Climate change impacts on Aotearoa New Zealand: A horizon scan approach

Many of the implications of climate change for Aotearoa (New Zealand) remain unclear. To identify so-far unseen or understudied threats and opportunities related to climate change we applied a horizon-scanning process. First, we collated 171 threats and opportunities across our diverse fields of research. We then scored each item for novelty and potential impact and finally reduced the list to ten threats and ten opportunities through a prioritisation process. Within the 20 items presented in this paper, we uncover a range of climate-related costs and benefits. Unexpected opportunities evolve from economic reorganisation and changes to perspectives. The threats we highlight include the overall failure to interconnect siloed policy responses, as well as those relating to extreme events and feedbacks, as well as pressures that undermine the coherence of society. A major theme of our work is that climate change effects in Aotearoa are likely to transgress the boundaries of research disciplines, industry sectors and policy systems, emphasising the importance of developing transdisciplinary methods and approaches. We use this insight to connect potential responses to climate change with Aotearoa’s culture and geography

The C:N:P:S stoichiometry of soil organic matter

The formation and turnover of soil organic matter (SOM) includes the biogeochemical processing of the macronutrient elements nitrogen (N), phosphorus (P) and sulphur (S), which alters their stoichiometric relationships to carbon (C) and to each other. We sought patterns among soil organic C, N, P and S in data for c. 2000 globally distributed soil samples, covering all soil horizons. For non-peat soils, strong negative correlations (p < 0.001) were found between N:C, P:C and S:C ratios and % organic carbon (OC), showing that SOM of soils with low OC concentrations (high in mineral matter) is rich in N, P and S. The results can be described approximately with a simple mixing model in which nutrient-poor SOM (NPSOM) has N:C, P:C and S:C ratios of 0.039, 0.0011 and 0.0054, while nutrient-rich SOM (NRSOM) has corresponding ratios of 0.12, 0.016 and 0.016, so that P is especially enriched in NRSOM compared to NPSOM. The trends hold across a range of ecosystems, for topsoils, including O horizons, and subsoils, and across different soil classes. The major exception is that tropical soils tend to have low P:C ratios especially at low N:C. We suggest that NRSOM comprises compounds selected by their strong adsorption to mineral matter. The stoichiometric patterns established here offer a new quantitative framework for SOM classification and characterisation, and provide important constraints to dynamic soil and ecosystem models of carbon turnover and nutrient dynamics

Ammonia volatilisation is not the dominant factor in determining the soil nitrate isotopic composition of pasture systems

© 2014 Elsevier B.V. Nitrate dual isotopes (δ¹⁵N-NO₃⁻ and δ¹⁸O-NO₃⁻) are increasingly used to assess the sources and sinks of nitrogen (N) pollution in freshwater systems. However, the application of this methodology to pasture agroecosystems is currently limited by the lack of information on how, or even if, the primary N inputs to the systems (livestock urine and urea fertiliser) are expressed in the isotopic signature of exported NO₃⁻. To remedy this gap, direct measurements of fractionation during ammonia volatilisation were linked with changes in the concentration and isotopic composition of the residual soil inorganic N pool (NO₃⁻, nitrite, and ammonium) following the addition of differing levels of bovine urine and urea fertiliser. Ammonia volatilisation, with a δ¹⁵N enrichment factor of +35±5‰, removed from 5 to 40% of N inputs from the different treatments, which should have enriched the residual inorganic N pool to 25‰ and 3‰, respectively. However, this fractionation did not propagate into the soil NO₃⁻ pool due to a combination of urine-induced mineralisation (up to 120 μg N g soil⁻¹ day⁻¹ in the high urine treatment) and on-going nitrification. Consequently, NO₃⁻ measured within the treatments was not as enriched in ¹⁵N as the values typically ascribed to excreta-N sources. Up-scaling these results, the whole-pasture NO₃⁻ isotopic composition primarily reflected time since fertilisation, regardless of urine inputs. These findings necessitate expanding the range of δ¹⁵N-NO₃⁻ values ascribed to livestock sources to encompass values as low as -10‰, highlighting the need to account for post-deposition soil N cycling in order to accurately define NO₃⁻ isotopic source ranges

Spatial and temporal variations in nitrogen export from a New Zealand pastoral catchment revealed by stream water nitrate isotopic composition

Viable indicators of nitrogen (N) attenuation at the catchment scale are needed in order to sustainably manage global agricultural intensification. We hypothesized that the dominance of a single land use (pasture production) and strong ground-to-surface water connectivity would combine to create a system in which surface water nitrate isotopes (δ15N and δ18O of NO3-) could be used to monitor variations in catchment-scale attenuation. Nitrate isotopes were measured monthly over a 2 year period in four reaches along a spring-fed, gaining stream (mean NO3 - N of 6 mg L-1) in Canterbury, New Zealand. The stream water NO3- pool indicated that the highest degree of denitrification occurred in the shallow upper reaches. Moving downstream through increasingly sandy soils, the isotopic signature of denitrification became progressively weaker. The lowest reaches fell into the expected range for NO3- produced from the nitrification of pasture N sources (urine and fertilizers), implying that the attenuation capacity of the groundwater and riparian systems was lower than the rate of N inputs. After excluding months affected by effluent spills or extreme weather (n = 4), variations in the degree of denitrification over stream distance were combined with the measured NO3- discharge to estimate N attenuation over time in the subcatchment. Attenuation was highly responsive to rainfall: 93% of calculated attenuation (20 kg NO3 - N ha-1yr-1) occurred within 48 h of rainfall. These findings demonstrate the potential for detailed NO3- stable isotope data to provide integrative measures of catchment NO3- loss pathways

