Influence of DCD and DMPP on soil N dynamics after incorporation of vegetable crop residues

We have investigated the effect of two nitrification inhibitors, 3,4-dimethylpyrazole phosphate (DMPP) and dicyandiamide (DCD), on the accumulation of and after incorporation of cauliflower residues in incubation experiments. Cauliflower leaves were incubated with soil and DCD or DMPP at two application rates [8.93 and 17.9 mg active component (ac) kg⁻¹ for DCD; 0.89 and 1.79 mg ac kg⁻¹ for DMPP]. Both doses of DCD and DMPP increased on average by 18.9 and 26.0 mg N kg⁻¹ for DCD1 (during 30 days) and DCD2 (during 45 days), respectively, and on average by 14.4 mg N kg⁻¹ for DMPP1 and DMPP2 during a period of at least 95 days. In DCD-treated soils, data followed an S-shaped curve, indicating that nitrification restarted during the experiment: inhibition was on average 24% during 35 days for DCD1 and on average 45% during 49 days for DCD2. Thereafter, amount in DCD-treated soils exceeded that of the cauliflower-only treatment by 31% for DCD1 and 78% for DCD2, probably due to a nitrogen release from DCD itself and a priming effect induced by DCD. In DMPP-treated soils, data followed a linear pattern since nitrification was inhibited during the complete incubation (95 days): inhibition was on average 56 and 64% for DMPP1 and DMPP2, respectively. DMPP did not affect the N mineralization of the crop residues. Under favourable conditions, DCD is able to inhibit the nitrification from crop residues for 50 days and DMPP for at least 95 days. Hence, especially DMPP shows a potential to reduce leaching after incorporation of crop residues.     

Reducing NH3, N2O and {\text{NO}}_3^ - –N losses from a pasture soil with urease or nitrification inhibitors and elemental S-amended nitrogenous fertilizers

A 3-month field experiment comparing nitrogen (N) losses from and the agronomic efficiency of various N fertilizers was conducted on a sandy loam (Typic Hapludand) soil at Ruakura AgResearch farm, Hamilton, New Zealand during October to December 2003. Three replicates of seven treatments: urea, urea + the urease inhibitor N-(n-butyl) thiophosphoric triamide (trade name Agrotain), urea + Agrotain + elemental sulphur (S), urea + double inhibitor [DI; i.e., Agrotain + dicyandiamide (DCD)], diammonium phosphate (DAP), DAP + S, each applied at 150 kg N ha⁻¹, and control (no N). After fertilizer application, soil ammonium () and nitrate () concentrations (7.5-cm soil depth), ammonia (NH₃) volatilization, nitrate () leaching, nitrous oxide (N₂O) emission, pasture dry matter, and N uptake were monitored at different timings. Urea applied with Agrotain or Agrotain + S delayed urea hydrolysis and released soil at a slower rate than urea alone or urea + DI. Urea applied with DI increased NH₃ volatilization by 29% over urea alone, while urea + Agrotain and urea + Agrotain + S reduced NH₃ volatilization by 45 and 48%, respectively. Ammonia volatilization losses from DAP were lower than those from urea with or without inhibitors. Total reduction in leaching losses for urea + DI and urea + Agrotain compared to urea alone were 89% and 47%, respectively. Application of S with urea + Agrotain reduced leaching losses by an additional 6%. Nitrous oxide emissions were higher from the DAP and urea alone treatments. Urea applied with DI and urea + Agrotain reduced N₂O emissions by 37 and 5%, respectively, over urea alone. Compared to urea alone, total pasture production increased by 20, 17, and 15% for urea + Agrotain + S, urea + Agrotain, and urea + DI treatments, respectively, representing 86, 71, and 64% increases in N response efficiency. Total N uptake in urea + Agrotain, urea + Agrotain + S, and urea + DI increased by 29, 22, and 20%, respectively, compared to urea alone. These results suggest that the combination of both urease and nitrification inhibitors may have the most potential to reduce N losses and improve pasture production in intensively grazed systems.     

Properties of anaerobically digested and composted municipal solid waste assessed by linking soil mesofauna dynamics and nitrogen modelling

We have studied the effect of anaerobically digested (ADMSW) and composted municipal solid waste (CMSW) on mineralization and foodweb dynamics to verify the hypothesis that ADMSW would immobilize N right after addition to soil in contrast to addition with CMSW. Another hypothesis was that the mesofauna (enchytraeids and microarthropods) would stimulate N release from the decomposer community. We measured excretion of -N and urea-N from the mesofauna and hypothesized that enchytraeids would release urea. ADMSW and CMSW were amended to pots with sandy loam and barley. The pots were divided into treatments with or without mesofauna. Mesofauna, plant N and biomass, soil N and ergosterol (fungal biomass) were measured over a 113-day period of four equidistant samplings. Soil respiration, N mineralization and N release by the mesofauna were modelled from concurrent studies. ADMSW- and CMSW-treated soils initially (<20 days) immobilized N. The amendments did not increase plant growth substantially, and this was probably due to N-limitation in the early stages of plant growth. Enchytraeid abundance was about three times higher in ADMSW- than CMSW-treated soils, indicating that ADMSW contained more labile compounds, bacteria, and microfauna. The mesofauna did not affect N-content, but the cumulated -N excreted by the mesofauna was estimated to be substantial and about one-fifth of total plant N in ADMSW. An explanation for this discrepancy might be that in the absence of the mesofauna, other members of the detrivore and microbivore community performed the mesofauna’s function. Neither enchytraeids nor microarthropods excreted urea.     

