatments than in STW-F in 2004. The STN at 0–20 cm was 0.39–0.96 Mg N ha⁻¹, PON 0.10–0.30 Mg N ha⁻¹, and PNM 4.6–9.4 kg N ha⁻¹ greater in other treatments than in STW-F. At 0–5 cm, STN, PON, and MBN were greater in STCW than in FSTW-B/P and STW-F. At 5–20 cm, STN and PON were greater in NTCW and STCW than in STW-F, PNM and MBN were greater in STCW than in NTCW and STW-F, and NO₃-N was greater in FSTW-B/P than in NTCW and FSTCW. Estimated N loss through leaching, volatilization, or denitrification at 0–20 cm depth increased with increasing tillage frequency or greater with fallow than with continuous cropping and ranged from 9 kg N ha⁻¹ yr⁻¹ in NTCW to 46 kg N ha⁻¹ yr⁻¹ in STW-F. Long-term no-till or spring till with continuous cropping increased dryland crop grain and biomass N, soil surface residue N, N storage, and potential N mineralization, and reduced N loss compared with the conventional system, such as STW-F, at the surface 20 cm layer. Greater tillage frequency, followed by pea inclusion in the last 5 out of 21 yr in FSTW-B/P, however, increased N availability at the subsurface layer in 2004.     

Effect of organic and inorganic nutrient sources on soil mineral nitrogen and maize yields in central highlands of Kenya

High population pressure in the central highlands of Kenya has led to continuous cultivation of land with minimal additional inputs leading to soil nutrient depletion. Research work has reported positive results from use of manure and biomass from Tithonia, Calliandra, Leucaena, Mucuna and Crotolaria for soil fertility replenishment. An experimental field was set up in Chuka Division to test different soil nutrient replenishment treatments. The experimental design was randomised complete block with 14 treatments replicated three times. At the beginning and end of the experiment, soil was sampled at 0–15 cm depth and analysed for pH, Ca, Mg, K, C, N and P. End of the 2000/2001 short rains (SR) season and 2001 long rains (LR) season, soil samples were taken at 0–30, 30–100 and 100–150 cm for nitrate and ammonium analysis. All the treatments received an equivalent of 60 kg N ha⁻¹, except herbaceous legume treatments, where N was determined by the amount of the biomass harvested and incorporated in soil and control treatment received no inputs. Results indicate soil fertility increased slightly in all treatments (except control) over the 2-year study period. Average maize grain yield across the treatments was 1.1, 5.4, 3.5 and 4.0 Mg ha⁻¹ during the 2000 LR, 2000/2001 SR, 2001 LR and 2001/2002 SR, respectively. The reduced yield in 2000 LR and 2001 LR are attributed to poor rainfall distribution during the two seasons. On average, Tithonia with half recommended rate of inorganic fertilizer recorded the highest (4.8 Mg ha⁻¹) maize yield followed by sole Tithonia (4.7 Mg ha⁻¹). Highest average concentration (144.8 and 115.5 kg N ha⁻¹) of mineral N was recorded at the 30–100 cm soil depth at the end of both 2000/2001 SR and LR, respectively. The lowest average concentration (67.1 kg N ha⁻¹) was recorded in the 100–150 cm soil depth in both seasons, while during the 2001 LR, the 0–30 cm soil depth recorded the lowest concentration (52.3 kg N ha⁻¹). The residual mineral N in the 100–150 cm soil depth doubled at the end of the LR 2001 compared to what was present and the end of the SR 2000/2001 season in all treatments. This shows that there is substantial amount of mineral N that is being leached below the rooting zone of maize in this region.     

