Graham P. Sparling

Estimation of soil microbial C by a fumigation-extraction method: use on soils of high organic matter content, and a reassessment of the kEC-factor.

The fumigation-extraction method to estimate microbial C was applied to a range of seven soils with organic-C contents up to 47%. The additional oxidizable organic-C released by chcl3 fumigation at −5 kPa water potential followed by extraction with 0.5 m K2SO4. was compared with the microbial C estimated by a modified substrate-induced-respiration (SIR) method, and by in situ labelling of cells with [14C]glucose. The extractable organic-C flush comprised 37–63% of the microbial C. The overall kecfactors, to convert from the organic-C flush to microbial C were 0.37 for the SIR method and 0.42 by 14C-labelling. The relationship between microbial C and the oxidizable C released by fumigation was similar for both organic and mineral soils. Accumulated data from 168 comparisons of the relationship between the organic-C flush and microbial C were re-examined. The kec-factors varied widely between soils and were strongly influenced by soil water content. It is recommended that soils should be rewetted. when required, to the -5kPa water potential, to ensure that the fumigation stage is fully effective. The reliability of the SIR and fumigation-incubation methods is discussed and a revised calibration for the SIR method: microbial C (/gmg g−1) = 50(/gmlCO2g−1h−1) is proposed, based on 14C-labelling and alternative methods of calcula the CO2-C flush when using the fumigation-incubation method. Taking account of possible underestimation of microbial C by these calibrations, a value of 0.35 is suggested as being representative of an overall kec-factor for New Zealand soils. Variability between soils means there is an error in applying an average factor to all soils, but a standard method should be adequate for most applications of microbial C measurements.

Decreases in organic C reserves in soils can reduce the catabolic diversity of soil microbial communities

An understanding of the main factors influencing microbial diversity in soils is necessary to predict the eAects of current landuse trends on terrestrial diversity. We used microbial catabolic evenness as a measure of one component of soil microbial diversity. Catabolic evenness was assessed by measuring the short-term respiration responses of soil to a range of simple organic compounds. DiAerences in catabolic evenness between pasture and other land-uses on matched soils were related to diAerences in organic C pools (total organic C, microbial biomass C, and potentially mineralizable C). This approach enabled comparison of land-use eAects on organic C pools in relation to catabolic evenness without the eAects of soil type. In general, microbial catabolic evenness was greatest in soils under pasture and indigenous vegetation (range: 19.7‐23.3), and least in soils under cereal/maize/horticultural cropping (range: 16.4‐19.6). Soils under mixed cropping land-uses had catabolic evenness that ranged between these extremes (range: 17.7‐20.5), but under pine forestry there was no characteristic level of evenness (range: 15.1‐ 22.3). Catabolic evenness correlated poorly with the absolute values of soil organic C pools (r 2 < 0.36). However, across a range of paired comparisons between pasture and other land-uses, greater diAerences in microbial catabolic evenness corresponded with greater diAerences in organic C (r 2 =0.76) and, to a lesser degree, with diAerences in microbial biomass C (r 2 < 0.45) or potentially mineralizable C (r 2 < 0.13). Therefore, land-uses that deplete organic C stocks in soils may cause declines in the catabolic diversity of soil microbial communities. Although the implications of this for microbial processes are unknown, maintenance of soil organic C may be important for preservation of microbial diversity. # 2000 Elsevier Science Ltd. All rights reserved.

Is the microbial community in a soil with reduced catabolic diversity less resistant to stress or disturbance

