752 research outputs found
Soil, Water and Land Use: II. Understanding Nitrogen Interactions
No full textSoil, Water and Land Use: II. Understanding Nitrogen Interactions was written by Angela Schipper, Louis Schipper and Art Hornsby of the Soil and Water Science Department, University of Florida. Reviewed June, 2002.
https://edis.ifas.ufl.edu/4h13
Long term nitrate removal in a denitrification wall
No full textNitrogen (N) inputs to groundwater are one of the most widespread environmental problems globally. However, as N is important for crop production to support the current global population, it is difficult to limit N input to an extent where groundwater contamination is completely avoided. Researchers have been testing new ways to remove N (in the form of nitrate (NO3-)) from groundwater, primarily through enhancing microbial denitrification. One technology utilizing this microbial process is a denitrification wall, which is an inexpensive, low-maintenance technology compared to other options to treat NO3--contaminated groundwater. Denitrification walls have been shown to be effective for removing NO3- from groundwater through denitrification for seven years in New Zealand, nine years in Iowa, and 15 years in Canada; however, long-term data on the efficacy of denitrification walls remain limited. In order to understand how these systems function in the long term, the performance of a New Zealand denitrification wall installed in 1996 was examined. Field sampling was carried out during the winter of 2010 at the denitrification wall at Bardowie Farm in Cambridge, New Zealand. This farm had received relatively high N inputs from spray-irrigation of effluent from the nearby Hautapu Dairy Factory for over 30 years. The denitrification wall was originally constructed by mixing 40 m3 Pinus radiata sawdust with soil down to a depth of 1.5 m where it intercepted groundwater flow. Groundwater samples were collected from wells installed upslope and within the wall and samples were analyzed for NO3- concentrations on five occasions. Soil samples were collected on four occasions from below the water table and analyzed for denitrifying enzyme activity (DEA), total carbon (C), available C, and microbial biomass C. Results were compared to previous measurements. Groundwater NO3- concentrations entering the wall averaged 2.6 mg N L-1, which was a decrease from 2002 where NO3- entered the wall at an average of 9 mg N L-1. Despite this decrease, NO3- concentrations within the wall averaged 0.2 mg N L-1, which corresponded to 92% NO3- removal. DEA rates in the wall were nearly as high as the first year of construction. In contrast, total C and microbial biomass C had decreased by half, while available C remained the same as measured two years after construction. Denitrification in the wall remained NO3- limited suggesting that C was still sufficiently available to the denitrifiers. These data indicated that the denitrification wall was still effective after 14 years. To predict denitrification wall longevity, a first-order decay curve was fitted to the total C data through time (R2 = 0.92; p < 0.05). The decay curve was used to predict the time until total C reached 0.1%, although it is unclear at what %C denitrification will become C limited. Using this decay curve, it was estimated that C in the wall would not be depleted for 66 years, although it is possible that C will become limiting to denitrifiers before that time. This long-term study suggested that denitrification walls are cost-effective solutions to removing NO3- from groundwater as they can be effective for a number of years without any maintenance
The temperature response of nitrate removal in denitrification beds
No full textThe addition of reactive nitrogen (Nᵣ) to agricultural systems has helped crop production match human population growth. However, the addition of Nᵣ comes at a cost to environment in the form of ozone destruction, habitat degradation and biodiversity loss. Denitrification beds represent an effective method for the removal of Nᵣ from a range of wastewaters and groundwater with high nitrate (NO₃¯) concentrations. Beds are lined containers filled with a carbon (C) source to enhance denitrification: the conversion of NO₃¯ to unreactive dinitrogen (N₂).
