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PUBLICATIONS Journal of Geophysical Research: Earth Surface RESEARCH ARTICLE 10.1002/2016JF003864 Key Points: • Soil chemistry and weathering vary nonlinearly across a large rainfall gradient (400–4700 mm/yr) in NZ • Climate control evidenced in detailed soil chemistry, Fe and Al mobilities, and cation leaching • Moisture availability can act as a “switch” to enable rapid chemical weathering in young soils Supporting Information: • Supporting Information S1 • Data Set S1 Correspondence to: J. L. Dixon, jean.dixon@montana.edu Citation: Dixon, J. L., O. A. Chadwick, and P. M. Vitousek (2016), Climate-driven thresholds for chemical weathering in postglacial soils of New Zealand, J. Geophys. Res. Earth Surf., 121, 1619–1634, doi:10.1002/2016JF003864. Received 18 FEB 2016 Accepted 12 AUG 2016 Accepted article online 17 AUG 2016 Published online 16 SEP 2016 Climate-driven thresholds for chemical weathering in postglacial soils of New Zealand Jean L. Dixon 1,2 , Oliver A. Chadwick 2 , and Peter M. Vitousek 3 Department of Earth Sciences and the Institute on Ecosystems, Montana State University, Bozeman, Montana, USA, Department of Geography, University of California, Santa Barbara, California, USA, 3 Department of Biology, Stanford University, Stanford, California, USA Abstract Chemical weathering in soils dissolves and alters minerals, mobilizes metals, liberates nutrients to terrestrial and aquatic ecosystems, and may modulate Earth’s climate over geologic time scales. Climate-weathering relationships are often considered fundamental controls on the evolution of Earth’s surface and biogeochemical cycles. However, surprisingly little consensus has emerged on if and how climate controls chemical weathering, and models and data from published literature often give contrasting correlations and predictions for how weathering rates and climate variables such as temperature or moisture are related. Here we combine insights gained from the different approaches, methods, and theory of the soil science, biogeochemistry, and geomorphology communities to tackle the fundamental question of how rainfall influences soil chemical properties. We explore climate-driven variations in weathering and soil development in young, postglacial soils of New Zealand, measuring soil elemental geochemistry along a large precipitation gradient (400–4700 mm/yr) across the Waitaki basin on Te Waipounamu, the South Island. Our data show a strong climate imprint on chemical weathering in these young soils. This climate control is evidenced by rapid nonlinear changes along the gradient in total and exchangeable cations in soils and in the increased movement and redistribution of metals with rainfall. The nonlinear behavior provides insight into why climate-weathering relationships may be elusive in some landscapes. These weathering thresholds also have significant implications for how climate may influence landscape evolution and the release of rock-derived nutrients to ecosystems, as landscapes that transition to wetter climates across this threshold may weather and deplete rapidly. 1. Introduction 1.1. Climate’s Elusive Control on Chemical Weathering Soils lie at the interface of air, water, life, and rock, and the weathering dynamics that transform minerals and water in soils are shaped by diverse processes. Climate has long been recognized to be one of the major dri- vers of these weathering processes [Jenny, 1941]. Temperature controls the kinetics of chemical reactions, and water has a role in nearly every chemical weathering reaction that directly results in mass loss from a rock or mineral. Therefore, warmer and wetter conditions should lead to higher weathering rate and intensity in soils. However, a coherent understanding of how climate controls soil chemical weathering remains elusive. Several reasons emerge for the lack of consensus across studies. 1.2. Competing Variables ©2016. The Authors. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distri- bution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. DIXON ET AL. First, while the conceptual framework for climate control on weathering rates is validated by laboratory experiments quantifying dissolution rates under different temperature or water-flow conditions [White et al., 1999; White and Brantley, 2003], field-based studies reveal significant complexity among climate- weathering linkages [Brantley, 2003; Drever et al., 1994; White and Brantley, 2003], which may be explained by time-dependent factors such as changing mineral surface area, pore water concentrations, and secondary precipitates. Similarly, climate’s control on soil weathering can be modified by the complex influence of other competing variables such as lithology, erosion rates, and/or dust deposition [e.g., Ferrier et al., 2012; Riebe et al., 2004]. Furthermore, field-based weathering rates are often measured in locations where multiple variables (including climate variables such as temperature and water availability) exert competing controls on mineral weathering and the fate of released ions [Chadwick and Chorover, 2001]. These competing climatic controls may be deconvolved using careful sampling designs and accounting for multiple variables [e.g., Dixon et al., 2009a; Rasmussen et al., 2011; White and Blum, 1995]; however, derived relationships and models may be site specific or have limited applicability. CLIMATE-DRIVEN WEATHERING THRESHOLDS

