Maja Vojvodic-Vukovic

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.

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.

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.

Restoring cut-over restiad peat bogs: A factorial experiment of nutrients, seed and cultivation

Abstract More than 75% of the original restiad peat bogs of the Waikato region, New Zealand, have been converted to agricultural use. Restiad bogs are also mined for peat but there is little information on methods for the post-harvest restoration of restiad bogs. We established a restoration trial on a mined bog with a full factorial experiment of four fertiliser additions (nitrogen, phosphorus, nitrogen plus phosphorus, and control), three seed additions ( Leptospermum scoparium , Sporadanthus ferrugineus , and no seed added [control] and three cultivation techniques (lightly-tilled, deeply-tilled and raised). All combinations were duplicated in 5×5 m plots. The first restoration goal was to establish rapidly a vegetation cover to minimise peat degradation. All plots in the raised cultivation treatments exceeded 88% cover by 2 years, whereas the other cultivation treatments had significantly less cover, ranging between 1 and 75%. The best combination within the raised treatments was nitrogen plus phosphorus fertiliser with L. scoparium seed, which reached 100% cover. A second goal was to establish the late successional species S. ferrugineus. Although cover of this species was greatest on the raised plots (up to 32%), the benefits of seed and fertiliser applications were not clear. Measured changes in nitrogen and phosphorus pools showed that nutrient status of the peat had returned to background levels within 2 years, minimising the potential for invasion by weeds with greater nutrient requirements.