In general, the rate of NO₃¯ removal in denitrification beds increases with increasing temperature. However, the temperature response of NO₃¯ removal in beds is poorly constrained as other controlling factors (e.g. NO₃¯ concentration and C source availability) can obscure the effect of temperature. The objective of this study was to measure the rates of NO₃¯ removal in three denitrification beds as temperature changed seasonally. The beds were located in the North Island of New Zealand and were loaded with NO₃¯ from wastewater from a hydroponic glasshouse (Karaka), domestic effluent from a campground (Motutere) and wastewater and domestic effluent from a research station (Newstead). Water samples were collected from wells installed along the length of each bed every month and were analysed for NO₃¯ concentration by ion chromatography. Rates of NO₃¯ removal were calculated using the change in NO₃¯ concentration and the flow rate. The temperatures of the beds were also measured at each sampling.
Nitrate concentrations declined along the length of each denitrification bed and rates of NO₃¯ removal were calculated to average 3.6, 4.3 and 1.7 g N m¯³ day¯¹ for Karaka, Motutere and Newstead, respectively. The rates of removal increased with increasing temperature at Karaka and Motutere and the Q₁₀ values (the factor by which the rate of removal increased for a 10 °C increase in temperature) were calculated as 4.1 and 2.2 for Karaka and Motutere, respectively. The rates of NO₃¯ removal and Q₁₀ values were similar to those reported in previous studies of denitrification beds both in New Zealand and overseas. However, the rate of NO₃¯ removal at Karaka was less than the rate of removal of 7.6 g N m¯³ day¯¹ previously measured at Karaka in a study 5 years ago. Similarly, the temperature response at Karaka was higher than the Q₁₀ of 2 reported in this previous study at Karaka. The decrease in removal and increase in Q₁₀ may have been due to a decline in C source quality.
There was no evidence of an increase in the rate of NO₃¯ removal with temperature at Newstead, with a Q₁₀ calculated as 1.0. The denitrification bed had been recently installed and was in a start-up phase. It was likely that the pretreatment system, in particular the nitrifying component responsible for converting ammonium (NH4+) in the effluent to NO₃¯, was not functioning effectively which resulted in low NO₃¯ concentrations entering the bed at Newstead. Nitrate was depleted within the beds at Motutere and Newstead which indicated that the rates of removal were NO₃¯ limited and that the temperature response may not have been adequately measured.
This study confirmed that the rate of NO₃¯ removal increased with increasing temperature in the denitrification beds at Karaka and Motutere. The temperature response of NO₃¯ removal was similar to the response reported in previous studies of denitrification beds. However, additional research is required to further constrain the range of Q₁₀ values from which future denitrification beds can be designed to optimise NO₃¯ removal. Whether Q₁₀ values increase as wood chips age and C quality decreases also requires further investigation
Microbial Processes and Nitrate removal in Denitrification Beds
No full textAbstract The anthropogenic abundance of reactive nitrogen (N) forms has increased in the last few decades, increasing food production, but also resulting in increased eutrophication, algae blooms, loss of biodiversity, and greenhouse gas (GHG) emissions, in aquatic and terrestrial ecosystems. Denitrification beds are one approach to return this reactive N back to the atmosphere. These beds are large containers filled with a carbon (C) substrate, often wood byproducts. This substrate acts as a C and energy source for denitrifiers to reduce nitrate (NO₃⁻) from point source discharges into non-reactive dinitrogen (N₂) gas. This study investigated the biological mechanisms, controlling factors and adverse effects of NO₃⁻ removal in a woodchip denitrification bed (176 m x 5 m x 1.5 m) treating glasshouse effluent, and in barrels (0.2 m³) testing alternative carbon substrates for use in denitrification beds (pine and eucalyptus woodchips, sawdust, green waste, maize cobs and wheat straw). Furthermore, different techniques for measuring denitrification rates were compared and an approach for determining reliable NO₃⁻ removal rates in denitrification beds was developed.
