The Influence of Nutrient Availability on Soil Organic Matter Turnover Estimated by Incubations and Radiocarbon Modeling

We investigated the decomposability of soil organic matter (SOM) along a chronosequence of rainforest sites in Hawaii that form a natural fertility gradient and at two long-term fertilization experiments. To estimate turnover times and pool sizes of organic matter, we used two independent methods: (1) long-term incubations and (2) a three-box soil model constrained by radiocarbon measurements. Turnover times of slow-pool SOM (the intermediate pool between active and passive pools) calculated from incubations ranged from 6 to 20 y in the O horizon and were roughly half as fast in the A horizon. The radiocarbon-based model yielded a similar pattern but slower turnover times. The calculation of the ¹⁴C turnover times is sensitive to the lag time between photosynthesis and incorporation of organic C into SOM in a given horizon. By either method, turnover times at the different sites varied two- or threefold in soils with the same climate and vegetation community. Turnover times were fastest at the sites of highest soil fertility and were correlated with litter decay rates and primary productivity. However, experimental fertilization at the two least-fertile sites had only a small and inconsistent effect on turnover, with N slowing turnover and P slightly speeding it at one site. These results support studies of litter decomposition in suggesting that while plant productivity can respond rapidly to nutrient additions, decomposition may respond much more slowly to added nutrients.     

Temperature response of litter and soil organic matter decomposition is determined by chemical composition of organic material

The global soil carbon pool is approximately three times larger than the contemporary atmospheric pool, therefore even minor changes to its integrity may have major implications for atmospheric CO₂ concentrations. While theory predicts that the chemical composition of organic matter should constitute a master control on the temperature response of its decomposition, this relationship has not yet been fully demonstrated. We used laboratory incubations of forest soil organic matter (SOM) and fresh litter material together with NMR spectroscopy to make this connection between organic chemical composition and temperature sensitivity of decomposition. Temperature response of decomposition in both fresh litter and SOM was directly related to the chemical composition of the constituent organic matter, explaining 90% and 70% of the variance in Q₁₀ in litter and SOM, respectively. The Q₁₀ of litter decreased with increasing proportions of aromatic and O‐aromatic compounds, and increased with increased contents of alkyl‐ and O‐alkyl carbons. In contrast, in SOM, decomposition was affected only by carbonyl compounds. To reveal why a certain group of organic chemical compounds affected the temperature sensitivity of organic matter decomposition in litter and SOM, a more detailed characterization of the ¹³C aromatic region using Heteronuclear Single Quantum Coherence (HSQC) was conducted. The results revealed considerable differences in the aromatic region between litter and SOM. This suggests that the correlation between chemical composition of organic matter and the temperature response of decomposition differed between litter and SOM. The temperature response of soil decomposition processes can thus be described by the chemical composition of its constituent organic matter, this paves the way for improved ecosystem modeling of biosphere feedbacks under a changing climate.     

Soil C and N availability determine the priming effect: microbial N mining and stoichiometric decomposition theories

The increasing input of anthropogenically derived nitrogen (N) to ecosystems raises a crucial question: how does available N modify the decomposer community and thus affects the mineralization of soil organic matter (SOM). Moreover, N input modifies the priming effect (PE), that is, the effect of fresh organics on the microbial decomposition of SOM. We studied the interactive effects of C and N on SOM mineralization (by natural ¹³C labelling adding C₄‐sucrose or C₄‐maize straw to C₃‐soil) in relation to microbial growth kinetics and to the activities of five hydrolytic enzymes. This encompasses the groups of parameters governing two mechanisms of priming effects – microbial N mining and stoichiometric decomposition theories. In sole C treatments, positive PE was accompanied by a decrease in specific microbial growth rates, confirming a greater contribution of K‐strategists to the decomposition of native SOM. Sucrose addition with N significantly accelerated mineralization of native SOM, whereas mineral N added with plant residues accelerated decomposition of plant residues. This supports the microbial mining theory in terms of N limitation. Sucrose addition with N was accompanied by accelerated microbial growth, increased activities of β‐glucosidase and cellobiohydrolase, and decreased activities of xylanase and leucine amino peptidase. This indicated an increased contribution of r‐strategists to the PE and to decomposition of cellulose but the decreased hemicellulolytic and proteolytic activities. Thus, the acceleration of the C cycle was primed by exogenous organic C and was controlled by N. This confirms the stoichiometric decomposition theory. Both K‐ and r‐strategists were beneficial for priming effects, with an increasing contribution of K‐selected species under N limitation. Thus, the priming phenomenon described in ‘microbial N mining’ theory can be ascribed to K‐strategists. In contrast, ‘stoichiometric decomposition’ theory, that is, accelerated OM mineralization due to balanced microbial growth, is explained by domination of r‐strategists.     