Microbial catabolic diversity can be reduced by intensive land-uses, which may have implications for the resistance of the soils to stress or disturbance. We tested the hypothesis that the microbial community in a soil where catabolic diversity has been reduced by cropping is less resistant to increasing stress or disturbance compared with a matched soil under pasture, where catabolic diversity was high. Increasing stress was imposed by reducing pH, increasing salinity (imposed by increasing soil electrical conductivity; EC) or increasing heavy metal contamination in the soils. Disturbance was simulated by a series of wet‐dry or freeze‐thaw cycles. After incubation of the soils under these regimes, catabolic evenness (a component of microbial functional diversity defined as the uniformity of substrate use) was calculated from catabolic response profiles. These profiles were determined by adding a range of simple C substrates to the soils and measuring shortterm respiration responses. Stress or disturbance caused much greater changes in catabolic evenness in the crop soil (low catabolic evenness) than the pasture soil (high catabolic evenness). Increasing Cu or salt stress caused increases in catabolic evenness at low intensities in both soils, but, in the crop soil, greater stress caused greater declines in catabolic evenness. Declines in pH also caused much greater decreases in catabolic evenness in the crop than the pasture soil. Catabolic evenness initially increased with increasing numbers of wet‐dry or freeze‐thaw cycles, but after four cycles, evenness declined in both soils. These changes in evenness could be attributed to significant changes (P , 0.05) in most catabolic responses. In contrast, there were generally few changes in microbial biomass C as a result of stress or disturbance treatments. Except for EC stress, all treatments caused slight increases in biomass C at low levels (only significant in the pH and Cu treatments) that subsequently diminished at the highest stress or disturbance levels. Microbial catabolic diversity generally followed the classical ‘hump-back’ responses of diversity to increasing stress or disturbance. We concluded that reduction in catabolic diversity and changes in soil properties due to land use could reduce the resistance of microbial communities to stress or disturbance. q 2001 Elsevier Science Ltd. All rights reserved.

Changes in microbial heterotrophic diversity along five plant successional sequences

Abstract Little is known about the changes in microbial diversity associated with ecosystem development. We measured microbial heterotrophic evenness (a component of diversity) and other soil/humus properties (including basal respiration, substrate-induced respiration, pH, total C, N and P) at different stages in the development of five different ecosystems, with plant assemblages being used to define the phase in the successional sequence. Our objectives were to determine whether there were common patterns in establishment of microbial heterotrophic evenness with ecosystem development and whether changes in evenness were correlated to soil properties. Samples were collected from five sequences: Gisborne land slips (a chronosequence of re-vegetating landslip scars); Mount Tarawera (primary succession on aerially-deposited ash from a volcanic eruption); Rangitoto island (primary succession on a lava flow from a volcanic eruption); Franz Josef (primary succession initiated on gravels after the retreat of a glacier); and Swedish islands (a series of islands of differing size supporting different stages of plant succession). Heterotrophic diversity was measured using the catabolic response profile technique where CO2 efflux is measured during a 4-h incubation of samples amended with 25 different carbon substrates. Heterotrophic evenness was calculated from the CO2 responses using the Simpson–Yule index (maximum possible is 25). For Tarawera and Gisborne sequences, heterotrophic evenness was significantly lower at the first stage of succession (11.5 and 19.9, respectively), but subsequently plateaued, ranging between 21 and 23. Heterotrophic evenness declined significantly with succession at Rangitoto and Franz Josef sequences, but there was no trend along the Swedish island sequence. Despite the lack of a common pattern of heterotrophic evenness along all the sequences, there were significant linear correlations between heterotrophic evenness and basal respiration for Rangitoto (r=0.51, P

Importance of soil water content when estimating soil microbial C, N and P by the fumigation-extraction methods

Abstract The influence of soil water content on the estimation of microbial C, N and P by the fumigation-extraction (FE) method and microbial C and N by the fumigation-incubation (FI) method was investigated using a range of air-dry soils. The estimates of microbial C were compared with those obtained by the substrate-induced respiration (SIR) method. Soils were fumigated overnight with CHCl3 when either air-dry, or rewetted to 50% w/w water content immediately before fumigation. The presence of water during fumigation greatly increased the C and N extracted by 0.5 M K2SO4 compared with soils fumigated while air-dry. Overnight rewetting of non-fumigated (control) soils decreased extractable-C but the effect on extractable-N was variable. Rewetting before fumigation also increased CO2-C production and net N-mineralization (during subsequent incubation) compared to soils fumigated while air dry. However, because of high variability the increases were often not significant. The flushes of extractable-C and N (the difference between fumigated and non-fumigated soils) were calculated in three ways. Comparison with the biomass C estimated on air-dry soils by the SIR method suggests the most appropriate way to estimate the flush is: flush = (extractable-C from wetted, fumigated soil) — (extractable-C from air-dry, non-fumigated soil). Estimates of microbial C varied by up to 5-fold depending on how the flush was calculated. The release of inorganic P (P1) by fumigation of air-dry soil was generally increased by rewetting. Releases were greatest from rewetted soil fumigated with CHCl3 vapour, lower when using liquid CHCl3 and usually lowest when air-dry soil was fumigated with CHCl3 vapour. On two soils, gradually air-dried in the laboratory, the estimates of microbial C by the FE method were affected by rewetting (to 50% w/w water content) once the soils had dried below 20% w/w water content, and the rewetting effects were highly significant at