The NO₃⁻-N removal rates of the large denitrification bed averaged 7.6 g N m⁻³ bed volume d⁻¹ and increased with increasing temperature (Q₁₀ = 2.1). Microbial denitrification was the main NO₃⁻ removal mechanism in the denitrification bed and was always limited by C, rather than by NO₃⁻ availability. Dissimilatory nitrate reduction to ammonium (DNRA) and anammox were likely minor processes due to low ammonia (NH₄⁺) and nitrite (NO₂⁻) concentrations throughout the bed. Sulfate (SO₄²⁻) reduction, and methanogenesis, could not compete with NO₃⁻ reduction for C due to continuously high NO₃⁻ concentrations in the bed (>37 mg N L⁻¹). Aerobic processes dominated in the first few meters of the bed and close to the surface, but dissolved oxygen (DO) concentrations decreased rapidly along the bed from the inlet and remained low throughout most of the bed. There were some adverse effects observed in the denitrification bed associated with NO₃⁻ removal. About 4.3% of NO₃⁻-N removed from the bed was released as nitrous oxide (N₂O), but methane (CH₄) emissions from the surface of the bed were very low. A total of 35.4 kg d⁻¹ of carbon dioxide (CO₂) was released from the bed, but was not considered to contribute to a net increase in CO₂ concentrations of the atmosphere as the substrate (woodchips) used in the bed would likely decayed to CO₂ if used for other purposes. A net dissolved organic carbon (DOC) loss from the outlet was not detected. Longevity of the C substrate of the denitrification bed to support denitrification was about 39 years as calculated from the total C losses (CO₂ emissions and release of dissolved CO₂ and DOC from the bed). In a barrel study of different carbon substrates, NO₃⁻ removal was predominantly limited by C availability and temperature (Q₁₀ = 1.2) when NO₃⁻-N concentrations were above 1 mg L⁻¹. All C substrates showed high numbers of denitrification genes (nitrite reductase, nirS and nirK; nitrous oxide reductase, nosZ), providing further support that microbial denitrification was responsible for NO₃⁻ removal. Substrates incubated at 27.1 °C had greater ratio of nir/nosZ genes than substrates incubated at 16.8 °C, which was possibly a partial explanation for higher N₂O production in the warmer barrels. Wheat straw released 10% of NO₃⁻-N removed as dissolved N₂O, while all other carbon substrates released on average about 1.4% of the removed NO₃⁻-N as dissolved N₂O. Methane production occurred when NO₃⁻ concentrations were below 2 mg L-1 in the barrels. Maize cobs removed about 2.5 times more NO₃⁻ than woodchips, but released total organic carbon (TOC) in the outflow and a substantial portion of C was likely consumed by non-denitrifiers. Woodchips had low adverse effects and provided ideal conditions for denitrifiers determined by the relatively high ratio of denitrification gene copies/16S rRNA copies compared to the other C substrates examined.
Investigating different approaches to determine denitrification rates revealed that both the acetylene inhibition method and the copy number of nitrite reductase genes (nirS, nirK) were useful for comparative estimations of NO₃⁻ removal rates between different carbon substrates and temperatures. However, neither approach could be used to quantify actual rates of denitrification. The acetylene inhibition method overestimated the actual NO₃⁻ removal rate by five fold. An in situ push-pull test using enriched ¹⁵NO₃⁻ was useful for determining denitrification rates at one specific point in a denitrification bed but would require multiple testing sites to obtain an average rate of NO₃⁻ removal for the bed. Comparing the ratio of the slopes of natural abundance ¹⁵N-N₂ and ¹⁵N- NO₃⁻ along the length of the bed determined the portion of NO₃⁻ removed by microbial denitrification, but not the denitrification rate. Measurements of dissolved N₂ concentration along the length of the bed were a useful approach to determine denitrification rates. This last approach was rapid and produced relatively accurate rates of NO₃⁻ removal compared to the other approaches conducted in this study. In summary, denitrification beds are an efficient approach for removing NO₃⁻ from point source discharges, but the beds do produce some N₂O. Woodchips could be combined with maize cobs to enhance NO₃⁻ removal rates while keeping adverse effects low in denitrification beds. Measurement of N₂ concentrations along the length and water flow of the bed was the most appropriate approach to determine denitrification rates of denitrifying bioreactors, and may also be useful in other ecosystems with high NO₃⁻ concentration and even flow
Impacts of conversion from forestry to pasture on soil physical properties of Vitrands (Pumice Soils) in central North Island, New Zealand
No full textTens of thousands of hectares of land have been converted from plantation forest to pasture in the central North Island of New Zealand between 2000 and 2010. The land use change was driven by the perceived better long term returns from dairy farming compared with forestry. Pumice Soils (NZ Soil Classification, equivalent to Vitrands in Soil Taxonomy) in the central North Island are formed on pumice deposited mainly from the AD 232 ± 5 Taupo volcanic eruption. The texture of Pumice Soils (Figure 1) varies from silt to coarse gravel and they have weak structure and erode easily when disturbed. Water holding capacity may be low but increases as the organic matter content of the topsoil is built up
Storing carbon in soil. Can we slow a revolving door?
