S. Saggar

A simplified resin membrane technique for extracting phosphorus from soils

A simplified procedure for determining the amount of phosphate (P) extracted from soils by ion exchange resin membranes is reported. Strips of anion (HCO3- form) and cation (Na+ form) exchange membrane were shaken with suspensions of soil in deionised water for 16–17 hours. After shaking, the strips were thoroughly rinsed in deionised water before the phosphate retained on the anion exchange resin strip was determined by shaking the strip directly with phosphate reagent. Compared to the common use of resin beads in nylon mesh bags, this resin membrane procedure is simpler, more convenient, and because an elution step is omitted, less time consuming.The mixed resin membrane method for soil phosphate extraction was compared to the use of resin bags on four New Zealand soils, contrasting in P sorbing capacity and exchangeable calcium. The soils were preincubated with and without 240 mg P kg−1 soil with three P sources of different solubilities. The resin strips extracted amounts of P which were closely correlated (R2 = 0.972) with that extracted by the resin bags. The amounts of P extracted by the mixed resin procedure were in proportion to the solubility of the P sources in each soil.

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.

Modelling nitrous oxide emissions from dairy-grazed pastures

Soil N2O emissions were measured during four seasons from two highly productive grass-clover dairy pastures to assess the influences of soil moisture, temperature, availability of N (NH4+ and NO3–) and soluble C on N2O emissions, and to use the emission data to validate and refine a simulation model (DNDC). The soils at these pasture sites (Karapoti fine sandy loam, and Tokomaru silt loam) differed in texture and drainage characteristics. Emission peaks for N2O coincided with rainfall events and high soil moisture content. Large inherent variations in N2O fluxes were observed throughout the year in both the ungrazed (control) and grazed pastures. Fluxes averaged 4.3 and 5.0 g N2O/ha/day for the two ungrazed sites. The N2O fluxes from the grazed sites were much higher than for the ungrazed sites, averaging 26.4 g N2O/ha/day for the fine sandy loam soil, and 32.0 g N2O/ha/day for the silt loam soil. Our results showed that excretal and fertiliser-N input, and water-filled pore space (WFPS) were the variables that most strongly regulated N2O fluxes. The DNDC model was modified to include the effects of day length on pasture growth, and of excretal-N inputs from grazing animals; the value of the WFPS threshold was also modified. The modified model ‘NZ-DNDC’ simulated effectively most of the WFPS and N2O emission pulses and trends from both the ungrazed and grazed pastures. The modified model fairly reproduced the real variability in underlying processes regulating N2O emissions and could be suitable for simulating N2O emissions from a range of New Zealand grazed pastures. The NZ-DNDC estimates of total yearly emissions of N2O from the grazed and ungrazed sites of both farms were within the uncertainty range of the measured emissions. The measured emissions changed with changes in soil moisture resulting from rainfall and were about 20% higher in the poorly drained silt loam soil than in the well-drained sandy loam soil. The model accounts for these climatic variations in rainfall, and was also able to pick up differences in emissions resulting from differences in soil texture.

A review of emissions of methane, ammonia, and nitrous oxide from animal excreta deposition and farm effluent application in grazed pastures

Abstract The agricultural sector in New Zealand is the major contributor to ammonia (NH3), nitrous oxide (N2O), and methane (CH4) emissions to the atmosphere. These gases cause environmental degradation through their effects on soil acidification, eutrophication, and stratospheric ozone depletion. With its strong agricultural base and relatively low level of heavy industrial activity, New Zealand is unique in having a greenhouse‐gas‐emissions inventory dominated by the agricultural trace gases, CH4 and N2O, instead of carbon dioxide which dominates in most other countries. About 96% of this anthropogenic CH4 is emitted by ruminant animals as a byproduct during the process of enteric fermentation. Methane is also produced by anaerobic fermentation of animal manure and many other organic substrates. In pastoral soils, NH3 and N2O gases are generated from N originating from dung, urine, biologically fixed N2, and fertiliser. The amount of these gaseous emissions depends on complex interactions between soil properties, climatic factors, and agricultural practices. In this review paper, the animal‐excretal inputs and farm‐effluent applications to New Zealand pastures are quantified. Data from overseas and New Zealand studies on CH4, NH3, and N2O emissions from excretal deposition and animal effluents, and the factors affecting these emissions, are synthesised with an aim to improve the New Zealand estimates of emissions from these sources. The practical implications of these emissions are described in relation to environmental impacts and management strategies for reducing these emissions.