An invasive wetland grass primes deep soil carbon pools

Understanding the processes that control deep soil carbon (C) dynamics and accumulation is of key importance, given the relevance of soil organic matter (SOM) as a vast C pool and climate change buffer. Methodological constraints of measuring SOM decomposition in the field prevent the addressing of real‐time rhizosphere effects that regulate nutrient cycling and SOM decomposition. An invasive lineage of Phragmites australis roots deeper than native vegetation (Schoenoplectus americanus and Spartina patens) in coastal marshes of North America and has potential to dramatically alter C cycling and accumulation in these ecosystems. To evaluate the effect of deep rooting on SOM decomposition we designed a mesocosm experiment that differentiates between plant‐derived, surface SOM‐derived (0–40 cm, active root zone of native marsh vegetation), and deep SOM‐derived mineralization (40–80 cm, below active root zone of native vegetation). We found invasive P. australis allocated the highest proportion of roots in deeper soils, differing significantly from the native vegetation in root : shoot ratio and belowground biomass allocation. About half of the CO₂ produced came from plant tissue mineralization in invasive and native communities; the rest of the CO₂ was produced from SOM mineralization (priming). Under P. australis, 35% of the CO₂ was produced from deep SOM priming and 9% from surface SOM. In the native community, 9% was produced from deep SOM priming and 44% from surface SOM. SOM priming in the native community was proportional to belowground biomass, while P. australis showed much higher priming with less belowground biomass. If P. australis deep rooting favors the decomposition of deep‐buried SOM accumulated under native vegetation, P. australis invasion into a wetland could fundamentally change SOM dynamics and lead to the loss of the C pool that was previously sequestered at depth under the native vegetation, thereby altering the function of a wetland as a long‐term C sink.     

Linking temperature sensitivity of soil organic matter decomposition to its molecular structure,accessibility, and microbial physiology

Temperature sensitivity of soil organic matter (SOM) decomposition may have a significant impact on global warming. Enzyme‐kinetic hypothesis suggests that decomposition of low‐quality substrate (recalcitrant molecular structure) requires higher activation energy and thus has greater temperature sensitivity than that of high‐quality, labile substrate. Supporting evidence, however, relies largely on indirect indices of substrate quality. Furthermore, the enzyme‐substrate reactions that drive decomposition may be regulated by microbial physiology and/or constrained by protective effects of soil mineral matrix. We thus tested the kinetic hypothesis by directly assessing the carbon molecular structure of low‐density fraction (LF) which represents readily accessible, mineral‐free SOM pool. Using five mineral soil samples of contrasting SOM concentrations, we conducted 30‐days incubations (15, 25, and 35 °C) to measure microbial respiration and quantified easily soluble C as well as microbial biomass C pools before and after the incubations. Carbon structure of LFs (<1.6 and 1.6–1.8 g cm^?3) and bulk soil was measured by solid‐state ¹³C‐NMR. Decomposition Q₁₀ was significantly correlated with the abundance of aromatic plus alkyl‐C relative to O‐alkyl‐C groups in LFs but not in bulk soil fraction or with the indirect C quality indices based on microbial respiration or biomass. The warming did not significantly change the concentration of biomass C or the three types of soluble C despite two‐ to three‐fold increase in respiration. Thus, enhanced microbial maintenance respiration (reduced C‐use efficiency) especially in the soils rich in recalcitrant LF might lead to the apparent equilibrium between SOM solubilization and microbial C uptake. Our results showed physical fractionation coupled with direct assessment of molecular structure as an effective approach and supported the enzyme‐kinetic interpretation of widely observed C quality‐temperature relationship for short‐term decomposition. Factors controlling long‐term decomposition Q₁₀ are more complex due to protective effect of mineral matrix and thus remain as a central question.     

Anthropogenic N deposition alters soil organic matter biochemistry and microbial communities on decaying fine roots

Fine root litter is a primary source of soil organic matter (SOM), which is a globally important pool of C that is responsive to climate change. We previously established that ~20 years of experimental nitrogen (N) deposition has slowed fine root decay and increased the storage of soil carbon (C; +18%) across a widespread northern hardwood forest ecosystem. However, the microbial mechanisms that have directly slowed fine root decay are unknown. Here, we show that experimental N deposition has decreased the relative abundance of Agaricales fungi (?31%) and increased that of partially ligninolytic Actinobacteria (+24%) on decaying fine roots. Moreover, experimental N deposition has increased the relative abundance of lignin‐derived compounds residing in SOM (+53%), and this biochemical response is significantly related to shifts in both fungal and bacterial community composition. Specifically, the accumulation of lignin‐derived compounds in SOM is negatively related to the relative abundance of ligninolytic Mycena and Kuehneromyces fungi, and positively related to Microbacteriaceae. Our findings suggest that by altering the composition of microbial communities on decaying fine roots such that their capacity for lignin degradation is reduced, experimental N deposition has slowed fine root litter decay, and increased the contribution of lignin‐derived compounds from fine roots to SOM. The microbial responses we observed may explain widespread findings that anthropogenic N deposition increases soil C storage in terrestrial ecosystems. More broadly, our findings directly link composition to function in soil microbial communities, and implicate compositional shifts in mediating biogeochemical processes of global significance.     