No full textThere is no doubt that soils are a vast store of carbon and partially control the carbon dioxide content of the atmosphere. Maintaining soil organic matter is also crucial for production and environmental protection. Land-use change and management practices are central to maintaining soil carbon, because these can both increase and decrease soil carbon. Pasture systems can store large amounts of soil carbon and there may be an opportunity to store more in New Zealand dairy systems with multiple benefits. Active research is investigating approaches to achieve this goal through the New Zealand Agricultural Greenhouse Gas Research Centre
Contribution of Dissolved Organic Carbon Leaching to the Annual Carbon Budget of a Dairy Farm
No full textSoils are the largest terrestrial store of carbon (C) and changes in this store of C can impact on soil quality and atmospheric CO2 concentrations. Research on C budgets at paddock to national scales has focused most attention on the processes of respiration and photosynthesis in determining the net loss or gain of carbon from an ecosystem. However, leaching of dissolved organic carbon (DOC) is a potentially important component of the carbon budget that is rarely measured when developing carbon budgets, and as a consequence, is often estimated or excluded. Much of the literature indicates that while DOC leaching is important, the loss of DOC from the terrestrial ecosystem may only be small. In the vasose zone DOC that is leached may be adsorbed on to soil and stabilised or may be mineralised, effectively preventing it from leaching from the ecosystem.
The objectives of this thesis were to determine if DOC leaching from the soil of a dairy farm was an important contribution to the carbon budget. To measure this, soil leachate was collected from five paddocks using 100 suction cup lysimeters. These were installed within the footprints of two eddy covariance towers on a dairy farm in Waharoa, Waikato, New Zealand. In general samples were bulked over paddocks, with 10 mL of water from each suction cup contributing to the overall bulked sample. Water extracted from the suction cups was analysed for DOC, total nitrogen, and nitrate. DOC concentration measurements were coupled to the volume of water draining through the soil. The volume of drainage was obtained from a water balance model using measurements of evaporation and precipitation. Leaching from the soil started in mid-May continuing through till mid-November. The total amount of water draining through the soil for the year was calculated to be 990 mm, with a mean concentration of 4.5 ± 0.8 mg L-1 (mean ± SE). The mass of DOC leached was 38 ± 4 kg C ha-1 yr-1 (mean ± SE). The concentration of DOC showed no monthly variation, while the mass of DOC showed a strong seasonal trend, with the greatest mass of DOC leaching during the wet winter period. Ultimately the main driver of DOC leaching at this site was the volume of water draining through the soil, because DOC concentration changed very little.
In order to understand the suite of processes that influence the fate of DOC the subsoil, internal cycling process including mineralisation and sorption of DOC were investigated in the laboratory. Results showed that DOC leached to a depth of 0.65 m could be mineralised by soil microbes lower in the profile, converting it to CO2. The total C respired over a week (12.81 µg CO2-C-1 g soil-1) was 11 times greater than the C added (1.18 µg C-1 g soil-1). In a repeat of the same study the amount of CO2 respired was 25 times greater than the addition of DOC. Additionally sorption experiments indicated that the concentration of DOC lost to the groundwater would be less than the concentration of DOC measured at 0.65 m. Soil water solution with a concentration of 7 mg L-1 DOC mixed with subsoil had a 50% reduction in concentration when shaken for four hours with Te Puninga soil. Similar results were found in the Piarere soil with a 34% reduction in DOC concentration. In contrast when both soils were shaken with DOC (4 mg L-1) in a second experiment, there was a small amount of net desorption. There was potential for the soils at this site to reduce the concentration of DOC leached from 0.65 m, through adsorption of DOC onto the soil. Subsequently sorption would have caused a reduction in the DOC mass lost. While results from laboratory studies were variable it was clear that both sorption and mineralisation in subsoils will moderate leaching losses of DOC to groundwater.
