Paolo D’Odorico

Hydrologic controls on soil carbon and nitrogen cycles. I. Modeling scheme

Abstract The influence of soil moisture dynamics on soil carbon and nitrogen cycles is analyzed by coupling an existing stochastic soil moisture model [Adv. Water. Resour. 24 (7) (2001) 707; Proc. R. Soc. Lond. A 455 (1999) 3789] to a system of eight nonlinear differential equations that describe the temporal evolution of the organic matter and the mineral nitrogen in the soil at the daily to seasonal time scales. Special attention is devoted to the modeling of the soil moisture control on mineralization and immobilization fluxes, leaching losses, and plant nitrogen uptake, as well as to the role played by the soil organic matter carbon-to-nitrogen ratio in determining mineralization and immobilization. The model allows a detailed analysis of the soil nitrogen cycle as driven by fluctuations in soil moisture at the daily time scale resulting from the stochastic rainfall variability. The complex ensuing dynamics are studied in detail in a companion paper [Adv. Water Resour. 26 (1) (2003) 59], which presents an application to the Nylsvley savanna in South Africa. The model accounts for the soil moisture control on different components of the nitrogen cycle on a wide range of time scales: from the high frequency variability of leaching and uptake due to the nitrate flushes after persistent rainfall following a period of drought, to the low frequency temporal dynamics of the soil organic matter pools. All the fluctuations in the various pools are statistically characterized in relation to their dependence on climate, soil, and vegetation characteristics.

Hydrologic controls on soil carbon and nitrogen cycles. II. A case study

The nitrogen and carbon cycles in the broad-leafed savanna at Nylsvley (S. Africa) are modeled using the stochastic approach presented by Porporato et al. [Adv Water Res (this issue)]. An accurate representation of the hydrological mechanisms that control the nitrogen cycle at the daily time scale is shown to be necessary to capture the impact of the high-frequency variability of the soil moisture on the nitrogen and carbon dynamics. The fluctuations of the random precipitation forcing propagate to soil moisture, carbon, and nitrogen dynamics, giving rise to a gamut of fluctuations at different time scales. Long simulations are carried out to achieve a probabilistic characterization of the dynamics of the state variables under different rainfall regimes.

Post-Fire Resource Redistribution in Desert Grasslands: A Possible Negative Feedback on Land Degradation

Desert grasslands, which are very sensitive to external drivers like climate change, are areas affected by rapid land degradation processes. In many regions of the world the common form of land degradation involves the rapid encroachment of woody plants into desert grasslands. This process, thought to be irreversible and sustained by biophysical feedbacks of global desertification, results in the heterogeneous distribution of vegetation and soil resources. Most of these shrub-grass transition systems at the desert margins are prone to disturbances such as fires, which affect the interactions between ecological, hydrological, and land surface processes. Here we investigate the effect of prescribed fires on the landscape heterogeneity associated with shrub encroachment. Replicated field manipulation experiments were conducted at a shrub-grass transition zone in the northern Chihuahuan desert (New Mexico, USA) using a combination of erosion monitoring techniques, microtopography measurements, infiltration experiments, and isotopic studies. The results indicate that soil erosion is more intense in burned shrub patches compared to burned grass patches and bare interspaces. This enhancement of erosion processes, mainly aeolian, is attributed to the soil–water repellency induced by the burning shrubs, which alters the physical and chemical properties of the soil surface. Further, we show that by enhancing soil erodibility fires allow erosion processes to redistribute resources accumulated by the shrub clumps, thereby leading to a more homogeneous distribution of soil resources. Thus fires counteract or diminish the heterogeneity-forming dynamics of land degradation associated with shrub encroachment by enhancing local-scale soil erodibility.