Tillage-induced changes to soil structure and organic carbon fractions in New Zealand soils

The effects of increasing cropping and soil compaction on aggregate stability and dry-sieved aggregate-size distribution, and their relationship to total organic C (TOC) and the major functional groups of soil organic carbon, were investigated on 5 soils of contrasting mineralogy. All soils except the allophanic soil showed a significant decline in aggregate stability under medium- to long-term cropping. Mica-rich, fine-textured mineral and humic soils showed the greatest increase in the mean weight diameter (MWD) of dry aggregates, while the oxide-rich soils, and particularly the allophanic soils, showed only a slight increase in the MWD after long-term cropping. On conversion back to pasture, the aggregate stability of the mica-rich soils increased and the MWD of the aggregate-size distribution decreased, with the humic soil showing the greatest recovery. Aggregate stability and dry aggregate-size distribution patterns show that soil resistance to structural degradation and soil resilience increased from fine-textured to coarse-textured to humic mica-rich soils to oxide-rich soils to allophanic soils. Coarse- and fine-textured mica-rich and oxide-rich soils under pasture contained medium amounts of TOC, hot-water soluble carbohydrate (WSC), and acid hydrolysable carbohydrate (AHC), all of which declined significantly under cropping. The rate of decline varied with soil type in the initial years of cropping, but was similar under medium- and long-term cropping. TOC was high in the humic mica-rich and allophanic soils, and levels did not decline appreciably under medium- and long-term cropping. 13C-nuclear magnetic resonance evidence also indicates that all major functional groups of soil organic carbon declined under cropping, with O-alkyl C and alkyl C showing the fastest and slowest rate of decline, respectively. On conversion back to pasture, both WSC and AHC returned to levels originally present under long-term pasture. TOC recovered to original pasture levels in the humic soil, but recovered only to 60–70% of original levels in the coarse- and fine-textured soils. Aggregate stability was strongly correlated to TOC, WSC, and AHC (P < 0.001), while aggregate-size distribution was moderately correlated to aggregate stability (P < 0.01) and weakly correlated to AHC (P < 0.05). Scanning electron microscopy indicated a loss of the binding agents around aggregates under cropping. The effect of the loss of these binding agents on soil structure was more pronounced in mica-rich soils than in oxide-rich and allophanic soils. The very high aggregate stabilities of the humic soil under pasture was attributed to the presence of a protective water-repellent lattice of long-chain polymethylene compounds around the soil aggregates.

14C-labelled glucose turnover in New Zealand soils

The influence of soil mineralogy, as well as texture, on organic-C turnover was determined with 14C-labelled glucose. Samples of 16 soils from major mineralogical classes of New Zealand pastures and providing a range of organic C, clay contents and surface area, were incubated with 14C-labelled glucose for 35 d. The amounts of 12CO2 and 14CO2 evolved during incubation were monitored and the residual 14C concentrations determined. Periodically, the samples were removed and microbial biomass 12C and 14C determined using the fumigation-extraction technique. System mean residence times (MRTs) were obtained by three independent methods: (i) a compartmental model using 14C microbial biomass data, (ii) a non-compartmental model using 14C microbial biomass data and (iii) a biexponential equation as an empirical equation from residual 14C data. The effect of soil characteristics on MRTs was compared. The 14CO2 respired, after 35 d incubation, accounted for 51 to 66% of the glucose 14C input to these soils. The soils differed significantly in their amounts of 14CO2 evolution and in the proportions of labelled 14C in the biomass. The extent of mineralization of 14C-labelled glucose was influenced by soil clay content and clay surface area. Soils of low clay content (3–12%) had high biophysical quotients (respired: residual 14C); the highest (1.93) was in the soil with least clay (3%) and lowest mineral surface area, suggesting that clay is effective in C stabilization immediately after substrate assimilation. A biexponential model was found to be suitable for describing changes in the residual 14C and microbial biomass 14C during the 35 d glucose decomposition for most of the soils. MRTs for microbial biomass 14C were correlated with clay content (P<0.001), surface area estimated by para-nitrophenol (pNP) (P<0.003) and pH (P<0.01). Our results also showed that the MRTs of microbially assimilated 14C are similar despite differences in the chemical nature of the applied 14C-labelled substrate. However, the MRT for humus 14C differed with the chemical nature of the applied substrate. Clay and surface area played a major role in controlling the decomposition of added substrate through the stabilization and protection of the microbial biomass.

Effects of heavy metal contamination on the short-term decomposition of labelled [14C]glucose in a pasture soil

The influence of heavy metal contamination on the efficiency of conversion of fresh substrates into new microbial biomass in a pasture soil was examined. Three soils covering a range of chromium, copper and arsenic concentrations, and an uncontaminated control soil, were amended with [U-14C]glucose and incubated for 28 days. During incubation, microbial biomass 14C was determined using the fumigation-extraction technique. The amounts of 14CO2 evolved during incubation were monitored, and residual 14C concentrations were determined. Throughout the incubation, the microbial biomass-14C formed following addition of glucose was consistently lower in the metal-contaminated soils than in the uncontaminated control soil. Soils differed significantly in their rates of 14CO2 evolution. More glucosederived 14CO2 was evolved from contaminated soil than from the uncontaminated control. The ratio of both (total respired C): (total biomass-C) and (respired 14CO2): (biomass-14C) was greater in the contaminated soils than in the uncontaminated soil. The results suggest that the microbial biomass in soils contaminated with heavy metals are less efficient in the utilization of substrates for biomass synthesis and need to expend more energy for maintenance requirements.

