R. W. Tillman

Denitrification and N2O:N2 production in temperate grasslands: processes, measurements, modelling and mitigating negative impacts.

In this review we explore the biotic transformations of nitrogenous compounds that occur during denitrification, and the factors that influence denitrifier populations and enzyme activities, and hence, affect the production of nitrous oxide (N2O) and dinitrogen (N2) in soils. Characteristics of the genes related to denitrification are also presented. Denitrification is discussed with particular emphasis on nitrogen (N) inputs and dynamics within grasslands, and their impacts on the key soil variables and processes regulating denitrification and related gaseous N2O and N2 emissions. Factors affecting denitrification include soil N, carbon (C), pH, temperature, oxygen supply and water content. We understand that the N2O:N2 production ratio responds to the changes in these factors. Increased soil N supply, decreased soil pH, C availability and water content generally increase N2O:N2 ratio. The review also covers approaches to identify and quantify denitrification, including acetylene inhibition, (15)N tracer and direct N2 quantification techniques. We also outline the importance of emerging molecular techniques to assess gene diversity and reveal enzymes that consume N2O during denitrification and the factors affecting their activities and consider a process-based approach that can be used to quantify the N2O:N2 product ratio and N2O emissions with known levels of uncertainty in soils. Finally, we explore strategies to reduce the N2O:N2 product ratio during denitrification to mitigate N2O emissions. Future research needs to focus on evaluating the N2O-reducing ability of the denitrifiers to accelerate the conversion of N2O to N2 and the reduction of N2O:N2 ratio during denitrification.

SURFACE CHARGE AND SOLUTE INTERACTIONS IN SOILS

Many soil physical and chemical properties are controlled by the nature and the amount of surface charge and the variation of surface charge with soil solution characteristics. These properties include dispersion and flocculation, electrophoretic mobility, solubility, and the adsorption and movement of solutes. The surface reactions of charged particles are essential to the biogeochemical cycling of nutrients and pollutants and the pathway of detoxification of the latter when present at hazardous concentrations. Surface charge can be manipulated to take advantage of solid phase interactions relating to the movement of nutrient and pollutant ions in soils, the degradation of pesticides, and the decontamination of soils. This chapter brings together fundamental aspects of surface charge and recent developments on the implications of surface charge in relation to other soil properties, particularly solute interactions in soils. We first outline the development of charge on both permanent- and variable-charge surfaces. Then we discuss the various methods used to measure surface charge and factors affecting this charge. An attempt has been made to compare current theories on the nature of the charged solid surface-solution interface. The manipulation of surface charge can be achieved through liming and the addition of fertilizers containing specifically adsorbed ions. The practical implications of surface charge to soil properties have been discussed in relation to the dispersion and the flocculation of soils and the adsorption and leaching of inorganic cations and anions. Future research should focus on the development of methods to measure surface charge under in situ conditions and to explore further the role of surface charge in remediating contaminated soils

Pesticide sorption by allophanic and non‐allophanic soils of New Zealand

Abstract We investigated the sorption of five pesticides (metsulfuron methyl, atrazine, 2,4‐D, phorate, and terbufos) in a range of allophanic and non‐allophanic soils of New Zealand, using surface and subsoil horizons from 10 soil series. Sorption of pesticides was measured by a batch equilibrium technique using 14C‐labelled pesticides. The effect of soil properties on pesticide sorption was also examined. Sorption of pesticides was adequately described by the Freundlich equation with an R 2 value > 0.97. The value of the exponent in the fitted Freundlich equation for the pesticide sorption varied from 0.79 to 0.98. The pesticide sorption, as measured by the distribution coefficient (Kd, sorption per unit concentration), was as follows: terbufos > phorate > 2,4‐D > atrazine > metsulfuron methyl (Kd = 20.7, 18.1, 4.88, 3.74, and 0.54 L/kg, respectively). Sorption of pesticides was higher for allophanic than for non‐allophanic soils and in general decreased with depth. Multiple regression analysis between ...

