Graeme Blair

Nutrient efficiency — what do we really mean?

There are many ways in which plants can adapt or respond to nutrient stress. These can be via alterations to root branching and root extension rates, rate of uptake per unit root length or root dry weight, partitioning between roots and shoots and between shoots and grain and the amount or concentration of the nutrient required for plants to function. Each of these can be altered to some extent by selection, breeding or biotechnology.

Measurement of decomposition and associated nutrient release from straw (Oryza sativa L.) of different rice varieties using a perfusion system.

Maintenance of soil carbon concentrations and synchronisation of nutrient release from crop residues to match crop demand is important if soil organic matter levels are to be maintained and nutrients are to be recycled efficiently. Two laboratory perfusion experiments were conducted to study the decomposition and nutrient release rates from straw of different rice varieties (Oryza sativa L.) which were collected from the field. Cumulative evolution of CO2 and nutrient release from the straw of 20 rice varieties was measured over a 6-week period. The CO2 release rate differed between varieties with the peak of CO2 release varying from 2 to 4 days. After 42 days, the percentage of C released from the straw ranged from 15.4% in variety PSBRc18 to 38.4% in variety Soc Nau. There was an inverse relationship (r2=0.60) between cumulative C release and C:N ratio and a direct relationship (r2=0.62) between digestible organic matter (DOM) and cumulative C release. A Straw Quality Index (SQI) was developed to describe the decomposition rate of the rice straw as follows:SQI= -56.85+(11.68 × % N) + (1.25 × % DOM) +(2.59 × % lignin) r2 = 0.81These findings indicate that SQI has a potential in assessing the quality of straw residue materials in predicting their usefulness in crop-residue management systems. Varieties, Soc Nau, IR67962 and PSBRc18, were used in a second perfusion experiment to determine C and nutrient release at 5, 10 and 20 days under aerobic and anaerobic conditions. Despite a two-fold difference between varieties in the amount of C evolved over 20 days, the proportion of nutrient release did not differ significantly between them. When perfusion was performed under anaerobic conditions, there were 45, 32 and 27% reductions in C evolution compared to aerobic conditions from Soc Nau, IR67962 and PSBRc18, respectively, but no differences in the proportion of N, P, S, K Mg and Na released between the aeration systems. These results indicate that under flooded conditions, depletion of O2 decreases decomposition rate of straw but the initial rate of nutrient release is unaffected. This uncoupling of C and nutrient release appears to be related to the more labile components of the nutrients present in the plants and their physiological role.

Tracing the nitrogen, sulfur, and carbon released from plant residues in a soil/plant system

Matching plant residue mineralisation rate to plant nutrient demand is one way of increasing the efficiency of nutrient cycling. A glasshouse experiment was conducted in a Soloth soil with a C4d13 C signature using drained pots to examine the effect on the yield of Japanese millet (Echinochloa frumentocea) and the fate of 15 N, 35 S, and C (using d 13 C shift) from the C 3 plants Flemingia macrophylla, Medicago truncatula hay, and wheat (Triticum aestivum) straw applied at 3 t/ha in the presence of N and NPKS fertiliser. The yield of Japanese millet at 91 days was highest where medic hay had been added (13.7 g/pot) and lowest where wheat straw was used (11.5 g/pot). Recovery of 35 S by the millet was highest in the wheat straw and medic hay treatments (mean 11.5%), whilst recovery of 15 N was highest from medic hay (15.8%). Leaching losses of 35 S were highest in the Flemingia and medic treatments (mean 8.1%), and 15 N loss in leachate was highest in the medic hay treatment (6.6%). A maximum of 1.5% of the C added in residues was recovered in the leachate of the medic hay treatment.

Phosphorus Use-Efficiency by Cotton Grown in an Alkaline Soil as Determined Using 32Phosphorus and 33Phosphorus Radio-Isotopes

ABSTRACT 32Phosphorus (P) and 33P radioisotopes were used to identify the contribution of soil and fertilizer P sources to P uptake by cotton (Gossypium hirsutum L.) grown in an alkaline soil representative of those used for growing cotton in Australia. Phosphorus fertilizer application only increased P concentration in the plants during leaf expansion, but had no effect on biomass production, P uptake at later growth stages sampled, or lint yield. However, at flowering, more than 50% of the P taken up was derived from within the fertilized band of soil, even though recovery of the 45 kg P ha−1 applied was low (1.2–3.4%). Phosphorus fertilizer application did not increase lint yield of cotton in this soil. Although P fertilizer recovery was poor, P fertilizer remained in an available form throughout the experiment indicating that pre-plant applications can remain effective. However, they will only be beneficial when soil P is limiting to crop growth.

Diet selection and productivity of sheep grazing contrasting pastures

A grazing experiment was conducted at the Big Ridge 2 site CSIRO, Chiswick (30˚31′S, 151˚39′E), 20 km south of Armidale, New South Wales, Australia. The site was established in 1955. In March 1966, phalaris and white clover were sown and pastures were fertilised annually with superphosphate until 1993. There were 3 pasture treatments, each with 2 replicates: degraded pasture (low phalaris content), phalaris-dominant, and phalaris–white clover. The effect of pasture type on animal production (liveweight gain and wool) was only significant in 1996, when there were large differences in pasture composition and production between the 3 pasture types. n-Alkane based estimates showed that pasture degradation affected diet selection and nutrient intake and thus sheep production. The estimates in this study also showed no clear preference for a single pasture species over time and lack of strong preferential selection for clovers when sheep were grazing 3 contrasting pastures. Preferential selection of a particular species varied over time depending on the presence and availability of alternative species. Although there were large differences in total N and S intake and faecal output between the 3 pastures, the proportion of the dietary nutrient used for production was similar. This observation reveals the importance of further improving pasture and grazing management, particularly in productive phalaris–white clover pasture with high nutrient flux, to improve nutrient recycling through plant uptake and retention by animals in the grazing ecosystem, and reduce losses.