The timing of alluvial deposition in arid and semiarid areas is tied to land-surface instability caused by regional climate changes. The distribution pattern of dated deposits provides maps of regional land-surface response to past climate change. Sensitivity to differences in surface roughness and composition makes remote sensing techniques useful for regional mapping of alluvial deposits. Radar images from the Spaceborne Radar Laboratory and visible wavelength images from the French SPOT satellite were used to determine remote sensing signatures of alluvial fan units for an area in the Kun Lun Mountains of northwestern China. These data were combined with field observations to compare surface processes and their effects on remote sensing signatures in northwestern China and the southwestern United States. Geomorphic processes affecting alluvial fans in the two areas include aeolian deposition, desert varnish, and fluvial dissection. However, salt weathering is a much more important process in the Kun Lun than in the southwestern United States. This slows the formation of desert varnish and prevents desert pavement from forming. Thus the Kun Lun signatures are characteristic of the dominance of salt weathering, while signatures from the southwestern United States are characteristic of the dominance of desert varnish and pavement processes. Remote sensing signatures are consistent enough in these two regions to be used for mapping fan units over large areas.

[1] Black carbon (BC) has long been considered a chemically resistant component of soil organic carbon (SOC). However, there is substantial evidence that the chemistry of most C compounds is less important for long-term storage than is physical protection (e.g., mineral sorption). We explored BC retention in grasslands that lie along a climate gradient that produces strong differences in short range order (SRO) minerals known to drive landscape-scale retention of SOC. We measured soil BC content using 13C nuclear magnetic resonance (NMR) spectroscopy, and used radiocarbon dating on a subset of samples to relate BC content to long-term soil C storage. Black C concentrations in soil ranged from 0.2 to 2.9%, representing 10–30% of SOC and spanning levels found in temperate grasslands around the world. Black C concentrations were significantly correlated with SRO minerals, but the strongest single predictor of BC content was simply SOC. The ratio of BC/OC was fairly insensitive to SRO minerals, suggesting that BC responds similarly to reactive minerals as does OC. Direct links between SOC and BC retention warrant further study. We found no evidence that BC is preferentially retained relative to OC, with soil radiocarbon ages apparently driven primarily by inputs of new OC. These results indicate that physical protection may play a strong role in BC retention, and that BC cycling in soils may be more similar to OC cycling than is generally thought.

[1] This study aimed to better understand the mechanisms for soil organic matter delivery to and accumulation in mineral horizons of tropical rain forest, volcanic soils. We used soil morphology, lysimetry, isotopes, and spectroscopy to investigate the role of preferential flow paths in the delivery of carbon (C) to the subsoil. High rainfall, high primary productivity, and the dominance of highly reactive, short-range-order minerals combine to sequester substantial stocks of soil C with long mean residence times. The soils have large peds, separated by wide cracks, which form a network of channels propagating downward through the top 40 to 60 cm, facilitating macropore flow. The channel infillings and crack surfaces were enriched in organic material (OM) with lower C:N ratios, and had higher ammonium oxalate-extractable Al, and lower ammonium oxalate-extractable Fe than the adjacent mineral bulk soil. CP MAS 13C-NMR spectra of OM accumulating at depth showed strong signal intensities in the carboxyl and carbonyl C regions, indicative of organic acids, while decaying roots showed greater contributions of aromatic and O-alkyl C. The ratios of alkyl-to-O-alkyl C in the organic infillings were more similar to those of the bulk Bh and to dissolved organic matter than to those of decaying roots. Radiocarbon-based ages of OM infillings at >50 cm depth were significantly younger than the mineral soil (2000 years versus 7000 years). Respired CO2 from incubated soils showed that OM accumulating at depth is a mixture of modern and much older C, providing further evidence for the downward movement of fresh C.