Nematode diversity,food web condition,and chemical and physical properties in different soil habitats of an organic farm

The aim of this paper was to assess biodiversity among different habitats of an organic farm and the relationships between some soil properties, nematode taxonomic diversity, and soil food web condition. Eight habitats were studied in the farm: ponds, ditches, a riparian corridor, hedgerows, and four agricultural fields (mustard, oats, fallow, and legumes). The undisturbed riparian corridor had higher soil and concentrations, and potentially mineralizable N and higher abundances of bacterivore nematodes and longer food webs. Canonical correlation analysis showed associations between habitats and nematode trophic groups: predatory and bacterial-feeding nematodes in the riparian corridor and hedgerows, omnivore nematodes in the ponds and ditches, and fungal-feeding nematodes in the legume field. Soil chemical and physical properties mirrored the aboveground farm patterns and were more similar among habitats that were or had been cultivated, compared to the riparian corridor. Soil food web indices, based on functional analysis of nematode faunal composition, reflected the aboveground landscape heterogeneity. Discriminant analysis indicated that soil food web indices separated the two most disturbed habitats (ponds and tailwater ditches) from the two least disturbed habitats (the riparian corridor and hedgerows). The indices correlated with soil functioning as inferred by soil properties. Abundance of nematode taxa was not associated with aboveground landscape patterns. The complexity of the soil food web may have been influenced by (1) environmental factors that differed between years, (2) different time periods since disturbance in the various habitats, and (3) movement of nutrients and organisms by water flow between habitats in the farmscale.     

Gross nitrogen mineralisation and fungi-to-bacteria ratios are negatively correlated in boreal forests

In terrestrial ecosystems, gross nitrogen mineralisation is positively correlated to microbial biomass but negatively to soil organic matter C-to-N ratios; the influence of the microbial community structure is less well known. Here, we relate rates of gross N mineralisation to fungi-to-bacteria ratios in three natural forest types of contrasting N availability and in a long-term N-loading experiment in a boreal forest. We report, for the first time, a strong negative correlation between gross N mineralisation and the fungi-to-bacteria ratio ( = 0.91, P = 0.0005, N = 7). There was also a negative correlation between gross N mineralisation and the C-to-N ratio ( = 0.89, P = 0.001, N = 7), but a weaker positive correlation between gross N mineralisation and soil pH ( = 0.64, P = 0.019, N = 7). Our analysis suggests that soil fungi-to-bacteria and C-to-N ratios are interrelated and that they exert strong influences on soil N cycling in boreal forests.     

Gross nitrogen mineralisation rates in pastural soils and their relationships with organic nitrogen fractions, microbial biomass and protease activity under glasshouse conditions

In this study, gross nitrogen (N) mineralisation rates were determined in six pasture soils (Fleming, Kairanga, Karapoti, Lismore, Templeton and Waikoikoi) from three different regions of New Zealand. The soils were kept under controlled soil water potential (–10 to –30 kPa) and temperature (12–20°C) conditions in a glasshouse. The gross N mineralisation rates ranged from 0.76 to 5.87 g N g^–1 soil day^–1 in the six soils and were positively correlated with the amount of amino acid-N (AA-N), ammonia-N (NH₃-N), total hydrolysable-N (TH-N), microbial biomass-carbon (MB-C), microbial biomass-N (MB-N), protease activity and organic C and N. A stepwise regression was used to generate equations that could best describe gross N mineralisation rates. Microbial biomass-carbon and AA-N were included in the equation that best described the gross N mineralisation rate:

Spraying of oxytetracycline and gentamicin onto field-grown coriander did not affect the abundance of resistant bacteria, resistance genes, and broad host range plasmids detected in tropical soil bacteria

Horticultural supplements containing oxytetracycline and gentamicin, two clinically relevant biocides, are widely marketed to prevent or control infections by bacterial phytopathogens. Despite their regular consumption in the world’s less developed countries, it is unknown whether exposure of tropical farmlands to these drugs results in an enrichment of resistant bacteria, resistance genes, and/or mobile genetic elements in the soil. These concerns were investigated under field conditions by repeatedly spraying recommended amounts of a commercial product containing oxytetracycline-HCl, and gentamicin- onto two coriander plots. Subsequent to five applications within 16 months, composite soil samples from control and treated sections were compared with respect to the abundance of resistant bacteria and the prevalence of conserved nucleotide sequences from tetracycline efflux proteins, tetracycline ribosomal protection proteins, four different families of gentamicin-modifying enzymes, and broad host range plasmids of the IncP-1 and IncQ incompatibility groups. The isolation frequency of oxytetracycline- and gentamicin-resistant bacteria and the detection rate of the aforementioned genes and elements were unrelated to application of the supplement. Despite the omnipresence of sequences from IncP-1 plasmids, conjugative plasmids conferring resistance to oxytetracycline or gentamicin were not captured in biparental matings. The widespread occurrence of resistant bacteria and resistance genes at the beginning of the trial emerges as a reasonable explanation for the lack of anticipated responses. Moreover, we assume that the biocides applied were inactivated by biotic and abiotic factors under tropical conditions.     

