Microbial decomposition of extracted and leached dissolved organic carbon (DOC) and nitrogen (DON) was demonstrated from three pasture soils in laboratory incubation studies. DOC concentration in ...water extracts ranged between 29 and 148 mg C L
−1
and DON concentration ranged between 2 and 63 mg N L
−1
. Between 17 and 61 % of the DOC in the water extracts were respired as CO
2
by microbes by day 36. DON concentrations in the extracts declined more rapidly than DOC. Within the first 21 days of incubation, the concentration of DON was near zero without any significant change in the concentration of NO
3
−
or NH
4
+
, indicating that microbes had utilized the organic pool of N preferentially. Decomposition of leached DOC (ranged between 7 and 66 mg C L
−1
) and DON (ranged between 6 and 11 mg N L
−1
) collected from large lysimeters (with perennial pasture; 50 cm diameter × 80 cm deep) followed a similar pattern to that observed with soil extracts. Approximately 28 to 61 % of the DOC in leachates were respired as CO
2
by day 49. The concentration of DON in the leachates declined to below 1 mg N L
−1
within 7–14 days of the incubation, consistent with the observations made with extractable DON. Our results clearly show that DOC and DON components of the dissolved organic matter in pasture soils, whether extracted or leached, are highly decomposable and bioavailable and will influence local ecosystem functions and nutrient balances in grazed pasture systems and receiving water bodies.
Nitrification inhibitors, such as dicyandiamide (DCD), have been shown to decrease leaching from urea- and ammonium-based fertilisers and from urine patches in grazed pastures. To date there have ...been few studies on effects of nitrification inhibitors on non-target soil microbiota. This pot trial examined the short-term effects of DCD on the activity and diversity of both target (ammonium-oxidising bacteria and archaea) and non-target soil microbial populations. Bovine urine at a rate equivalent to 600kgurine-Nha−1 with or without DCD at 30kgha−1 was applied to pots planted with perennial ryegrass. This rate of DCD was typical of the amount applied to pasture in New Zealand, although this annual rate may be spread over several applications carried out over 2–3 months. The single high rate application was used to provide a “worst case scenario” to assist detection of potential impacts of DCD application to non-target soil microflora. Treatments also included DCD alone and untreated control pots. Soil used was a Horotiu sandy loam and pots were maintained at 80% WHC in a controlled-environment room at 12°C/16h (day) and 8°C/8h (night). Soil mineral N, hot water extractable C and N concentrations, soil pH, microbial biomass C and N, and DCD persistence were measured at regular intervals. Diversity and composition of the overall soil bacterial community were analysed by serial analysis of ribosomal sequence tags (SARST). Effects on ammonium-oxidising bacterial and archaeal communities were monitored more closely by determining the size of these populations using real-time PCR and their transcriptional activity by comparing RNA-denaturing gradient gel electrophoresis (DGGE) profiles following RT-PCR of the amoA gene. Changes in soil pH and mineral N following application of urine in the pot trial reflected patterns typically demonstrated in the field. Application of DCD to soil did not change the diversity of the soil bacterial community, with the four predominant phyla (Proteobacteria, Actinobacteria, Acidobacteria and Firmicutes) remaining in proportions that were similar to control soils. In contrast, urine application to soil resulted in a significant increase in members of Firmicutes, some of which are relatively stress tolerant. In line with the SARST results, shifts in the structure of the active component of the general soil bacterial community were detected in the urine and urine+DCD treatments only, further suggesting DCD had little impact on the overall soil bacterial activity. In contrast the microbes targeted by DCD, the ammonium-oxidising bacteria, were significantly affected by DCD with reductions in population size and altered activity. Ammonium-oxidising archaea, however, showed no response to application of DCD to soil, and were only minimally affected by application of urine. The results suggest that application of DCD to pasture is a relatively benign intervention that has an important role to play in mitigating the environmental hazards imposed by ongoing land use intensification.
In this study, the genera, abundance, and activities of endophytic bacteria in field-grown white clover (Trifolium repens) and the fate of introduced antibiotic-tolerant bacteria in white clover ...tissues were investigated. Pseudomonas, Pantoea, and Corynebacterium were the most frequently isolated endophytic bacteria genera, whereas Xanthomonas, Microbacterium, and Cellulomonas occurred less frequently. The average bacterial populations in stolons and roots were approximately 100 000 colony-forming units (CFU) (g wet mass)
-1
. Of the 28 strains tested for activity, none were chitinolytic or able to inhibit the root pathogen Codinaea fertilis in vitro. However, Fusarium oxysporum and Cylindrocladium scoparium were inhibited by one and five strains, respectively. Four of seven strains tested depressed clover seedling growth. In pot experiments, colonization and recovery of spontaneous rifampicin-tolerant mutants (Rif
+
) of bacteria were studied in clover plants for periods up to 20 weeks. The strains used, sourced from white clover (endophytic and rhizoplane) and organic compost, had previously shown growth promotion potential of white clover seedlings by increasing plant mass and decreasing nematode numbers. In one experiment in this present study, five Rif
+
strains were individually inoculated onto white clover seedlings, all five were re-isolated from shoots after 6 weeks and four strains were re-isolated after 20 weeks (numbers of Rif
+
bacteria ranged from 51 to 200 CFU (g wet mass)
-1
). No Rif
+
bacteria were isolated from root tissue at either time. In the second experiment, conducted with two strains of Rif
+
bacteria, the population was highest in the shoots (range >500 CFU of Rif
+
bacteria (g shoot fresh mass)
-1
) in weeks 2 and 3, declining to <200 CFU in week 5. Again, no Rif
+
bacteria could be detected in roots. No Rif
+
bacteria were recovered after 14 weeks for one of the strains. It appears that the main route of bacterial entry into seedlings was through stomata and that bacteria remained in the aerial parts of plants rather than migrating to the roots.Key words: endophytic bacteria, clover, microbial biological control, nematode.