most accurate technique, direct polarization solid-state 13 C NMR with the molecular mixing model, agrees with elemental analyses to ±0.036 Cox units (±0.009 OR units). Using this technique, we show a large natural variability in soil Cox and OR values. Soil Cox values have a mean of � 0.26 and a range from � 0.45 to 0.30, corresponding to OR values of 1.08 ± 0.06 and a range from 0.96 to 1.22. We also estimate the OR of the carbon flux from a boreal forest fire. Analysis of soils from nearby intact soil profiles imply that soil carbon losses associated with the fire had an OR of 1.091 (±0.003). Fire appears to be a major factor driving the soil C pool to higher oxidation states and lower OR values. Episodic fluxes caused by disturbances like fire may have substantially different ORs from ecosystem respiration fluxes and therefore should be better quantified to reduce uncertainties associated with our understanding of the global atmospheric carbon budget.

Degree of oxidation of organic carbon (Cox) is a fundamental property of the carbon cycle, reflecting the synthesis and decomposition of natural organic matter. Cox is also related to ecosystem oxidative ratio (OR), the molar ratio of O2 to CO2 fluxes associated with net ecosystem exchange (NEE). Here we compare two methods for measuring Cox and OR: (1) %C, %H, %N, and %O elemental analysis, and (2) heat of combustion (ΔHc) measured by means of bomb calorimetry coupled with %C elemental analysis (hereafter referred to as calorimetry). Compared with %C, %N, %H, and %O elemental analysis, calorimetry generates Cox and OR data more rapidly and cheaply. However, calorimetric measurements yield less accurate Cox and OR data. We additionally report Cox and OR data for a pair of biomass standards and a suite of biomass samples. The OR values we measured in these samples were less variable than OR data reported in the literature (generated by simultaneous measurement of ecosystem O2 and CO2 gas mixing ratios). Our biomass OR values had a mean of 1.03 and range of 0.99–1.06. These estimates are lower than the OR value of 1.10 that is often used to partition uptake of fossil fuel CO2 between the ocean and the terrestrial biosphere.

[1] The dynamics of biologic Si cycling in grassland ecosystems are largely unknown and likely to impact mineral weathering rates regionally and diatom productivity globally; key regulatory processes in the global Si cycle are closely tied to the global carbon cycle. Across a bioclimatic sequence spanning major grassland ecosystems in the Great Plains, soil biogenic silica depth distributions are similar to that of soil organic carbon; however, unlike soil organic carbon, quantities of soil biogenic silica decrease with increasing precipitation, despite an increase in annual biogenic inputs through litterfall across the same gradient. Though comprising only 1–3% of the total Si pool, faster turnover of biogenic Si and annual cycling by grasses should positively impact mineral dissolution. Our results suggest that the largest reservoir of biogenic Si in terrestrial ecosystems resides in soils, and emphasize the potential significance of grasslands in the global biogeochemical cycle of Si.

[1] Studies of tropical soil organic carbon (SOC) response to deforestation present conflicting results, confounding estimates of the regional effects of land-cover change on carbon storage. We calculated the change in SOC stocks due to deforestation through 1996 for the state of Rondonia, Brazil, in the southwestern Amazon basin. Whereas the net change in SOC for the state as a whole was slightly negative (−0.5% or −5012 Gg), spatially explicit maps suggest dramatic local changes, ranging from −76% to +74%, with outliers as high as +330%. The direction and magnitude of change in SOC following forest clearing is related to original forest soil carbon and pH, which in turn provides a general measure for overall nutrient availability and possible toxicities. When native soil carbon is high, SOC decreases in response to land-cover conversion from forest to pasture; conversely, low soil carbon and low soil fertility lead to gains in carbon under pasture. Mapping variability, rather than relying on large-area averages, illustrates why results from individual field sites have been contradictory.