Impacts of nitrogen fertilization on biomass production of switchgrass (Panicum Virgatum L.) and changes in soil organic carbon in Ohio

Growing switchgrass (Panicum virgatum, L.), a promising bioenergy crop, needs finely-tuned nitrogen (N) fertilization to improve biomass yields depending on soil types and site characteristics. N fertilization can also affect the soil organic carbon (SOC) pool. Therefore, this study was conducted to assess the effects of N fertilization on switchgrass biomass production and the SOC stock in Ohio. Switchgrass was established at three research stations (Northwest, Jackson, and Western sites) of the Ohio Agricultural Research and Development Center (OARDC) in spring 2004. N fertilizer was applied at four different rates (0, 50, 100, and 200 kg N ha⁻¹) in 2008 and 2009. Aboveground and root biomass and the carbon (C) and N concentrations in plant tissues, SOC concentrations up to 30 cm depth were measured at the end of the growing season in 2009. Aboveground biomass at the Western site was the highest as 26 Mg ha⁻¹ with 200 kg N ha⁻¹ application, but there were no significant effects of N fertilization on aboveground biomass at two other sites and on root biomass across all sites. The amount of N export due to harvesting aboveground biomass increased with increase in N rates but did not vary among sites. With increasing N rates, the SOC stock linearly increased from 102 to 123 and from 55 to 70 Mg C ha⁻¹ at the Northwest and the Jackson sites, respectively. However, this positive correlation was not observed for the Western site (a range of 59 to 67 Mg C ha⁻¹). This study showed a potential of growing switchgrass as a bioenergy crop in Ohio and positive responses of the SOC stock to N fertilization.     

Sources and rates of nitrous oxide emissions from grazed grassland after application of N-labelled mineral fertilizer and slurry

Nitrogen (N) fertilizer application and grazing are known to induce nitrous oxide (N₂O) emissions from grassland soils. In a field study, general information on rates of N₂O emission, the effect of cattle grazing and the type (mineral fertilizer, cattle slurry) and amount of N supply on the flux of N₂O from a sandy soil were investigated. N₂O emissions from permanent grassland managed as a mixed system (two cuts followed by two grazing cycles) were monitored over 11 months during 2001-2002 in northern Germany using the closed chamber method. The field experiment consisted of four regionally relevant fertilizer combinations, i.e. two mineral N application rates (0 and 100 kg N ha⁻¹ yr⁻¹) and two slurry levels (0 and 74 kg N ha⁻¹ yr⁻¹).Mean cumulative N₂O-N loss was 3.0 kg ha⁻¹ yr⁻¹, and the cumulative ¹⁵N-labelled N₂O emissions varied from 0.03% to 0.19% of the ¹⁵N applied. ¹⁵N labelling indicated that more N₂O was emitted from mineral N than from slurry treated plots, and in all treatments the soil N pool was always clearly the major source of N₂O. Regarding the total cumulative N₂O losses, differences among treatments were not significant, which was caused by: (i) a high variance in emissions during and after cattle grazing due to the random distribution of excrements and by (ii) high N₂ fixation of white clover in the 0 kg N ha⁻¹ treatments, which resulted in similar N status of all treatments. However before grazing started, treatments showed significant differences. After cattle grazing in summer, N₂O emission rates were higher than around the time of spring fertilizer application, or in winter. Grazing resulted in N₂O flux rates up to 489 μg N₂O-N m⁻² h⁻¹ and the grazing period contributed 31-57% to the cumulative N₂O emission. During freeze-thaw cycles in winter (December-February) N₂O emission rates of up to 147 μg N₂O-N m⁻² h⁻¹ were measured, which contributed up to 26% to the annual N₂O flux. The results suggest that N fertilizer application and grazing caused only short-term increases of N₂O flux rates whereas the major share of annual N₂O emission emitted from the soil N pool. The significantly increased N₂O fluxes during freeze-thaw cycles show the importance of emission events in winter which need to be covered by measurements for obtaining reliable estimates of annual N₂O emissions.     

Simulation of nitrogen rhizodeposition and assimilation back into corn (Zea mays L.) roots