In the context of a paddock scale C budget, where the atmospheric exchange of C through respiration and photosynthesis was about -880 kg C ha-1yr-1, leaching of 38 kg C ha-1 yr-1, represents 4.5% of the total exchange. Compared to the net ecosystem carbon budget, which included farm inputs and outputs, of a similar intensive grazed system, DOC leaching is equal to 3-15% of the total. However as DOC leaching at 0.65 m does not accurately represent a leaching loss from the system, as sorption and mineralisation can further alter the mass leached, the contribution of DOC loss through leaching to the carbon budget is comparably small and does not represent a significant component of the C budget at this site
Comparison of soil carbon and nitrogen stocks of adjacent dairy and drystock pastures
No full textThe largest terrestrial store of carbon (C) is in soil and research has shown that anthropogenic land use change and management practices can alter soil C stocks. A concern is that small losses of soil C can contribute to large increases in atmospheric CO2. Research has focused on identifying which land use conversions modify soil C dynamics and more recently, how management practices influence soil C stocks, with particular emphasis on croplands and forests but less on grazed pasture systems. The soil nitrogen (N) cycle has also been modified with increased N inputs, especially under agriculture where N fertilisers and N-fixing plants are used.
About 33% of New Zealand’s total land area is used for grazing. A previous study observed that between the 1980s and 2000s soils on flat land under dairy farming had lost significant amounts of C and N, while soils under drystock farming on flat land had not. A conclusion drawn from the previous study was that a dairy farm was likely have a lower soil C stock than an adjacent drystock farm on the same soil, on flat land. The reasons for the reported soil C and N losses from dairy farm soils are not well understood and require further testing and verification using other approaches.
The objectives of this thesis were to firstly, determine if there was a difference in soil C and N between adjacent dairy and drystock farms on the same soil and secondly, if differences were detected whether they were dependant on differences in farming intensity, as defined by stocking rate.
A synthesis of recent literature showed that when differences in soil C have been observed under various grazing intensities, soil C was generally always lower under higher stocking rates. However, many of the grazing intensity studies were based in semi-arid regions and not particularly applicable to New Zealand’s pastoral grazing systems.
I sampled 25 adjacent dairy and drystock farms (paired sites) on flat land in the Waikato Region to 0.6 m depth and analysed samples for C, N and soil dry bulk density by horizon. Sampling sites at each paired site were an average of 108 m apart and located on the same soil with a similar slope, aspect and topography. The estimated average stocking rate for dairy farms (24 ± 0.8 SU ha-¹) was higher (P<0.01) than drystock farms (14 ± 2.0 SU ha-¹). The mean soil C and N stocks for the whole soil profile (0–0.6 m) were 173.1 ± 12.4 t C ha-¹ and 18.5 ± 0.9 t N ha-¹ for the dairy farms and 182.7 ± 15.0 t C ha-¹ and 19.1 ± 5.7 t N ha-¹ for the drystock farms. The soil C and N stocks for the whole soil profile were not significantly different between dairy and drystock farms. However, when soil horizons were considered separately there was a significant difference in C stocks of the A horizon (P<0.05). The mean soil C in the A horizon under dairying was 94.7 ± 5.7 t C ha-¹ and 103.3 ± 6.1 t C ha-¹ under drystock, with dairy farms having an average of 8.6 ± 4.1 t C ha-¹ less than the drystock farms (P<0.05). No significant difference in soil N stock of the A horizons was detected. The increased variability of soil C and N with depth meant that the significant difference in soil C of the A horizon was not evident when the whole soil profile was considered. The A horizon thickness under dairy farming was shallower (P<0.05) and the soil dry bulk density was higher (P<0.05) than the drystock farms, indicating soil compaction. The total mass of soil sampled from the A horizons was similar for both types of grazing (0.14 ± 0.01 t m-²). Therefore, the significant difference in soil C of the A horizon was likely to be a consequence of land management rather than sampling different masses of soil.
