B. L. Williams

Extractable N and P in relation to microbial biomass in UK acid organic soils

SummarySamples of peat and mor humus were treated with CHCl3 to kill microbial cells and the amounts of C as CO2, N as soluble- and mineral-N, and P as inorganic-P released by the treatment were compared with estimates of the microbial biomass by the Anderson and Domsch test and ATP determination. Increased amounts of soluble-N and inorganic-P, extracted with 1M KCl and 0.01M CaCl2 respectively, were detected immediately after the fumigation treatment. However, the subsequent rates of production of CO2−C and mineral-N measured during a 10-day incubation period at 25°C were low and variable, resulting in anomalously low estimates of microbial biomass. The flush of inorganic-P was more consistent and, in mor humus, generally related to biomass-C as measured by the Anderson and Domsch test.

Nitrogen mineralization and water-table height in oligotrophic deep peat

SummaryPeat cores, 0–60 cm depth, were taken on 14 occasions from three experimental plots where the water levels in the surrounding ditches had been artificially controlled for 14 years at 0, 20 and 50 cm below the surface. Numbers of aerobic and anaerobic ammonifying bacteria in the profile were significantly increased (P< 0.05) by lowering the water level from 0 to 50 cm. These increases occurred mainly in the surface 20 cm horizon, where 80%–90% of the ammonifying bacteria in the profile occurred. Mineral N in fresh samples, which was present almost entirely as ammonium, decreased sharply with depth below 20 cm, and on two occasions concentrations were significantly greater (P<0.05) in plots with water levels at 20 and 50 cm than in the flooded peat. Readily mineralized N, produced during waterlogged incubation at 30°C for 9 weeks, was significantly greater (P<0.05) on eight occasions in samples from plots with water levels at 20 or 50 cm than in those where the water level was at the surface. Calculations showed that the increases in N availability as a result of lowering the water-table could be attributed mainly to deeper rooting.

Temporal changes in C, P and N concentrations in soil solution following application of synthetic sheep urine to a soil under grass

We have determined the temporal changes in the concentration of dissolved organic carbon (DOC) and P and N components in soil solution following application of synthetic sheep urine (500 kg N ha-1) to a brown forest soil in boxes sown with Agrostis capillaris. Three contrasting defoliation treatments (no cutting, single cut before urine application and regular cutting twice per week) plus a fallow soil were studied. The synthetic urine contained 15N labelled urea and was P-free. Intact soil cores were taken after 2, 7, 14, 21 and 56 d and centrifuged to obtain soil solution. The urea in the synthetic urine was rapidly hydrolysed in the soil, increasing soil solution pH, DOC and total dissolved phosphorus (TDP) concentrations. For the regularly defoliated sward, DOC and P reached maximum concentrations (4000 mg DOC L-1 and 59 mg TDP L-1) on day 7. From their peak values, pH and DOC and P concentrations generally decreased with time and at day 56 were near those of the control. Concentrations of NH4+ and NO3- in the no-urine treatments fluctuated and the greatest treatment differences were between the fallow soil and the soil sown with grass. Adding synthetic urine increased NH4+ concentrations during the first week, but NO3- concentrations decreased. This was consistent with the 15N labelling of the NO3- pool which required 3 weeks to reach that of 15NH4+. Dissolved organic nitrogen (DON) reached a maximum value at day 7 with a concentration of 409 mg N L-1. The DON in soil solution contained no detectable amounts of 15N label indicating that it was derived from sources in the soil. Differences in soil solution composition related to the effect of the other cutting treatments and the fallow treatment were small compared to the effect of synthetic urine addition.

Seasonal dynamics of N in two Sphagnum moss species and the underlying peat treated with 15NH415NO3

Uptake of 15N labelled NH4NO3 by two Sphagnum mosses on a raised bog in north east Scotland was measured at different times of the year. In a field experiment, fortnightly additions of NH4NO3 at natural abundance, equivalent to 3 g N m-2 yr-1, were made over 14 months to cores of Sphagnum capillifolium occupying hummocks and S. recurvum colonizing hollows. Pre-harvested cores were treated with 15NH415NO3 two weeks before harvesting and 15N abundance determined for the total N in the moss, inorganic and dissolved organic N (DON) in the moss water and extractable inorganic, organic and microbial N in the underlying peat. The proportion of added 15N taken up by the mosses two weeks after each addition averaged 72% and ranged between 11 and 100%, tending to be least during October when the rising water table reached the surface, particularly for S. recurvum. A small proportion of the 15N was detected in the moss water as NH4+ (0.01%) and as DON (0.03%) and on occasions a large proportion remained unaccounted for. In waters from S. capillifolium, DON was proportional to the amount of inorganic N added, but this was not the case for S. recurvum. Little or no 15N was detected in the underlying peat partly because of the large size and variability of the NH4+, DON and microbial N pools.

