Bryan A. Stevenson

Convergence of soil nitrogen isotopes across global climate gradients

Quantifying global patterns of terrestrial nitrogen (N) cycling is central to predicting future patterns of primary productivity, carbon sequestration, nutrient fluxes to aquatic systems, and climate forcing. With limited direct measures of soil N cycling at the global scale, syntheses of the 15N:14N ratio of soil organic matter across climate gradients provide key insights into understanding global patterns of N cycling. In synthesizing data from over 6000 soil samples, we show strong global relationships among soil N isotopes, mean annual temperature (MAT), mean annual precipitation (MAP), and the concentrations of organic carbon and clay in soil. In both hot ecosystems and dry ecosystems, soil organic matter was more enriched in 15N than in corresponding cold ecosystems or wet ecosystems. Below a MAT of 9.8°C, soil δ15N was invariant with MAT. At the global scale, soil organic C concentrations also declined with increasing MAT and decreasing MAP. After standardizing for variation among mineral soils in soil C and clay concentrations, soil δ15N showed no consistent trends across global climate and latitudinal gradients. Our analyses could place new constraints on interpretations of patterns of ecosystem N cycling and global budgets of gaseous N loss.

Nitrogen inputs and outputs for New Zealand from 1990 to 2010 at national and regional scales

Abstract Reactive nitrogen (N) is increasingly added to the New Zealand environment because of increased sales of N fertilizer and increased human population. The Greenhouse Gas Inventory now reports in detail on changes for N losses from grazing animals from 1990 to 2010. Using animal numbers, we made assessments of N inputs and outputs for the 16 regions of New Zealand for 1990, 2001 and 2010 to assess temporal trends. Fertilizer sales have increased from 46 Gg N in 1990 to 329 Gg N in 2010, which leads to reduced biological N fixation by pastures. The import of oil-palm kernel has increased from zero to about 28 Gg N in 2010. Total N inputs are estimated to have increased from 689 Gg to 951 Gg N. The outputs of produce, leachate, gasses and sediment have increased from 771 to 866 Gg N; outputs to rivers may increase further if increases in outputs lag behind increases in inputs. Many of the inputs and outputs are well constrained because animal numbers have been used rather than land area, but uncertainties do exist for specific land-use classes. For example, the area of lifestyle blocks is approaching 800,000 ha and there is uncertainty regarding N inputs and outputs in this land use. There are also uncertainties in the amount of N fixation, the N loss by leaching in any one year, the amounts and fate of dissolved organic N, and the N content of eroded sediment. These uncertainties need to be resolved so that the amount of N stored in soils can be assessed. It seems likely that the N concentration of soils under dairying is increasing relative to the carbon concentration (i.e. soil C/N ratios are declining) but there is conflicting evidence as to whether the total N (and C) in these soils is increasing or decreasing.

Microbial biomass and community structure changes along a soil development chronosequence near Lake Wellman, southern Victoria Land

Abstract Four pedons on each of four drift sheets in the Lake Wellman area of the Darwin Mountains were sampled for chemical and microbial analyses. The four drifts, Hatherton, Britannia, Danum, and Isca, ranged from early Holocene (10 ka) to mid-Quaternary (c. 900 ka). The soil properties of weathering stage, salt stage, and depths of staining, visible salts, ghosts, and coherence increase with drift age. The landforms contain primarily high-centred polygons with windblown snow in the troughs. The soils are dominantly complexes of Typic Haplorthels and Typic Haploturbels. The soils were dry and alkaline with low levels of organic carbon, nitrogen and phosphorus. Electrical conductivity was high accompanied by high levels of water soluble anions and cations (especially calcium and sulphate in older soils). Soil microbial biomass, measured as phospholipid fatty acids, and numbers of culturable heterotrophic microbes, were low, with highest levels detected in less developed soils from the Hatherton drift. The microbial community structure of the Hatherton soil also differed from that of the Britannia, Danum and Isca soils. Ordination revealed the soil microbial community structure was influenced by soil development and organic carbon.

Denitrification and availability of carbon and nitrogen in a well-drained pasture soil amended with particulate organic carbon

