The effects of short term, long term and reapplication of biochar on soil bacteria

Biochar has been shown to affect soil microbial diversity and abundance. Soil microbes play a key role in soil nutrient cycling, but there is still a dearth of knowledge on the responses of soil microbes to biochar amendments, particularly for longer-term or repeated applications. We sampled soil from a field trial to determine the individual and combined effects of newly applied (1 year ago), re-applied (1 year ago into aged biochar) and aged (9 years ago) biochar amendments on soil bacterial communities, with the aim of identifying the potential underlying mechanisms or consequences of these effects. Soil bacterial diversity and community composition were analysed by sequencing of 16S rRNA using a Miseq platform. This investigation showed that biochar in soil after 1 year significantly increased bacterial diversity and the relative abundance of nitrifiers and bacteria consuming pyrogenic carbon (C). We also found that the reapplication of biochar had no significant effects on soil bacterial communities. Mantel correlation between bacterial diversity and soil chemical properties for four treatments showed that the changes in soil microbial community composition were well explained by soil pH, electrical conductivity (EC), extractable organic C and total extractable nitrogen (N). These results suggested that the effects of biochar amendment on soil bacterial communities were highly time-dependent. Our study highlighted the acclimation of soil bacteria on receiving repeated biochar amendment, leading to similar bacterial diversity and community structure among 9-years old applied biochar, repeated biochar treatments and control.

Quantifying pasture dry matter responses to applications of potassium fertiliser for an intensively grazed, rain-fed dairy pasture in south-western Australia with or without adequate nitrogen fertiliser

Rain-fed dairy pastures on sandy soils common in the high rainfall (>800 mm annual average) Mediterranean-type climate of south-western Australia comprise the annual species subterranean clover (Trifolium subterraneum L.) and annual and Italian ryegrass (Lolium rigidum Gaud. and L. multiflorum Lam.). In wet years, clover becomes potassium (K) deficient and shows large dry matter (DM) responses to applied fertiliser K due to leaching of K in soil by rainfall. In contrast, ryegrass rarely shows DM responses to applied K. Many dairy pastures in the region are now intensively grazed to maximise pasture use for milk production, and nitrogen (N) fertiliser is applied after each grazing. It is not known if frequent applications of fertiliser N to these pastures changes pasture DM responses to applied K. Therefore, a long-term (2002–07) field experiment was undertaken on an intensively grazed dairy pasture in the region to quantify pasture DM responses to applied fertiliser K with or without applications of adequate fertiliser N (141–200 kg N/ha per year). Soil samples (top 10 cm of soil) were collected from each plot of the experiment each February to measure soil test K by the standard Colwell sodium bicarbonate procedure used for both K and phosphorus soil testing in the region. When no N was applied, pasture comprised ~70% (dry weight basis) clover and 25% ryegrass, compared with ~70% ryegrass and 25% clover when adequate N was applied. Significant linear responses of pasture DM to applied K occurred in 3 of the 6 years of the experiment only when no N was applied and clover dominated the pasture. The largest response varied from ~1.7 to 2.0 t/ha DM consumed by dairy cows at all grazings in each year, giving a K response efficiency of between 8 and 10 kg DM/ha per kg K/ha applied. Significant pasture DM responses to applied N occurred at all grazings in each year, with ~2–3 t/ha extra DM consumed by dairy cows at all grazings in each year being produced when a total of 141–200 kg N/ha was applied per year, giving an N response efficiency of ~7–19 kg DM/ha per kg N/ha applied. Soil test K values were very variable, attributed to varying proportions of soil samples per plot collected between and within cow urine patches, containing much K, arbitrarily deposited on experimental plots during grazing. Soil test K values were not significantly affected by the rates of K applied per year. A re-evaluation of results from the major soil K test study conducted for pastures in the region confirm that ryegrass rarely showed DM responses to applied K, and that for clover, soil K testing poorly predicted the likelihood of K deficiency in the next growing season.

Responses of intensively grazed dairy pastures to applications of fertiliser nitrogen in south-western Australia

For the first time, we quantified pasture dry matter (DM) responses to applied fertiliser nitrogen (N) for intensively grazed, rain-fed, dairy pastures on sandy soils common in the Mediterranean-type climate of south-western Australia. The pastures are composed of subterranean clover (Trifolium subterraneum L.) and annual and Italian ryegrass (Lolium rigidum Gaud. and L. multiflorum Lam.). Six rates of N, as urea (46% N), were applied to 15 m by 15 m plots four times during 2002 and after each of the first 5–7 grazings in 2003 and 2004, throughout the typical April–October growing season. Total rates of N applied in the first year of the experiments were 0, 60, 120, 160, 200 and 320 kg N/ha, which were adjusted in subsequent years as detailed in the ‘Materials and methods’ section of this paper. The pastures in the experiments were rotationally grazed, by starting grazing when ryegrass plants had 2–3 leaves per tiller. The amount of pasture DM on each plot was measured before and after each grazing and was then used to estimate the amount of pasture DM consumed by the cows at each grazing for different times during the growing season. Linear increases (responses) of pasture DM to applied N occurred throughout the whole growing season when a total of up to 320 kg N/ha was applied in each year. No maximum yield plateaus were defined. Across all three experiments and years, on average in each year, a total of ~5 t/ha consumed DM was produced when no N was applied and ~7.5 t/ha was produced when a total of 200 kg N/ha was applied, giving ~2.5 t/ha increase in DM consumed and an N response efficiency of ~12.5 kg DM N/kg applied. As more fertiliser N was applied, the proportion of ryegrass in the pasture consistently increased, whereas clover content decreased. Concentrations of nitrate-N in the DM consistently increased as more N was applied, whereas concentrations of total N, and, therefore, concentration of crude protein in the DM, either increased or were unaffected by applied N. Application of N had no effect on concentrations of other mineral elements in DM and on dry matter digestibility and metabolisable energy of the DM. The results were generally consistent with findings of previous pasture N studies for perennial and annual temperate and subtropical pastures. We have shown that when pasture use for milk production has been maximised in the region, it is profitable to apply fertiliser N to grow extra DM consumed by dairy cows; conversely, it is a waste of money to apply N to undergrazed pastures to produce more unused DM.