Influence of synthetic sheep urine on the microbial biomass, activity and community structure in two pastures in the Scottish uplands

The impact of urine on the microbial biomass, activity and community structure was compared in the soil beneath two pastures in the Scottish uplands; Fasset, a natural Agrostis capillaris–Festuca ovina–Galium saxatile grassland and Strathfinella, a semi-natural grassland, improved with fertiliser addition. Community level physiological profiles (CLPP) were used to characterise the microbial communities. The utilisation of sugars, oligosaccharides, alcohols, carboxylic acids, long chain aliphatic acids, acidic, basic and neutral amino acids, amide N, phenolic acids and long chain aliphatic acids was used to compare the soils and the impact of synthetic urine addition. In the untreated soils, the utilisation of all the substrates decreased from the first week in May through to October. Averaged over all times and urine treatment, the potential utilisation of all substrates except for phenolic acids, long chain aliphatic acids and carboxylic acids was greater in the improved and more intensively grazed Strathfinella site. When averaged over all sample times, urine increased the utilisation of sugars, oligosaccharides, basic amino acids and amide N and the increases were greater in the unimproved, less intensively grazed, Fasset soil than that at Strathfinella. The effect of urine tended to be greatest during the period between 2 and 5 weeks after urine addition when utilisation of alcohols, acidic and neutral amino acids was also increased. Microbial biomass C in the control soils was 155.9 and 112.7 g C m−2 at Fasset and Strathfinella, respectively. Values did not change significantly with time and were unchanged by the addition of urine. However, urine addition significantly increased basal respiration rates at Fasset and decreased them at Strathfinella. Urine also increased bacterial numbers in both soils, but had no consistent effect on fungi or yeasts. The significance of these findings for studies of soil microbial community structure and activity in grazed upland grasslands is discussed.

The fate of NH4NO3 added to Sphagnum magellanicum carpets at five European mire sites

Nitrogen additions as NH4NO3 corresponding to 0 (N0), 1 (N1), 3 (N3) and 10 (N10) g N m-2 yr-1 were made to Sphagnum magellanicum cores at two-week intervals in situ at four sites across Europe, i.e. Lakkasuo (Finland), Männikjärve (Estonia), Moidach More (UK) and Côte de Braveix (France). The same treatments were applied in a glasshouse experiment in Neuchâtel (Switzerland) in which the water table depth was artificially maintained at 7, 17 and 37 cm below the moss surface. In the field, N assimilation in excess of values in wet deposition occurred in the absence of growth, but varied widely between sites, being absent in Lakkasuo (moss N:P ratio 68) and greatest in Moidach More (N:P 21). In the glasshouse, growth was reduced by lowering the water table without any apparent effect on N assimilation. Total N content of the moss in field sites increased as the mean depth of water table increased indicating growth limitation leading to increased N concentrations which could reduce the capacity for N retention. Greater contents of NH4+ in the underlying peat at 30 cm depth, both in response to NH4NO3 addition and in the unamended cores confirmed poor retention of inorganic N by the moss at Lakkasuo. Nitrate contents in the profiles at Lakkasuo, Moidach More, and Côte de Braveix were extremely low, even in the N10 treatment, but in Männikjärve, where the mean depth of water table was greatest and retention absent, appreciable amounts of NO3- were detected in all cores. It is concluded that peatland drainage would reduce the capture of inorganic N in atmospheric deposition by Sphagnum mosses.

Impact of synthetic sheep urine on N and P in two pastures in the Scottish uplands

Synthetic sheep urine additions (59 g N m−2) were made to pastures on two soils, at Fasset, a semi-natural grassland, and Strathfinella, an improved pasture. Urine was applied to microplots in May and the soil, grass and soil solution analyzed 1, 2, 4, 5, 12 and 23 weeks after the addition. At Fasset, the grass was scorched by urine and the standing biomass decreased compared to the control, increasing only after 5 weeks. The absence of scorching at the Strathfinella site was attributed to a greater biomass of root mat to buffer the roots from exposure to NH3and a greater rainfall immediately following treatment. Scorching reduced the uptake of N and it was not clear if the greater contents of NH4+ and the increases in soil pH at Fasset compared with Strathfinella were the causes or symptoms of the scorch effect. Amounts of extractable organic N (DON) were similar in both soils and increased during the first 4 weeks and then decreased. Urine addition both increased and decreased DON at different times, but the overall mean values were unchanged. Urine application changed the distribution of P in the two soils, increasing the soil solution P at Fasset by 80 mg P m−2 and raising the P content of herbage at Strathfinella by 600 mg P m−2. In the soil solution, dissolved forms of molybdate reactive P, organic P and condensed P fractions were all increased by the urine addition. After 23 weeks, condensed P made the greatest contribution to soil solution P in both soils indicating that this fraction was the least available for plant uptake.

