Core Ideas This new technology is suitable for field‐scale quantification of CO2 in soil. The measurement scale ranges from decimeters up to decameters. The concentrations from the soil water and air phases are averaged. Transient CO2 production and transport reflect plant growth. Biological activity in soil causes fluxes of O2 into and CO2 out of the soil with significant global relevance. Hence, the dynamics of CO2 concentrations in soil can be used as an indicator for biological activity. However, there is an enormous spatial and temporal variability in soil respiration, which has led to the notion of hotspots and hot moments. This variability is attributed to the spatiotemporal heterogeneity of both plant–soil–microbiome interactions and the local conditions governing gas transport. For the characterization of a given soil, the local heterogeneities should be replaced by some meaningful average. To this end, we introduce a line sensor based on tubular gas‐selective membranes that is applicable at the field scale for a wide range in water content. It provides the average CO2 concentration of the ambient soil along its length. The new technique corrects for fluctuating external conditions (i.e., temperature and air pressure) and the impact of water vapor without any further calibration. The new line sensor was tested in a laboratory mesocosm experiment where CO2 concentrations were monitored at two depths during the growth of barley (Hordeum vulgare L.). The results could be consistently related to plant development, plant density, and changing conditions for gas diffusion toward the soil surface. The comparison with an independent CO2 sensor confirmed that the new sensor is actually capable of determining meaningful average CO2 concentrations in a natural soil for long time periods.
<p class="p1">Losses in above- and belowground biodiversity are linked to changes in land use practices and immediately affect processes in the upper soil horizons. Theoretically, these superficial changes are reversible unless a tipping point on an ecosystem level was crossed. This safety net of functional redundancy facilitates the return of vital soil functions and processes to the initial state. Little is known about how deep these changes have reached into the subsoil over time, because early warning indicators of a tipping point being about to be crossed are still sparse. This is especially important for the tropics, as soils are typically intensively weathered and nutrient depleted, therefore, plants relying largely on nutrients from below. Net nitrous oxide (N<span class="s1"><sub>2</sub></span>O) emission rates from the soil surface have proved to be valid proxies for detecting the crossing of tipping points in soil biogeochemistry. Here, we advanced this approach by looking deeply into the soil and reveal that potential greenhouse gas (GHG) production and consumption are useful proxies to estimate how deep the loss of aboveground biodiversity has already impacted soil microbial processes. We performed incubation experiments on forest and pasture soils stemming from shallow as well as 1 m deep profiles from the Peruvian Amazon Basin to determine the production and consumption potential of the GHGs at different water holding capacities. We expected pasture soils to have lost direct carbon(C)-related connections to deeper soil horizons. In forests, roots with different and greater depths also connect deeper soil layers. Therefore, in greater depth, carbon dioxide (CO<span class="s1"><sub>2</sub></span>) production declined faster under pastures than under forests because C limitations are reached sooner. In the surface, grasses are well known for their input of C and increased CO<span class="s1"><sub>2</sub></span> production. Since old pastures are limited in nitrogen (N), almost no N<span class="s1"><sub>2</sub></span>O should be produced, possibly increasing the potential to take up N<span class="s1"><sub>2</sub></span>O. However, denitrification is a heterotrophic process also dependent on available C, therefore, the potentials of N<span class="s1"><sub>2</sub></span>O production and consumption in deeper forest soils were larger with increasing water holding capacity. These findings could indicate that losses in aboveground biodiversity due to forest conversion can have profound impacts on soil microbial processes, extending also to deeper soil layers while altering the connection of the subsoil to the top.</p>
<p>Most biogeochemical models commonly obtain their soil input from pedotransfer functions based on soil texture and other crude but widely available soil data. However, soil texture based on single grain size distribution neglects the impact of actual soil structures in the field. Consequently, scientific efforts are being made to correct for this systematic bias in predicting soil functioning. Pronounced discrepancies between field measurements and model predictions occur for tropical soils: overestimated N<sub>2</sub>O emissions is a prominent example of this mismatch. A well-known characteristic of tropical soils, potentially responsible for the systematic error, are stable aggregates called pseudo-sands. In the field they are perceived as sand, but in the lab measured as clay and silt. The simple assumption that pseudo-sands act just like sands in the field seems to work satisfactorily for certain hydrological predictions, so models were easily adjusted to it. However, here we pursue the hypothesis that, biogeochemically, pseudo-sands do not act like sands. Due to their high internal surface and rough structure, pseudo-sands, unlike sands, provide a wide variety of ecological niches for a diverse community of microorganisms to establish. We will present first evidence for pseudo-sands to act more like a biophysical reactor than just another grain of sand. The long-term goal is to develop a transfer function related to the properties of pseudo-sands that will lead to improved models for tropical soils.</p>
<p>The western Amazon is particularly sensitive to drought since precipitation is common even during "dry season". The combination of increasing land use pressure and droughts due to climate change makes the scenario of this ecosystem likely to cross or having crossed tipping points. We argue that nitrous oxide (N<sub>2</sub>O) emissions can be used to identify the crossing of tipping points in soils, particularly those related to N-cycling. This hypothesis is being tested within the BMBF funded Project PRODIGY, which will show that under stress microbial functional diversity in soils are a safety-net for ecosystems. The survey area (MAP) spreads across three countries (Peru, Brazil and Bolivia).&#160;Lab and field experiments are used to test our hypothesis based on the observations that N<sub>2</sub>O emission under tropical pasture shift after 10 years in use. Pre-measurement modeling is used to optimize measurement designs. Replicated above-ground biodiversity levels (n=4) will be sampled in each country. The soil will also be used for lab drought manipulation experiments to unravel underlying mechanisms. Measured values have shown to be lower than expected and simulated rates. Maybe because tipping points at different spacial and temporal scales are crossed faster than in temperate regions and biogeochemistry is less understood? Results from this investigation will allow the improvement of N<sub>2</sub>O models for tropical soils.</p>
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