Changes in soil organic C, microbial C and aggregate stability under continuous maize and cereal cropping, and after restoration to pasture in soils from the Manawatu region, New Zealand

Abstract Soil compaction can be a major limitation to continuous maize production on the heavier-textured soils of the Manawatu region, New Zealand. A survey was undertaken of commercially-operated farms that were producing continuous maize using conventional mouldboard ploughing and cultivation methods. The effects of cultivation on the proportions of water-stable macroaggregates, total organic C content, microbial biomass C and soil respiration were measured in the predominant soil type, a poorly-drained Kairanga silty clay loam (Typic Haplaquept). Three other regional soil types, a Manawatu silt loam (well drained), Kairanga silt loam (poorly drained) and Moutoa humic clay (very poorly drained) were incluced for comparison. The effects of cropping with wheat or barley, and the recovery of the soils after restoration to pasture, were also measured. Continuous cultivation with maize on the Kairanga silty clay loam for up to 11 years decreased the total C content in the top 20 cm of soil by 21%, microbial C by 49% and water-stable aggregates (greater than 2 mm) by 54% compared with the levels under long-term permanent pasture. Losses on cultivation of pastures were greater on Manawatu silt loam with decreases of 49% in organic C, 60% in microbial C and 98% of the greater than 2 mm aggregates. Changes were much less pronounced on the Moutoa soil with a greater initial organic matter content, and the soil rapidly established new, only slightly lower, equilibria. Cultivation for barley or wheat had a less detrimental effect than cultivation for maize on the total organic and microbial C contents and aggregate stability of Kairanga silty clay loam. There was a strong linear relationship between the decline in the proportion of stable aggregates and the loss of microbial C and organic C in the top 10 cm of cultivated Kairanga soil. The relationship was stronger between microbial C and aggregate stability (R2 = 77.8%) than total organic C and aggregate stability (R2 = 64%), but was only valid when soil organic C was declining, and was not significant in the soil at the 10–20 cm depth. Re-establishment of pasture caused a more rapid recovery in microbial biomass C than total C, and increased the proportion of organic C comprised of microbial C. However, recovery of the total organic and microbial C pools and aggregate stability was very variable, and after 4 years of pasture none of the sites had re-established the levels found under permanent pasture. The previous cropping history of these soils before being returned to pasture, rather than the organic or microbial C content, appeared to be of greater importance in controlling the aggregate stability characteristics.

A comparison of soil and microbial carbon, nitrogen, and phosphorus contents, and macro-aggregate stability of a soil under native forest and after clearance for pastures and plantation forest

Total, extractable, and microbial C, N, and P, soil respiration, and the water stability of soil aggregates in the F-H layer and top 20 cm of soil of a New Zealand yellow-brown earth (Typic Dystrochrept) were compared under long-term indigenous native forest (Nothofagus truncata), exotic forest (Pinus radiata), unfertilized and fertilized grass/clover pastures, and gorse scrub (Ulex europaeus). Microbial biomass C ranged from 1100 kg ha-1 (exotic forest) to 1310kg ha-1 (gorse scrub), and comprised 1–2% of the organic C. Microbial N and P comprised 138–282 and 69–119 kg ha-1 respectively, with the highest values found under pasture. Microbial N and P comprised 1.8–7.0 and 4.9–18% of total N and P in the topsoils, and 1.8–4.4 and 23–32%, respectively, in the F-H material. Organic C and N were higher under gorse scrub than other vegetation. Total and extractable P were highest under fertilized pasture. Annual fluxes through the soil microbial biomass were estimated to be 36–85 kg N ha-1 and 18–36 kg P ha-1, sufficiently large to make a substantial contribution to plant requirements. Differences in macro-aggregate stability were generally small. The current status of this soil several years after the establishment of exotic forestry, pastoral farming, or subsequent reversion to scrubland is that, compared to levels under native forest, there has been no decline in soil and microbial C, N, and P contents or macro-aggregate stability.