The effects of anion sorption on sorption and leaching of cadmium

The effect of chloride, sulfate, nitrate, and phosphate anions on the sorption and leaching of cadmium was examined in 2 soils (Manawatu silt loam and Egmont clay loam) which differ in their variable charge components. There was a larger sorption of cadmium in the presence of phosphate than in the presence of sulfate, nitrate, and chloride, and the difference was more pronounced in the Egmont soil. In soils, specific sorption of phosphate increases the negative charge. The increase in negative charge per unit amount of phosphate sorbed decreased with increasing phosphate sorption. The sorption of cadmium increased in response to phosphate sorption. The phosphate-induced cadmium sorption resulted from the increase in negative charge due to phosphate sorption. Column studies indicated that cadmium was less susceptible to leaching in the presence of phosphate than in the presence of nitrate.

Release of phosphorus from sheep faeces on grazed, hill country pastures

Abstract The breakdown of sheep faeces enclosed in mesh bags and placed on 3 slopes of varying steepness on grazed hill country was monitored in both winter and summer. Samples had completely decomposed within 28 days in winter, but lasted for over 75 days in summer. The rate of breakdown was most rapid on the flat camp site areas and slowest on steep slopes. The concentration of both total phosphorus (P) and water-extractable P in the dung samples remained relatively constant with time. Thus the major mechanism controlling movement of P from faeces into the soil was the rate of physical breakdown of the dung rather than the leaching of P from the dung sample. This contrasted with laboratory studies which demonstrate a high water-extractable P content in the dung samples at all stages of the trial. There was no detectable increase in water-extractable P levels in soil samples collected from immediately beneath the decomposing dung.

The effects of nitrogen fertilizer form on the plant availability of phosphate from soil, phosphate rock and mono-calcium phosphate

A glasshouse trial using lettuce as the test crop, and laboratory incubations were used to evaluate the influence of various nitrogen fertilizers on the availability of phosphate from an unfertilized loamy sand soil and from the same soil fertilized with Sechura phosphate rock or monocalcium phosphate. The order in which nitrogen fertilizer form increased plant yield and P uptake from soil alone and from soil fertilized with the rock was ammonium sulphate > sulphurised urea > ammonium nitrate > urea > potassium nitrate. For each rock application (both 30 and 60 mg/pot) and for soil alone, increased P uptake by the plant correlated well with decreased soil pH. In soil fertilized with the soluble P form, monocalcium phosphate, the form of the nitrogen fertilizer had little effect on plant P uptake. Subsequent laboratory incubation studies showed that increased dissolution of soil-P or Sechura phosphate rock did not occur until acidity, generated by nitrification or sulphur oxidation of the fertilizer materials, had lowered soil pH to below 5.5. A sequential phosphate fractionation procedure was used to show that in soils treated with the acidifying nitrogen fertilizers, ammonium sulphate and urea, there was considerable release of Sechura phosphate rock P to the soil, amounting to 42% and 27% of the original rock P added, respectively.

