The mineral protection of microbial necromass is critical for soil carbon stabilization. The water impact on the interactions between minerals and microbial necromass, however, remains unclear due to a lack of proper methods to quantify the mineral-protected microbial necromass. Here, we used offline hydrous pyrolysis to release the archaeal necromass—isoprenoid glycerol dialkyl glycerol tetraethers (isoGDGTs) bound to clay minerals by heating samples at increasing temperatures (150, 200, 250, 300, and 350 °C) under argon atmosphere. The content of extractable isoGDGTs in soils reached the maximum at 200–250 °C, indicating the release of bound isoGDGTs. By investigating the free and bound isoGDGTs along a soil transect with increasing soil water content (SWC), we found that the portion of bound isoGDGTs reached up to 97% of total isoGDGTs in dry soils, and decreased with increased SWC, suggesting that SWC controls isoGDGT stabilization on clay minerals. This relationship was confirmed in the soils from the Northeast China Transect, where the proportion of bound isoGDGTs decreased with increased mean annual precipitation from 160 to 814 mm. Such a connection was further supported by the water-saturated samples of the lake and marine sediments, which have a much lower proportion of bound lipids than the soils. The decrease of bound lipid portion with increasing water content is related to the lower attachment of microbial cells to minerals in wetter soils. Microbial cells are more strongly attached to the mineral phase under drier conditions due to the thin water film on the mineral surfaces. In addition, microorganisms secrete more extracellular polymeric substances under drier conditions, which reinforces the interactions between microbial-derived organic matter and minerals. The increased microbe-mineral interactions under drier conditions offer more stable microbial legacies and stabilize organic matter in the (semi)arid regions.
Die Genese von Boden aus vulkanischen Gesteinen ist nicht nur wegen der spezifischen chemischen und mineralogischen Prozesse besonders interessant; mit Blick darauf, dass diese Boden oft auserordentlich hohe Gehalte an organischem Kohlenstoff (Corg) enthalten, ist sie auch hochrelevant. Die hier vorgestellte Studie zielt auf eine quantitative Analyse des Einflusses der Faktoren Zeit (t), mittlerer Jahresniederschlag (MAP) und Jahresmitteltemperatur (MAT) auf die Pedogenese aus basaltischen Pyroklastika unter feucht-tropischen Bedingungen ab. Mount Cameroon bietet optimale Voraussetzungen dazu, den Einfluss dieser Faktoren auf die Pedogenese unabhangig voneinander zu quantifizieren. Der Einfluss des Faktors t wird an Bodenbildungsstadien auf Basaltstromen unterschiedlichen Alters quantifiziert, vom jungsten Basaltstrom aus dem Jahr 2000 bis zu einige Tausend Jahre alten Basaltstromen. Der Einfluss des Faktors MAP wird entlang eines MAP-Gradienten von >9000 mm a-1 im Westen (an der Kuste) bis 2000 mm a-1 im Osten erfasst, der des Faktors MAT entlang des vertikalen MAT-Gradienten von 26-29 °C am Fus des Mount Cameroon bis 0 °C auf dem Gipfel in 4095 m u. NN. Die Pedogenese fuhrt uberwiegend zu Silandic Andosols, die sich spater zu Nitisols weiterentwickeln. Erste Daten von Boden mit bekannter Faktorenkombination geben Hinweise darauf, wie schnell mit zunehmendem Bodenalter die Fed/Fet-Verhaltnisse, die Gehalte an Ton und Corg sowie Sio, Alo und Feo zunehmen. Gesamt- und Tonmineralanalysen stutzen die Interpretation dieser Daten im Hinblick auf die Bildung von short-range order minerals. Die Rontgendiffraktogramme zeigen jedoch auch erhebliche Gehalte an Magnetit und Gibbsit an, sodass bei der Interpretation der Feo- und Alo-Werte Vorsicht geboten ist. Die Abnahme der molaren (Ca+Mg+K+Na)/Al- und Si/Al-Verhaltnisse aufgrund fortschreitender Verwitterung, Entbasung und Desilifizierung unter dem Einfluss des Faktors t wird ebenso quantifiziert. Diese Indices werden zusatzlich durch die Faktoren MAP und MAT beeinflusst. Mit fortschreitendem Bodenalter sind zudem zunehmende Quarzgehalte in den Boden zu beobachten. Da das Ausgangsgestein quarzfrei ist, ist dies ein Hinweis auf Eintrag von Saharastaub, der fur Boden des Mount Cameroon in der Literatur bereits beschrieben wurde. Sowohl die Magnetit- und Gibbsitgehalte als auch der Staubeintrag sind daher bei der Beurteilung der Indices im Hinblick auf fortschreitende Bodenentwicklung zu berucksichtigen.
