Louis A. Schipper

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.

Nitrate removal, communities of denitrifiers and adverse effects in different carbon substrates for use in denitrification beds.

Denitrification beds are containers filled with wood by-products that serve as a carbon and energy source to denitrifiers, which reduce nitrate (NO(3)(-)) from point source discharges into non-reactive dinitrogen (N(2)) gas. This study investigates a range of alternative carbon sources and determines rates, mechanisms and factors controlling NO(3)(-) removal, denitrifying bacterial community, and the adverse effects of these substrates. Experimental barrels (0.2 m(3)) filled with either maize cobs, wheat straw, green waste, sawdust, pine woodchips or eucalyptus woodchips were incubated at 16.8 °C or 27.1 °C (outlet temperature), and received NO(3)(-) enriched water (14.38 mg N L(-1) and 17.15 mg N L(-1)). After 2.5 years of incubation measurements were made of NO(3)(-)-N removal rates, in vitro denitrification rates (DR), factors limiting denitrification (carbon and nitrate availability, dissolved oxygen, temperature, pH, and concentrations of NO(3)(-), nitrite and ammonia), copy number of nitrite reductase (nirS and nirK) and nitrous oxide reductase (nosZ) genes, and greenhouse gas production (dissolved nitrous oxide (N(2)O) and methane), and carbon (TOC) loss. Microbial denitrification was the main mechanism for NO(3)(-)-N removal. Nitrate-N removal rates ranged from 1.3 (pine woodchips) to 6.2 g N m(-3) d(-1) (maize cobs), and were predominantly limited by C availability and temperature (Q(10) = 1.2) when NO(3)(-)-N outlet concentrations remained above 1 mg L(-1). The NO(3)(-)-N removal rate did not depend directly on substrate type, but on the quantity of microbially available carbon, which differed between carbon sources. The abundance of denitrifying genes (nirS, nirK and nosZ) was similar in replicate barrels under cold incubation, but varied substantially under warm incubation, and between substrates. Warm incubation enhanced growth of nirS containing bacteria and bacteria that lacked the nosZ gene, potentially explaining the greater N(2)O emission in warmer environments. Maize cob substrate had the highest NO(3)(-)-N removal rate, but adverse effects include TOC release, dissolved N(2)O release and substantial carbon consumption by non-denitrifiers. Woodchips removed less than half of NO(3)(-) removed by maize cobs, but provided ideal conditions for denitrifying bacteria, and adverse effects were not observed. Therefore we recommend the combination of maize cobs and woodchips to enhance NO(3)(-) removal while minimizing adverse effects in denitrification beds.

Regulators of denitrification in an organic riparian soil

Abstract We investigated microbial denitrification in an organic riparian zone and identified factors which regulated its rate. The riparian zone received nitrate from incoming groundwater draining an upslope forest which was spray irrigated with treated effluent. Soil cores were taken from the riparian zone and the following variables were measured: KCl-extracted nitrate, water soluble carbon concentration, organic matter content, moisture content, denitrifying enzyme activity, on-site denitrification rates and natural N 2 O production. Five sampling surveys were made at a range of field temperatures (12–21°C). The riparian soil was continually water-saturated and contained an average organic matter content of 26%. Nitrate concentration in groundwater entering the upslope edge of the riparian zone was generally greater than 5 mg N l −1 . In combination, these factors resulted in an ideal environment for denitrification. Mean and median denitrification rates were found to be 1.12 and 0.95g N m −2 day −1 ; while mean and median N 2 O production rates were 73 and 84 mg N m −2 day −1 These rates were 1–3 orders of magnitude greater than those reported in previous studies of upland soils. Up to 77% of the variation in on-site denitrification rate could be explained by nitrate concentration and denitrifying enzyme activity. Temperature may also have regulated the rate of denitrification; however, insufficient observations at different temperatures were made to fully establish a temperature effect. N 2 O production was found to be most highly correlated to on-site denitrification rate. Rates of denitrifying enzyme activity were also greater than those generally found in upland soils, the mean and median rates were 810 and 740 ng N g −1 h −1

Five years of nitrate removal, denitrification and carbon dynamics in a denitrification wall.

