Soil Sensor Technology: Life within a Pixel

摘要

Soil organisms undertake every major ecosystem process, from primary production to decomposition to carbon sequestration, and those processes they catalyze have a bearing on the management of issues from agriculture to global climate change. Nonetheless, until recently, research to measure the dynamics of microscopic organisms living belowground has largely been limited to infrequent field sampling and laboratory extrapolation. Now, however, new sensor technologies can measure and monitor soil organisms and processes at rapid and continuous temporal scales. In this article, we describe these technologies and how they can be arrayed for an integrated view of soil dynamics.nnSoil organisms are the catalysts that link elemental exchange among the lithosphere, biosphere, and atmosphere. Understanding the rates of these exchanges, and the sequestration of elements within any pool, is becoming increasingly crucial to understanding soil processes and to sustainable management of local processes that are linked to the global climate. Indeed, scaling may be the single most difficult task in the study of soil ecological processes. The nutrient transformations that take place on the surfaces of soil particles, roots, and soil microbes must be defined and scaled up for managing soil nutrient and energy transformation at the ecosystem level.nnThe greatest challenges for predicting soil processes are learning what to measure and how frequently, and organizing individual measurements into units that correspond to a remote-sensing pixel of information. Today, pixels at scales of meters to kilometers provide composite estimates of the effects of complex soil processes, but these composites are blind to the small-scale processes that contribute to larger-scale phenomena. To fully understand these phenomena, we need to be able to measure soil processes in situ to determine which organisms participate and, simultaneously, to aggregate measurements in spatially and temporally meaningful ways.nnA key driver of biogeochemical processes and the most readily measured soil parameter is the energy stored in carbon (C) compounds. Soil C is derived largely from plant photosynthesis and allocated to the soil either directly from plant roots or from leaf litter and decomposition. The kinetics of soil processes have been estimated in terms of respiration rates, which depend on temperature, water, and a number of other variables that vary at microscopic scales. To address these challenges of scale, we implemented a networked array of sensors designed to measure small-scale soil dynamics and correlate these spatially and temporally with larger-scale measurements.nnDescribing the respiration process is relatively simple. Glucose (C6H12O6) is oxidized and broken down to carbon dioxide (CO2) and water (H2O), releasing ATP (adenosine triphosphate, or energy): C6H12O6 + 6O2 →6CO2 + 6H2O +ATP; measuring CO2 fluxes in the laboratory for a cell or an organism is relatively straightforward. Unfortunately, a vast number of organisms and processes (in addition to respiration) contribute to gains and losses of C in soil. Much of the difficulty with measuring soil respiration is because of the variation in production and diffusion of CO2 in the soil profile (Hirano et al. 2003, Jassal et al. 2005). Any soil respiration measurement taken at one point in time and space may have little relationship to one taken at the next moment or at a nearby location (Belnap et al. 2003). However, because of the complexity of this problem and limiting technologies, most studies have been forced to assume spatial and temporal homogeneity.nnGeneral CO2 models are often correct in terms of physical and biogeochemical processes, and the parameters that determine their outputs, but these attributes are usually assumed to be independent of the scale at which the model is used. A common assumption is that soil respiration can be predicted using abiotic variables, including temperature, precipitation, and clay content (e.g., using models such as DAY-CENT; Parton et al. 1998). However, soil respiration is a result of complex interactions among the biotic, chemical, and physical constituents within very small regions of soil, which cause the observed variability of soil respiration (Stoyan et al. 2000, Davidson et al. 2006). Thus, because of spatial and temporal variation, regional estimates could easily be off by an order of magnitude or more. Eddy-covariance techniques can be employed to measure CO2 fluxes from the canopy boundary layer to the atmosphere. This approach measures CO2 along a concentration gradient between the atmosphere and the canopy at frequent (10 to 20 times per second) intervals, coupled with vertical wind speeds to integrate turbulence. However, this technique integrates a large footprint (up to several hectares) that is dependent on the height of measurement and the canopy structure.nnThe variation of CO2 flux within the understory due to any single parameter such as temperature or water potential (ψ) can vary two- to fourfold (Baldocchi et al. 2000). Q10 ratios (the respiration rate at temperature t + 10 divided by the rate at temperature t [in degrees Celsius]) are used widely to assess microbial or root respiration of individual entities, such as root tips (e.g., Burton et al. 2002), but values can vary from tip to tip depending on water and nitrogen (N). When Burton and colleagues (2002) evaluated respiration at sites across the North American continent, the range of Q10 values for mycorrhizal root-tip respiration varied from 2.4 to 3.1. Root respiration in vivo became more predictable when the N concentration of individual tips was integrated into the model. However, because the tip included mycorrhizal fungi, which made up as much as 25 percent of the mass and 40 percent of the N, relative contributions become another question (Allen et al. 2002). In addition, root and soil respiration in situ were highly variable, especially when the systems were subject to drought, as with the Georgia oaks and New Mexico pinyon and juniper (Burton et al. 2002).nnWater regulates respiration and soil CO2 both directly and indirectly: directly, in that root and microbial growth require water (but the rates decline as water content exceeds a thresh-old at which oxygen [O2] becomes limiting); and indirectly, in that as soil water content increases, it fills pore space and reduces CO2 amounts and diffusivity in the soil. Although water entering the soil system is generally measured at a single point and reported as monthly precipitation, snow and rainfall can be highly variable over short distances, resulting in a complex spatial pattern of soil moisture distribution. Following precipitation, water moves chaotically downward through the soil profile through soil pores and along routes formed by earthworms and decayed roots (Jury et al. 2003, Wang et al. 2003a, 2003b), and live roots and mycorrhizal hyphae move water horizontally (Dawson 1993, Ryel et al. 2002, Allen 2007). Nonsaturating precipitation leads to spatial and temporal complexity in nutrient pulses (Belnap et al. 2003, Ivans et al. 2003) and absorbs some of the gaseous O2 and CO2. Subsequent soil drying patterns beneath complex canopies are driven by further spatial variation in solar radiation (e.g., Martens et al. 2000). All of these small-scale moisture-driven processes result in complex temporal and spatial variations that influence soil respiration and CO2 production (Davidson et al. 1998).nnProduction and turnover of roots and microbes are highly variable; lab observations and field minirhizotrons showed that absorbing networks of arbuscular mycorrhizae (AM) are produced and disappear within about a week (Friese and Allen 1991, Allen et al. 2003, Staddon et al. 2003). Ecto-mycorrhizae (EM) tips have life spans that vary from a few days to years, depending on N concentrations, fungal species, and the environment (Majdi et al. 2001,Allen et al. 2003, Ruess et al. 2003, Treseder et al. 2004). Rhizomorphs, structures containing a mass of intertwined hyphae, can survive for several months and persist through a growing season (Treseder et al. 2005). Many nodules formed between host plants and N-fixing bacteria persist only a matter of days, but some can also be perennial (Nygren and Ramirez 1995, Nygren et al. 2000). Turnover of severed nodules occurs within three to five days, depending on moisture and herbivory. Although on the basis of lab studies, the life spans of single bacterial colonies and hyphae are presumed to be short, there are few real data to test this idea. Allen (1993) reported that microbial biomass doubled and then dropped by 75 percent within two days after a watering event, and that after seven days, no differences between watered and unwatered treatments existed.nnSpatial variation is as great as temporal variation, but it is rarely addressed. Measuring points as close as 2 centimeters (cm), Allen and MacMahon (1985) found fungal distribution patterns differed with soil C and nutrient composition, which indicated different functional time-space processes. In a maize field, tillage dictated the primary scale of distribution in process and species composition (Robertson and Freckman 1995). However, in a wildland ecosystem, species composition and functional units varied across small but distinctive spatial patches (Klironomos et al. 1999). Just as important, each process, such as nutrient uptake by mycorrhizae and ammonification, scaled differently. Unfortunately, all of these studies were based on destructive sampling, which does not allow for repeatable measurement through time. Thus, although spatial structure can be characterized instantaneously, simultaneous measurement of space and time, and the association of activity and composition changes with changes to soil environments, has remained impossible (Ettema and Wardle 2002). Despite our best efforts, we are still unable to capture complex, real-time responses of respiration to organism activity and those caused by fluctuations in physical conditions (precipitation, temperature) or biological conditions (animal grazing and bioturbation