Showing papers by "Nicolas Brüggemann published in 2016"

Modeling Soil Processes: Review, Key Challenges, and New Perspectives

Abstract: The remarkable complexity of soil and its importance to a wide range of ecosystem services presents major challenges to the modeling of soil processes. Although major progress in soil models has occurred in the last decades, models of soil processes remain disjointed between disciplines or ecosystem services, with considerable uncertainty remaining in the quality of predictions and several challenges that remain yet to be addressed. First, there is a need to improve exchange of knowledge and experience among the different disciplines in soil science and to reach out to other Earth science communities. Second, the community needs to develop a new generation of soil models based on a systemic approach comprising relevant physical, chemical, and biological processes to address critical knowledge gaps in our understanding of soil processes and their interactions. Overcoming these challenges will facilitate exchanges between soil modeling and climate, plant, and social science modeling communities. It will allow us to contribute to preserve and improve our assessment of ecosystem services and advance our understanding of climate-change feedback mechanisms, among others, thereby facilitating and strengthening communication among scientific disciplines and society. We review the role of modeling soil processes in quantifying key soil processes that shape ecosystem services, with a focus on provisioning and regulating services. We then identify key challenges in modeling soil processes, including the systematic incorporation of heterogeneity and uncertainty, the integration of data and models, and strategies for effective integration of knowledge on physical, chemical, and biological soil processes. We discuss how the soil modeling community could best interface with modern modeling activities in other disciplines, such as climate, ecology, and plant research, and how to weave novel observation and measurement techniques into soil models. We propose the establishment of an international soil modeling consortium to coherently advance soil modeling activities and foster communication with other Earth science disciplines. Such a consortium should promote soil modeling platforms and data repository for model development, calibration and intercomparison essential for addressing contemporary challenges.

A review of chemical reactions of nitrification intermediates and their role in nitrogen cycling and nitrogen trace gas formation in soil

Abstract: Summary Soil is a major source of nitrogen trace gases (NTGs). Microbial denitrification has long been identified as a source of NTGs under reducing conditions, whereas the production of NTGs during nitrification is far from being completely understood. This review updates information about the role of abiotic processes in the formation of gaseous N products in soil and brings attention to the potential interplay of microbial and chemical soil processes that tend to be neglected in research on NTG emissions. Several reactions that involve the nitrification intermediates, nitrite (NO2−) and hydroxylamine (NH2OH), are known to produce the NTGs nitric oxide (NO) and nitrous oxide (N2O). These abiotic reactions are: the self-decomposition of NO2−, reactions of NO2− with reduced metal cations, nitrosation of soil organic matter (SOM) by NO2−, the reaction between NO2− and NH2OH, and the oxidation of NH2OH by Fe3+ or MnO2. These reactions can occur over a broad range of soil characteristics, but they are disregarded in most current research on NTG studies in favour of biological processes only. Relatively few studies have tried to quantify the contribution of abiotic processes to total NTG emissions, which results in uncertainty in emission models and mitigation strategies. It is difficult to discriminate between biological and abiotic sources because both processes can proceed at the same time in the same soil layer. The potential of stable isotope techniques to disentangle the different processes in soil and to constrain budgets of atmospheric NTGs better are highlighted. Recent advances in stable isotope technologies, such as infrared real-time laser spectroscopy, provide considerable potential for both natural abundance and tracer studies in this field.

Reducing soil CO2 emission and improving upland rice yield with no-tillage, straw mulch and nitrogen fertilization in northern Benin

Abstract: To explore effective ways to decrease soil CO 2 emission and increase grain yield, field experiments were conducted on two upland rice soils (Lixisols and Gleyic Luvisols) in northern Benin in West Africa. The treatments were two tillage systems (no-tillage, and manual tillage), two rice straw managements (no rice straw, and rice straw mulch at 3 Mg ha −1 ) and three nitrogen fertilizers levels (no nitrogen, recommended level of nitrogen: 60 kg ha −1 , and high level of nitrogen: 120 kg ha −1 ). Potassium and phosphorus fertilizers were applied to be non-limiting at 40 kg K 2 O ha −1 and 40 kg P 2 O 5 ha −1 . Four replications of the twelve treatment combinations were arranged in a randomized complete block design. Soil CO 2 emission, soil moisture and soil temperature were measured at 5 cm depth in 6–10 days intervals during the rainy season and every two weeks during the dry season. Soil moisture was the main factor explaining the seasonal variability of soil CO 2 emission. Much larger soil CO 2 emissions were found in rainy than dry season. No-tillage significantly reduced soil CO 2 emissions compared with manual tillage. Higher soil CO 2 emissions were recorded in the mulched treatments. Soil CO 2 emissions were higher in fertilized treatments compared with non-fertilized treatments. Rice biomass and yield were not significantly different as a function of tillage systems. On the contrary, rice biomass and yield significantly increased with application of rice straw mulch and nitrogen fertilizer. The highest response of rice yield to nitrogen fertilizer addition was obtained for 60 kg N ha −1 in combination with 3 Mg ha −1 of rice straw for the two tillage systems. Soil CO 2 emission per unit grain yield was lower under no-tillage, rice straw mulch and nitrogen fertilizer treatments. No-tillage combined with rice straw mulch and 60 kg N ha −1 could be used by smallholder farmers to achieve higher grain yield and lower soil CO 2 emission in upland rice fields in northern Benin.

