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Micronutrients copper (Cu) and zinc (Zn) have the potential to inhibit soil urease activity (UA) and reduce ammonia (NH3) emissions over long duration (8–12 weeks) but have not been tested for reducing NH3 losses from cattle urine deposited in dairy-grazed pasture soils. The objective of this study was to assess the effectiveness and longevity of Cu and Zn in reducing soil UA, for the use of these metals to reduce NH3 emissions from deposited urine by grazing cattle. A series of experiments were conducted to (i) assess the relationship between inherent Cu and Zn status and soil UA of New Zealand dairy-grazed pasture soils, (ii) determine the impact of Cu and Zn addition to pasture soils on soil UA and (iii) investigate how soil organic carbon (C) and other C-related textural and mineralogical properties such as clay content and cation exchange capacity influence the effectiveness of added Cu and Zn in reducing urea hydrolysis. The results showed significant positive correlations of soil total C and total nitrogen (N) with soil UA. However, there were no significant negative correlations of soil UA with inherent Cu and Zn levels. Similarly, addition of Cu and Zn to soil did not significantly reduce soil UA. However, when Cu was added to two different soil supernatants there was a significant reduction in hydrolysis of urea applied at 120 and 600 mg urea-N kg–1 soil. Additions of Zn achieved negligible or small reductions in urea hydrolysis after 120 and 600 mg urea-N kg–1 soil applications to soil supernatants. This result suggests that Cu can inhibit soil UA and urea hydrolysis in soil supernatants with potentially low C, clay and cation exchangeable base contents. However, the interaction of bioavailable Cu with labile soil organic C and clay particles leads to its inactivation, resulting in ineffectiveness in organic C-rich pasture soils. Although most of the added Zn did not complex and remained bioavailable, the observed levels of bioavailable Zn had limited effect on soil UA.

The objective of this work was to study the degradation kinetics of a nitrification inhibitor (NI), dicyandiamide (DCD), and evaluate its effectiveness in reducing nitrous oxide (N2O) emissions in different types of soils. Three soils contrasting in texture, mineralogy, and organic carbon (C) content were incubated alone (control) or with urine at 600 mg N/kg soil with 3 levels of DCD (0, 10, and 20 mg/kg). Emissions of N2O and carbon dioxide (CO2) were measured during the 58-day incubation. Simultaneously, subsamples were collected periodically from the incubating soils (40-day incubation) and the amounts of DCD, NH4+, and NO3− were determined. Our results showed that the half-life of DCD in these laboratory incubating soils at 25°C was 6–15 days and was longer at the higher rate of DCD application. Of the 3 soils studied, DCD degradation was fastest in the brown loam allophanic soil (Typic orthic allophanic) and slowest in the silt loam non-allophanic soil (Argillic-fragic Perch-gley Pallic). The differences in DCD degradation among these soils can be attributed to the differences in the adsorption of DCD and in the microbial activities of the soils. Among the 3 soils the highest reduction in N2O emissions with DCD from the urine application was measured in the non-allophanic silt loam soil followed by non-allophanic sandy loam soil and allophanic brown loam soil. There was no adverse impact of DCD application on soil respiratory activity or microbial biomass.

The objectives of this study were to improve our understanding of seasonal variation in the biophysical disappearance of the nitrification inhibitor dicyandiamide (DCD) in soil and the key regulatory factors. Changes in DCD concentrations in the soil and plant canopy were measured following application to dairy-grazed pasture soil and non-grazed pasture soil. Treatments included two levels of DCD alone (10 and 20 kg ha–1) applied to non-grazed pasture field plots, and DCD (10 kg ha–1) applied with urine and with urea fertiliser. DCD (10 kg ha–1) was also applied in grazed farmlets following grazing. About 4–40% of the DCD applied was intercepted and stayed on the plant canopy from