My result that dairy farms had less topsoil C than adjacent drystock farms aligned with the conclusion drawn from a previous study of New Zealand pastoral grazing systems. The result also supported the general trend of less soil C under higher stocking rates than lower stocking rates observed in the literature synthesis. Further work is required to understand what has driven the difference in topsoil C under dairy and drystock farming on flat land in New Zealand. Future research should include exploring how important stocking rates and the type of livestock being grazed are on soil C and N dynamics, as this may be useful information for future farming management decision making
Manipulation of carbon media, temperature and hydraulic efficiency to increase nitrate removal rate in denitrification beds
No full textThe accumulation of reactive nitrogen (Nr) in terrestrial and aquatic environments is a global environmental issue that causes or contributes to climate change, stratospheric ozone depletion, and deterioration of coastal and terrestrial waters. Point source discharges of Nr from municipal and septic treatment systems, agricultural tile drainage, and industrial discharges contribute to these issues. Practical, low-cost methods are needed to reduce the Nr load into the environment from small-volume point source discharges. Denitrification beds are one such method. Improving the nitrate removal rate of denitrification beds will lead to reduced bed volumes, lower construction costs that likely facilitate greater uptake of the technology and reduced accumulation of Nr in the environment. The main objective of this thesis was to test a number of approaches that might increase the rate of nitrate removal rate in a denitrification bed under non-nitrate limiting conditions, including: manipulation of carbon source, temperature and hydraulic flow. To date, operational denitrification beds have used wood media as the carbon source which sustains nitrate removal rates of between 2–10 g N m-3 of media d-1 and relatively high permeability. While previous laboratory experiments have investigated the potential of alternative carbon sources, these studies were typically of short duration and small scale and did not necessarily provide reliable information for denitrification bed design purposes. To address this issue, nitrate removal, hydraulic and nutrient leaching characteristics of nine different carbon substrates were compared in 0.2 m3 barrels, at 14oC and 23.5oC over a 23 month period. The relationship between hydraulic efficiency and nitrate removal of the different media was also investigated. Findings from the barrel trial were field tested in pilot scale (2.9 m3) denitrification beds receiving municipal effluent dosed with KNO3, over a 15 month period. The pilot scale trial tested whether nitrate removal could be improved by using an alternative carbon media (maize cobs) and increasing bed temperature through passive solar heating. The influence of bed flow regime (horizontal-point, horizontal-diffuse, downflow and upflow) on hydraulic efficiency and nitrate removal was also investigated. This thesis demonstrated that more labile carbon sources, such as maize cobs, had significantly higher nitrate removal rates (15.0 to 21.8 g N m-3 d-1) than wood media (3.0 to 4.9 g N m-3 d-1) over the duration of the barrel trial. Nitrate removal rates increased with increasing temperature with mean Q10 of 1.6 for all media. The hydraulic efficiency of fragmented wood media decreased with increasing grain-size. However, nitrate removal rate was not dependent on hydraulic efficiency of the media, which was attributed to the significant secondary porosity of the media allowing denitrification to occur both on the surface and within the media particle. In the pilot scale trial, bed temperature increased by 3.4oC due to passive solar heating, but did not cause a measureable increase in nitrate removal rate due to variability in removal rates and possibly low temperature responsiveness of maize cobs for removing nitrate. Flow regime affected the hydraulic efficiency of denitrification beds and nitrate removal rates were lower in flow regimes with poor hydraulic efficiency. This was attributed to short-circuit flow reducing the bed volume that contributed to nitrate removal. The results indicate that a four-fold reduction in denitrification bed size could potentially be achieved by using maize cobs as the carbon substrate, as opposed to wood fragments, and increasing bed temperature by incorporating passive solar heating techniques. The findings of this thesis indicate that future research on improving the nitrate removal rate of denitrification beds under non-nitrate limiting conditions should focus on carbon substrates, increasing bed temperature, and hydraulic design of beds rather than on hydraulic efficiency of media. For example, research on coupling improved solar heating design with an appropriate inlet/outlet structure and location
