Carbon in soils

Carbon is the fourth most common element in the galaxy(by mass) but does not even rank in the twelve most abundant elements on Earth. By far the most abundant source of carbon on Earth is in the crust as inorganic rocks such as calcite and limestone in marine and sedimentary deposits. These rocks have taken many millions of years to form. Other major inorganic sources are in the oceans and atmosphere

Nitrogen, soils and environment

The article discusses the risk of damaging the environment brought by nitrogen fertilisers which are used for increasing agricultural productivity. The oxidation of ammonium allows for the formation of nitrate. Troposphere ozone and aerosols are produced through the increase of reactive nitrogen in the atmosphere

Multiple small monthly doses of dicyandiamide (DCD) did not reduce denitrification in Waikato dairy pasture

The effectiveness of multiple small doses of the nitrification inhibitor dicyandiamide (DCD) to decrease denitrification under warm moist conditions was tested in a 1-year field trial on a grazed dairy pasture. DCD was applied approximately every 4 weeks as an aqueous spray onto ten replicate plots 3 days after rotational grazing by dairy cows. Each application was at the rate of 3 kg DCD ha⁻¹, with a total annual application of 33 kg ha⁻¹. Denitrification was assessed 5 days after each DCD application using the acetylene block method. At the end of the trial, the rate of degradation of DCD under summer conditions was measured. DCD significantly decreased the mean annual nitrate concentration by about 17%. Denitrification and denitrification enzyme activity were highly variable and no significant effect of DCD in decreasing denitrification was detected. In the summer month of December, DCD degraded rapidly with an estimated half-life of 5 ± 3 days (mean and standard deviation)

Anti-self-dual conformal structures with null Killing vectors from projective structures

Using twistor methods, we explicitly construct all local forms of four--dimensional real analytic neutral signature anti--self--dual conformal structures $(M,[g])$ with a null conformal Killing vector. We show that $M$ is foliated by anti-self-dual null surfaces, and the two-dimensional leaf space inherits a natural projective structure. The twistor space of this projective structure is the quotient of the twistor space of $(M,[g])$ by the group action induced by the conformal Killing vector. We obtain a local classification which branches according to whether or not the conformal Killing vector is hyper-surface orthogonal in $(M, [g])$. We give examples of conformal classes which contain Ricci--flat metrics on compact complex surfaces and discuss other conformal classes with no Ricci--flat metrics.Comment: 43 pages, 4 figures. Theorem 2 has been improved: ASD metrics are given in terms of general projective structures without needing to choose special representatives of the projective connection. More examples (primary Kodaira surface, neutral Fefferman structure) have been included. Algebraic type of the Weyl tensor has been clarified. Final version, to appear in Commun Math Phy

3-dimensional Cauchy-Riemann structures and 2nd order ordinary differential equations

The equivalence problem for second order ODEs given modulo point transformations is solved in full analogy with the equivalence problem of nondegenerate 3-dimensional CR structures. This approach enables an analog of the Feffereman metrics to be defined. The conformal class of these (split signature) metrics is well defined by each point equivalence class of second order ODEs. Its conformal curvature is interpreted in terms of the basic point invariants of the corresponding class of ODEs

Accumulation of soil organic C and change in C:N ratio after establishment of pastures on reverted scrubland in New Zealand