Recent History and Geography of Virtual Water Trade

The global trade of goods is associated with a virtual transfer of the water required for their production. The way changes in trade affect the virtual redistribution of freshwater resources has been recently documented through the analysis of the virtual water network. It is, however, unclear how these changes are contributed by different types of products and regions of the world. Here we show how the global patterns of virtual water transport are contributed by the trade of different commodity types, including plant, animal, luxury (e.g., coffee, tea, and alcohol), and other products. Major contributors to the virtual water network exhibit different trade patterns with regard to these commodity types. The net importers rely on the supply of virtual water from a small percentage of the global population. However, discrepancies exist among the different commodity networks. While the total virtual water flux through the network has increased between 1986 and 2010, the proportions associated with the four commodity groups have remained relatively stable. However, some of the major players have shown significant changes in the virtual water imports and exports associated with those commodity groups. For instance, China has switched from being a net exporter of virtual water associated with other products (non-edible plant and animal products typically used for manufacturing) to being the largest importer, accounting for 31% of the total water virtually transported with these products. Conversely, in the case of The United states of America, the commodity proportions have remained overall unchanged throughout the study period: the virtual water exports from The United States of America are dominated by plant products, whereas the imports are comprised mainly of animal and luxury products.

An Analysis of the Soil Moisture Feedback on Convective and Stratiform Precipitation

Abstract Land–atmosphere interactions in midlatitude continental regions are particularly active during the warm season. It is still unclear whether and under what circumstances these interactions may involve positive or negative feedbacks between soil moisture conditions and rainfall occurrence. Assessing such feedbacks is crucially important to a better understanding of the role of land surface conditions on the regional dynamics of the water cycle. This work investigates the relationship between soil moisture and subsequent precipitation at the daily time scale in a midlatitude continental region. Sounding data from 16 locations across the midwestern United States are used to calculate two indices of atmospheric instability—namely, the convective available potential energy (CAPE) and the convective inhibition (CIN). These indices are used to classify rainfall as convective or stratiform. Correlation analyses and uniformity tests are then carried out separately for these two rainfall categories, to asse...

Water-controlled wealth of nations

Population growth is in general constrained by food production, which in turn depends on the access to water resources. At a country level, some populations use more water than they control because of their ability to import food and the virtual water required for its production. Here, we investigate the dependence of demographic growth on available water resources for exporting and importing nations. By quantifying the carrying capacity of nations based on calculations of the virtual water available through the food trade network, we point to the existence of a global water unbalance. We suggest that current export rates will not be maintained and consequently we question the long-run sustainability of the food trade system as a whole. Water rich regions are likely to soon reduce the amount of virtual water they export, thus leaving import-dependent regions without enough water to sustain their populations. We also investigate the potential impact of possible scenarios that might mitigate these effects through (1) cooperative interactions among nations whereby water rich countries maintain a tiny fraction of their food production available for export; (2) changes in consumption patterns; and (3) a positive feedback between demographic growth and technological innovations. We find that these strategies may indeed reduce the vulnerability of water-controlled societies.Population growth is in general constrained by food production, which in turn depends on the access to water resources. At a country level, some populations use more water than they control because of their ability to import food and the virtual water required for its production. Here, we investigate the dependence of demographic growth on available water resources for exporting and importing nations. By quantifying the carrying capacity of nations on the basis of calculations of the virtual water available through the food trade network, we point to the existence of a global water unbalance. We suggest that current export rates will not be maintained and consequently we question the long-term sustainability of the food trade system as a whole. Water-rich regions are likely to soon reduce the amount of virtual water they export, thus leaving import-dependent regions without enough water to sustain their populations. We also investigate the potential impact of possible scenarios that might mitigate these effects through (i) cooperative interactions among nations whereby water-rich countries maintain a tiny fraction of their food production available for export, (ii) changes in consumption patterns, and (iii) a positive feedback between demographic growth and technological innovations. We find that these strategies may indeed reduce the vulnerability of water-controlled societies.