Crop straw is often incorporated into soil, but how agricultural practices affect straw decomposition and the involved biological mechanisms are not fully understood. We conducted a half-year straw decomposition assay in a 16-year experiment with tillage (conventional tillage or no-tillage) and fertilization (single or combined nitrogen, phosphorus and/or cow manure addition) regimes in a dryland agroecosystem. Straw decomposition and extracellular enzyme activities as well as bacterial, fungal, protistan and metazoan communities in the straw residues were analyzed. Straw mass loss was accelerated by tillage, whereas it was slowered by all but not single nitrogen fertilization treatments and correlated negatively with soil fertility. Enzyme activities were sensitive to tillage and fertilization but not correlated with straw decomposition. Straw mass loss was positively correlated with the relative abundances of a dominant saprotrophic fungus (related to Apodus deciduus) and an ecological cluster of the inter-kingdom co-occurrence network, whose abundances were increased by tillage and decreased with increasing soil fertility. This study shows strong effects of agricultural practices on straw decomposition, and highlights the importance of key decomposers and inter-kingdom interactions in regulating straw decomposition. We propose that straw incorporation should be applied with appropriate tillage and reduced fertilizers in dryland agroecosystems.
Deforestation and land-use change in tropics have increased over the past decades, driven by the demand for agricultural products. Although phosphorus (P) is one of the main limiting nutrients for agricultural productivity in the tropics, the effect of land-use change on P availability remains unclear. The objective was to assess the impacts of land-use change on soil inorganic and organic P fractions of different availability (Hedley sequential fractionation) and on P stocks in highly weathered tropical soils. We compared the P availability under extensive land-use (rubber agroforest) and intensive land-use with moderate fertilization (rubber monoculture plantations) or high fertilization (oil palm monoculture plantations) in Indonesia. The P stock was dominated by inorganic forms (60 to 85%) in all land-use types. Fertilizer application increased easily-available inorganic P (i.e., H2O-Pi, NaHCO3-Pi) in intensive rubber and oil palm plantations compared to rubber agroforest. However, the easily-available organic P (NaHCO3-extractable Po) was reduced by half under oil palm and rubber. The decrease of moderately available and non-available P in monoculture plantation means that fertilization maintains only the short-term soil fertility that is not sustainable in the long run due to the depletion of P reserves. The mechanisms of this P reserve depletion are: 1) soil erosion (here assessed by C/P ratio), 2) mineralization of soil organic matter (SOM) and 3) P export with yield products. Easily-available P fractions (i.e., H2O-Pi, NaHCO3-Pi and Po) and total organic P were strongly positively correlated with carbon content, suggesting that SOM plays a key role in maintaining P availability. Ecologically based management is therefore necessary to mitigate SOM losses and thus increase the sustainability of agricultural production in P-limited, highly weathered tropical soils.
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The soil volume affected by roots – the rhizosphere – is one of the most important microbial hotspots determining the processes, dynamics and cycling of carbon (C), nutrients and water in terrestrial ecosystems. Rhizosphere visualization is necessary to understand, localize and quantify the ongoing processes and functions, but quantitative conclusions are very uncertain because of: 1) the continuum of the parameters between the root surface and root-free soil, i.e., there are no sharp borders, 2) differences in the distributions of various parameters (C, nutrients, pH, enzyme and microbial activities, gases, water etc.) across and along roots, 3) temporal changes of the parameters and processes with root growth as well as with water and C flows. In situ techniques: planar optodes, zymography, sensitive gels, 14C and neutron imaging as well as destructive approaches (thin layer slicing) have been used to analyze the rhizosphere extent and the gradients of various physico-chemical and biological characteristics: pH, CO2, O2, redox potential, enzyme activities, content of water, nutrients and excess elements, and organic compounds. A literature analysis allows the conclusion that: i) the rhizosphere extent for most of the parameters assessed by non-destructive visualization techniques is 0.5–4 mm but exceeds 4 mm for gases, nitrate, water and redox potential. ii) The rhizosphere extent of nutrients (N, P) is decoupled from the extent of the corresponding enzyme activities. iii) The imbalance between element flows to and uptake by roots may lead to accumulation of excess elements and formation of root carapaces (e.g. CaCO3 rhizoliths, Fe plaque) ranging up to a few cm. iv) All destructive approaches show a much (3–5 times) larger rhizosphere extent compared to visualization techniques. These conclusions are crucial for a mechanistic understanding of rhizosphere properties and functioning, estimation of the nutrient stocks available to roots, and for rhizosphere modelling considering root growth and architecture. Overall, roots function as ecosystem engineers and build their environment, serving as the main factors shaping rhizosphere extent. Sharp gradients are formed within a few days for nutrients and enzymes, but more time is necessary for the establishment of specific microbial communities. Despite the very strong dynamics of most parameters, their stationarity is reached within a few days because the release of C and enzymes or nutrient uptake are very quickly compensated by utilization by surrounding microorganisms or/and sorption and diffusion processes. We conclude that despite the dynamic nature of each property, the rhizosphere gradients, their extent and shape are quasi-stationary because of the opposite directions of their formation processes.