Denitrification walls are a useful approach for removing nitrate from shallow groundwater, but little is known about the sustainability of nitrate removal, which is dependent on the continued supply of organic carbon to denitrifying bacteria. To address this question, we monitored nitrate removal, denitrification and carbon dynamics in a pilot-scale denitrification wall for 5 yr. The wall continuously removed more than 95% of the incoming nitrate in groundwater, which ranged from 5 to 15 mg N L(-1). We did not detect decreases in total carbon during the 5-yr study. Available carbon declined for the first 200 days after the wall was constructed but has since remained relatively constant. While microbial biomass has varied between 350 and 550 microg C g(-1) there was no downward trend, suggesting that carbon availability was not limiting the size of the microbial population. However, there was a large decrease in denitrifying population, as indicated by declines in denitrifying enzyme activity. Despite this decrease, denitrification rates have remained high enough to remove nitrate from groundwater and denitrification was limited by nitrate rather than by carbon. Our data demonstrates that there was sufficient available carbon in this denitrification wall to support denitrification and nitrate removal for at least 5 yr.

Nitrate removal from groundwater and denitrification rates in a porous treatment wall amended with sawdust.

Porous treatment walls are increasingly used for remediating contaminated groundwater. These walls are constructed below the water table and perpendicular to the groundwater flow. Successful nitrate removal from groundwater has been demonstrated in porous walls amended with sawdust but the mechanism responsible has not been identified. The objective was to determine whether denitrification rates in such a wall were high enough to account for observed nitrate removal. During a year-long field trial, the rate of nitrate removal from groundwater was measured as it passed through a 1.5 m wide wall. Concurrently, denitrification rates were measured in samples taken from the wall using an acetylene-inhibition technique. Denitrification rates (0.6–18.1 ng cm−3 h−1) were generally high enough to account for the nitrate losses in groundwater (0.8–12.8 ng N cm−3 h−1), except on one occasion, when nitrate loss in groundwater was greater than 50 ng N cm−3 h−1. When the water table dropped below the wall, nitrate inputs were decreased, and there were concurrent declines in denitrification rates. Rates subsequently increased once the water table rose. Laboratory incubations also demonstrated that denitrification was highly responsive to nitrate inputs. Denitrification rates increased by an order of magnitude within 7 h of nitrate addition. This treatment wall has removed nitrate from groundwater for more than 2.5 years and denitrification rates were high enough to account for nitrate removal.

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

Hydraulic constraints on the performance of a groundwater denitrification wall for nitrate removal from shallow groundwater

Denitrification walls are a practical approach for decreasing non-point source pollution of surface waters. They are constructed by digging a trench perpendicular to groundwater flow and mixing the aquifer material with organic matter, such as sawdust, which acts as a carbon source to stimulate denitrification. For efficient functioning, walls need to be permeable to groundwater flow. We examined the functioning of a denitrification wall constructed in an aquifer consisting of coarse sands. Wells were monitored for changes in nitrate concentration as groundwater passed through the wall and soil samples were taken to measure microbial parameters inside the wall. Nitrate concentrations upstream of the wall ranged from 21 to 39 g N m(-3), in the wall from 0 to 2 g N m(-3) and downstream from 19 to 44 g N m(-3). An initial groundwater flow investigation using a salt tracer dilution technique showed that the flow through the wall was less than 4% of the flow occurring in the aquifer. Natural gradient tracer tests using bromide and Rhodamine-WT confirmed groundwater bypass under the wall. Hydraulic conductivity of 0.48 m day(-1) was measured inside the wall, whereas the surrounding aquifer had a hydraulic conductivity of 65.4 m day(-1). This indicated that during construction of the wall, hydraulic conductivity of the aquifer had been greatly reduced, so that most of the groundwater flowed under rather than through the wall. Denitrification rates measured in the center of the wall ranged from 0.020 to 0.13 g N m(-3) day(-1), which did not account for the rates of nitrate removal (0.16-0.29 g N m(-3) day(-1)) calculated from monitoring of groundwater nitrate concentrations. This suggested that the rate of denitrification was greater at the upstream face of the wall than in its center where it was limited by low nitrate concentrations. While denitrification walls can be an inexpensive tool for removing nitrate from groundwater, they may not be suitable in aquifers with coarse textured subsoils where simple inexpensive construction techniques result in major decreases in hydraulic conductivity.