Combining no-tillage, rice straw mulch and nitrogen fertilizer application to increase the soil carbon balance of upland rice field in northern Benin

Abstract: Agricultural management practices are frequently non conservative and can lead to substantial loss of soil organic carbon and soil fertility, but for many regions in Africa the knowledge is very limited. To study the effect of local agricultural practices on soil organic carbon content and to explore effective ways to increase soil carbon storage, field experiments were conducted on an upland rice soil (Lixisol) in northern Benin in West Africa. The treatments comprised two tillage systems (no-tillage, and manual tillage), two rice straw managements (no rice straw, and rice straw mulch at 3 Mg ha ⿿1 ) and three nitrogen fertilizer levels (no nitrogen, 60 kg ha ⿿1 , 120 kg ha ⿿1 ). Phosphorus and potassium fertilizers were applied to be non-limiting at 40 kg P 2 O 5 ha ⿿1 and 40 kg K 2 O ha ⿿1 per cropping season. Heterotrophic respiration was higher in manual tillage than no-tillage, and higher in mulched than in non-mulched treatments. Under the current management practices (manual tillage, with no residue and no nitrogen fertilization) in upland rice fields in northern Benin, the carbon added as aboveground biomass and root biomass was not enough to compensate for the loss of carbon from organic matter decomposition, rendering the upland rice fields as net sources of atmospheric CO 2 . With no-tillage, 3 Mg ha ⿿1 of rice straw mulch and 60 kg N ha ⿿1 , the soil carbon balance was approximately zero. With no other changes in management practices, an increase in nitrogen level from 60 kg N ha ⿿1 to 120 kg N ha ⿿1 resulted in a positive soil carbon balance. Considering the high cost of inorganic nitrogen fertilizer and the potential risk of soil and air pollution often associated with intensive fertilizer use, implementation of no-tillage combined with application of 3 Mg ha ⿿1 of rice straw mulch and 60 kg N ha ⿿1 could be recommended to the smallholder farmers to compensate for the loss of carbon from organic matter decomposition in upland rice fields in northern Benin.

N2O source partitioning in soils using 15N site preference values corrected for the N2O reduction effect

Abstract: RationaleThe aim of this study was to determine the impact of isotope fractionation associated with N2O reduction during soil denitrification on N2O site preference (SP) values and hence quantify the potential bias on SP-based N2O source partitioning. MethodsThe N2O SP values (n=431) were derived from six soil incubation studies in N-2-free atmosphere, and determined by isotope ratio mass spectrometry (IRMS). The N-2 and N2O concentrations were measured directly by gas chromatography. Net isotope effects (NIE) during N2O reduction to N-2 were compensated for using three different approaches: a closed-system model, an open-system model and a dynamic apparent NIE function. The resulting SP values were used for N2O source partitioning based on a two end-member isotopic mass balance. ResultsThe average SP0 value, i.e. the average SP values of N2O prior to N2O reduction, was recalculated with the closed-system model, resulting in -2.6 (+/- 9.5), while the open-system model and the dynamic apparent NIE model gave average SP0 values of 2.9 parts per thousand (+/- 6.3) and 1.7 parts per thousand (+/- 6.3), respectively. The average source contribution of N2O from nitrification/fungal denitrification was 18.7% (+/- 21.0) according to the closed-system model, while the open-system model and the dynamic apparent NIE function resulted in values of 31.0% (+/- 14.0) and 28.3% (+/- 14.0), respectively. ConclusionsUsing a closed-system model with a fixed SP isotope effect may significantly overestimate the N2O reduction effect on SP values, especially when N2O reduction rates are high. This is probably due to soil inhomogeneity and can be compensated for by the application of a dynamic apparent NIE function, which takes the variable reduction rates in soil micropores into account. Copyright (c) 2016 John Wiley & Sons, Ltd.

Isotopic composition of plant water sources

Abstract: In their study, Evaristo et al.1 collected an extensive data set on the basis of which they statistically determined the isotopic compositions of the plant water source (δ Ointersect and δ Hintersect, called respectively δ Ointercept and δ Hintercept in their paper) as the x and y coordinates in (δ 18O, δ 2H) space of the intersection between the local meteoric water line (LMWL) and the plant xylem water ‘evaporation line’ (EL) for a range of climates and vegetation types. Evaristo et al.1 showed that for 80% of their referenced sampling sites, the mean value of the ground­ water hydrogen isotopic composition (δ HGW) was statistically differ­ ent from δ Hintersect, supporting their hypothesis that the precipitation sources for groundwater recharge are different from the precipitation sources for plant water uptake. However, we consider that Evaristo et al.1 made a mistake in their equation (2), such that their analysis of rainfall segregation between plant transpiration and groundwater is not valid. This result does however not bring into question the paper’s con­ clusion1 of hydrological separation formulated on the basis of the pre­ cipitation offset. There is a Reply to this Brief Communication Arising by Evaristo, J., Jasechko, S. & McDonnell, J.J. Nature 536, http://dx.doi. org/10.1038/nature18947 (2016). The equation used in ref. 1 for the LMWL at a given sampling site is:

