Effects of long-term inputs of fertiliser and irrigation on soil carbon under grazed pasture

Objective: To investigate and quantify the effects of long-term increases in pasture production caused by variable inputs of fertiliser and irrigation on soil profile carbon under permanent grazed pasture.Funding provided by the Agricultural and Marketing Research and Development Trust, The New Zealand Ministry of Agriculture and Forestry, and Environment Canterbury

Effects of past and current crop management on leaching losses, soil microbial community composition and activity

A lysimeter experiment was conducted to investigate the effect of past and current management practices on soil microbial biomass, activity and community composition. Intact monolith lysimeters were taken from sites of the same soil type that had been under long-term organic and conventional crop management. They were subjected to the same crop rotation and managed according to best organic and conventional practices. Mineral N loss was determined in leachates and soil samples taken before the experiment and after 30 months were analysed for biological soil properties, including microbial diversity. Results indicated that leaching losses were similar for all treatments and that crop was the main influence on microbial biomass composition and size, while microbial activity was determined by management history

Critical evaluation of methods for determining total organic phosphorus in tropical soils

The determination of total organic phosphorus (TOP) in soils presents several methodological problems, particularly on strongly weathered and tropical soils. We reviewed the application of several methods for TOP determination to soils from different zones of the globe and evaluated the applicability of one ignition and two extraction methods to tropical soils from Brazil and Ghana. Reproducibility (coefficients of variation) was within 6% for the ignition method, and 13% for the extraction methods, due to the simplicity of the former. The two extraction methods produced results similar to each other, while the ignition method generally gave higher TOP contents. Unusually low C to organic P ratios indicate that the ignition method overestimated TOP in several soils. The Bowman extraction method, developed on weakly weathered soils, appeared also suitable for a variety of tropical soils

Influence of organic and mineral amendments on microbial soil properties and processes

Microbial diversity in soils is considered important for maintaining sustainability of agricultural production systems. However, the links between microbial diversity and ecosystem processes are not well understood. This study was designed to gain better understanding of the effects of short-term management practices on the microbial community and how changes in the microbial community affect key soil processes. The effects of different forms of nitrogen (N) on soil biology and N dynamics was determined in two soils with organic and conventional management histories that varied in soil microbial properties but had the same fertility. The soils were amended with equal amounts of N (100 kg ha⁻¹) in organic (lupin, Lupinus angustifolius L.) and mineral form (urea), respectively. Over a 91-day period, microbial biomass C and N, dehydrogenase enzyme activity, community structure of pseudomondas (sensu stricto), actinomycetes and α proteobacteria (by denaturing gradient gel electrophoresis (DGGE) following PCR amplification of 16S rDNA fragments) and N mineralisation were measured. Lupin amendment resulted in a two- to five-fold increase in microbial biomass and enzyme activity, while these parameters did not differ significantly between the urea and control treatments. The PCR–DGGE analysis showed that the addition of mineral and organic compounds had an influence on the microbial community composition in the short term (up to 10 days) but the effects were not sustained over the 91-day incubation period. Microbial community structure was strongly influenced by the presence or lack of substrate, while the type of amendment (organic or mineral) had an effect on microbial biomass size and activity. These findings show that the addition of green manures improved soil biology by increasing microbial biomass and activity irrespective of management history, that no direct relationship existed among microbial structure, enzyme activity and N mineralisation, and that microbial community structure (by PCR–DGGE) was more strongly influenced by inherent soil and environmental factors than by short-term management practices

The phosphorus composition of temperate pasture soils determined by NaOH-EDTA extraction and solution 31P NMR spectroscopy

Information on the composition and dynamics of soil phosphorus (P) remains limited, but is integral to understanding soil biogeochemical cycles. We used solution 31P nuclear magnetic resonance (NMR) spectroscopy to characterise NaOH—EDTA extractable P in 29 permanent pasture soils from England and Wales (total carbon 29-80 g kg- 1 soil, clay 219-681 g kg- 1 soil, pH 4.4-6.8). Total P ranged between 376 and 1981 mg P kg- 1 soil, of which between 45 and 88% was extracted with NaOH—EDTA. The extracts were dominated by orthophosphate monoesters (29-60% extracted P) and inorganic orthophosphate (21-55% extracted P), with smaller concentrations of orthophosphate diesters (2-10% extracted P), pyrophosphate (1-7% extracted P), phosphonates (0 - 3% extracted P), and traces of polyphosphates. Orthophosphate diesters were subclassified into phospholipids (1- 7% extracted P) and DNA (1-6% extracted P). Signals slightly downfield of inorganic orthophosphate were tentatively assigned to aromatic orthophosphate diesters similar in structure to R-(—)-1,1'-binaphthyl-2,2'-diyl hydrogen phosphate. Such signals are rarely detected in soil extracts, but were present in relatively large concentrations in the samples analysed here (2-5% extracted P). Relationships between functional P groups and soil properties suggested that the various functional groups are involved in the soil P cycle to different extents. In particular, concentrations of orthophosphate monoesters appeared to be controlled by the potential for chemical stabilisation in soil, whereas DNA and pyrophosphate were strongly correlated with the microbial biomass, suggesting an active involvement in biological nutrient turnove

Quantification of myo-inositol hexakisphosphate in alkaline soil extracts by solution 31P NMR spectroscopy and spectral deconvolution

