Phosphorus (P) limitations to agricultural productivity commonly occur in Australian soils and have largely been overcome by the use of inorganic fertilisers. However, studies have shown that most of the P taken up by plants is from native P pools. The turnover of P and native soil organic matter may be strongly affected by drying and rewetting (DRW). Rewetting dry soil results in a pulse of respiration activity and available nutrients. In Mediterranean-type climates surface soils naturally undergo recurrent DRW cycles. In southern Australia, soils experience DRW due to erratic rainfall within the growing season, and short, high intensity thunderstorms also during summer periods. The principal objective of this thesis was to determine the significance of dry-rewet events, for altering P availability and cycling in agricultural soils in Australia. Soils representing a wide range of soil types and climatic zones of southern Australia, showed large flushes in carbon (C) mineralisation after a single DRW event. For some soils these were comparable with reported values, however large variability in flush size between soils was observed. Soils that commonly experience DRW did not appear to be more resilient to DRW than soils from areas with fewer DRW events. Even when soils had relatively small respiration flushes, as a result of low soil organic matter, a high proportion of the soil C was mineralised after rewetting. Soil physiochemical properties (total C, total N, organic C, humus, microbial biomass P, organic P, sand and silt) were correlated to the size of the flush, hence nutrient availability and soil texture appear to primarily determine flush size. Therefore, the influence of climate on DRW may relate to determining the quantity of organic matter and microbial biomass that is available for turnover. Different size and composition of the microbial biomass within the same soil matrix were achieved by adding three different C substrates (glucose, starch and cellulose at 2.5 g kg-¹) at 5 times over 25 weeks. The treatments showed disparate responses to DRW, due to greater biomass (larger flushes) and effects of community composition, highlighting the central role of the soil microbes in DRW processes. When subjected to multiple DRW events these soils showed smaller rewetting respiration flushes with subsequent rewetting events. In contrast, the amount of P released after rewetting was the same. This study showed that increases in P after rewetting were transient and rapid immobilisation of P by microbes occurred, which may limit the availability to plants. The composition of the microbial community was changed by DRW with a reduction in fungi and gram negative bacteria, showing that certain species are more susceptible to DRW than others. Closer investigation at 2 hourly intervals after rewetting confirmed the transient nature of P flushes. The response in microbial respiration after rewetting was immediate, with the highest activity occurring within the first 2 h. Phosphorus availability was increased by DRW but remained stable over the following 48 h incubation period. The study highlights the rapid nature of changes in available nutrients after rewetting. Furthermore, while potentially only a small component of the P flush that occurred, the DRW soil had higher levels of P than most incubated soil at 48 h, this would be potentially available for plant uptake or movement with the soil solution. Long-term water regimes (continuously moist or air-dry, or DRW occurring at different times during incubation) that were imposed on two soils from different climatic regions over a 14 wk period, did not alter available nutrient (P and C) pools or the size of the microbial biomass. However, these long-term water regimes determined the respiration response of the soils to experimental DRW. The largest flushes occurred in the treatment with the longest dry period, and confirm findings of reported studies that the response of a soil at rewetting is determined by the length of the period that it is dried. Microbial biomass was little affected by experimental DRW, but showed large changes in C:P ratio. Thus, changes in physiological state or community composition may be more affected by DRW than the size of the microbial biomass. Microbial communities were altered by DRW irrespective of climatic history (warm wet summer and temperate Mediterranean), however these changes were not related to specific groups of organisms. In addition, the disparate respiration responses and inhibition of phosphatase by DRW, indicate that functional changes may be induced by DRW but can not be sufficiently explained by quantifying available nutrient pools or the microbial biomass. The use of wheat seedlings bio-indicators of P availability after the long-term water regimes, confirmed that plant available P was altered by DRW, indicated by differences in growth, although the large variability in seedling growth made it difficult to quantify these differences. However, the distribution of labile P, available at planting, in soil and plant pools at harvest, showed that long-term water regimes increased P allocation in plant tissue in one soil and decreased it in another. Furthermore, only a small fraction of the labile P present at planting was taken up by plants, which confirms the superior ability of soil microbes to immobilise P that is released by DRW. Nevertheless, since the long-term water regimes increased P availability, this may be transported via surface water or leaching. DRW is important for C and P turnover in soils of southern Australia. However, P flushes occur rapidly after rewetting and are transient. Therefore, DRW appears to have only minor consequences for P availability to plants. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1321018 / Thesis (Ph.D.) -- University of Adelaide, School of Earth and Environmental Sciences, 2008