[1] Soil carbon (C), nitrogen (N), and phosphorus (P) are lost from hillslopes in particulate forms through soil erosion. The fate of the eroded C (e.g., sequestration or oxidation) may affect the global C budget, and delivery of N and P to waterbodies can lead to eutrophication. Whereas the magnitude of particulate nutrient losses may be similar to or greater than dissolved losses, it is rarely estimated. We couple a sediment delivery model with measurements of C, N, and P in soil to account explicitly for hillslope sediment transport processes that yield sediment-bound nutrients to fluvial networks. The model is applied to a site in California dominated by coastal sage scrub and gopher-rich grasslands. Although the magnitude of sediment delivery predicted by the model has been tested with reservoir sedimentation records, no data exist to test the predicted rates of nutrient delivery. Nevertheless, the model results are provocative; it predicts that losses of particulate C from sage covered hillslopes (23 kg/ha/yr) are nearly double that from grassland hillslopes (13 kg/ha/yr), despite a lower annual sediment yield from the sage hillslopes. The model predicts similar average annual N and P losses for sage and grasslands but dramatic differences in the frequency and magnitude of delivery events. Nutrient delivery from grasslands is chronic whereas delivery from the coastal sage is highly episodic, with large pulses driven by fire frequency. These results suggest that changes in the vegetation community can alter the delivery regime of sediment-bound C, N, and P.

[1] Biogeochemical processes fractionate Ca isotopes in plants and soils along a 4 million year developmental sequence in the Hawaiian Islands. We observed that plants preferentially take up 40Ca relative to 44Ca, and that biological fractionation and changes in the relative contributions from volcanic and marine sources produce a significant increase in 44Ca in soil exchangeable pools. Our results imply moderate fluxes enriched in 44Ca from strongly nutrient-depleted old soils, in contrast with high 40Ca fluxes in young and little weathered environments. In addition, biological fractionation controls divergent geochemical pathways of Ca and Sr in the plant-soil system. While Ca depletes progressively with increasing soil age, Sr/Ca ratios increase systematically. Sr isotope ratios provide a valuable tracer for provenance studies of alkaline earth elements in forested ecosystems, but its usefulness is limited when deciphering biogeochemical processes involved in the terrestrial Ca cycle. Ca isotopes in combination with Sr/Ca ratios reveal more complex processes involved in the biogeochemistry of Ca and Sr.

Author(s): Masiello, C.A.; Chadwick, O.A.; Southon, J.; Torn, M.S.; Harden, J.W. | Abstract: On a sequence of soils developed under similar vegetation, temperature, and precipitation conditions, but with variations in mineralogical properties, we use organic carbon and 14C inventories to examine mineral protection of soil organic carbon. In these soils, 14C data indicate that the creation of slow-cycling carbon can be modeled as occurring through reaction of organic ligands with Al3+ and Fe3+ cations in the upper horizons, followed by sorption to amorphous inorganic Al compounds at depth. Only one of these processes, the chelation of Al3+ and Fe3+ by organic ligands, is linked to large carbon stocks. Organic ligands stabilized by this process traverse the soil column as dissolved organic carbon (both from surface horizons and root exudates). At our moist grassland site, this chelation and transport process is very strongly correlated with the storage and long-term stabilization of soil organic carbon. Our 14C results show that the mechanisms of organic carbon transport and storage at this site follow a classic model previously believed to only be significant in a single soil order (Spodosols), and closely related to the presence of forests. The presence of this process in the grassland Alfisol, Inceptisol, and Mollisol soils of this chronosequence suggests that this process is a more significant control on organic carbon storage than previously thought.

[1] We examined the pedogenic behavior of lithium (Li) and its isotopes in Hawaii by sampling same-age lava flows under mean annual rainfall ranging from 18 to 300 cm. Lithium concentrations in these soils vary from 1 to 29 ppm. Whereas Na, K, and Ca are completely leached from the soil at the most humid and severely weathered site, Li, Mg, Si, and Al show significant retention due to their association with secondary clay minerals. In these soils, allochthonous Li delivered in marine and mineral aerosol mixes with basalt-derived Li, modifying the isotopic composition of the Li pool. The ability of soil to retain Li is related to its effective cation exchange capacity, which in turn is governed by rainfall and leaching intensity and their resulting effect on mineralogy. Lithium isotope ratios have a large range (δ7Li of −0.4 to 14‰), well beyond the intershield or temporal variation of the lavas of Hawaiian volcanoes (2.5 to 5.7‰). The most unweathered samples have δ7Li (∼5–6‰) comparable to that of Mauna Kea and Mauna Loa lavas. The Li concentrations and isotope ratios together suggest that in arid to subhumid sites there is a net addition of isotopically heavier lithium from marine aerosol but that under greater rainfall there is a net loss of Li, and isotopically light Li is preferentially retained. Thus, during incongruent weathering of primary minerals, Li, especially 6Li, is adsorbed and sequestered by the clay mineral fraction of high cation exchange capacity soils in drier regions but lost from highly weathered, low cation exchange capacity acidic soils in the wetter regions.