Non-Equilibrium Sorption during the Movement of Pesticides in Soils

Sorption and movement of two ionic herbicides (2,4-D and atrazine) and two non-ionic insecticides (phorate and terbufos) in an allophanic (Patua silt loam) and a non-allophanic (Tokomaru silt loam) soil were examined using 14C- labelled pesticides. For sorption measurements, a range of concentrations of pesticide solutions in 0.1 M calcium chloride were shaken with soil samples at a soil: solution ratio of 1 : 10 for 4 h at 25°C. The movement of pesticides was examined using re-packed soil columns following a step input of 2,4-D and tri- tiated water (3HzO) and a pulse input of 2,4-D, atrazine, phorate and terbufos. A convection-dispersion equation (CDE), either with an equilibrium or a bicon- tinuum non-equilibrium sorption process, was used to simulate the measured effluent breakthrough curves (BTCs) obtained by simultaneous displacement of a non-sorbed solute (3H20) and a sorbed solute (2,4-D). The Patua soil sorbed more pesticide than did the Tokomaru soil. This is attributed to the larger amounts of organic matter and the presence of short- range order clays (allophane) in the former soil compared to the latter. Kinetic sorption data for pesticides showed an initial rapid rate followed by a slower rate of sorption. In column experiments, the pesticide in the leachate appeared later in the Patua than in the Tokomaru soil. Movement of pesticides in soils decreased with an increase in K, values. The step-function experiments showed a symmetrical BTC for the non-sorbed solute (3HzO) with a sigmoidal shape, whereas there was an asymmetrical BTC with extensive tailing for the sorbed solute (2,4-D). The CDE with an equilibrium sorption process adequately described the 3H,0 BTC, but failed to simulate the BTC for 2,4-D. The CDE with a bicontinuum non-equilibrium sorption process provided a good descrip- tion of the BTC for 2,4-D. Diffusion of pesticides into sorbent organic matter was considered to be the likely mechanism for the observed sorption non- equilibrium during the movement of pesticides in soils.

Effects of season and fertiliser rate on phosphorus concentrations in pasture and sheep faeces in hill country

Abstract The effects of different rates of superphosphate fertiliser (10, 20, 30, 50, and 100 kg/ha of P) and of slope (0 – 10, 11 – 20, 21 – 30, 31 – 40° and 41° +) on P concentrations in pasture were monitored on a seasonal basis for 3 years in hill country. Faecal P concentrations were also measured. Pasture P decreased with increasing slope. Pasture and faecal P concentrations increased with increasing fertiliser rate; both were highest in winter and spring and lowest in summer. A highly significant relationship (r = 0.94) existed between the P concentration in pasture on offer for grazing within a paddock and P concentration of the faeces deposited subsequently during the grazing period. This close relationship will facilitate attempts to model P return via the grazing animal and to assess the effects on P losses by animal transfer.

Solute movement during intermittent water flow in a field soil and some implications for irrigation and fertilizer application

Abstract The movement of a bromide tracer in response to intermittent water flow was investigated in the field. Two experiments were conducted. The first involved application of a 5 mm pulse of dilute potassium bromide solution to pasture plots of contrasting initial water content. All plots then received a further application of 50 mm of water. Twenty-four hours later core samples of soil were collected and the distribution of water and bromide measured. The final bromide distributions were found to be dependent on the initial water content of the soil. Bromide applied to initially dry soil was very resistant to movement by subsequent leaching water. The second experiment also involved the application of a 5 mm pulse of potassium bromide solution followed by some leaching water. The time scale was however much greater. There was a 12 day period between solute application and final sampling, with a leaching treatment applied at different times. Coupling a mobile-immobile variant of the convection-dispersion model with a description of the transient water flow and root water extraction provided a mechanistic model. This model could successfully describe the main features of the solute movement under the four different regimes of water application. The assumed depth of water extraction by roots strongly affected the predicted final solute distribution. Some practical implications for the scheduling of fertiliser applications and irrigation events are discussed.

Uptake zones for phosphorus in spring by pasture on different strata within a hill paddock

Abstract Approximately 85,92, and 90% of P uptake, as 32P, in spring by mixed pasture on campsites, 25° slopes, and 45° slopes respectively was from within the surface 7 cm of soil; most uptake was from the 0–3 cm soil zone. Significant P uptake from depths greater than 30 cm was unlikely. Approximately 90% of P uptake by pasture on all strata was from within 13 cm upslope and downslope of the P source. The extent of P uptake by pasture from the upper 3 cm of soil was similar both upslope and downslope from a P source, but uptake from greater depths was affected by slope. The angle of predominant root activity was between the vertical and a line normal to the soil surface, an angle which departed more from the vertical as surface slope increased. 32P placed at 11 cm depth on the outside edge of stock tracks was available to plants further downslope than when placed at shallower depths.