Sulfur soil testing

A wide range of extractants have been used to extract S from soils but correlations with plant response have generally been poor. Direct evidence that ester sulfates contribute to plant S supply lead to the development of the 0.25 M KCl extract heated at 40°C for 3 hours. Sequential extraction showed that this extractant removed 5.9% of ester sulfates from unfertilised soil. Support of the efficacy of the extract was obtained using specific radioactivity data from plants and soil extract.

Modelling of sulfur oxidation from elemental sulfur

An elemental S oxidation model has been developed which combines a maximum S release rate with modifiers for temperature and soil moisture conditions. This model has been combined with a pasture growth and CNSP nutrient cycling model to match S oxidation rate to pasture S demand. In two Southern Australian enviroments, 100μm elemental S was superior to 200μm particles whilst in Northern Australia the 200μm particles were superior. These models can be used to match S release to plant demand.

Measurement of decomposition and associated nutrient release from barrel medic (Medicago truncatula) hay and chickpea (Cicer arietinum) straw using an in vitro perfusion system

Two in vitro laboratory experiments were conducted to evaluate the effects of nutrient additions, soil inoculation, and sampling times on the decomposition rates and nutrient release patterns of leguminous crop residues. Cumulative CO2 evolution and the percentage nutrient release from barrel medic hay (Medicago truncat- ula) and chickpea straw (Cicer arietinum) were determined over a 6-week period using a 0.005 M CaCl2 recycling system (UNE in vitro perfusion apparatus). The decomposition study was carried out at 25°C in a controlled tem- perature laboratory. There were no significant differences between the decomposition rates of the 2 residues. The percentage C release was not significantly affected by nutrient addition or by soil inoculation. More than 50% of the total C that was released from the residues occurred in the first 7 days with peak release at 3-4 days. Because of an accumulation of nutrients in a muscilaginous substance in the apparatus, and adsorption of P to the plastic, it was not possible to measure the release of nutrients from the residues by sampling the perfusion solution. This had to be achieved by direct measurement of loss of nutrients from the residues. Except for K, the nutrient released from residues was not affected by residue or sampling time.

Enhancing carbon sequestration in soil with coal combustion products: a technology for minimising carbon footprints in coal-power generation and agriculture

Coal-fired power generation and agriculture account for more than half of global greenhouse gas emissions, but the coal fly ash (CFA) produced in the former can be a resource for reducing emissions from agriculture to minimise environmental footprints in both industries. Our aim in this study was to test how acidic and alkaline CFA addition could minimise loss of C and N from acidic soil, with or without added manure. We determined composition and structural characteristics of acidic and alkaline CFA for their capacity to adsorb organic carbon, but observed poor adsorption because of low concentrations of cenospheres and unburnt carbon as the primary absorbents in the ash. Addition of CFA had no impact on the loss of carbon or nitrogen from unmanured soil in which concentrations of these nutrients were low. Loss of carbon from manured soil was reduced by 36xa0% with alkaline ashes and by 3-fold with acidic ashes; while loss of N was 30–50xa0% lower with acidic ashes, but 28xa0% higher with alkaline ashes, compared with no ash treatment. The increases in C sparing with CFA addition were achieved not by direct C absorption but by restraining microbial population and respiration, and potentially emissions. Alkaline CFA increased soil pH and if used to substitute just 10xa0% of lime for ameliorating soil acidity would reduce CO2 emission associated with the mining of the lime and its eventual dissolution in soil byu2009~u20092.66 Tg or 2.8xa0% of Australia’s annual agricultural emissions. High concentrations of oxides of phosphorus, silicon, titanium and clay particles in acidic ashes, and oxides of cations in alkaline ashes, were associated with potential for promoting C storage and acidity amelioration in soil.

In Situ Acidulation of Rock Phosphate

ABSTRACT This study was undertaken to study the impact of adding <75 μm elemental sulfur (ES) on P availability from a range of <250 μm ground rock phosphates (RP) namely; Minjingu (Mi) from Tanzania, Khouribga from Morocco (Mo) and Duchess (D) from Australia. Italian ryegrass (Lolium multiflorum, Thumpa tetraploid) grown in a glasshouse was used as the test crop and tops were harvested 5 times over 27 weeks. Co-granulating RP and 10.7% ES yielded 30–70% more ryegrass tops than RP alone, with the greatest effect with Mi. Fertilizer P recovery in the tops from the RP was 5.2% with D and 6.5% and 7.8% from Mi and Mo, respectively. ES addition increased this by 51% from Mo, 98% from D and 194% from Mi. Co-granulation of RP with ES has been shown to be an effective means of releasing plant available P to crops from RP with minimal fertilizer processing.