Irrigation of an allophanic soil with dairy factory effluent for 22 years: responses of nutrient storage and soil biota

Long-term application of wastewater adds large amounts of carbon (C), nitrogen (N), and phosphorus (P) to soils, and their effects on soil quality are not fully known. We compared the distribution of C, N, P, and Olsen P in the top 0.75 m of an allophanic soil after 22 years irrigation with dairy factory effluent with that in a non-irrigated soil. Earthworm species, biomass and distribution, microbial biomass, microbial activity, and relative use of substrates were measured to evaluate the contribution of biological processes to cycling and redistribution of total C. Total C did not differ between irrigated and non- irrigated soil, although there was less total C in the 0–0.1 m layer and more total C at 0.1–0.5 m in the effluent-irrigated soil. Microbial biomass C and basal respiration activity were increased by 4- and 1.6- fold, respectively, in the 0–0.1 m layer of the irrigated soil. Measurements of relative use of substrates indicated that the greater microbial biomass in the effluent-irrigated soil was supported by the inputs of available C (particularly lactose) in the effluent rather than by greater decomposition of the organic C in the soil. Irrigation increased total N storage by 2.1 t/ha and total P was increased by 11.5 t/ha. Most of the increase in total N occurred in the 0.1–0.5 m layers, whereas total P was greater at all depths. Olsen P also increased at all depths by 1.3- to 23-fold. Approximately 8% of the N and 91% of the P applied during the past 22 years was stored in the 0–0.75 m layer of the profile, with the potential for further P storage. Effluent irrigation increased the total soil nutrient stores, without detrimental effects on total C storage. Changes in nutrient distribution at the irrigated site can be partially attributed to leaching and the dominance (155 g/m2 ) of the earthworm Aporrectodea longa, which forms permanent burrows to lower depths.

Vegetation and peat characteristics in the development of lowland restiad peat bogs, North Island, New Zealand

A chronosequence of restiad peat bogs (dominated by Restionaceae) in the lowland warm temperate zone of the Waikato region, North Island, New Zealand, was sampled to identify the major environmental determinants of vegetation pattern and dynamics. Agglomerative hierarchical classification of vegetation data from 69 plots in nine different-aged bogs, initiated from c. 600 to c. 15,000 cal yr BP, identified eight groups. Six of these groups formed a sequence from sedges through Empodisma minus, the main peatforming restiad species, to phases dominated by a second restiad species, Sporadanthus ferrugineus. The sequence reflected bog age and paralleled patterns of temporal succession over the last 15,000 years (from early successional sedges through mid-successional Empodisma to late successional Sporadanthus) derived from previous studies of plant macrofossils and microfossils in peat cores. This indicated that different-aged bogs in the Waikato region could be used to interpret temporal succession. The remaining two classificatory groups comprised plots from sites modified by drainage, fire, or weed invasion and currently dominated by non-restiad species. The relationships between environmental variables and the six groups representing restiad bog succession indicated that, as succession proceeds, von Post decomposition index and nutrients in the top 7.5 cm peat zone decrease. The most useful indicators of successional stage were von Post, total P, total N, and % ash. Environmental response curves of the dominant plant species separated the species along nutrient and peat decompositional gradients, with early successional species having wider potential environmental ranges than late successional species. Empodisma minus, a mid-successional species, also had a relatively wide environmental range, which probably contributes to its key role in restiad bog development.