Biogeochemistry and community ecology in a spring-fed urban river following a major earthquake

In February 2011 a Mw 6.3 earthquake in Christchurch, New Zealand inundated urban waterways with sediment from liquefaction and triggered sewage spills. The impacts of, and recovery from, this natural disaster on the stream biogeochemistry and biology were assessed over six months along a longitudinal impact gradient in an urban river. The impact of liquefaction was masked by earthquake triggered sewage spills (∼20,000 m³ day⁻¹ entering the river for one month). Within 10 days of the earthquake dissolved oxygen in the lowest reaches was <1 mg l⁻¹, in-stream denitrification accelerated (attenuating 40–80% of sewage nitrogen), microbial biofilm communities changed, and several benthic invertebrate taxa disappeared. Following sewage system repairs, the river recovered in a reverse cascade, and within six months there were no differences in water chemistry, nutrient cycling, or benthic communities between severely and minimally impacted reaches. This study highlights the importance of assessing environmental impact following urban natural disasters

A hydrochemically guided landscape classification system for modelling spatial variation in multiple water quality indices: Process-attribute mapping

Spatial variation in landscape attributes can account for much of the variability in water quality relative to land use on its own. Such variation results from the coupling between the dominant processes governing water quality, namely hydrological, redox, and weathering and gradients in key landscape attributes, such as topography, geology, and soil drainage. Despite the importance of ‘process-attribute’ gradients (PAG), few water quality models explicitly account for their influence. Here a processes-based water quality modelling framework is presented that more completely accounts for the role of landscape variability over water quality – Process-Attribute Mapping (PoAM). Critically, hydrochemical measures form the basis for the identification and mapping of effective landscape attributes, producing PAG maps that attempt to replicate the natural landscape gradients governing each dominant process. Application to the province of Southland (31,824 km²), New Zealand, utilised 12 existing geospatial datasets and a total of 28,626 surface water, groundwater, spring, soil water, and precipitation analyses to guide the identification and mapping of 11 individual PAG. The ability of PAGs to replicate regional hydrological, redox, and weathering gradients was assessed on the accuracy with which the hydrochemical indicators of each dominant process (e.g. hydrological tracers, redox indicators) were estimated across 93 long-term surface water monitoring sites (cross-validated R² values of 0.75–0.95). Given hydrochemical evidence that PAGs replicate actual landscape gradients governing the dominant processes, they were combined with a land use intensity layer and used to estimate steady-state surface water quality. Cross-validated R² values ranged between 0.81 and 0.92 for median total nitrogen, total oxidised nitrogen, total phosphorus and dissolved reactive phosphorus. Models of particulate species E. coli and total suspended sediment, although reasonable (R² 0.72–0.73), were less accurate, suggesting finer-grained land use, landscape attribute, and/or flow normalised measures are required to improve estimation

Long-term organic carbon turnover rates in natural and semi-natural topsoils

We combined published and new radiocarbon and ancillary data for uncultivated topsoils (typically 15 cm depth), to make two databases, one for the United Kingdom (133 sites), and one global (114 sites). Forest topsoils are significantly higher in radiocarbon than non-forest soils, indicating greater enrichment with ‘‘bomb carbon’’ and therefore faster C turnover, if steady-state conditions are assumed. Steady-state modelling, taking into account variations in atmospheric 14CO2, including the effects of 20th century nuclear weapons testing and radioactive decay, was used to quantify soil carbon turnover rates. Application of a model with variable slow (20 year mean residence time, MRT) and passive (1,000 year MRT) carbon pools partitioned the topsoil C approximately equally, on average, between the two pools when the entire data set was considered. However, the mean slow:passive ratio of 0.65:0.35 for forest soil was highly significantly different (p\0.001) from the 0.40:0.60 ratio for non-forest soils. Values of the slow and passive fractions were normally distributed, but the non-forest fractions showed greater variation, with approximately twice the relative standard deviations of the forest values. Assuming a litter input of 500 g C m-2 a-1, average global C fluxes (g C m-2 a-1) of forest soils are estimated to be 298 (through a fast pool ofMRT1 year), 200 (slow pool) and 2.0 (passive pool), while for non-forest soils, respective average fluxes of 347, 150 and 3.3 g C m-2 a-1 are obtained. The results highlight the widespread global phenomenon of topsoil C heterogeneity, and indicate key differences between forest and non-forest soils relevant for understanding and managing soil C