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Context. Understanding the dynamics of soil organic matter (SOM) requires that it be divided into fractions with contrasting behaviour. Aim. Using soils from a field trial with a diverse set of treatments (pasture to arable conversion, ex-pasture soil maintained fallow for 13 years and arable cropping with contrasting tillage treatments), we evaluated two approaches to characterise SOM: particle-size fractionation and hot water extraction. Methods. Soils were separated into four size fractions, including the >50 [micro]m fraction (particulate organic matter, considered the most labile fraction) and the 50%) of the C lost under cropping and bare fallow was derived from the clay fraction, which was also the source of 59-77% of HWC. Conclusions and Implications. Our results indicate that hot water is specific for the labile component of SOM while SOM in size fractions is a composite of labile and stable components, the proportions of which may vary depending on land use and other factors. Ideally, data on quantity of SOM in size fractions should be complemented by information on its lability. Keywords: hot water extraction, labile C, land use effects, long-term fallow, particle size fractionation, pasture, soil organic matter, tillage., Introduction Soil organic matter (SOM) is vital to the health and functioning of soils. It also plays a central role in the global cycles of C and nutrients, including nitrogen [...]

Evidence is emerging that the solubility of soil organic matter (SOM) in water is a key factor regulating the turnover of carbon (C) and nitrogen (N). We used data from a field trial with a wide range of treatments in a case study to: (1) examine the link between SOM solubility and bioavailability and (2) evaluate whether low water-solubility is a factor contributing to the persistence of refractory SOM. The trial was established in 2000 on a silt loam (Udic Dystocrept) at Lincoln, New Zealand to identify management practices that maintain SOM following the conversion of long-term pasture to arable cropping. The following land use treatments were sampled (0-7.5, 7.5-15 and 15-25 cm) in 2013: (1) long-term ryegrass-white clover pasture; (2) arable cropping rotation, managed using either intensive, minimum, or no tillage; and (3) continuous bare fallow (plots maintained plant-free using herbicide; not cultivated). The bioavailability of SOM was determined by measuring C and N mineralisation in a 98-day incubation at 25[degrees]C (soil maintained near field capacity) and water solubility was assessed by measuring hot-water-extractable C and N (16-h extraction at 80[degrees]C). After 13 years of arable cropping, C stocks (to 25 cm) were 11 t [ha.sup.-1] less than in pasture soil (decrease of 14%). Tillage 'intensity' had no effect on C stocks in the top 25 cm. Large losses of C were observed in the bare fallow treatment (19 t C [ha.sup.-1] less than pasture soil). The bioavailability of SOM (C[O.sub.2]-C mineralised as a proportion of soil C) also declined under arable cropping and bare fallow. The relationship between total C and mineralised C had a significant (P < 0.001) intercept, indicating that part of the organic matter (13 g C [kg.sup.-1]) did not contribute to C mineralisation (it was biologically inert). Across treatments and sampling depths, SOM mineralised in 98 days generally corresponded well with that extracted in hot water. A significant fraction of SOM (~9 g C [kg.sup.-1]) did not release C to hot water. Water-insoluble organic matter, including compounds that are strongly bonded to mineral surfaces, may comprise a significant part of the refractory SOM. Keywords: bioavailability, land use effects, long-term fallow, recalcitrance, soil organic matter solubility., Introduction Soil organic matter (SOM) plays a central role in the global cycles of carbon (C) and nutrients, including nitrogen (N) (Powlson 1993; Janzen 2004; Stockmann el al. 2013). Knowledge [...]

