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Soil organic carbon (SOC) is both a source and sink of atmospheric carbon dioxide (C[O.sub.2]), with important implications for global climate change. Irrigation of grazed pastures has reportedly increased, reduced or made no difference to SOC stocks relative to dryland management. This study examined, over an annual plant growth cycle, the persistence of photosynthate-derived carbon (C) previously allocated to the plant-soil system during summer, under irrigated or dryland conditions. A continuous [sup.13]C[O.sub.2] pulse labelling method was used to label ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) mesocosms under simulated dryland or irrigated conditions. Plant and soil [sup.13]C was traced over 349 days using destructive sampling on days 1, 12, 125, 237 and 349 (T1-T5 respectively). After the cessation of labelling and summer irrigation, the mesocosms were maintained under the same seasonal soil moisture conditions. The persistence of [sup.13]C in the aboveground plant C pools was lower than in the root C pool. Approximately 50% of the initial mass of [sup.13]C recovered in roots at T1 remained by T5. There was no difference between the summer irrigated and dryland treatments in terms of the [sup.13]C recovered from the soil over the subsequent annual growing season. There was also no significant change in the [sup.13]C recovered in the soil between T1 and T5. However, summer irrigation did affect the spatial and temporal distribution of the photosynthate-derived C within the soil size fractions relative to summer dryland conditions. Summer irrigation promoted the transfer and storage of [sup.13]C in the fine particulate organic matter and clay size fractions. Keywords: irrigation, pasture, photosynthate C, soil particle size fractionation., Introduction Increasing or maintaining soil organic carbon (SOC) has been proposed as an option to offset atmospheric carbon dioxide (C[O.sub.2]) emissions, and therefore mitigate climate change (Chabbi et al. 2017; [...]

Soil rewetting can induce a flush of organic matter mineralisation, but the factors underpinning this mineralisation response are poorly understood. We investigated the effects of antecedent soil water content, before rewetting, on the quantity, quality and biodegradability of dissolved organic matter present in the leachate pore volumes from a soil under two different management histories: arable and grassland. Soils were collected at field capacity (FC) and dried to give four soil gravimetric water contents ([[theta].sub.g]): 22% (not dried, left at FC), 15%, 8% and Additional keywords: arable, dissolved organic carbon, biodegradation, drying and rewetting, grassland, [SUVA.sub.254]. Received 18 June 2018, accepted 21 February 2019, published online 29 March 2019, Introduction Dissolved organic matter (DOM) in soil solution is recognised as the most mobile (von Lutzow et al. 2007) and reactive (Haynes 2005) soil organic matter (SOM) fraction, and its [...]

Developing strategies to sequester carbon (C) in soils requires an understanding of the key factors that influence C stabilisation. Although fine mineral particles, especially clay, play a key role in stabilising soil organic matter (SOM), the relationship between SOM and texture is often not strong. We examined the role of the fine mineral fraction in C storage in sedimentary soils in New Zealand. Soils, representing two soil Orders (Brown and Recent) and different land use histories (total of 58 soils; 0-15 cm depth) were sampled. The concentration of C (and N) in four particle size fractions (50 µm) was determined (soils fractionated after dispersion by sonication). The soils had a wide range of textures and SOM; the mass proportion of clay (0.05) between the C concentration in whole soil and clay content. The concentration of C in the clay fraction, which varied over a wide range (35 to 135 g [kg.sup.-1] clay), decreased as the mass proportion of clay increased. A similar trend in C concentration was observed for the fine (5-20 µm) silt fraction. Because of this inverse relationship between the mass of the fine fractions and their C concentration, there was little change in amount of stable C (defined as C in the Additional keywords: C stabilisation by mineral particles, extractable Al & Fe, organic matter fractions., Introduction Soils play an important role in the global C cycle. Management practices that enhance soil C storage are frequently advocated as possible mitigation options to offset C[O.sub.2] emissions from [...]

