Moira Dexter

Effects of dairy factory effluent application on nutrient transformation in soil

Abstract Dairy factory effluent (DFE) contains significant amounts of nutrients such as nitrogen (N), phosphorus (P), potassium (K), and sulphur (S) which are beneficial to plant growth. It also contains high amounts of carbon (C). Lately, there has been some concern that DFE application to pastoral land is adversely affecting plant growth in some regions of New Zealand. In this study, we determined the mineralisation and immobilisation of nutrients particularly C, N, S, and cations, in a DFE‐treated Omeheu sandy loam soil. We report findings from laboratory‐based open incubation studies carried out at 10, 20, and 30°C, with four rates of DFE application (0, 150 000, 300 000, and 450 000 litres ha–1) alone and with added NO3 – (100 kg N ha–1). The DFE was applied at two‐weekly intervals into packed soil columns which were leached with 0.01 MCaCl2 solution. Leachates were analysed for total C, total N, SO4 2–, NO3 ‐, NH4 +, K+, Na+, and Mg2+. Effects of DFE application on soil microbial bio‐mass‐C, hot‐water extractable‐C, and anaerobically mineralisable‐N were also determined. Addition of DFE increased the size of the microbial biomass pool and thereby enhanced immobilisation of nutrients, mainly N and S. The immobilisation was greater at higher temperature. At 10°C, microbes were unable to utilise all of the added C, even at the lowest rate of DFE application, and 40–50% of the C was leached from soil columns. However, at 30°C soil microbes either immobilised or respired between 95–97% of the C added from DFE, and only small amounts of C were measured in the leachates. Addition of NO3 –‐N had no significant influence on the C immobilisation or respiration. Most of the added N (92–97%) from DFE remained immobilised in the soils throughout the study. A high proportion of the NO3 –‐N added with DFE was immobilised in soils at 10 and 20°C, showing the dominating influence of soluble C, added through the two‐weekly application of DFE, in stimulating microbial activity and causing a prolonged immobilisation of N. There was a net mineralisation of about 100 μg NO3 –‐N g–1 soil at 30°C, indicating faster metabolic use of soluble C from DFE by microbes at this temperature. Between 15–35% of the SO4 2–‐S applied from DFE was either immobilised by soil microbes or was adsorbed on soil organic matter. The presence of significant amounts of NH4 + in DFE‐treated soils suggests that parts of the soil columns may have become anaerobic during incubation, causing mineralisation of N from the death of aerobic microbes or decomposition of soil organic matter. A high proportion of the cations (K+, Na+, and Mg2+) that were added with DFE leached out, indicating that DFE application would have very little effect on the availability of these cations for plant uptake. This study, in part, explains that the poor performance of DFE application on pastoral soils predominantly arises through its effects on the availability of N for plant growth.

Addition of straw or sawdust to mitigate greenhouse gas emissions from slurry produced by housed cattle: a field incubation study.

The use of housed wintering systems (e.g., barns) associated with dairy cattle farming is increasing in southern New Zealand. Typically, these wintering systems use straw or a woodmix as bedding material. Ammonia (NH) and greenhouse gas (GHG) emissions (nitrous oxide [NO] and methane [CH]) associated with storage of slurry + bedding material from wintering systems is poorly understood. A field incubation study was conducted to determine such emissions from stored slurry where bedding material (straw and sawdust) was added at two rates and stored for 7 mo. During the first 4 mo of storage, compared with untreated slurry, the addition of sawdust significantly reduced NH and CH emissions from 29 to 3% of the initial slurry nitrogen (N) content and from 0.5 to <0.01% of the initial slurry carbon (C) content. However, sawdust enhanced NO emissions to 0.7% of the initial slurry-N content, compared with <0.01% for untreated slurry. Straw generally had an intermediate effect. Extending the storage period to 7 mo increased emissions from all treatments. Ammonia emissions were inversely related to the slurry C:N ratio and total solid (TS) content, and CH emissions were inversely related to slurry TS content. Mitigation of GHG emissions from stored slurry can be achieved by reducing the storage period as much as possible after winter slurry collection, providing ground conditions allow access for land spreading and nutrient inputs match pasture requirements. Although adding bedding material can reduce GHG emissions during storage, increased manure volumes for carting and spreading need to be considered.

Nitrous oxide, ammonia and methane emissions from dairy cow manure during storage and after application to pasture

Housing dairy cattle off-paddock in animal confinement facilities provides an alternative to grazing winter crops in southern New Zealand, but the associated greenhouse gas (GHG) emissions from such systems are poorly understood. Nitrous oxide (N2O), methane (CH4) and ammonia (NH3) emissions were measured from stored and land-applied dairy manures collected from winter-housed cows. Emissions increased with storage period, suggesting that losses could be minimised by reducing the storage period, provided ground conditions are suitable for manure spreading and there is minimal risk of nutrient run-off/leaching. After land application, NH3 emissions varied according to manure type, whereas N2O and CH4 emissions were negligible. The rate of manure application did not influence emission factors for any of the three GHGs. When manure emissions were converted to carbon dioxide equivalents (CO2e) per wintered cow, it was found that relative GHG emissions from manure were greatest during storage compared with land application, although these results require verification on a farm scale.

Predicting nitrogen supply from dairy effluent applied to contrasting soil types

ABSTRACT A closed incubation assay was conducted to identify attributes that can be used to predict nitrogen (N) supply from dairy cattle effluents applied to contrasting soil types. The experimental design included six slurry (DM 7.9–13%) and six solid (DM 15.9–42.5%) effluents applied to a Horotiu sandy loam or a Templeton silt loam with treatments incubated at 70% of field capacity at 20°C for 2, 5, 14, 21, 35, 42, 63, 96, 119 and 175 days. After 175 days, inorganic N supply (INS) ranged from −38.6 to 82.8% of total effluent N applied while net N mineralisation (NNM) ranged from −107.4 to 65.1% of organic effluent N applied. The best predictor of INS at day 175 was the log ratio of total C to water extractable N (P < 0.001, r = −0.85) and the log ratio of DM to water extractable N (P < 0.001, r = −0.85) (water extractable N = water soluble N + hot water extractable N). These measures were inversely related to INS and effective at explaining both net mineralisation and immobilisation effects. Importantly, correlations between INS and C:N ratio measures were improved considerably when total N was substituted for a labile N measure, in this case water extractable N. We were unable to identify an effluent measure to consistently predict NNM.