Hot water is believed to extract bio-available soil organic matter (SOM), including organic compounds from the biomass of soil microbes. The role of soil physico-chemical factors in relation to extractability of SOM in hot water is not well understood. We evaluated the influence of exchangeable sodium (Na) on the quantity and quality of organic matter extracted in hot water from soils with a range of total and extractable C (total C 19-60 g [kg.sup.-1]; hot water extractable C (HWC) 659-3292 mg [kg.sup.-1]). The soils were pre-treated with different rates of Na (0-156 [cmol.sub.c] [kg.sup.-1], as NaCl) to establish a range of exchangeable Na percentages (ESP), and then extracted with hot water (80[degrees]C) for 16 h. Hot water extractable C increased linearly as ESP increased, but the rate of increase differed between soils (the increase in HWC per unit increase in ESP ranged within 19-71 mg [kg.sup.-1]). At ESP 15, a threshold used to separate sodic and nonsodic soils, HWC was 30-60% greater than that measured without added Na. Ultraviolet absorbance (260 nm) data indicated that aromatic organic matter was preferentially released following Na pre-treatment. The proportion of HWC in phenolic form was generally little affected by Na treatment but there was a consistent increase in protein in response to Na (hot water extractable organic N in protein form increased from an average of 5.5 [+ or -] 2.2% without added Na to 11.0 [+ or -] 3.6% at the highest Na rate). The Na-induced increases in UV absorbance may be largely attributable to release of proteins containing aromatic amino acids (tryptophan and tyrosine). Our results suggest that organic matter desorbed from mineral surfaces may be an important contributor to hot water extractable C and N, and factors that affect the adsorption-desorption process may significantly influence organic matter extractability in hot water. Additional keywords: chemical controls, dissolved protein and phenols, extractable organic matter, ultraviolet absorbance. Received 13 August 2019, accepted 18 February 2020, published online 31 March 2020, Introduction Dissolved organic matter is a labile component of soil organic matter (SOM) and a key driver of soil biological processes. The quantity of dissolved organic matter in soil is [...]

In New Zealand, pastoral soils have substantial organic carbon (OC) stocks, which may be vulnerable to loss from disturbance and environmental perturbations. We assessed OC vulnerability using two approaches. For the first approach, we postulated that the OC deficit of continuously cropped soils relative to nearby pastoral soils would provide a measure of the quantity of potentially vulnerable OC in pastures. As a test, soils were sampled to a depth of 15 cm at 149 sites and the total organic carbon (TOC) and particulate organic carbon (POC) contents were measured. The second approach involved measurement of OC mineralisation in a laboratory assay (98 day aerobic incubation at 25[degrees]C). For the pastoral soils, the mean TOC and POC was about twice that of the cropped soils. On average, 89% more OC was mineralised from the pastoral soils compared with the cropped counterparts. However, the quantity of OC mineralised in pasture soils was small relative to the potential for OC loss inferred from the difference in TOC between pastoral and cropped soils. Carbon mineralisation was explained using a two-pool exponential model with rate constants of the 'fast' and 'slow' pools equating to 0.36 [+ or -] 0.155 and 0.007 [+ or -] 0.003 [day.sup.-1] respectively. The larger, slow OC pool correlated strongly with hot water extractable OC whereas the fast pool was related to OC extracted using cold water. Our results suggest that water extraction (using cold and hot water) can provide a rapid estimate of the quantity of mineralisable OC across a wide range of New Zealand soils. Additional keywords: C mineralisation, cropping, pasture SOC, vulnerability., Introduction Increasing soil organic carbon (SOC) has been identified as a potential strategy to off-set the increase in global greenhouse gas emissions (Lai 2004). Depending on a range of factors [...]

Cold (room temperature) and hot water (70-80[degrees]C) extraction methods have been used to assess the availability of organic matter and N in soil; however, the influence of temperature on their extractability is largely unknown. Twenty-one mineral soils from New Zealand and eastern Canada were incubated for 16 h in water (1:6 soil/water ratio) at temperatures ranging from 20 to 80[degrees]C. After centrifugation and removal of the supernatant solution, the soil residue was extracted with 2 tool [L.sup.-1] KCl to recover N[H.sub.4]-N released during incubation. In all cases, water-extractable organic C (WEOC) and N (WEON) increased exponentially with temperature. The temperature dependence of organic matter solubility differed considerably among the soils (estimated increase in WEOC and WEON: 1.1-5.3% [degrees][C.sup.-1]). These differences were not explained by management history or soil physicochemical properties, although there was evidence that organic matter extractability in some clay soils was less sensitive to temperature. Ammonium N was released in significant amounts at all temperatures. Peak N[H.sub.4]-N release typically occurred at 50[degrees]C, where N[H.sub.4]-N comprised an average of 49% of the total recovered N (WEON plus N[H.sub.4]-N). Factors that may account for N[H.sub.4]-N production during the 16-h incubation include mineralization at the lower temperatures and abiotic processes (e.g., release of clay-fixed N[H.sub.4] and thermal degradation of organic N) at higher temperatures. The possibility that mineralization made a significant contribution to N[H.sub.4]-N production at temperatures as high as 50[degrees]C warrants further investigation. Abbreviations: WEOC, water-extractable organic carbon; WEOM, water-extractable organic matter; WEON, water-extractable organic nitrogen. doi:10.2136/sssaj2008.0347

