Radiocarbon Evidence for Contrasting Soil Carbon Dynamics in a Andisol and Non-Andisol Pasture Soil Comparison

Symposium Pape

Can oxygen stable isotopes be used to track precipitation moisture

Variations in the isotopic composition of precipitation are determined by fractionation processes which occur during temperature- and humidity-dependent phase changes associated with evaporation and condensation. Oxygen stable isotope ratios have therefore been frequently used as a source of palaeoclimate data from a variety of proxy archives, which integrate this signal over time. Applications from ombrotrophic peatlands, where the source water used in cellulose synthesis is derived solely from precipitation, have been mostly limited to Northern Hemisphere Sphagnum-dominated bogs, with few in the Southern Hemisphere or in peatlands dominated by vascular plants. New Zealand (NZ) provides an ideal location to undertake empirical research into oxygen isotope fractionation in vascular peatlands because single taxon analysis can be easily carried out, in particular using the preserved root matrix of the restionaceous wire rush (Empodisma spp.) that forms deep Holocene peat deposits throughout the country. Furthermore, large gradients are observed in the mean isotopic composition of precipitation across NZ, caused primarily by the relative influence of different climate modes. Here, we test whether δ18O of Empodisma α-cellulose from ombrotrophic restiad peatlands in NZ can provide a methodology for developing palaeoclimate records of past precipitation δ18O. Surface plant, water and precipitation samples were taken over spatial (six sites spanning >10◦ latitude) and temporal (monthly measurements over one year) gradients. A link between the isotopic composition of root-associated water, the most likely source water for plant growth, and precipitation in both datasets was found. Back-trajectory modelling of precipitation moisture source for rain days prior to sampling showed clear seasonality in the temporal data that was reflected in root-associated water. The link between source water and plant cellulose was less clear, although mechanistic modelling predicted mean cellulose values within published error margins for both datasets. Improved physiological understanding and modelling of δ18O in restiad peatlands should enable use of this approach as a new source of palaeoclimate data to reconstruct changes in past atmospheric circulation

Convergence of soil nitrogen isotopes across global climate gradients

Quantifying global patterns of terrestrial nitrogen (N) cycling is central to predicting future patterns of primary productivity, carbon sequestration, nutrient fluxes to aquatic systems, and climate forcing. With limited direct measures of soil N cycling at the global scale, syntheses of the (15)N:(14)N ratio of soil organic matter across climate gradients provide key insights into understanding global patterns of N cycling. In synthesizing data from over 6000 soil samples, we show strong global relationships among soil N isotopes, mean annual temperature (MAT), mean annual precipitation (MAP), and the concentrations of organic carbon and clay in soil. In both hot ecosystems and dry ecosystems, soil organic matter was more enriched in (15)N than in corresponding cold ecosystems or wet ecosystems. Below a MAT of 9.8°C, soil δ(15)N was invariant with MAT. At the global scale, soil organic C concentrations also declined with increasing MAT and decreasing MAP. After standardizing for variation among mineral soils in soil C and clay concentrations, soil δ(15)N showed no consistent trends across global climate and latitudinal gradients. Our analyses could place new constraints on interpretations of patterns of ecosystem N cycling and global budgets of gaseous N loss

Redefining the inert organic carbon pool

Radiocarbon measurements reveal that soil carbon is often hundreds to thousands of years old; significantly older than the annual flux of carbon through the soil would suggest. Models deal with this discrepancy by conceptualizing soil carbon as having fast and slow cycling pools. The Rothamsted Soil Carbon Model contains an inert pool for this reason. Here we use a unique record of time-series radiocarbon measurements from long-term trials to demonstrate that the inert pool is hardly inert, and that its mean age varies from 2000 to as little as 90 years depending on carbon flow through the soil. This finding suggests that the concept of truly inert organic matter requires redefinition to account for the enhanced probability that microorganisms will overcome barriers to previously inaccessible organic matter as their activity increases

Using nutrient balance to estimate net C balance in landslide-prone pastoral hill country: testing the ‘‘dynamic equilibrium’’ hypothesis in New Zealand soft rock landscapes