Rates of organic carbon accumulation and changes in C:N ratio are reported for 10 New Zealand soils converted to pastures from scrub. The data were derived from archive papers originally published in 1964, but which did not report on changes in the C contents of the soils. The soils had been sampled to 0–7.5, 7.5–15, and 15–30 cm depths and chronosequences of up to 66 years obtained by selecting sites with differing times since pasture establishment. We calculated changes in the mass of C and N in the 0–7.5 cm depth and compared that to the mass in the 0–30 cm depth of soil. The shortest time over which organic matter change was assessed was 18 years and the longest was 66 years. Nine of the ten soils showed increases in the C contents of the 0–7.5 cm depth soil, and a natural logarithmic curve generally gave a better fit to the time course data than a linear fit. However, when the full 0–30 cm depth was considered, only two soils showed a significant increase in total C, changes in the C contents of other soils were non-significant, and two soils showed a decline in total C. The rates of change in the C contents were averaged over 0–5 years, 5–25 years and 25–50 years. Across all 10 soils, the mean rates of accumulation of C in the 0–7.5 cm depth were 1.07 (between 0 and 5 years), 0.27 (between 5 and 25 years) and 0.09 Mg C ha−1 year−1 (between 25 and 50 years) and significantly (P < 0.05) greater than zero. Very similar rates were obtained for the 0–30 cm depth of soil with mean rates across all soils 1.01 (0–5 years), 0.25 (5–25 years) and 0.09 Mg C ha−1 year −1 (25–50 years), respectively. In the 0–7.5 cm depth of soil, total Kjeldahl N (TKN) increased significantly in seven of the 10 soils. When expressed for the 0–30 cm depth of soil, only five soils still showed significant increases in TKN contents over time. Using the data for the 0–7.5 cm depth, the predicted time (mean and standard error) for the soils to reach a C:N ratio of <10 was 46 ± 17 years. The soils were originally sampled over 44 years ago, suggesting that currently (2009), very few of them could be expected to have capacity for further N storage in organic matter in the surface soil unless there was an increase in soil C. Changes in soil C and N in the shallow upper soil layers are easily masked by the relatively small changes in C and N contents and much greater masses of soil at lower depths

Rates of accumulation of cadmium and uranium in a New Zealand hill farm soil as a result of long-term use of phosphate fertilizer

In New Zealand, phosphate (P) fertilisers used in agriculture are the main sources of the potentially toxic elements cadmium (Cd) and uranium (U), which occur as unwanted contaminants. New Zealand is developing draft soil guideline values (SGV) for maximum concentrations of Cd. To assess when soils under pasture for sheep production might reach a particular SGV, we analysed archived soil samples from a 23 yr P fertiliser trial. The pasture sites were at Whatawhata, North Island, New Zealand, and had received P fertiliser at the rates of 0, 30, 50 and 100 kg P ha⁻¹ yr⁻¹. From 1983 to 1989, P was applied as single superphosphate, from 1989 to 2006, P was applied as triple superphosphate. Soils from replicate paddocks were sampled annually to a depth of 75 mm on easy (10–20°) and steep (30–40°) slope classes. Total P, Cd and U were analysed by ICP-MS after acid digestion. Data were analysed by fitting trend lines using linear mixed models for two slope classes and for two sampling periods 1983–1989 and 1989–2006 when the soil sampling method and fertiliser type had been changed. The changes in total P, Cd and U were directly related to the type and amount of P fertiliser applied, the control treatment showed no significant change in P, Cd or U. At 50 and 100 kg P ha⁻¹ yr⁻¹ there were generally linear increases in total P and total U, and the same trend line applied to both time periods, but the rate of increase in P was greater on the easy slope class. For Cd, a “broken stick” model was needed to explain the data. Pre-1989, Cd increased in the 50 and 100 kg P ha⁻¹ yr⁻¹ treatment (0.036–0.045 mg kg⁻¹ yr⁻¹, respectively): post 1988 the rate of increase declined markedly on those two treatments (0.005–0.015 mg kg⁻¹ yr⁻¹, respectively), and declined absolutely in the 30 kg P ha⁻¹ yr⁻¹ treatments. The maximum content of Cd was in the 100 kg P ha⁻¹ yr⁻¹ treatment which reached 0.931 mg Cd kg⁻¹ on the easy slope. For U there were steady linear increases for the 30, 50 and 100 kg P ha⁻¹ treatments, and no significant difference between the steep and easy slopes, nor the two sampling periods, the maximum concentration obtained was 2.80 mg U kg⁻¹ on the 100 kg P ha⁻¹ treatment. The results suggest that at rates of P fertiliser likely to be applied to hill farms (<50 kg P ha⁻¹ yr⁻¹ ), and using P fertiliser with low Cd content, then the Cd concentration in this soil will never reach a SGV of 1 mg kg⁻¹