Nitrous oxide (N2O) from agricultural soils is a major source of greenhouse gas emissions in New Zealand. Nitrification inhibitors are seen as a potential technology to reduce these N2O emissions from agricultural soils. In previous studies on the effect of dicyandiamide (DCD) on N2O emissions from animal excreta, DCD was directly applied to urine. However, farmers apply DCD to grazed pastures shortly before or after grazing rather than applying it specifically to the urine patches. Accordingly, the objectives of this study were: (1) to test, using chamber measurements, whether the same level of N2O reduction is achieved under grazed conditions where excretal N is non-uniformly deposited, (2) to apply the process-based NZ-DNDC model to simulate the effect of DCD on emission reductions, and (3) to perform a sensitivity analysis on the NZ-DNDC model to investigate how uncertainties in the input parameters affect the modelled N2O emissions. Two circular 1260-m2 treatment plots were grazed simultaneously for 5 h, by 20 cattle on each plot. The following day, DCD was applied in 800 L of water to one of the plots at 10 kg/ha and N2O emissions were measured periodically for 20 days. The cumulative N2O emissions were 220 ± 90 and 110 ± 20 g N2O-N/ha for the untreated and DCD-treated plots, respectively (based on the arithmetic mean and standard error of the chambers). This suggests a reduction in N2O emission from DCD application of ~50 ± 40% from a single grazing event. However, this result should be treated with caution because the possibility of sampling error due to the chamber distribution cannot be excluded. NZ-DNDC simulated N2O emissions of 169 and 68 g N2O-N/ha for the untreated and DCD-treated areas, respectively, corresponding to a reduction of 60% in N2O emissions from DCD application. This level of reduction is consistent with that found in experiments with individual urine patches. N2O emissions found through use of NZ-DNDC were sensitive to uncertainties in the input parameters. The combined effect of varying the initial soil NO3– and NH4+, soil moisture, soil organic carbon, bulk density, clay content, pH, and water-filled pore-space at field capacity inputs within plausible ranges was to change the simulated N2O emissions by –87% to +150%.

Pasture and wetland soils are regarded as the major source of nitrous oxide (N 2 O) and dinitrogen (N 2 ) emissions as they receive regular inputs of N from various sources. To understand the factors affecting N 2 O and N 2 emissions and their ratio as influenced by soil amendments (zeolite or lime), we conducted laboratory experiments using 10-L plastic containers at 25°C for 28 days. Soil samples (0-0.1 m soil depth) collected from pasture and adjacent wetland sites were treated with nitrate-N (NO 3 - ) at 200 kg N/ha with and without added lime or zeolite. Nitrous oxide and N 2 emissions were measured periodically from soil subsamples collected in 1-L gas jars using acetylene (C 2 H 2 ) inhibition technique, and soil ammonium (NH 4 + ) and NO 3 - concentrations were determined to assess the changes in N transformation. Soil NO 3 - -N disappeared relatively faster in wetland soil than that in pasture soil. In the presence of added NO 3 - , wetland soils emitted significantly more N 2 O and N 2 than pasture soils, while the reverse trend was observed in the absence of NO 3 - . Total N 2 O emitted as percentage of the applied N was 25% for wetland and 5.7% for pasture soils. Total N 2 emissions expressed as a percentage of the applied N from wetland and pasture soils were 5-9% and 0.29-0.74%, respectively. Higher N 2 O and N 2 emissions and lower N 2 O : N 2 ratios from wetland soils than pasture soils were probably due to the higher water content and greater availability of soluble C in wetland. Zeolite applied to wetland soils reduced N 2 O emissions but had little effect on N 2 O emissions from pasture soils. Liming appeared to exacerbate N 2 O emissions from fertilised lands and treatment wetlands and shift the balance between N 2 O and N 2 , and may be considered as one of the potential management tools to reduce the amount of fertiliser N moving from pasture and wetland into waterways.

Information on the dynamics of recently photo-assimilated carbon (C) allocated to roots and root-derived exudates in soils is scarce and experimentally difficult to obtain. We used Rhizon Soil Moisture SamplersTM (RSMS) placed at different depths in soil (20, 40, 80, 120 mm) to monitor short-term dynamics of root and root-derived C at the root–soil interface after 14CO2 pulse-labelling of pasture cores. At the 20 mm depth, 14C activity in soil solution peaked within 2 h of 14CO2 application. The peak of 14C activity took longer to appear and slower to disappear with increased depth. Negligible amounts of 14C as soluble exudates were found in the soil solution. The pattern of initial 14C activity in soil solution, allocation of recently assimilated 14C in roots, and root mass distribution with depth were closely related to each other. This suggested that the rapid appearance of 14CO2 in soil solution is more closely linked to root respiration of recent 14C-assimilates (transferred via shoots to roots) and/or to microbial decomposition of root-released 14C-assimilates than to transfer by diffusion of atmospheric 14CO2 through open soil surface to different depths in soil. The use of RSMS was an effective, simple, and non-destructive method to monitor the dynamics of root-derived 14C by in situ sampling of soil solution.