The objectives were to quantify the dynamics of N exchange between corn root and soil under field conditions and to compute the amount of N-rhizodeposition that is recycled back into corn by maturity. In a previous publication the simulation model NCSWAP/NCSOIL was used to quantify the release of organic-C from corn and its incorporation into soil organic matter. In this publication, results from the same simulations but pertinent to N are presented. The model was calibrated against measured N concentration in corn and soil and ¹⁵N enrichment data obtained from a long-term field experiment located near St Paul, MN. Field management included rototillage, the removal of stover-residue and grain and 4 fertilizer N treatments: 200 and 20 Kg N ha⁻¹ added yearly from 1980 to 1992, with ¹⁵N added with the fertilizer from 1980 until either 1985 or 1992. The same total amount of ¹⁵N was added with the 200 and 20 Kg N ha⁻¹ fertilizer: 4 and 40 ¹⁵N at.%, respectively. Corn roots released 24% of total N uptake. This loss was mitigated by the recycling of 14% of N-rhizodeposition into corn by maturity. ¹⁵N enrichment in corn and soil was higher for the 200 than the 20 Kg N ha⁻¹ treatment. This resulted from the rapid N mineralization-immobilization turnover that channelled N through the inorganic N pool whose ¹⁵N enrichment was fixed yearly to that of the fertilizer. Tracer N enrichment decreased more rapidly in corn than in soil from 1986 to 1992 when tracer N was no longer added with the fertilizer, and by 1992, ¹⁵N was localized in the stable pool and flushed from the more labile pools.     

Natural N abundance of plants and soils under different management practices in a montane grassland

The natural ¹⁵N abundance (δ¹⁵N) of different ecosystem compartments is considered to be an integrator of nitrogen (N) cycle processes. Here we investigate the extent to which patterns of δ¹⁵N in grassland plants and soils reflect the effect of different management practices on N cycling processes and N balance. Investigations were conducted in long-term experimental plots of permanent montane meadows with treatments differing in the amount and type of applied fertilizer (0-200 kg N ha⁻¹ yr⁻¹; mineral fertilizer, cattle slurry, stable manure) and/or the cutting frequency (1-6 cuts per season). The higher δ¹⁵N values of organic fertilizers compared to mineral fertilizer were reflected by higher δ¹⁵N values in soils and harvested plant material. Furthermore, δ¹⁵N of top soils and plant material increased with the amount of applied fertilizer N. N balances were calculated from N input (fertilization, atmospheric N deposition and symbiotic N₂ fixation) and N output in harvest. ‘Excess N’—the fraction of N input not harvested—was assumed to be lost to the environment or accumulated in soil. Taking fertilizer type into account, strong positive correlations between δ¹⁵N of top soils and the N input-output balance were found. In plots receiving mineral N fertilizer this indicates that soil processes which discriminate against ¹⁵N (e.g. nitrification, denitrification, ammonia volatilization) were stimulated by the increased supply of readily available N, leading to loss of the ¹⁵N depleted compounds and subsequent ¹⁵N enrichment of the soils. By contrast, in plots with organic fertilization this correlation was partly due to accumulation of ¹⁵N-enriched fertilizer N in top soils and partly due to the occurrence of significant N losses. Cutting frequency appeared to have no direct effect on δ¹⁵N patterns. This study for the first time shows that the natural abundance of ¹⁵N of agricultural systems does not only reflect the type (organic or mineral fertilizer) or amount of annual fertilizer amendment (0-200 kg ha⁻¹ yr⁻¹) but that plant and soil δ¹⁵N is better described by N input-output balances.     

Nitrate and Phosphate Leaching under Turfgrass Fertilized with a Squid-based Organic Fertilizer

Consumer demand for cleaned squid generates a substantial amount of waste that must be properly disposed of, creating an economic burden on processors. A potential solution to this problem involves converting squid by-products into an organic fertilizer, for which there is growing demand. Because fertilizer application to lawns can increase the risk of nutrient contamination of groundwater, we quantified leaching of NO₃–N and PO₄–P from perennial ryegrass turf (Lolium perenne L.) amended with two types of fertilizer: squid-based (SQ) and synthetic (SY). Field plots were established on an Enfield silt loam, and liquid (L) and granular (G) fertilizer formulations of squid and synthetic fertilizers were applied at 0, 48, 146, and 292 kg N ha⁻¹ year⁻¹. Levels of NO₃–N and PO₄–P in soil pore water from a depth of 60 cm were determined periodically during the growing season in 2008 and 2009. Pore water NO₃–N levels were not significantly different among fertilizer type or formulation within an application rate throughout the course of the study. The concentration of NO₃–N remained below the maximum contaminant level (MCL) of 10 mg L⁻¹ until midSeptember 2009, when values above the MCL were observed for SQG at all application rates, and for SYL at the high application rate. Annual mass losses of NO₃–N were below the estimated inputs (10 kg N ha⁻¹ year⁻¹) from atmospheric deposition except for the SQG and SYL treatments applied at 292 kg N ha⁻¹ year⁻¹, which had losses of 13.2 and 14.9 kg N ha⁻¹ year⁻¹, respectively. Pore water PO₄–P levels ranged from 0 to 1.5 mg P L⁻¹ and were not significantly different among fertilizer type or formulation within an application rate. Our results indicate that N and P losses from turf amended with squid-based fertilizer do not differ from those amended with synthetic fertilizers or unfertilized turf. Although organic in nature, squid-based fertilizer does not appear to be more—or less—environmentally benign than synthetic fertilizers.     