Recolonization of methyl bromide sterilized soils under four different field conditions

SummaryThe course of recovery in biological activity was assessed in the top 5 cm of undisturbed soil cores (29.7 cm diameter, 30 cm deep) that had been fumigated in the laboratory with methyl bromide. The cores were returned to their original pasture and forest sites, two with a moderate and two with a high rainfall, and untreated soils at all sites served as baselines. Sampling took place over 166 days (midsummer to midwinter). Microbial biomass (as measured by fumigation-extraction and substrate-induced respiration procedures) and dehydrogenase activity both recovered rapidly, but remained consistently lower in the fumigated than in untreated samples at both forest sites and at the moister of the two pasture sites. Bacterial numbers also recovered rapidly. Fungal hyphal lengths were, on average over 166 days, 25% lower in the fumigated soils. Levels of mineral N were initially highest in the fumigated soils, but declined with time. Fumigation generally had no detectable effects on the subsequent rates of net N mineralization and little effect on nitrification rates. Fumigation almost totally eliminated protozoa, with one to three species being recovered on day 0; the numbers recovered most rapidly in the moist forest soil and slowly in the dry pasture soil. The recoionization rate of protozoan species was similar in all soils, with species numbers on day 110 being 33 and 34 in the fumigated and untreated soils, respectively. Nematodes were eliminated by fumigation; recolonization was first detected on day 26 but by day 166, nematode numbers were still lower in fumigated than in untreated soils, the abundance being 10 and 62 g-1 soil and diversity 10 and 31 species, respectively. Overall, the results suggest that protozoan and nematode populations and diversities could provide a useful medium-term ecological index of the recovery in comprehensive soil biological activity following major soil pollution or disturbance.

Changes in soil properties after application of dairy factory effluent to New Zealand volcanic ash and pumice soils

In field studies, we assessed the effects of irrigation with dairy factory effluent on the surface (0–10 cm) properties of 3 rhyolytic tephra soils. Horotiu and Te Kowhai soils had been irrigated for 22 years and Taupo soil for 2 years. Soil properties measured were: total organic C, total N, pH, bulk density, unsaturated hydraulic conductivity, moisture release characteristics, readily and totally available water, particle density, microbial C, soil respiration, mineralisable N, denitrification enzyme activity, and nitrification potential. Matched, non-irrigated areas were sampled for comparison. Average annual loadings (kg/ha) of C, N, and P since 1990 were 1120 kg N, 560 kg P, and 36 300 kg C onto Horotiu soil, and 760 kg N, 380 kg P, and 25 200 kg C onto the Te Kowhai soil. These post-1990 loadings were, on average, 23% less than pre-1990 values. Irrigation for 22 years onto the Horotiu and Te Kowhai soils caused no change, or a slight decrease, in total C and N in the topsoil, but microbial C and mineralisable N contents were more than doubled, and N cycling activity much increased. Soil pH was increased by up to 1.8 units. Unsaturated hydraulic conductivity was increased from 8.5 to 49.8 mm/h on the Horotiu soil, and from 6.5 to 29.0 mm/h on the Te Kowhai soil. Irrigation increased the volumetric water content between 10 and 100 kPa. Most of the changes in soil properties suggest current application rates and pasture production can be maintained or increased. The high loading and mineralisation of N in the irrigated soils raises concerns about potential leaching of nitrate; current management practices are targeted towards minimising N loadings from effluent.

Effects of microwave radiation on the microbial biomass, phosphatase activity and levels of extractable N and P in a low fertility soil under pasture

Abstract Microwave irradiation was investigated as a controlled soil biocidal treatment which could selectively kill microbial biomass. Plate counts indicated that fungi were more susceptible to irradiation than were bacteria, but selective-inhibition of substrate-induced respiration (SIR) indicated that eukaryotes and prokaryotes were generally equally susceptible. Under the experimental conditions chosen, irradiation of the soil sample for 90 s gave a kill of microbial biomass equal to that achieved by CHCl3 fumigation. Extractable mineral N was increased after incubation of irradiated soil, and after 90s irradiation was only slightly lower than that of fumigated soil. At intermediate irradiation times, the increase in extractable mineral N was closely related to the decrease of SIR biomass C, indicating that the N was largely of microbial origin. It was not possible to determine microbial P content of irradiated soil because heat denaturation of intracellular enzymes resulted in releases of extractable inorganic and total P considerably lower than would be expected. Soil phosphatase activity was more resistant to microwave irradiation than was the microbial biomass. The heat stability of soil extracellular enzymes may make it possible to use the controlled heating of microwave irradiation to apportion activity into intra- and extracellular components.