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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 [...]