A well-drained soil in N-fertilized dairy pasture was amended with particulate organic carbon (POC), either sawdust or coarse woody mulch, and sampled every 4 wk for a year to test the hypothesis that the addition of POC would increase denitrification activity by increasing the number of microsites where denitrification occurred. Overall mean denitrifying enzyme activity (DEA), on a gravimetric basis, was 100% greater for the woody mulch treatment and 50% greater for the sawdust treatment compared with controls, indicating the denitrifying potential of the soil was enhanced. Despite differences in DEA, no difference in denitrification rate, as measured by the acetylene block technique, was detected among treatments, with an average annual N loss of ∼22 kg N ha yr Soil water content overall was driving denitrification in this well-drained soil as regression of the natural log of volumetric soil water content (VWC) against denitrification rate was highly significant ( = 0.74, < 0.001). Addition of the amendments, however, had significant effects on the availability of both C and N. An additional 20 to 40 kg N ha was stored in POC-amended treatments as a result of increases in the microbial biomass. Basal respiration, as a measure of available C, was 400% greater than controls in the sawdust treatment and 250% greater than controls in the mulch. Net N mineralization, however, was significantly lower in the sawdust treatment, resulting in significantly lower nitrate N levels than in the control. We attribute the lack of measured response in denitrification rate to the high temporal variability in denitrification and suggest that diffusion of nitrate may ultimately have limited denitrification in the amended treatments. Our data indicate that manipulation of denitrification by addition of POC may be possible, particularly when nitrate levels are high, but quantifying differences in the rate of denitrification is difficult because of the temporal nature of the process (particularly the complex interaction of N availability and soil water content).

Removal of contaminant organic gases from air in closed systems by soil

Abstract The effect of air flow rate and prior conditioning of soil on the removal of contaminant organic gases from air by soil bed reactors (SBRs) was experimentally tested in closed systems. Removal rates of methane, ethane and propane were not significantly different from controls indicating that no net removal of these gases occurred during the experiments. Carbon monoxide removal was complete and rapid, while the removal of ethylene appeared to be accelerated with prior exposure of the soil to ethylene (conditioning), suggesting that the induction of microbial populations may be involved. The amount of contaminant gases removed from the air did not significantly change with changes in air flow rate.

Changes in pH, bicarbonate-extractable-P, carbon and nitrogen in soils under pasture over 7 to 27 years

We examined changes in soil pH and total carbon (C), nitrogen (N) and bicarbonate phosphorus (bicarbonate-P) in resampled sites under grazed pastures, using two data sets on commercial farms. These were a soil quality (SQ) data set (0–10 cm soil depth; samples collected along transects) and a soil profile data set from National Soils Database (NSD) sites. For 158 topsoils from the SQ data set, the number of sites increasing and decreasing in pH was almost equal, and the mean pH increased slightly after about 7 years. Soil C increased under drystock, especially in hill country, but there was no significant change under dairying; the C:N ratio decreased more rapidly in soils under drystock than under dairy. Although there was no strong direct relationship between pH and C loss or gain, those sites decreasing in pH had significant gains in C, whereas sites increasing in pH had no significant gains in C. Bicarbonate-P increased in flatter land, but not in hill country. Resampling of 89 NSD soil profiles after about 27 years showed an average increase in pH for all horizons; the data also showed a decrease in pH upon storage of archived soils. Bicarbonate-P generally increased in soils in flatter land, as did the N status of soils, probably as a result of intensification of land use. A subset of 22 NSD profiles initially sampled in the 1980s and resampled in 2004 and 2009 showed C and N decreased in allophanic soils in dairy farms up to c. 2004, but no significant trend was detected from 2004 to 2009, although longer-term monitoring is required to confirm this trend.

Effect of nitrogen fertilizer on nitrogen pools and soil communities under grazed pastures

Abstract Nitrogen (N) fertilizer increases pasture production in New Zealand in a near linear manner and affects pasture composition, soil below-ground communities and N losses. We monitored N fertilized plots established in long-term low-fertility pasture over different time periods to compare changes in N availability on below-ground soil communities (particularly ammonia-oxidizing archaea [AOA] and ammonia-oxidizing bacteria [AOB] as they appear to be sensitive to change). Although the most significant effects were seen in the 30 gN (300 kg/ha) treatment, there were indications that even 5 gN had effects on archaea. AOA gene copies dominated in control and 5 gN treatments but decreased in the 10 gN treatment. The ratio of AOA to AOB in 10 gN was lower and more similar to 30 gN suggesting that the AOA/AOB ratio may be a sensitive indicator of change in N status. In the 30 gN treatment, both the soil C/N ratio and the fungal phospholipid fatty acids were reduced (consistent with changes in DNA profiles) and microbes were suppressed. The number of AOB gene copies significantly increased in this treatment and corresponded to a switch from ammonium-N to nitrate-N as the dominant inorganic form of N in the 56-day incubation. This was consistent with increased soil ammonium-N and nitrate-N concentrations, leading to increased nitrate-N leaching that occurred at a threshold of between 10 gN and 30 gN, and suggesting that, with 30 gN, nitrification and nitrate leaching are influenced more by AOB than archaea. Generally there was no significant change in mesofauna, microfauna or bacterial DNA profiles with the treatments. In a high-fertility pasture, DNA profiles for bacterial, fungal and archaeal groups clustered away from low-fertility pasture suggesting that changes in soil communities, with increased soil fertility, take more time to be fully expressed.