Influence of crop residue management, cropping system and N fertilizer on soil N and C dynamics and sustainable wheat (Triticum aestivum L.) production

Management of N is the key for sustainable and profitable wheat production in a low N soil. We report results of irrigated crop rotation experiment, conducted in the North West Frontier Province (NWFP), Pakistan, during 1999–2002 to evaluate effects of residue retention, fertilizer N application and mung bean (Vigna radiata) on crop and N yields of wheat and soil organic fertility in a mung bean–wheat sequence. Treatments were (a) crop residue retained (+residue) or (b) removed (−residue), (c) 120 kg N ha⁻¹ applied to wheat, (d) 160 kg N ha⁻¹ to maize or (e) no nitrogen applied. The cropping system was rotation of wheat with maize or wheat with mung bean. The experiment was laid out in a spit plot design. Postharvest incorporation of crop residues significantly (p < 0.05) increased the grain and straw yields of wheat during both years. On average, crop residues incorporation increased the wheat grain yield by 1.31 times and straw yield by 1.39 times. The wheat crop also responded strongly to the previous legume (mung bean) in terms of enhanced grain yield by 2.09 times and straw yield by 2.16 times over the previous cereal (maize) treatment. Application of fertilizer N to previous maize exerted strong carry over effect on grain (1.32 times) and straw yield (1.38 times) of the following wheat. Application of N fertilizer to current wheat produced on average 1.59 times more grain and 1.77 times more straw yield over the 0 N kg ha⁻¹ treatment. The N uptake in wheat grain and straw was increased 1.31 and 1.64 times by residues treatment, 2.08 and 2.49 times by mung bean and 1.71 and 1.86 times by fertilizer N applied to wheat, respectively. The soil mineral N was increased 1.23 times by residues, 1.34 times by mung bean and 2.49 times by the application of fertilizer N to wheat. Similarly, the soil organic C was increased 1.04-fold by residues, 1.08 times by mung bean and 1.00 times by the application of fertilizer N. We concluded that retention of residues, application of fertilizer N and involvement of legumes in crop rotation greatly improves the N economy of the cropping system and enhances crop productivity in low N soils.     

Mechanisms of soil N dynamics following long-term application of organic fertilizers to subtropical rain-fed purple soil in China

In this study, a ¹⁵N tracing incubation experiment and an in situ monitoring study were combined to investigate the effects of different N fertilizer regimes on the mechanisms of soil N dynamics from a long-term repeated N application experiment. The field study was initiated in 2003 under a wheat-maize rotation system in the subtropical rain-fed purple soil region of China. The experiment included six fertilization treatments applied on an equivalent N basis (280 kg N ha⁻¹), except for the residue only treatment which received 112 kg N ha⁻¹: (1) UC, unfertilized control; (2) NPK, mineral fertilizer NPK; (3) OM, pig manure; (4) OM-NPK, pig manure (40% of applied N) with mineral NPK (60% of applied N); (5) RSD, crop straw; (6) RSD-NPK, crop straw (40% of applied N) with mineral NPK (60% of applied N). The results showed that long-term repeated applications of mineral or organic N fertilizer significantly stimulated soil gross N mineralization rates, which was associated with enhanced soil C and N contents following the application of N fertilizer. The crop N offtake and yield were positively correlated with gross mineralization. Gross autotrophic nitrification rates were enhanced by approximately 2.5-fold in the NPK, OM, OM-NPK, and RSD-NPK treatments, and to a lesser extent by RSD application, compared to the UC. A significant positive relationship between gross nitrification rates and cumulative N loss via interflow and runoff indicated that the mechanisms responsible for increasing N loss following long-term applications of N fertilizer were governed by the nitrification dynamics. Organic fertilizers stimulated gross ammonium (NH₄⁺) immobilization rates and caused a strong competition with nitrifiers for NH₄⁺, thus preventing a build-up of nitrate (NO₃⁻). Overall, in this study, we found that partial or complete substitution of NPK fertilizers with organic fertilizers can reduce N losses and maintain high crop production, except for the treatment involving application of RSD alone. Therefore, based on the N transformation dynamics observed in this study, organic fertilizers in combination with mineral fertilizer applications (i.e. OM, OM-NPK, and RSD-NPK treatments) are recommended for crop production in the subtropical rain-fed purple soils in China.     