Most studies showing potential organic nitrogen uptake were conducted with amino acids. They conclude that, in some ecosystems, amino acids significantly contribute to the N demand of plants and that roots have special transporters to re-uptake amino acids released into the rhizosphere. However, the relevance of the uptake of organic N compounds can only be evaluated by comparing the uptake of N-containing and N-free organic substances. We compared the uptake of alanine, glucose and acetate labelled with 14C by maize. Additionally, the N uptake was estimated by 15N labelled alanine and KNO3. We found a similar uptake of 14C from alanine, glucose and acetate, amounting for the whole plant less than 1% of 14C input. These results show that maize did not prefer N-containing to N-free organic substances. The uptake of 15N by maize exceeded that of 14C (10- to 50-fold), irrespective of the 15N source. However, plant uptake of nitrate (23.6–35.2% of 15N input) always exceeded the uptake of N from alanine (9.6–28.8%). The uptake of organically bound N by maize growing in soil occurred mainly by transpiration flow – as dissolved organics. The contribution of specific amino acid transporters was minor.
In 1994, the 14 hectare plateau of a coal refuse bank in Landsweiler-Reden (Southwest-Germany) was covered with a mixture consisting of 80% (v/v) refuse materials, 10% (v/v) composted wood and 10% (v/v) of sewage sludge as part of a reclamation project. The amount of sludge dry matter applied was approximately 450 Mg ha−1 to a depth of 2 m. The approximate amount of nitrogen (N) applied with the substrate was 20 Mg ha−1 (total N). From April 1996 until November 1997, contents of mineral nitrogen and nitrogen mineralization were monitored down to a depth of 2 m. Nitrogen mineralization was monitored by means of a modified buried bag procedure using a retrievable cylindrical receptacle. The contents of NH4+- and NO3−-N were largest at the beginning of the observation period, reaching a peak value of 650 kg ha−1 in May 1996. Then, mineral N stabilized in 1997 at a level of 200 kg N ha−1, with the soil profile below 150 cm contributing about 75% to this amount. Net nitrogen mineralization was characterized by the same depth distribution. Other than in surface horizons, mineralization activity at the bottom of the profile continued into 1997 with the same intensity as in 1996. Variability among replicate buried bag incubations was high (CV > 100% on several occasions). Nitrogen loss through leaching was estimated at 630 kg N ha−1 over the observation period, averaging at 360 kg N ha−1 a−1. The reclamation procedure used in this study may have the potential to contaminate ground water in hydrologically sensitive areas. Stickstoffumsatz in einem Rekultivierungssubstrat aus Bergematerial, Holzkompost und Klärschlamm 1994 wurde das 14 Hektar große Plateau der Bergehalde Landsweiler-Reden im Zuge einer Rekultivierungsmaßnahme mit einer Mischung aus 80 Vol.-% Bergematerial, 10 Vol.-% Holzkompost und 10 Vol.-% Klärschlamm bedeckt. Dabei wurden etwa 450 Mg ha−1 Klärschlamm bis in eine Tiefe von 2 m eingebaut. Die dabei ausgebrachte Gesamtstickstoffmenge betrug etwa 20 Mg ha−1. In diesem Substrat wurden Mineralstickstoffgehalte und die Stickstoffmineralisation während des Zeitraumes von April 1996 bis November 1997 bis in eine Tiefe von 2 m beobachtet. Dies geschah mit Hilfe der „buried bag”︁-Methode unter Verwendung einer herausnehmbaren zylindrischen Brutkapsel. Die Gehalte an NH4+ und NO3− waren zu Beginn der Beobachtungsphase am höchsten, der Spitzenwert betrug 650 kg N ha−1 im Mai 1996. Der Mineralstickstoffgehalt stabilisierte sich während des Jahres 1997 bei 200 kg N ha−1. Der Profilbereich tiefer als 150 cm hatte daran einen Anteil von 75%. Die Nettostickstoffminera lisation war durch einen gleichgerichteten Trend gekennzeichnet. Im Gegensatz zu den Oberflächenhorizonten war die Mineralisa tions aktivität in den tiefen Bereichen des Profils im Jahr 1997 ebenso hoch wie im Jahr 1996. Die Variabilität zwischen den Einzelinkubaten war hoch (Variationskoeffizient an mehreren Terminen > 100%, n = 20 pro Tiefe). Stickstoffverluste mit dem Sickerwasser wurden für die gesamte Beobachtungsphase mit 630�kg N ha−1 (entsprechend 360 kg ha−1 a−1) geschätzt. Das hier benutzte Rekultivierungsverfahren kann daher nicht zur Anwendung in hydrologisch sensitiven Bereichen empfohlen werden.
