Storing carbon in soil. Can we slow a revolving door?

There 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

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

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⁻¹ yr⁻¹ 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 (r ² = 0.74, P < 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⁻¹ 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)

In situ mixing of organic matter decreases hydraulic conductivity of denitrification walls in sand aquifers

In a previous study, a denitrification wall was constructed in a sand aquifer using sawdust as the carbon substrate. Ground water bypassed around this sawdust wall due to reduced hydraulic conductivity. We investigated potential reasons for this by testing two new walls and conducting laboratory studies. The first wall was constructed by mixing aquifer material in situ without substrate addition to investigate the effects of the construction technique (mixed wall). A second, biochip wall, was constructed using coarse wood chips to determine the effect of size of the particles in the amendment on hydraulic conductivity. The aquifer hydraulic conductivity was 35.4 m/d, while in the mixed wall it was 2.8 m/d and in the biochip wall 3.4 m/d. This indicated that the mixing of the aquifer sands below the water table allowed the particles to re-sort themselves into a matrix with a significantly lower hydraulic conductivity than the process that originally formed the aquifer. The addition of a coarser substrate in the biochip wall significantly increased total porosity and decreased bulk density, but hydraulic conductivity remained low compared to the aquifer. Laboratory cores of aquifer sand mixed under dry and wet conditions mimicked the reduction in hydraulic conductivity observed in the field within the mixed wall. The addition of sawdust to the laboratory cores resulted in a significantly higher hydraulic conductivity when mixed dry compared to cores mixed wet. This reduction in the hydraulic conductivity of the sand/sawdust cores mixed under saturated conditions repeated what occurred in the field in the original sawdust wall. This indicated that laboratory investigations can be a useful tool to highlight potential reductions in field hydraulic conductivities that may occur when differing materials are mixed under field conditions

Use of shallow samples to estimate the total carbon storage in pastoral soils

Using data from pastoral soils sampled by horizon at 56 locations across New Zealand, we conducted a meta-analysis. On average, the total depth sampled was 0.93 ± 0.026 m (± SEM), and on a volumetric basis, the total C storage averaged 26.9 ± 1.8, 13.9 ± 0.6 and 9.2 ± 1.4 kg C m⁻² for allophanic (n=12), non-allophanic (n=40) and pumice soils (n=4), respectively. We estimated the total C storage, and quantified the uncertainty, using the data for samples taken from the uppermost A-horizon whose depth averaged 0.1 ± 0.003 m. For A-horizon samples of the allophanic soils, the mean C content was 108 ± 6 g C kg⁻¹ and the bulk density was 772 ± 29 kg m⁻³, for non-allophanic soils they were 51 ± 4 g C kg⁻¹ and 1055 ± 29 kg m⁻³, and for pumice soils they were 68 ± 9 g C kg⁻¹ and 715 ± 45 kg m⁻³. The C density —a product of the C content and bulk density —of the A-horizon samples was proportional to their air-dried water content, a proxy measure for the mineral surface area. By linear regression with C density of the A-horizon, the total C storage could be estimated with a standard error of 3.1 kg C m⁻², 19% of the overall mean

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)

Evaluating the character and preservation of DNA within allophane clusters in buried soils on Holocene tephras, northern New Zealand

Clay minerals possess sorptive capacities for organic and inorganic matter, including DNA (Lorenz and Wackernagel, 1994), and hence reduce the utilization and degradation of organic matter or DNA by microorganisms. Buried allophane-rich soils on tephras (volcanic-ash beds) on the North Island, dated using tephrochronology, provide a valuable paleobiological ‘laboratory’ for studying the preservation of ancient DNA (aDNA) (Haile et al., 2007). Allophane comprises Al-rich nanocrystalline spherules ~3.5-5 nm in diameter (Fig. 1) with extremely large surface areas (up to 1000 m2 g-1). Moreover, allophanic soils are strongly associated with organic matter (Parfitt, 2009), and so we hypothesize that allophane also plays an important role for DNA protection within such soils

Use of cadmium isotopes to distinguish sources of cadmium in New Zealand agricultural soil: Preliminary results

In New Zealand’s agricultural soils, phosphate fertiliser applications are the main source of cadmium (Cd). In 1997, the NZ fertiliser industry discontinued sourcing rock phosphate from Nauru (about 450 mg Cd/ Kg P) and began producing superphosphate from other rock phosphate sources (such as Morocco), which have generally lower concentrations of Cd. Research on the concentration of Cd in soils from the long-term irrigation trials at the Winchmore research farm (Canterbury) indicates that Cd accumulation rates have started to slow in the period since 1997 (Fig. 1) (McDowell 2012)

Tracing sources of cadmium in agricultural soils using cadmium stable isotopes

The application of phosphate fertilizers has, on a global basis, resulted in long-term accumulation of cadmium (Cd) in agricultural soils [1]. While this accumulation has led to concern over potential environmental consequences, we currently lack a viable tool to track fertilizer- derived Cd in terrestrial environments. In 1997, the main source of phosphate fertilizers in New Zealand (NZ) was changed from Nauru to a mixed product sourced from other phosphorites with lower concentrations of Cd. Around the same time, Cd accumulation in a 66-year-long field trial (Winchmore Farm, South Island, NZ) showed an apparent plateau [2]. In this study, Cd isotope ratios (ɛ114/110Cd) were used to trace Cd sources in Winchmore soil and determine the cause of this plateau. The ɛ114/110Cd was measured in archived phosphate fertilizer, phosphorite and topsoil (0-7.5 cm) samples from Winchmore. The ɛ114/110Cd of fertilized topsoils and fertilizers was distinct from control (unfertilized) subsoils by around +0.6‰. Bayesian isotope modelling using pre- and post-2000 fertilizers and control soil as the endmembers, confirmed the dominant contribution of Cd is from pre-2000 fertilizers (ɛ114/110Cd=2.48±0.37) with signature comparable to source rocks (ɛ114/110Cd=2.19± 0.39) but distinct from control subsoil (ɛ114/110Cd=-3.33 ±0.41). The decline in Cd concentration after 2000 followed the reduction in fertilizer Cd concentration. The ɛ114/110Cd of soil remained quite constant following the source change, confirming that soil Cd represents the historical burden of Cd (originating from Nauru phosphorites) and concurrent applications of fertilizer are not resulting in further accumulation of Cd