Interactive effects from combining fertilizer and organic residue inputs on nitrogen transformations

Concerns about sustainability of agroecosystems management options in developed and developing countries warrant improved understanding of N cycling. The Integrated Soil Fertility Management paradigm recognizes the possible interactive benefits of combining organic residues with mineral fertilizer inputs on agroecosystem functioning. However, these beneficial effects may be controlled by residue quality. This study examines the controls of inputs on N cycling across a gradient of (1) input, (2) residue quality, and (3) texture. We hypothesized that combining organic residue and mineral fertilizers would enhance potential N availability relative to either input alone. Residue and fertilizer inputs labeled with ¹⁵N (40–60 atom% ¹⁵N) were incubated with 200 g soil for 545 d in a microcosm experiment. Input treatments consisted of a no-input control, organic residues (3.65 g C kg⁻¹ soil, equivalent to 4 Mg C ha⁻¹), mineral N fertilizer (100 mg N kg⁻¹ soil, equivalent to 120 kg N ha⁻¹), and a combination of both with either the residue or fertilizer ¹⁵N-labeled. Zea mays stover inputs were added to four differently textured soils (sand, sandy loam, clay loam, and clay). Additionally, inputs of three residue quality classes (class I: Tithonia diversifolia, class II: Calliandra calothyrsus, class III: Z. mays stover) were applied to the clay soil. Available N and N₂O emissions were measured as indicators for potential plant N uptake and N losses. Combining residue and fertilizer inputs resulted in a significant (P < 0.05) negative interactive effect on total extractable mineral N in all soils. This interactive effect decreased the mineral N pool, due to an immobilization of fertilizer-derived N and was observed up to 181 d, but generally became non-significant after 545 d. The initial reduction in mineral N might lead to less N₂O losses. However, a texture effect on N₂O fluxes was observed, with a significant interactive effect of combining residue and fertilizer inputs decreasing N₂O losses in the coarse textured soils, but increasing N₂O losses in the fine textured soils. The interactive effect on mineral N of combining fertilizer with residue changed from negative to positive with increasing residue quality. Our results indicate that combining fertilizer with medium quality residue has the potential to change N transformations through a negative interactive effect on mineral N. We conclude that capitalizing on interactions between fertilizer and organic residues allows for the development of sustainable nutrient management practices.     

Pasture type and fertilization effects on N2 fixation, N budgets and external energy inputs in western Canada

A grazing experiment was conducted in Brandon, Manitoba, Canada. The objectives were to examine the effects of including alfalfa and fertilizer management on N₂ fixation by alfalfa and plant N dynamics, and to compare N budgets in the four contrasting pasture systems and external energy inputs between fertilizer-N-based and legume-based pasture systems. Estimates of annual amounts of N₂ fixed, based on shoot herbage production in grazed mixed alfalfa/grass pastures, ranged from 40 to 118 kg N ha⁻¹ y⁻¹. The amounts would be in the range of 52-153 kg N ha⁻¹ y⁻¹, if the amounts of fixed N stored in the roots, were included. Compared to grass-only pastures, total amounts of N₂ fixed in the mixed pastures should be sufficient to improve total external N inputs, replace N fertilizer and sustain plant protein for grazing. The reliance of alfalfa (Medicago sativa L.) on N₂ fixation for growth was high (70-95%), and %N derived from the atmosphere by alfalfa (%Ndfa) was not affected by P fertilizer management. Thus, the amounts of N₂ fixed were predominantly regulated by alfalfa dry matter productivity. The data also indicated that alfalfa fixed 27 kg N t⁻¹ dry matter produced. In mixed alfalfa/grass pastures, high soil mineral N uptake by companion grasses, was essential to effectively utilize N that was fixed by alfalfa and returned to soils through the decomposition of alfalfa litter and roots. Compared to grass-only pastures with or without N fertilizer, alfalfa-based pastures could supply sufficient plant protein for grazing animals through N₂ fixation, and at same time, sustain animal productivity with only 28% of the external energy input of the grass-only pasture with N fertilizer.     