Genetic diversity for root-knot nematode resistance in white clover and related species

A total of 237 Plant Introduction in eleven Trifolium species were evaluated for resistance to Meloidogyne arenaria (Neal) Chitwood race 1, M. hapla Chitwood, M. incognita (Kofoid & White) Chitwood race 3, and M. javanica (Treub) Chitwood. Plants were infected with 1500 nematode eggs collected from 'Rutgers' tomato (Lycoperiscon esculentum Mill.) roots with 0.5% NaOCl. Ratings of galling severity and egg mass production were assigned to each plant 8 wk after inoculation. Host plant reaction was classified as immune, highly resistant, resistant, moderately resistant, intermediate, moderately susceptible, susceptible, and highly susceptible according to the resistance index . More than 95% of 171 white clover accessions were moderately to highly susceptible to all four nematodes species. The best white clover accessions were only moderately resistant to either M. arenaria (PI 291843 and PI 306286) or M. hapla (PI 100250 and PI 204930). Accessions with moderate resistance or resistance to root-knot nematodes were found among relatives of white clover, with T. ambiguum M. Bieb. exhibiting the greatest resistance level. Among the other Trifolium species evaluated, T. carolinianum Michx. PI 516273 was immune or highly resistant to all four nematode species while accessions of T. hirtum All. showed a wide range of reaction to root-knot nematodes. Identified germplasm of white clover relatives with resistance to root knot nematodes should be useful for the selection of parents in white clover breeding programs.     

Distribution and activity of hypolithic soil crusts in a hyperarid desert (Baja California, Mexico)

Widespread and ecologically important, biological soil crusts include those microbial communities living on the surface of the soil and those that live beneath semitranslucent rocks (a.k.a. hypolithic crusts). We examined the distribution, abundance, physiology, and potential soil N contributions of hypolithic, biological crusts in hyperarid ecosystems of the Baja California peninsula and islands in the midriff region of the Gulf of California, Mexico (Sonoran desert). Crusts were limited in distribution to areas with translucent quartz rocks less than 3 cm thick, were not found on areas of islands with seabird guano deposition, but covered as much as 1% (12,750 m²) of the surface area of one island. The percent of available rocks colonized by crusts was similar between the mainland (38%) and islands without seabird guano (26%). Carbon fixation rates in the field, which have not been previously reported, ranged between 0 and 1.23 μmol m⁻² s⁻¹, and in the lab ranged between 0.66 and 0.94 μmol m⁻² s⁻¹. Evidence of low rates of N fixation was inferred from δ ¹⁵N values of crust and soil. Hypolithic crusts were found to have minimal, if any, influence on soil salinity, pH, and , but may represent up to 14% of the biomass of primary producers on these islands and provide C and N to the belowground and possibly aboveground heterotrophic communities where crusts exist. The results of this study suggest a limited but potentially important contribution of hypolithic soil crusts to hyperarid ecosystems.     

Nitrous oxide emissions from cattle-impacted pasture soil amended with nitrate and glucose

There is little information concerning N₂O fluxes in the pasture soil that has received large amounts of nutrients, such as urine and dung, for several years. The aims of this study were to (1) experimentally quantify the relationship between mineral N input and N₂O emissions from denitrification, (2) describe the time course of N₂O fluxes resulting in N inputs, and (3) find whether there exists an upper limit of the amount of nitrogen escaping the soil in the form of N₂O. The study site was a grassland used as a cattle overwintering area. It was amended with KNO₃ and glucose corresponding to 10–1,500 kg N and C per hectare, covering the range of nutrient inputs occurring in real field conditions. Using manual permanent chambers, N₂O fluxes from the soil were monitored for several days after the amendments. The peak N₂O emissions were up to 94 mg N₂O–N m⁻² h⁻¹, 5–8 h after amendment. No upper limit of N₂O emissions was detected as the emissions were directly related to the dose of nutrients in the whole range of amendments used, but the fluxes reflected the soil and environmental conditions, too. Thus, in three different experiments performed during the season, the total cumulative losses of N₂O–N ranged from 0.2 to 5.6% of the applied 500kg ha⁻¹. Splitting of high nutrient doses lowered the rate of N₂O fluxes after the first amendment, but the effect of splitting on the total amount of N₂O–N released from the soil was insignificant, as the initial lower values of emissions in the split variants were compensated for by a longer duration of gas fluxes. The results suggest that the cattle-impacted soil has the potential to metabolize large inputs of mineral nitrogen over short periods (∼days). Also, the emission factors for did not exceed values reported in literature.