Detection of differences in soil carbon and nitrogen stocks between paired dairy and drystock pastures
No full textSoil is the largest terrestrial store of carbon (C) with some 2000 Pg to a depth of 1 m compared to 500 Pg in the atmosphere. Maximizing storage of C in soil is not only important for reducing atmospheric CO2 concentrations but also for maintaining soil quality. Recent research has shown that land use management is a key factor in determining the storage of C in pastoral systems. Barnett et al. (2014, AEE 185:34-40) used a paired pit approach to sample 25 adjacent dairy and drystock pastures to a fixed depth of 0.6 m and showed that soils under drystock sites had about 8.6 t.ha-1 more C in the top soil than adjacent dairy sites (P<0.05). However, there was no significant difference between land uses when C was accumulated to 0.6 m.
The main objective of this research was to test a potentially more accurate method for estimating differences in C stocks between sites sampled by Barnett et al. (2014), with a second objective being to better understand the effect of dairy and drystock grazed pastures on soil C and N stocks. A third objective was to investigate the effect of dairy and drystock managed pastures on earthworm abundance and biomass.
A synthesis of recent literature showed that measuring differences in soil C stocks is difficult, given the high variability of soil C over small spatial scales. However, careful consideration to sampling methodology and statistical analysis can greatly improve the detection of differences in soil C stocks.
Twenty three paired dairy and drystock sites were sampled to a depth of 0.6 m by taking 5 soil cores from each of two plots (5x5 m) within a paddock of each land use and soil C/N and soil mass were determined. Seventeen of the paired dairy and drystock farms were sampled from 3 points in each paddock between August and November 2013 for earthworms. Samples were sorted and earthworms were classified to species level.
To a depth of ~60 cm (C stocks adjusted for equivalent soil mass), drystock sites had 1.6 t ha-1 more C than dairy sites but this was not significant. However, when soil layers were analysed separately, drystock sites contained more C (4.1 ± 2.1 t C ha-1) in the top 10 cm (P=0.06) and dairy farms had significantly more C (3.7 ± 1.7 t C ha-1) in the 25-60 cm layer (P=0.04). The difference in the relative distribution of soil C in dairy and drystock sites may be due to the greater size and concentration of dairy urine patches which can solubilise C in the top-soil and redeposit dissolved C lower in the profile.
When comparing whole-profile C stocks between dairy and drystock sites, the two-plot coring approach would have been able to detect a true difference of 9.3 t C ha-1, had it occurred, compared to 13.6 t C ha-1 for the pit approach (P<0.05). For the purpose of providing information for future sampling, power analysis was also conducted and revealed that with 23 paired sites, the pit approach could detect a significant difference (P<0.05) of 16 t C ha-1 with 66% certainty. In contrast, the coring approach could detect the same difference of 16 t C ha-1 with 90% certainty. These results supported the literature synthesis which demonstrated that sampling methodologies that include spatial variability of soil C can greatly improve the detection of differences. Furthermore, the coring approach reduced cost and increased efficiency compared to the single-pit approach.
Earthworm abundance and biomass were not significantly different between dairy and drystock farms despite the significantly higher grazing intensity and top soil bulk density of dairy sites. Total earthworm abundance and biomass averaged 193 ± 30 ind m-2 and 77 ± 12 g m-2 for dairy farms compared to 188 ± 26 ind m-2 and 75 ± 13 g m-2 for drystock farms. These results suggested that for Allophanic Soils in the Waikato Region, the effects of varying grazing management on earthworm abundance and biomass is negligible
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