[1] Leaching, biomass removal, and partitioning of phosphorus (P) into reservoirs not available to plants can limit the long-term productivity of terrestrial ecosystems. We evaluate the importance of atmospheric P inputs to the world's soils by estimating the total soil P turnover time with respect to dustborne P additions. Estimated turnover times range from ∼104 to ∼107 years. Our estimates provide a unique perspective on the importance and patterns of aeolian deposition to terrestrial landscapes. Dust source regions are areas of intense soil P cycling on large scales, but are too water-limited for this rapid cycling to have a major influence on ecosystem dynamics. By contrast, semiarid desert margins receive significant aeolian P from neighboring deserts and are likely influenced by dustborne P additions for the long-term maintenance of productivity. This is particularly true for the semiarid steppes of Africa and Eurasia. The prevalence of large dust sources in Africa and Eurasia indicates that these areas may generally be more influenced by dustborne P additions than soils in the Americas. Significant western hemisphere exceptions to this pattern occur on very old landscapes, such as the forests of the southeastern United States and the Amazon Basin. The Amazon Basin is highly dependent on aeolian deposition for the maintenance of long-term productivity. Dust deposition to terrestrial environments has not been constant with time. Variability in past P deposition related to geologically recent climate change may provide the strongest controls on present and future soil P in the Amazon and elsewhere.

[1] We describe spatiotemporal variation in land cover over 80,000 km2 in central Rondonia. We use a multistage process to map primary forest, pasture, second growth, urban, rock/savanna, and water using 33 Landsat scenes acquired over three contiguous areas between 1975 and 1999. Accuracy of the 1999 classified maps was assessed as exceeding 85% based on digital airborne videography. Rondonia is highly fragmented, in which forests outside of restricted areas consist of numerous, small irregular patches. Pastures in Rondonia persist over many years and are not typically abandoned to second growth, which when present rarely remains unchanged longer than 8 years. Within the state, annual deforestation rates, pasture area, and ratio of second growth to cleared area varied spatially. Highest initial deforestation rates occurred in the southeast (Luiza), at over 2%, increasing to 3% by the late 1990s. In this area, the percentage of cleared land in second growth averaged 18% and few pastures were abandoned. In central Rondonia (Ji-Parana), deforestation rates rose from 1.2% between 1978 and 1986 to a high of 4.2% in 1999. In the northwest (Ariquemes), initial deforestation rates were lowest at 0.5% but rose substantially in the late 1990s, peaking at 3% in 1998. The ratio of second growth to cleared area was more than double the ratio in Luiza and few pastures remained unchanged beyond 8 years. Land clearing was most intense close to the major highway, BR364, except in Ariquemes. Intense forest clearing extended at least 50 km along the margins of BR364 in Ji-Parana and Luiza. Spatial differences in land use are hypothesized to result from a combination of economic factors and soil fertility.

At the Earth's surface, a complex suite of chemical, biological, and physical processes combines to create the engine that transforms bedrock into soil (Figure 1). Earth's weathering engine provides nutrients to nourish ecosystems and human society mediates the transport of toxic components within the biosphere, creates water flow paths that carve and weaken bedrock, and contributes to the evolution of landscapes at all temporal and spatial scales. At the longest time scales, the weathering engine sequesters CO2, thereby influencing long-term climate change. Despite the importance of soil, our knowledge of the rate of soil formation is limited because the weathering zone forms a complex, ever-changing interface, and because scientific approaches and funding paradigms have not promoted integrated research agendas to investigate such complex interactions. No national initiative has promoted a systems approach to investigation of weathering science across the broad array of geology, soil science, ecology and hydrology. Such a program is certainly needed, and this article describes a platform on which to build the initiative to answer the following question: How does the Earth weathering engine break down rock to nourish ecosystems, carve errestrial landscapes, and control carbon dioxide in the global atmosphere?