Nitrous oxide emissions from a New Zealand cropped soil: tillage effects, spatial and seasonal variability

Agricultural practices are believed to be the major anthropogenic source of enhanced nitrous oxide (N 2O) gas emissions in New Zealand. Studies conducted in New Zealand generally suggest low N2O emission from pasture; however, there is little information for arable farming systems. This paper evaluates tillage and land use effects on N 2O emissions using a closed chamber technique at an Ohakea silt loam (Gleyic luvisol) where winter oats (Avena sativa L.)/fodder maize (Zea mays L.) was double-cropped for 5 years. The tillage types included conventional tillage (CT) and no-tillage (NT) systems, and a permanent pasture (PP) was used as a control. Spatial variability in all treatments showed large inherent variations in N 2O fluxes (a mean CV = 119%), which reflected natural soil heterogeneity, and perhaps the measurement technique used rather than the real differences due to the tillage and cropping systems evaluated. On an annualised basis, N2O emissions measured from December 1998 to September 1999 from the PP (1.66 kg N2O-N/ha per year or 19 gN 2O-N/(m 2 h)) were significantly lower than the CT and NT fields averaging at 9.20 (or 105) and 12.0 (or 137) kg N2O-N/ha per year (or gN 2O-N/(m 2 h)), respectively. However, there were no differences in N2O emission rates between the CT and NT treatments. Seedbed preparation using a power harrow which followed within a few days of first ploughing the CT field reduced N 2O emissions by 65% within the first hour after power harrowing. However, N2O emission rates returned to the pre-power harrowing levels at the next sampling period, which was 1 month later. There was a strong relationship between log-transformed data of soil water content (SWC) and N2O emissions in all treatments with r = 0.73, 0.75 and 0.86 for the PP, CT and NT treatments, respectively. Seasonal variations in N2O emission from the PP were in the order of winter = autumn > summer. Although fluxes in the CT were higher in winter than in the autumn season, there were no differences between the summer and autumn data. The seasonal variations in N2O emission in the NT treatment were in the order of winter > autumn = summer.

BIOGEOCHEMICAL IMPACT OF HIERACIUM INVASION IN NEW ZEALAND'S GRAZED TUSSOCK GRASSLANDS: SUSTAINABILITY IMPLICATIONS

The establishment and spread of invasive plants could be enhanced by plant–soil feedbacks that alter the cycling of biologically important elements. In New Zealand, overgrazing of tussock grasslands in the South Island has led to land degradation and simultaneous invasion of exotic weeds (primarily Hieracium spp.) over large areas. While Hieracium continues to spread rapidly, little is known about variation in the impact of Hieracium across landscapes characterized by a range of environmental conditions. We examined the impact of Hieracium invasion on soil and ecosystem processes first at the scale of individual patches under one disturbance regime and uniform “environment” (i.e., one aspect and elevation), and then under different environmental conditions (aspects) and disturbance regimes (long-term grazing, no grazing since 1978). Around individual plants on heavily grazed north-facing slopes with significant bare ground, Hieracium invasion increased total soil C and N and lowered soil pH and mineral N ...

Partitioning and translocation of photosynthetically fixed 14C in grazed hill pastures

Abstract Information on carbon (C) flows and transformations in the rhizosphere is vital for understanding soil organic matter dynamics and modelling its turnover. We followed the translocation of photosynthetically fixed C in three hill pastures that varied in their phosphorus (P) fertility, using a 14C-CO2 pulse-labelling chamber technique. Pasture shoot, root and soil samples were taken after 4h, 7 days and 35 days chase periods to examine the fluxes of 14C in the pasture plant-root-soil system. Shoot growth over 35 days amounted to 114, 179 and 182gm–2 at the low (LF), medium (MF) and high (HF) fertility pasture sites, respectively. The standing root biomass extracted from the soil did not differ significantly between sampling periods at any one level of fertility, but was significantly different across the three levels of fertility (1367, 1763 and 2406gm–2 at the LF, MF and HF pastures, respectively). The above- and below-ground partitioning of 14C was found to vary with the length of the chase period and fertility. Although most 14C (74%, 65% and 57% in the LF, MF and HF pastures, respectively) was in the shoot biomass after 4h, significant translocation to roots (23–39%) was also detected. By day 35, about 10% more 14C was partitioned below-ground in the LF pasture compared with the HF pasture. This is consistent with the hypothesis that, at limiting fertility, pasture plants allocate proportionally more resource below-ground for the acquisition of nutrients. In the LF site, with an annual assimilated C of 7064kgha–1, 2600kg was respired, 1861kg remained above-ground in the shoot and 2451kg was translocated to roots. In the HF pasture, of the 17313kgha–1 C assimilated, 7168kg was respired, 5298 remained in the shoot and 4432kg was translocated to the roots. This study provides, for the first time, data on the fluxes and quantities of C partitioned in a grazed pasture. Such data are critical for modelling C turnover and for constructing C budgets for grazed pasture ecosystems.