As soils under permanent pasture and grasslands have large topsoil carbon (C) stocks, the scope to sequester additional C may be limited. However, because C in pasture/grassland soils declines with depth, there may be potential to sequester additional C in the subsoil. Data from 247 continuous pasture sites in New Zealand (representing five major soil Orders and ~80% of the grassland area) showed that, on average, the 0.15–0.30 m layer contained 25–34 t ha−1 less C than the top 0.15 m. High‐production grazed pastures require periodic renewal (re‐seeding) every 7–14 years to maintain productivity. Our objective was to assess whether a one‐time pasture renewal, involving full inversion tillage (FIT) to a depth of 0.30 m, has potential to increase C storage by burying C‐rich topsoil and bringing low‐C subsoil to the surface where C inputs from pasture production are greatest. Data from the 247 pasture sites were used to model changes in C stocks following FIT pasture renewal by predicting (1) the C accumulation in the new 0–0.15 m layer and (2) the decomposition of buried‐C in the new 0.15–0.30 m layer. In the 20 years following FIT pasture renewal, soil C was predicted to increase by an average of 7.3–10.3 (Sedimentary soils) and 9.6–12.7 t C ha−1 (Allophanic soils), depending on the assumptions applied. Adoption of FIT for pasture renewal across all suitable soils (2.0–2.6 M ha) in New Zealand was predicted to sequester ~20–36 Mt C, sufficient to offset 9.6–17.5% of the country's cumulative greenhouse gas emissions from agriculture over 20 years at the current rate of emissions. Given that grasslands account for ~70% of global agricultural land, FIT renewal of pastures or grassland could offer a significant opportunity to sequester soil C and offset greenhouse gas emissions. [ABSTRACT FROM AUTHOR]

This study examined the role of earthworms in the stabilisation of soil organic carbon from livestock dung in a long-term mesocosm study, including its incorporation in deeper soil layers and association with particle size fractions. Two earthworm treatments were established, with the abundant and diverse treatment having 390 m−2 less epigeic Lumbricus rubellus, 2400 m−2 more endogeic Aporrectodea caliginosa and 220 m−2 more anecic Aporrectodea longa at the final sampling than the treatment dominated by epigeic species. The abundant and diverse earthworm treatment increased the decomposition of dung in the surface 20 mm, and doubled the amount of dung-derived C within the <250 µm size fractions at 20–75 mm depth compared to the epigeic treatment (4.8 vs. 9.8 g/kg soil, respectively). Earthworms have the ability to increase sequestration of C into smaller, and potentially more stable, size fractions. [ABSTRACT FROM AUTHOR]

Knowledge of soil biological and physical interactions with respect to N2O and N2 fluxes is essential to ensure that agricultural land management is environmentally and economically sustainable. This study determined how varying soil relative gas diffusivity (Dp/Do) affected cumulative N2O and N2 fluxes under simulated ruminant urinary-N deposition. Using repacked soil cores, the effects of varying soil bulk density (pb; from 1.1 to 1.5 Mg m-3) and soil matric potential (ψ; -10 to -0.2 kPa) on Dp/Do were examined in a Templeton silt loam soil (Udic Haplustept) following the application of simulated ruminant urine (700 kg N ha-1). Fluxes of N2O and N2, soil inorganic N, pH and dissolved organic C (DOC) dynamics were monitored over 35 d. Soil Dp/Do declined as soil bulk density and soil moisture increased. Soil N2O emissions increased exponentially as Dp/Do decreased until Dp/Do equaled 0.005, where upon N2O fluxes decreased rapidly due to complete denitrification, such that N2 fluxes reached a maximum of 60% of N applied at a Dp/Do of <0.005. Regression analysis showed that Dp/Do was better able to explain the variation in N2O and N2 fluxes than water-filled pore space (WFPS) because it accounted for the interaction of soil pb and ψ. This study demonstrates that soil Dp/Do can explain cumulative N2O and N2 emissions from agricultural soils. Under grazed pasture systems, potential exists to reduce the emissions of the greenhouse gas N2O and significant economic losses of N as N2 if soil management and irrigation can be maintained to maximize Dp/Do. [ABSTRACT FROM AUTHOR]