Le phosphore (P) est un nutriment essentiel dans le contrôle de l'eutrophisation des eaux de surface. La majorité du P causant cette eutrophisation dans les pays occidentaux est aujourd'hui issu des sols agricoles, ce qui explique pourquoi les recherches actuelles sur l'eutrophisation se focalisent sur la compréhension des mécanismes par lequel le P est relargué de ces sols. Dans cette thèse, nous étudions ces mécanismes en nous focalisant sur la fraction dissoute (DP) du P, fraction la plus menaçante du point de vue de l'eutrophisation. Une double approche a été utilisée, combinant le suivi de la composition d'eaux du sol et d'eaux de ruisseau dans un petit bassin versant (BV) agricole représentatif (BV de Kervidy-Naizin, France) et des simulations expérimentales au laboratoire. Les suivis de terrain ont révélé que les zones humides ripariennes (RW) étaient les principales zones de relargage de DP dans le BV étudié, via deux mécanismes essentiellement déclenchée par les fluctuations des hauteurs de nappe, i) la réhumectation des sols (DRW) et ii) la dissolution réductrice des oxydes de fer du sol dans de périodes d'anoxie. Ces mêmes suivis ont révélé la présence de fortes variations spatiales de la nature chimique (inorganique et organique/colloïdale) du DP relargué. Les variations saisonnières et interannuelles de l'hydroclimat, combinées aux variations locales de topographie ont été démontrées être les deux facteurs principaux contrôlant i) la fréquence des épisodes DRW, et ii) la durée des périodes anoxiques, entraînant au final de fortes variations saisonnières et interannuelles de la dynamique de relargage du DP. Comme indiqué dans un modèle conceptuel général, la topographie est sans doute le facteur clé de contrôle des variations observées, en raison de son rôle sur i) le transfert de P à partir des parcelles agricoles amont, ii) le taux de minéralisation du P organique du sol P et iii), le déclenchement des deux mécanismes de relargage précités. Les expériences en laboratoire ont confirmé le rôle des événements DRW comme processus clé causant le relargage de DP dans les RWs. Les résultats ont démontré que le DP relargué consistait non seulement de "vrai" DP inorganique et organique, mais aussi de P colloïdal, le P colloïdal et le DP organique étant les plus réactifs aux événements DRW. Les données ont aussi révélées que ces différentes formes de P provenaient de différentes sources dans le sol (méso et macroporosité pour P colloïdal et le DP organique; microporosité pour DP inorganique), et que la quantité de P colloïdal relargué était positivement corrélée avec la teneur en matière organique et la taille de biomasse microbienne du sol. Ces mêmes expériences ont confirmé le rôle des conditions anoxiques comme conditions favorisant la libération de DP dans les RWs. La dissolution réductive de sol Fe-oxyhydroxydes n'est cependant pas le seul processus impliqué, un autre processus étant la hausse du pH causée par des réactions de réduction. Les résultats obtenus démontrent que l'augmentation de pH contrôle la libération de DP dans les sols riches en matière organique, alors que ce relargage est contrôlé principalement par la réduction des oxydes de fer dans les sols pauvres en matière organique. Les données expérimentales démontrent également que l'apport de sédiments issus des sols agricoles amont accroit le relargage de DP dans les RW, probablement en raison de la dissolution des oxydes de fer de ces mêmes sédiments par les bactéries ferroréductrices des Rws. Au final, cette thèse permet de mieux contraindre les mécanismes et facteurs responsables du relargage de DP dans les bassins versants agricoles. Une conséquence très pratique de ce travail est que la conception de stratégies pour limiter les fuites de DP dans ces bassins ne peut se faire sans une prise en compte des rôles de l'hydroclimat, de la topographie locale et des propriétés du sol sur ce relargage. / Phosphorus (P) is a key nutrient in controlling surface water eutrophication. Because of the decrease of urban and industrial P emissions, most of the P nowadays causing surface water eutrophication in western countries consists of P transferred from agricultural soils, explaining why current eutrophication research focused on understanding the mechanisms by which P is released from soils. In this thesis, we studied these release mechanisms for dissolved P (DP) – i.e. the most bioavailable P component for algae - using an approach combining field monitoring of soil and stream water compositions in a small, headwater catchment typical of western countries agricultural catchments (the Kervidy-Naizn catchment, France), and laboratory experimental simulations. Field monitoring data revealed that riparian wetlands (RW) are the main zones of DP release and DP production in the studied catchment, through essentially two mechanisms triggered by groundwater table fluctuations, namely i) rewetting of dry soils (DRW), and ii) reductive dissolution of soil Fe (hydr)oxides during anaerobic periods. Field monitoring data also revealed the presence of strong spatial variations in the chemical nature (inorganic vs. organic/colloidal) of the released DP, which was in relation to differences in soil properties and local topography. Seasonal and inter-annual hydroclimate variations, combined with variations in local topography were found to control the frequency of soil DRW events and duration of anaerobic periods, resulting in strong seasonal and inter-annual variations of DP release dynamics. As shown in a conceptual model, topography is likely to be the key driver of the observed spatial and temporal variations, because of its combined control on i) the transfer of P from upland fields to RW zones, ii) the mineralization rates of soil organic P and iii) the triggering of the above two release mechanisms. Laboratory leaching experiments on the same soils confirmed the role of DRW events as a major process causing DP release pulses in RWs. The data demonstrated that the released DP consisted not only of true dissolved inorganic and organic P but also of colloidal P, the latter phase being the most reactive to DRW events. The data also revealed that the different P forms came from different P sources in the soil (soil macro/mesopores for colloidal P and organic DP; soil micropores for inorganic DP) and that the amount of released colloidal P correlated positively with the organic matter contents and soil microbial biomass size of the soil. Anaerobic incubation experiments, on their hand, confirmed the role of anoxic conditions as conditions favoring the release of DP in RW. Reductive dissolution of soil Fe-oxyhydroxide was, however, not the sole process involved in that release, another process being the rise in pH caused by reduction reactions. Experimental data showed that the pH rise controlled the DP release in organic-rich soils, this release being on the contrary mainly controlled by soil Fe-oxyhydroxides reductive dissolution in organic-poor soils. Experimental data also showed that the input of soil sediments from upland fields enhanced the release of DP in RW, most likely due to the enhanced dissolution of sediment Fe-oxyhydroxides by RW Fe-reducing bacteria. Overall, this thesis allowed new constraints to be placed on the release mechanisms of DP in headwater agricultural catchments. One very practical output is that great care should be taken of hydroclimate variability, local topography, and soil property when designing and implementing management options to reduce DP release and transfer in agricultural catchments.