Effect of urease and nitrification inhibitors on N transformation, gaseous emissions of ammonia and nitrous oxide, pasture yield and N uptake in grazed pasture system

Nitrogen (N) losses via nitrate (NO₃⁻) leaching, ammonia (NH₃) volatilization and nitrous oxide (N₂O) emissions from grazed pastures in New Zealand are one of the major contributors to environmental degradation. The use of N inhibitors (urease and nitrification inhibitors) may have a role in mitigating these N losses. A one-year field experiment was conducted on a permanent dairy-grazed pasture site at Massey University, Palmerston North, New Zealand to quantify these N losses and to assess the effect of N inhibitors in reducing such losses during May 2005-2006. Cow urine at 600 kg N ha⁻¹ rate with or without urease inhibitor N-(n-butyl) thiophosphoric triamide (nBTPT) or (trade name “Agrotain”) (3 L ha⁻¹), nitrification inhibitor dicyandiamide (DCD) (7 kg ha⁻¹) and the use of double inhibitor (DI) containing a combination of both Agrotain and DCD (3:7) were applied to field plots in autumn, spring and summer. Pasture production, NH₃ and N₂O fluxes, soil mineral N concentrations, microbial biomass C and N, and soil pH were measured following the application of treatments during each season. All measured parameters, except soil microbial biomass C and N, were influenced by the added inhibitors during the three seasons. Agrotain reduced NH₃ emissions over urine alone by 29%, 93% and 31% in autumn, spring and summer respectively but had little effect on N₂O emission. DCD reduced N₂O emission over urine alone by 52%, 39% and 16% in autumn, spring and summer respectively but increased NH₃ emission by 56%, 9% and 17% over urine alone during those three seasons. The double inhibitor reduced NH₃ by 14%, 78% and 9% and N₂O emissions by 37%, 67% and 28% over urine alone in autumn, spring and summer respectively. The double inhibitor also increased pasture dry matter by 10%, 11% and 8% and N uptake by the 17%, 28% and 10% over urine alone during autumn, spring and summer respectively. Changes in soil mineral N and pH suggested a delay in urine-N hydrolysis with Agrotain, and reduced nitrification with DCD. The combination of Agrotain and DCD was more effective in reducing both NH₃ and N₂O emissions, improving pasture production, controlling urea hydrolysis and retaining N in NH₄⁺ form. These results suggest that the combination of both urease and nitrification inhibitors may have the most potential to reduce N losses if losses are associated with urine and improve pasture production in intensively grazed systems.     

Phosphorus fertilisation management modifies the biodiversity of AM fungi in a tropical savanna forage system