An improved 15N tracer approach to study denitrification and nitrogen turnover in soil incubations

Abstract: Rationale Denitrification (the reduction of oxidized forms of inorganic nitrogen (N) to N2O and N2) from upland soils is considered to be the least well-understood process in the global N cycle. The main reason for this lack of understanding is that the terminal product (N2) of denitrification is extremely difficult to measure against the large atmospheric background. Methods We describe a system that combines the 15N-tracer technique with a 40-fold reduced N2 (2% v/v) atmosphere in a fully automated incubation setup for direct quantification of N2 and N2O emissions. The δ15N values of the emitted N2 and N2O were determined using a custom-built gas preparation unit that was connected to a DELTA V Plus isotope ratio mass spectrometer. The system was tested on a pasture soil from sub-tropical Australia under different soil moisture conditions and combined with 15N tracing in extractable soil N pools to establish a full N balance. Results The method proved to be highly sensitive for detecting N2 (1.12 μg N h−1 kg−1 dry soil (ds)) and N2O (0.36 μg N h−1 kg−1 ds) emissions. The main end product of denitrification in the investigated soil was N2O for both water contents, with N2 accounting for only 3% to 13% of the total denitrification losses. Between 90 and 95% of the added 15N fertiliser could be recovered in N gases and extractable soil N pools. Conclusions The high and N2O-dominated denitrification rates found in this study are pointing at both the high ecological and the agronomic importance of denitrification in subtropical pasture soils. The new system allows for a direct and highly sensitive detection of N2 and N2O fluxes from soils and may help to significantly improve our mechanistic understanding of N cycling and denitrification in terrestrial agro-ecosystems. Copyright © 2016 John Wiley & Sons, Ltd.

An improved 15N tracer approach to study denitrification and nitrogen turnover in soil incubations

Abstract: Rationale: Denitrification (the reduction of oxidized forms of inorganic N to N2O and N2) from upland soils is considered to be the least well understood process in the global N cycleDenitrification from upland soils is considered to be the least well understood process in the global N cycle due to methodical constraints of existing methods. . The main reason for this lack of understanding is that the terminal product (N2) of denitrification is extremely difficult to measure against the hugelarge atmospheric background. Methods: Here wWe describe a system that combines the 15N-tracer technique with a 40-fold reduced N2 (2 % v/v) atmosphere in a fully automated incubation set up for direct quantification of N2 and N2O emissions. The system was tested on a pasture soil from sub-tropical Australia under different soil moisture conditions and combined with 15N tracing in extractable soil N pools to establish a full N balance. Results: The method proved to be highly sensitive for detecting N2 (1.12 µg N h-1 kg-1ds1 dry soil (ds)) and N2O (0.36 µg N h-1 kg 1 ds) emissions. The main end-product of denitrification in the investigated soil was N2O for both water contents with N2 accounting for only 3% to13% of the total denitrification losses. Between 90-95% of the added 15N fertiliser could be recovered in N gases and extractable soil N pools. Conclusions: These high and N2O-dominated denitrification rates found in this study are pointing at both a high ecological and agronomic importance of denitrification in subtropical pasture soils. The new system allows for a direct and highly sensitive detection of N2 and N2O fluxes from soils and may help to significantly improve our mechanistic understanding of N cycling and denitrification in terrestrial agro-ecosystems.

Modeling soil processes

Abstract: The remarkable complexity of soil and its importance to a wide range of ecosystem services presents major challenges to the modeling of soil processes. Although major progress in soil models has occurred in the last decades, models of soil processes remain disjointed between disciplines or ecosystem services, with considerable uncertainty remaining in the quality of predictions and several challenges that remain yet to be addressed. First, there is a need to improve exchange of knowledge and experience among the different disciplines in soil science and to reach out to other Earth science communities. Second, the community needs to develop a new generation of soil models based on a systemic approach comprising relevant physical, chemical, and biological processes to address critical knowledge gaps in our understanding of soil processes and their interactions. Overcoming these challenges will facilitate exchanges between soil modeling and climate, plant, and social science modeling communities. It will allow us to contribute to preserve and improve our assessment of ecosystem services and advance our understanding of climate-change feedback mechanisms, among others, thereby facilitating and strengthening communication among scientific disciplines and society. We review the role of modeling soil processes in quantifying key soil processes that shape ecosystem services, with a focus on provisioning and regulating services. We then identify key challenges in modeling soil processes, including the systematic incorporation of heterogeneity and uncertainty, the integration of data and models, and strategies for effective integration of knowledge on physical, chemical, and biological soil processes. We discuss how the soil modeling community could best interface with modern modeling activities in other disciplines, such as climate, ecology, and plant research, and how to weave novel observation and measurement techniques into soil models. We propose the establishment of an international soil modeling consortium to coherently advance soil modeling activities and foster communication with other Earth science disciplines. Such a consortium should promote soil modeling platforms and data repository for model development, calibration and intercomparison essential for addressing contemporary challenges.