Tidal influences on carbon assimilation by a salt marsh

Salt marshes are among the most productive ecosystems on Earth, and play an important role in the global carbon cycle. Net carbon dioxide (CO2) ecosystem exchanges in coastal salt marshes remain poorly investigated. In Spartina alterniflora dominated North American Atlantic coast marshes, the lack of a clear understanding of how Spartina alterniflora responds to flooding limits our current ability to understand and predict salt marsh response to sea-level rise. Here we investigate the processes influencing ecosystem-level carbon exchanges between a S. alterniflora dominated salt marsh on the eastern shore of Virginia and the atmosphere. We examined the impacts of tidal inundation on the marsh–atmosphere carbon exchanges through a combination of eddy covariance measurements and in situ photosynthetic measurements. Maximum daytime carbon fluxes were observed during the middle of the growing season (July and August) and amounted to −10 μmol CO2 m−2 s−1, and the marsh assimilated 130 gC m−2 during the 2007 growing season. Our study is the first to quantify the effects of tidal inundation on marsh plants, which caused anywhere from 3% to 91% reductions in atmospheric carbon fluxes, with a mean reduction of 46 ± 26%, when compared to non-flooded conditions.

Resilience and reactivity of global food security

Significance The past few decades have seen an intensification of international food trade and the increase in the number of countries that depend on food imports. As an effect of the associated globalization of food, local shocks in food production, combined with the adoption of new national or regional energy and trade policies, have recently led to global food crises. Here we develop a framework to investigate the coupled global food–population dynamics, and evaluate the effect of international trade on global food security. We find that, as the dependency on trade increases, the global food system is losing resilience and is becoming increasingly unstable and susceptible to conditions of crisis. The escalating food demand by a growing and increasingly affluent global population is placing unprecedented pressure on the limited land and water resources of the planet, underpinning concerns over global food security and its sensitivity to shocks arising from environmental fluctuations, trade policies, and market volatility. Here, we use country-specific demographic records along with food production and trade data for the past 25 y to evaluate the stability and reactivity of the relationship between population dynamics and food availability. We develop a framework for the assessment of the resilience and the reactivity of the coupled population–food system and suggest that over the past two decades both its sensitivity to external perturbations and susceptibility to instability have increased.

The water-land-food nexus of first-generation biofuels.

Recent energy security strategies, investment opportunities and energy policies have led to an escalation in biofuel consumption at the expenses of food crops and pastureland. To evaluate the important impacts of biofuels on food security, the food-energy nexus needs to be investigated in the context of its linkages with the overall human appropriation of land and water resources. Here we provide a global assessment of biofuel crop production, reconstruct global patterns of biofuel crop/oil trade and determine the associated displacement of water and land use. We find that bioethanol is mostly produced with domestic crops while 36% of biodiesel consumption relies on international trade, mainly from Southeast Asia. Altogether, biofuels rely on about 2-3% of the global water and land used for agriculture, which could feed about 30% of the malnourished population. We evaluate the food-energy tradeoff and the impact an increased reliance on biofuel would have on the number of people the planet can feed.

Post-fire resource redistribution and fertility island dynamics in shrub encroached desert grasslands: a modeling approach

A common form of land degradation in desert grasslands is associated with the relatively rapid encroachment of woody plants, a process that has important implications on ecosystem structure and function, as well as on the soil hydrological and biogeochemical properties. Until recently this grassland to shrubland transition was thought to be highly irreversible. However recent studies have shown that at the early stages of shrub encroachment in desert grasslands, there exists a very dynamic shrub–grass transition state with enough grass connectivity between the shrub islands to allow for fire spread. In this state fire could play a major role in determining the dominance of grasses and their recovery from the effects of overgrazing. Using a spatially explicit cellular automata model, we show how the patch-scale feedbacks between fires and soil erosion affects resource redistribution and vegetation dynamics in a mixed grass–shrub plant community at landscape to regional scales. The results of this study indicate that at its early stages, the grassland-to-shrubland transition can be reversible and that the feedbacks between fire and soil erosion processes may play a major role in determining the reversibility of the system.