In the present study we investigated how the community of arbuscular mycorrhizal fungi (AMF) in roots of Centrosema macrocarpum responded to different doses and sources of phosphorus (40 kg ha⁻¹ of P as rock phosphate, 150 kg ha⁻¹ of P as rock phosphate and 75 kg ha⁻¹ of P as diammonium phosphate together with 75 kg ha⁻¹ of P as rock phosphate) in a Venezuelan savanna ecosystem. We also related AMF diversity to soil parameters (total N, total P, available P, extractable K, total Ca, total Mg, total Fe, total Cu, total Zn, total Mn, glomalin-related soil protein, microbial biomass C, dehydrogenase, urease and acid phosphatase activities, water-soluble carbon and carbohydrates and aggregate stability) at different doses of P. The AM fungal small-subunit (SSU) rRNA genes were subjected to PCR, cloning, SSCP, sequencing and phylogenetic analyses. Nine fungal types were identified: six belonged to the genus Glomus and three to Acaulospora. The majority of fungal types showed high similarity to sequences of known glomalean isolates: Aca 1 to Acaulospora mellea, Aca 2 to Acaulospora rugosa, Aca 3 to Acaulospora spinosa, Glo 1 to Glomus intraradices and Glo 3 to Glomus fasciculatum. The control treatment was dominated by species belonging to the genus Acaulospora. However, when the soil was fertilised with low doses of P, the colonisation of roots increased and there was a change in the AMF diversity, the genus Glomus dominating. The AM development and the abundance of AM fungal types in roots were decreased dramatically by the fertilisation with high doses of P, without differences between the sources of P used. The available P in soil was negatively correlated with the AMF diversity. In conclusion, the application of low doses of P as rock phosphate stimulated mycorrhization and enhanced the soil quality parameters except water-soluble carbohydrates, helping to offset a loss of fertility in P-poor tropical savanna soils.     

Denitrification potential of intermittently saturated floodplain soils from a subtropical perennial stream and an ephemeral tributary

Denitrification has the potential to remove excess nitrogen from groundwater passing through riparian buffers, thus improving water quality downstream. In regions with markedly seasonal precipitation, transient stream flow events may be important in saturating adjacent floodplain soils and intermittently providing the anaerobic conditions necessary for denitrification to occur. In two experiments we characterised the denitrification potential of soils from two contrasting floodplains that experience intermittent saturation. We quantified under controlled laboratory conditions: 1) potential rates of denitrification in these soils with depth and over time, for a typical period of saturation; and 2) the influences on rates of nitrate and organic carbon. Treatments differed between experiments, but in each case soil-water slurries were incubated anaerobically with differing amendments of organic carbon and nitrate; denitrification rates were measured at selected time intervals by the acetylene-block technique; and slurry filtrates were analysed for various chemical constituents. In the first experiment (ephemeral tributary), denitrification was evident in soils from both depths (0-0.3 m; 0.3-1.1 m) within hours of saturation. Before Day 2, mean denitrification rates at each depth were generally comparable, irrespective of added substrates; mean rates (Days 0 and 1) were 5.2 ± 0.3 mg N kg dry soil⁻¹ day⁻¹ (0-0.3 m) and 1.6 ± 0.2 mg N kg dry soil⁻¹ day⁻¹ (0.3-1.1 m). Rates generally peaked on Days 2 or 3. The availability of labile organic carbon was a major constraint on denitrification in these soils. Acetate addition greatly increased rates, reaching a maximum in ephemeral floodplain soils of 17.4 ± 1.8 mg N kg dry soil⁻¹ day⁻¹ on Day 2: in one deep-soil treatment (low nitrate) this overcame differences in rates observed with depth when acetate was not added, although the rate increase in the other deep-soil treatment (high nitrate) was significantly less (P ≤ 0.01). Without acetate, peak denitrification rates in this experiment were 6.9 ± 0.4 and 2.8 ± 0.2 mg N kg dry soil⁻¹ day⁻¹ in surface and deep soils, respectively. Differences in rates were observed with depth on all occasions, despite similar initial concentrations of dissolved organic carbon (DOC) at both depths. Levels of substrate addition in the second experiment (perennial stream) more closely reflected natural conditions at the site. Mean denitrification rates were consistently much higher in surface soil (P ≤ 0.001), while the source of water used in the slurries (surface water or groundwater from the site) had little effect on rates at any depth. Mean rates when all treatments retained nitrate were: 4.5 ± 0.3 mg N kg dry soil⁻¹ day⁻¹ (0-0.3 m depth); 0.8 ± 0.3 mg N kg dry soil⁻¹ day⁻¹ (0.3-1.0 m); and 0.6 ± 0.1 mg N kg dry soil⁻¹ day⁻¹ (1.8-3.5 m). For comparable treatments and soil depths, denitrification potentials at both sites were similar, apart from higher initial rates in the ephemeral floodplain soils, probably associated with their higher DOC content and possibly also their history of more frequent saturation. The rapid onset of denitrification and the rates measured in these soils suggest there may be considerable potential for nitrate removal from groundwater in these floodplain environments during relatively short periods of saturation.