Dairy factory effluent (DFE) contains significant amounts of nutrients such as nitrogen (N), phosphorus (P), potassium (K), and sulphur (S) which are beneficial to plant growth. It also contains high ...amounts of carbon (C). Lately, there has been some concern that DFE application to pastoral land is adversely affecting plant growth in some regions of New Zealand. In this study, we determined the mineralisation and immobilisation of nutrients particularly C, N, S, and cations, in a DFE-treated Omeheu sandy loam soil. We report findings from laboratory-based open incubation studies carried out at 10, 20, and 30°C, with four rates of DFE application (0, 150 000, 300 000, and 450 000 litres ha
-1
) alone and with added NO
3
-
(100 kg N ha
-1
). The DFE was applied at two-weekly intervals into packed soil columns which were leached with 0.01 MCaCl
2
solution. Leachates were analysed for total C, total N, SO
4
2-
, NO
3
-
, NH
4
+
, K
+
, Na
+
, and Mg
2+
. Effects of DFE application on soil microbial bio-mass-C, hot-water extractable-C, and anaerobically mineralisable-N were also determined. Addition of DFE increased the size of the microbial biomass pool and thereby enhanced immobilisation of nutrients, mainly N and S. The immobilisation was greater at higher temperature. At 10°C, microbes were unable to utilise all of the added C, even at the lowest rate of DFE application, and 40-50% of the C was leached from soil columns. However, at 30°C soil microbes either immobilised or respired between 95-97% of the C added from DFE, and only small amounts of C were measured in the leachates. Addition of NO
3
-
-N had no significant influence on the C immobilisation or respiration. Most of the added N (92-97%) from DFE remained immobilised in the soils throughout the study. A high proportion of the NO
3
-
-N added with DFE was immobilised in soils at 10 and 20°C, showing the dominating influence of soluble C, added through the two-weekly application of DFE, in stimulating microbial activity and causing a prolonged immobilisation of N. There was a net mineralisation of about 100 μg NO
3
-
-N g
-1
soil at 30°C, indicating faster metabolic use of soluble C from DFE by microbes at this temperature. Between 15-35% of the SO
4
2-
-S applied from DFE was either immobilised by soil microbes or was adsorbed on soil organic matter. The presence of significant amounts of NH
4
+
in DFE-treated soils suggests that parts of the soil columns may have become anaerobic during incubation, causing mineralisation of N from the death of aerobic microbes or decomposition of soil organic matter. A high proportion of the cations (K
+
, Na
+
, and Mg
2+
) that were added with DFE leached out, indicating that DFE application would have very little effect on the availability of these cations for plant uptake. This study, in part, explains that the poor performance of DFE application on pastoral soils predominantly arises through its effects on the availability of N for plant growth.
A pot experiment was conducted to investigate the impact of high carbon dairy factory effluent application on the growth of perennial ryegrass (
Lolium perenne L.), plant nutrient uptake, soil ...microbial biomass carbon and nitrogen, populations of soil-microorganisms, root colonising fungi and the microbial functional diversity. The effluent was added at rates of 0, 100,000, 200,000 and 300,000 l ha
–1. These rates are equivalent to 0,
×
1,
×
2 and
×
3 normal field application rates. The added effluent contained (g l
–1), C; 19.42, total P; 0.65; S, 0.75, K; 1.33, Na; 4.55, Mg; 0.11, NH
4; 0.073, total N; 0.073 and had a pH of 4.33. Replicate pots (incubated in a controlled-environment room at 20 °C, with 16 h light/8 h dark) were harvested at 32, 61, and 130 days after setting up of the experiment. In the first sampling, shoot dry matter levels declined significantly (
P
<
0.01) with increased effluent. By the third sampling the trend was reversed with treated pots having greater amounts of shoot dry matter. The initial depression of growth was possibly due to a combination of factors including excess levels of available carbon (C) for microbes leading to immobilisation of nutrients, particularly nitrogen (N) and sulphur (S). Shoot N and S concentrations were lower (
P
<
0.001) and the phosphorus concentrations were higher in effluent-treated samples. Soil microbial biomass-C (480 and 770 μg g
−1 of biomass C in untreated and treated soil, respectively) and microbial-N (81 and 123 μg g
–1 of microbial-N in untreated and treated soil, respectively) were significantly (
P
<
0.001) greater in effluent-treated pots at all times. Populations of total culturable bacteria were higher (
P
<
0.01) in the treated pots in the first sample (log
10 populations g
–1 were 7.3 in untreated pots compared to 8.0 averaged across three treatments) but there were no differences in the subsequent two samples. Effluent also increased yeast populations (log
10 numbers g
–1 were 0.6 in untreated pots and 3.1 in treated pots averaged across treatments and times
P
<
0.01) at all three sampling times. The Shannon-Weiner Diversity Index of root fungi decreased with increasing effluent application (
P
<
0.01) while the species richness decreased with effluent as well as with time (
P
<
0.1). Potential root pathogens
Fusarium oxysporum, total
Fusarium spp. and
Pythium spp. significantly increased (
P
<
0.05) in treated samples but in the final sampling,
Codinaea fertilis significantly (
P
<
0.05) decreased with effluent treatment. The microbial functional diversity pattern and the average well colour development (AWCD) in soil were significantly changed by the effluent application but effects were not detectable after 130 days.