Conventional, fixed-depth sampling may lead to errors in quantifying temporal changes in C stocks if soil bulk density (BD) changes. When surface elevation is altered due to a change in BD, sampling depth should be adjusted so that the sampled soil mass remains constant. We evaluated the ability of a topographic survey method (precise level) to quantify temporal changes (2000-2007) in surface elevation following conversion of long-term pasture to arable cropping using conventional tillage (plowing to 20 cm), minimum tillage, or no-till. Measured elevations were compared with values estimated from BD determined for fixed-depth increments (0-7.5, 7.5-15,15-25, and 25-30 cm). In the first season, soil mass to 25 cm decreased by ~270 Mg/ha under conventional cultivation (relative to precultivation mass of 3345 Mg/ha) as BD decreased in the 7.5- to 15- and 15- to 25-cm layers. This represented an upward movement of the soil surface by an estimated 22 mm. Minimum tillage and no-till treatments moved 9 and 2 mm upward, respectively. Subsequently, the soil consolidated and surface elevation declined by an estimated 8 mm in all cultivated treatments. Elevation changes measured by the precise level were very similar to these BDbased estimates. Changes in elevation in conventional, minimum tillage, and no-till treatments averaged 13, 1.5, and -6 mm, respectively (positive values indicate upward movement, negative values subsidence). Corresponding BD-estimated values were 17, 1.1, and -3 mm. Using topographic survey information, sampling depth can be adjusted so that a constant mass is sampled from soils where BD has been changed by tillage or natural processes. doi: 10.2136/sssaj2009.0251

Specific surface area can be a strong predictor of organic carbon (SOC) contents in soils. Specific surface area can be estimated reliably and cost‐effectively from water adsorption by air‐dry soil samples, but SOC itself can also adsorb water. For estimating the mineral component of specific surface area, it is, therefore, necessary to exclude water‐adsorption by SOC. Here, we refer to "apparent specific surface area" for measurements that include water adsorption by both mineral soil and SOC. We used a mathematical approach to estimate water adsorption by SOC so that this component can be subtracted from measurements of apparent specific surface area. We used a dataset of apparent specific surface area and soil carbon at seven depths from 50 soil cores collected from a research farm in the Manawatu region in New Zealand. Both apparent specific surface area and SOC content decreased with soil depth with very high correlation (r2 = 0.98). We estimated the SOC contribution to apparent specific surface area from the slope of the relationship between changes in apparent specific surface area and SOC content. For our soils, the SOC contribution to apparent specific surface area was estimated as 0.43 ± 0.02 m2 mgC−1. This parameter allows apparent specific surface area measurements to be corrected for the water adsorption by SOC to calculate the functionally relevant mineral specific surface area. Highlights: Soil surface area can be estimated from the H2O content of air‐dry soil but SOC also adsorbs H2O.We developed a mathematical approach to estimate water adsorption by SOC.We estimated the contribution of SOC to apparent specific surface area as 0.43 ± 0.02 m2 mgC−1.Mineral specific surface area can be inferred by subtracting SOC‐based H2O adsorption. [ABSTRACT FROM AUTHOR]

Changes in soil physical conditions can have an impact on plant root growth. However, the underlying functional responses of root growth to variations in soil physical properties still remain poorly understood. We assessed the responses of barley (Hordeum vulgare L.) roots to a typical range of soil physical properties created by applying different degrees of loosening or compaction to subsurface soil layers. A replicated field experiment was established on a Templeton silt loam soil (Dystric Ustochrept) in Canterbury, New Zealand. Following removal of the top 15 cm of soil, five mechanical treatments were applied: loosened by cultivation, three degrees of compaction using a roller (one pass, eight passes, and eight passes with vibration), and an untreated control. Subsequently, the top 15 cm of soil was repositioned. The impact of these treatments on soil physical properties in the subsurface soil layer (15-30 cm) produced a discernible gradient that was well characterized by shifts in pore-size distribution, in particular macroporosity (radii > 30 µm). Macroporosity values were well represented throughout the range from 0.035 up to 0.20 m³ m-3 (n = 50), and these values were negatively correlated with both penetration resistance and bulk density measurements (r ≤ -0.80; P < 0.001). We found clear responses of root growth to this established gradient. As macroporosity decreased, root diameter increased by 42% (r = -0.63), while root length density decreased by 65% (r = +0.66). However, these divergent responses of root diameter and length density to changes in macroporosity were not linear over the whole range but exhibited specific response thresholds. The thresholds of macroporosity, where root responses inflected and became particularly pronounced, were <0.077 m³ m-3 for root diameter and >0.12 m³ m-3 for root length density, suggesting that macroporosity values above 0.13 m³ m-3 can facilitate root growth. Although these belowground responses were clearly evident, we detected no effects of these varying conditions on aboveground plant productivity. Moreover, the results also indicate that moderate-to-high soil water contents during the early stages of plant development allow annual roots to overcome potentially restricting soil physical conditions. [ABSTRACT FROM AUTHOR]