Inositol phosphates are the dominant class of organic phosphorus (P) compounds in most soils, but they are poorly understood because they are not easily identified in soil extracts. This study reports a relatively simple technique using solution 31P NMR spectroscopy and spectral deconvolution for the quantification of myo-inositol hexakisphosphate (phytic acid), the most abundant soil inositol phosphate, in alkaline soil extracts. An authentic myo-inositol hexakisphosphate standard added to a re-dissolved soil extract gave signals at 5.85, 4.92, 4.55, and 4.43 ppm in the ratio 1:2:2:1. Spectral deconvolution quantified these signals accurately (102 ± 4%) in solutions containing a mixture of model P compounds by resolving the envelope of signals in the orthophosphate monoester region. In NaOH-EDTA extracts from a range of lowland permanent pasture soils in England and Wales, concentrations of myo-inositol hexakisphosphate determined by spectral deconvolution ranged between 26 and 189 mg P kg- 1 soil, equivalent to between 11 and 35% of the extracted organic P. Concentrations were positively correlated with oxalate-extractable aluminum and iron but were not correlated with total carbon, total nitrogen, clay, or the microbial biomass. This suggests that myo-inositol hexakisphosphate accumulates in soils by mechanisms at least partially independent of those controlling organic matter stabilization and dynamics. Furthermore, myo-inositol hexakisphosphate concentrations were positively correlated with plant-available inorganic P and negatively correlated with the carbon-to-organic P ratio, suggesting that biological P availability may, in part, regulate myo-inositol hexakisphosphate concentrations in soils, perhaps because organisms capable of degrading this compound are favored in more P-limited environments. Solution 31P NMR spectroscopy and spectral deconvolution offers a relatively simple method of quantifying myo-inositol hexakisphosphate in soil extracts

Using organic phosphorus to sustain pasture productivity: A perspective

Organic phosphorus (P) in grazed pastures/grasslands could sustain production systems that historically relied on inorganic P fertiliser. Interactions between inorganic P, plants and soils have been studied extensively. However, less is known about the transformation of organic P to inorganic orthophosphate. This paper investigates what is known about organic P in pasture/grassland soils used for agriculture, as well as the research needed to utilise organic P for sustainable plant production. Organic P comprises > 50% of total soil P in agricultural systems depending on location, soil type and land use. Organic P hydrolysis and release of orthophosphate by phosphatase enzymatic activity is affected by a range of factors including: (a) the chemical nature of the organic P and its ability to interact with the soil matrix; (b) microorganisms that facilitate mineralisation; (c) soil mineralogy; (d) soil water electrolytes; and (e) soil physicochemical properties. Current biogeochemical knowledge of organic P processing in soil limits our ability to develop management strategies that promote the use of organic P in plant production. Information is particularly needed on the types and sources of organic P in grassland systems and the factors affecting the activity of enzymes that mineralise organic P. Integrated approaches analysing the soil matrix, soil water and soil biology are suggested to address this knowledge gap

Effects of past and current management practices on crop yield and nitrogen leaching - a comparison of organic and conventional cropping systems

Farming practices can have significant effects on important soil processes, including nitrogen (N) dynamics and nitrate leaching. A lysimeter experiment was conducted to determine differences in N leaching resulting from past and current crop management practices. Intact monolith lysimeters (50 cm diam. × 70 cm deep) were taken from sites of the same soil type that had either been under long-term organic or conventional crop management. These were then managed according to established organic and conventional practices over 2 1/2 years using the same crop rotation (barley (Hordeum vulgare L.), maize (Zea mays L.), rape (Brassica napus L. ssp. oleifera) plus a lupin green manure (Lupinus angustifolius L.)) and two fertiliser regimes, resulting in four treatments based on soil management history and current fertilisation strategy. Dry matter yield of each crop was determined after harvest and leachates were collected after significant rainfall events and analysed for total mineral N concentrations. Mineral fertilisation had a clear positive effect on yields of the first crop, whereas there were no considerable differences between treatments for the last crop owing to a significant positive effect of green manure incorporation on yields. Although there was a trend of lower mineral N leaching from organically fertilised soils (organic management: 24.2 kg N ha–1; conventional management: 28.6), differences in N losses were not statistically significant between treatments. This shows that under the experimental conditions, leaching losses and crop yields were more strongly influenced by crop rotation and green manuring than by the presence or absence of mineral fertilisation. Overall, the study highlights the benefits of including a green manure in the crop rotation of any farming system

Influence of organic and mineral amendments on soil microbial properties and processes

Microbial diversity in soils is considered important for maintaining sustainability of agricultural production systems. However, the links between microbial diversity and ecosystem processes are not well understood. This study was designed to gain better understanding of the effects of short-term management practices on the microbial community and how changes in the microbial community affect key soil processes. The effects of different forms of nitrogen (N) on soil biology and N dynamics was determined in two soils with organic and conventional management histories that varied in soil microbial properties but had the same fertility. The soils were amended with equal amounts of N (100 kg ha−1) in organic (lupin, Lupinus angustifolius L.) and mineral form (urea), respectively. Over a 91-day period, microbial biomass C and N, dehydrogenase enzyme activity, community structure of pseudomondas (sensu stricto), actinomycetes and α proteobacteria (by denaturing gradient gel electrophoresis (DGGE) following PCR amplification of 16S rDNA fragments) and N mineralisation were measured. Lupin amendment resulted in a two- to five-fold increase in microbial biomass and enzyme activity, while these parameters did not differ significantly between the urea and control treatments. The PCR–DGGE analysis showed that the addition of mineral and organic compounds had an influence on the microbial community composition in the short term (up to 10 days) but the effects were not sustained over the 91-day incubation period. Microbial community structure was strongly influenced by the presence or lack of substrate, while the type of amendment (organic or mineral) had an effect on microbial biomass size and activity. These findings show that the addition of green manures improved soil biology by increasing microbial biomass and activity irrespective of management history, that no direct relationship existed among microbial structure, enzyme activity and N mineralisation, and that microbial community structure (by PCR–DGGE) was more strongly influenced by inherent soil and environmental factors than by short-term management practices