To understand the effects of agroforestry on soil biological processes we assessed the conditions in Pinus radiata plantations of 50, 100, 200, and 400 stems/ha after 25 years of growth, and in a grassland. Agroforestry resulted in a 15–25% decline in soil organic C and N compared with grassland, and had a significant negative influence on soil microbial biomass. There was less microbial C and N in soils under 50–400 stems/ha of P. radiata than in soils under grassland (0 stems/ha). Soil carbon decomposition and microbial activity were measured by trapping the carbon dioxide produced by incubating soils over a 60-week period. The results showed that soil C decomposition rates were ~1.5 times as much (c. 15 mg CO2-C/kg soil) in soil from grassland as in that from plots with 50 or100 stems/ha (c. 10 mg CO2-C/kg soil), and were further reduced to one half (c. 5.5 mg CO2-C/kg soil) in the plots with 200 or 400 stems/ha. The soils under P. radiata gave off less carbon dioxide per unit of biomass (the metabolic quotient) than soils under grassland. These shifts in microbial biomass and its metabolic quotients appear to be associated with differences in the quantity and ‘quality’ of inputs and soil organic matter decomposition rates, and to reflect the land use change from grassland to forest. Given the general ability of soil microbial biomass to recolonise depopulated areas after tree harvest, we see no problem in restoring populations of these soil organisms vital in controlling nutrient cycling after tree felling, provided adequate adjustments to soil pH are made.

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.

We conducted a 7-week laboratory incubation experiment to evaluate the effect of leaching on net C and N mineralisation in soils. The soils were collected from adjacent fields of long-term pasture and maize, where each field contained an Inceptisol and an Andisol. The concentration of clay mineral was 200 g/kg halloysite in the Inceptisol and 120 g/kg allophane in the Andisol. Half the samples were leached weekly with 0.002 M CaCl2 at a suction of 20 kPa to remove soluble products, and half were not leached. Carbon mineralisation was determined from CO2-C evolved each week. Net N mineralisation was measured for the leached samples from the NH4-N and NO3-N in the CaCl2 extracts, and for the batch of non-leached samples by extraction in 0.5 M K2SO4. Carbon and net N mineralisation were greater in the soils under pasture than in soils under maize. The proportion of total C mineralised as CO2-C, and of total N mineralised as NH4-N and NO3-N, followed the order Inceptisol-pasture > Inceptisol-maize > Andisol-pasture > Andisol-maize, suggesting that allophane and Al ions reduced net mineralisation. Dissolved organic carbon (DOC) produced during incubation, as a proportion of total C, was greatest for the Inceptisol-maize sample and least for the Andisol-pasture sample. Non-leaching resulted in the accumulation of acids and solutes, and decreased C mineralisation for the Inceptisol samples.

The specific surface area (SSA) of a range of soils has been measured by adsorption of para-nitrophenol (pNP). These surface soils are representative of the major soil groups of New Zealand, varying in mineralogy, clay and organic carbon contents, and cation exchange capacity (CEC). All of the soils are under pastures of introduced grasses and legumes that have been regularly fertilised and grazed. The SSAs measured by pNP are compared with the values calculated from the clay content, clay mineral composition, and organic carbon content of the soils. Measured SSAs are also related to the air-dry soil moisture contents. There is a good 1:1 relation between measured and calculated SSAs. This correspondence improves when allophanic and smectitic soils are omitted from the relation. The SSAs measured by pNP are also well correlated with the air-dry moisture content and CEC of the soils. When allophanic soils are excluded, a highly significant correlation (r = 0.894; P < 0.001) is obtained between pNP surface area and moisture content of the air-dry soils. When the same relation is applied to an independent set of soils, 89% of the variations in SSA can be accounted for. We suggest that the SSAs of many soils can be reasonably deduced from their air-dry moisture content.