Nitrogen dynamics in humus and soil beneath Sitka spruce (Picea sitchensis (Bong.) Carr.) planted in pure stands and in mixture with Scots pine (Pinus sylvestris L.)

Sitka spruce planted on nutrient-poor soils in mixture with pine or larch, unlike pure spruce, does not become N deficient and does not require N fertilizer. To test the hypothesis that N availability in the soil is enhanced beneath mixed species, the seasonal changes in different N forms were compared in humus (L+F+H) and soil beneath 15-year-old Sitka spruce (SS) and mixed Sitka spruce-Scots pine (SS and SP) planted on a gleyed heathland soil. Amounts of mineral and organic N extracted from humus in spring were significantly (p < 0.05) higher in SS and SP than in SS. Larger amounts were measured in the underlying soil, which favoured the deeper-rooting spruce and pine in SS and SP plots. Annual net N mineralization, measured by in-situ incubation, was 32 and 47 kg N ha-1 in the surface 10 cm beneath SS and (SS and SP), respectively. In spring, readily mineralized organic N (waterlogged incubation at 30°C) was higher in humus and soil from (SS and SP) than from SS by 15 kg N ha-1. The larger N pools beneath (SS and SP) were consistent with the higher total N content of the humus beneath (SS and SP), 446 compared with 255 kg N ha-1 beneath SS. This indicated that beneath (SS and SP) N had been transferred from the underlying soil.

Determination of N-species in soil extracts using microplate techniques

Colourimetric methods for the determination of NO(3)(-), NH(4)(+) and total N in water extracts of soils using 96-well microplate techniques are described. Nitrate was determined by azo dye formation after reduction to NO(2)(-) using a solution of hydrazine sulphate. Ammonium in the soil extracts was purified and concentrated by diffusion as NH(3) from small volumes (750microL) of extract treated with MgO into a H(2)SO(4) collector using a double-plate, MicroResp method and subsequently determined by the Berthelot reaction. For the determination of total N, samples were oxidised with K(2)S(2)O(8) at 110 degrees C in a 96x1.1mL polytetrafluoroethylene block with a lid that closed individual wells. The oxidised solutions were transferred to standard plates for colourimetric analysis of NO(3)(-). The recovery of N, measured as NO(3)(-), from NH(4)NO(3) and a range of organic-N compounds was >95%. The limits of quantitation of the colourimetic assays were 0.020mgNL(-1) for NO(3)(-) and 0.051mgNL(-1) for NH(4)(+). The methods were tested on water extracts derived from a range of 10 nutrient poor soils from Scotland. There were acceptable linear correlations between the results obtained by established methods. For soil extracts analysed by the microplate method, the relationship for NO(3)(-) was 1.03x result from ion chromatography+0.0055 (R(2)=0.9961); for NH(4)(+) determined by the microplate method, the relationship was 0.9696xresult from a discrete analyser-0.0169 (R(2)=0.9757) and for total N determined by oxidation in the PTFE microplate the relationship was 0.9435xresult obtained by combustion+0.0489 (R(2)=0.9743). Purification of the NH(4)(+) in water extracts from the 10 different soils by the diffusion method did not result in any systematic difference (paired t-test, p=0.05) between measured concentration values determined before and after diffusion.

Mineral nitrogen dynamics in poorly drained blanket peat

SummaryMineral-N dynamics have been measured over a period of 3 years in PK- and NPK-treated plots (4 m2) laid out on an area of poorly drained, reseeded, blanket peat in the north of Scotland. Mineral-N, present in the peat almost entirely as NHin4sup+, accumulated in winter, reaching 42 kg N ha−1 in the surface 10 cm in April before the application of 112.5 kg N ha−1 as NH4NO3 or urea. In situ incubation of peat cores isolated to prevent leaching, and with grass tops removed, confirmed that net mineralization occurred between November and April, with the greatest rate, 1.2 kg N ha−1 day−1, recorded between March and April. During the period May to early June, immobilization of N predominated and rates of net immobilization ranged between 0.2 and 0.8 kg N ha−1 day−1. This coincided with a poor uptake into herbage, less than 16% of soil mineral N and fertilizer NH4NO3 in June of the first 2 years. The largest counts (most probable number) of ammonifying bacteria in the surface 5 cm were recorded in July for aerobes (27.1×109 litre−1) and August for anaerorbes (7.1×109 litre−1). N fertilizer increased these counts significantly (P<0.05) to 56×109 aerobes and 13×109 anaerobes. During July and August, in 2 out of the 3 years, mineralization predominated over immobilization and mean net rates of up to 0.9 kg N ha−1 were recorded.