Abstract: Given recent negotiations of the Committee of Parties to the United Nations Framework Convention on Climate Change, soil C accumulation related to soil conservation efforts may be counted as C credits in national C balance calculations under the Kyoto Protocol. It has been proposed that in some instances, erosion can establish a dynamic equilibrium that results in a C sink corresponding to ongoing recovery of C in eroded soils combined with ongoing C accumulation in terrestrial sediments. Given this hypothesis, full accounting for erosion and soil conservation in the C budgets of dynamic landscapes may represent a significant challenge due to indirect effects of burial and nutrient dynamics on the balance of plant production and decomposition of soil organic matter (SOM). As a method for evaluating the 'dynamic equilibrium' hypothesis, we examine the burial and availability of eroded C and nutrients using a combination of field data and models. Landslides represent a model system for studying erosional effects on C and N dynamics because they offer the ability to study easily identifiable events representing a known proportion of the landscape. We therefore investigate the C and N balance of pastoral land on soft-rock landscapes in New Zealand that commonly undergo shallow landslides. Soil cores driven to the bedrock interface indicate that 23-and 37-year-old landslides have recovered to -50% and 72% soil C stock, respectively, when compared with cores from uneroded sites on similar slopes and aspects. Locally, this upland soil loss represents the removal of 36 Mg/ha-90 Mg/ha. However, a portion of the eroded C may be retained on land if landslide debris is not fully evacuated by streams, and the eroded C can also become buried in marine environments. To determine whether the net effect of erosion can represent a sink for atmospheric CO 2 , we will combine the rate of upland soil C recovery with representations of the proportion of eroded C sequestered in sediments

Assessing effects of changes to nutrient loads on Lake Tarawera water quality: Model simulations for 2010 to 2020

Lake Tarawera is a nationally significant lake that is highly valued by tangata whenua, local residents and the regional community. Monitoring shows that lake water quality does not presently meet the target, based on a Trophic Level Index (TLI) value of 2.6 identified in the Tarawera Lakes Restoration Plan1. Between 2010 and 2020 annual observed TLI was frequently as high as 2.9. Managing nutrient loads to the lake is necessary to achieve desired lake water quality and interim nutrient reduction targets have been established. An objective of this study was to evaluate these targets by improving estimates of ‘sustainable nutrient loads’, which are the external loads of nitrogen and phosphorus that would result in meeting the TLI target

Greater soil carbon stocks and faster turnover rates with increasing agricultural productivity

Devising agricultural management schemes that enhance food security and soil carbon levels is a high priority for many nations. However, the coupling between agricultural productivity, soil carbon stocks and organic matter turnover rates is still unclear. Archived soil samples from four decades of a long-term crop rotation trial were analyzed for soil organic matter (SOM) cycling-relevant properties: C and N content, bulk composition by nuclear magnetic resonance (NMR) spectroscopy, amino sugar content, short-term C bioavailability assays, and long-term C turnover rates by modeling the incorporation of the bomb spike in atmospheric 14C into the soil. After > 40 years under consistent management, topsoil carbon stocks ranged from 14 to 33MgCha-1 and were linearly related to the mean productivity of each treatment. Measurements of SOM composition demonstrated increasing amounts of plant- and microbially derived SOM along the productivity gradient. Under two modeling scenarios, radiocarbon data indicated overall SOM turnover time decreased from 40 to 13 years with increasing productivity - twice the rate of decline predicted from simple steady-state models or static three-pool decay rates of measured C pool distributions. Similarly, the half-life of synthetic root exudates decreased from 30.4 to 21.5 h with increasing productivity, indicating accelerated microbial activity. These findings suggest that there is a direct feedback between accelerated biological activity, carbon cycling rates and rates of carbon stabilization with important implications for how SOM dynamics are represented in models.We would like to thank our predecessors at the Waite Research Institute for having the foresight to archive soil samples from this long-term trial, T. Carter for laboratory assistance, and J. McGowan for assistance in running NMR analyses. Funds for this research were provided by the CSIRO Sustainable Agriculture National Research Flagship and New Zealand public research funding through GNS Science (540GCT82). Mark Farrell was supported by a CSIRO Julius Career Award