A question of economic, social, and land-use importance in the predominantly steep South Island high country tussock grasslands of New Zealand is whether these lands can be sustainably farmed by oversowing introduced grasses and legumes and using fertilisers. To help answer this question, we compared vegetation and soil chemical, biochemical, and biological properties on Brown soils (Dystrudepts) on adjacent land areas which have been differently managed since 1978. One area had never been fertilised or oversown. The other had been oversown with grasses and clovers and received about 1100 kg/ha of sulfur-superphosphate between 1979 and 1997. Oversowing and fertilising reduced the amount of bare ground and transformed the vegetation to a species composition dominated by the introduced adventives Anthoxanthum odoratum and Agrostis capillaris. Fertilising raised soil carbon (C) and nitrogen (N) content, increased A-horizon thickness, and raised exchangeable cation values. All of the phosphorus (P) applied to the fertilised area was accounted for in the top 15 cm of soil, but has accumulated in the relatively unavailable organic form. Oversowing and fertilising significantly (P < 0 . 05) increased microbial biomass C, N, and P relative to values in unfertilised soils. The microbial biomass C: N and C: P ratios were significantly (P < 0 . 05) lower in fertilised soils. Fertilised soils had significantly more (P < 0 . 05) mineralised N than unfertilised soils. Populations of 5 groups of soil fauna (Scarabaeidae, nematodes, enchytraeids, rotifers, tardigrades) were higher in fertilised soils. Nematode species parasitic or pathogenic for clovers were present in greater numbers on fertilised soils. A combination of fertility decline (in particular, P immobilisation as organic P) and nematode damage may be the reason for the low clover cover on fertilised sites, and may explain the widely observed clover ‘flush’ and decline common to oversown high country. We conclude that there is no simple measure of ‘soil quality’ that can be used as a sustainability indicator in moist New Zealand high country. Nor can sustainability be judged purely on biological, biochemical, and chemical critieria. Although many of the effects associated with oversowing and fertilising, such as increased organic matter content of A horizons and increased biological activity, are positive, these effects must be balanced against the economic risk associated with being committed to a high-input farming system, the loss of low-producing but resilient native species, and increase of plant-pathogenic nematodes.

New Zealand hill-country farms consist of an amalgamation of land units in different slope and aspect categories, each with unique production potentials. Information on the influence these slope categories have on carbon (C) partitioning is imperative for more accurate and complete understanding of C inputs and fluxes through a grazed hill pasture ecosystem. The effects of 3 slope categories [representing 1–12°, 13–25°, and ≥26° microtopographical units corresponding to low (L), medium (M), and steep (S) slopes] on the vertical translocation of photosynthetically fixed C was studied by using a 14 C-CO2 pulse-labelling chamber technique. Pasture and soil samples were taken after 4-h, 7-day, and 35-day chase periods, to examine the fluxes of 14C in the pasture plant{root–soil system. Total C and 14C were determined in the pasture shoot, root, and soil components. Microbial biomass C and 14C contents in each soil were also determined using the chloroform fumigation{extraction technique. Pasture composition and growth varied with slope category. High fertility grasses (90%) were dominant on the L slope while low fertility grasses (≥60%) were dominant on the M and S slopes. Shoot growth over 35 days amounted to 4470, 2045, and 1308 kg/ha at the L, M, and S slopes, respectively. The standing root biomass did not differ signifficantly among the slopes. Allocation of the 14C-labelled assimilate below-ground was rapid, with 23–35% detected in the roots within 4 h of pulse-labelling. The above- and below-ground partitioning of 14C varied with the length of the chase period, and was strongly influenced by slope. Pasture plants allocated more C below-ground in the M and S slope categories. During the study period, 173 kg C/ha was assimilated daily at the L slope site, with 73 kg being respired, 50 kg remaining above-ground in the shoot, and 43 kg being partitioned into the root. In comparison, at the S slope, of the 56 kg/ha C assimilated daily, 22 kg was respired, 14 kg remained in the shoot, and 18 kg was partitioned into the root, and the daily input to the soil varied between 2 and 7 kg C/ha. By using annual growth measurements from adjacent areas, the amounts of C translocated annually to roots and soil at each slope category were also estimated from the 14C distribution of spring growth. At the L slope site, 9340 kg C/ha was respired, 6375 kg remained above-ground in the shoot, and 5510 kg was translocated to roots and 930 kg to soil. At the S slope site, 5710 kg C/ha was respired, 3490 kg remained in the shoot, and 4490 kg was translocated to the roots and 555 kg to soil.