Experimental evidence for the attenuating effect of SOM protection on temperature sensitivity of SOM decomposition

The ability to predict C cycle responses to temperature changes depends on the accurate representation of temperature sensitivity (Q₁₀) of soil organic matter (SOM) decomposition in C models for different C pools and soil depths. Theoretically, Q₁₀ of SOM decomposition is determined by SOM quality and availability (referred to here as SOM protection). Here, we focus on the role of SOM protection in attenuating the intrinsic, SOM quality dependent Q₁₀. To assess the separate effects of SOM quality and protection, we incubated topsoil and subsoil samples characterized by differences in SOM protection under optimum moisture conditions at 25 °C and 35 °C. Although lower SOM quality in the subsoil should lead to a higher Q₁₀ according to kinetic theory, we observed a much lower overall temperature response in subsoil compared with the topsoil. Q₁₀ values determined for respired SOM fractions of decreasing lability within the topsoil increased from 1.9 for the most labile to 3.8 for the least labile respired SOM, whereas corresponding Q₁₀ values for the subsoil did not show this trend (Q₁₀ between 1.4 and 0.9). These results indicate the existence of a limiting factor that attenuates the intrinsic effect of SOM quality on Q₁₀ in the subsoil. A parallel incubation experiment of ¹³C‐labeled plant material added to top‐ and subsoil showed that decomposition of an unprotected C substrate of equal quality responds similarly to temperature changes in top‐ and subsoil. This further confirms that the attenuating effect on Q₁₀ in the subsoil originates from SOM protection rather than from microbial properties or other nutrient limitations. In conclusion, we found experimental evidence that SOM protection can attenuate the intrinsic Q₁₀ of SOM decomposition.     

Evidence of a strong coupling between root exudation, C and N availability, and stimulated SOM decomposition caused by rhizosphere priming effects

Increased temperatures and concomitant changes in vegetation patterns are expected to dramatically alter the functioning of northern ecosystems over the next few decades. Predicting the ecosystem response to such a shift in climate and vegetation is complicated by the lack of knowledge about the links between aboveground biota and belowground process rates. Current models suggest that increasing temperatures and rising concentrations of atmospheric CO(2) will be partly mitigated by elevated C sequestration in plant biomass and soil. However, empirical evidence does not always support this assumption, as elevated temperature and CO(2) concentrations also accelerate the belowground C flux, in many cases extending to increased decomposition of soil organic matter (SOM) and ultimately resulting in decreased soil C stocks. The mechanism behind the increase has remained largely unknown, but it has been suggested that priming might be the causative agent. Here, we provide quantitative evidence of a strong coupling between root exudation, SOM decomposition, and release of plant available N caused by rhizosphere priming effects. As plants tend to increase belowground C allocation with increased temperatures and CO(2) concentrations, priming effects need to be considered in our long-term analysis of soil C budgets in a changing environment. The extent of priming seems to be intimately linked to resource availability, as shifts in the stoichiometric nutrient demands of plants and microorganisms will lead to either cooperation (resulting in priming) or competition (no priming will occur). The findings lead us on the way to resolve the varying response of primary production, SOM decomposition, and release of plant available N to elevated temperatures, CO(2) concentrations, and N availability.     

Stabilization mechanisms of soil organic matter: Implications for C-saturation of soils

The relationship between soil structure and the ability of soil to stabilize soil organic matter (SOM) is a key element in soil C dynamics that has either been overlooked or treated in a cursory fashion when developing SOM models. The purpose of this paper is to review current knowledge of SOM dynamics within the framework of a newly proposed soil C saturation concept. Initially, we distinguish SOM that is protected against decomposition by various mechanisms from that which is not protected from decomposition. Methods of quantification and characteristics of three SOM pools defined as protected are discussed. Soil organic matter can be: (1) physically stabilized, or protected from decomposition, through microaggregation, or (2) intimate association with silt and clay particles, and (3) can be biochemically stabilized through the formation of recalcitrant SOM compounds. In addition to behavior of each SOM pool, we discuss implications of changes in land management on processes by which SOM compounds undergo protection and release. The characteristics and responses to changes in land use or land management are described for the light fraction (LF) and particulate organic matter (POM). We defined the LF and POM not occluded within microaggregates (53–250 m sized aggregates as unprotected. Our conclusions are illustrated in a new conceptual SOM model that differs from most SOM models in that the model state variables are measurable SOM pools. We suggest that physicochemical characteristics inherent to soils define the maximum protective capacity of these pools, which limits increases in SOM (i.e. C sequestration) with increased organic residue inputs.     

The Microbial Efficiency‐Matrix Stabilization (MEMS) framework integrates plant litter decomposition with soil organic matter stabilization: do labile plant inputs form stable soil organic matter?

The decomposition and transformation of above‐ and below‐ground plant detritus (litter) is the main process by which soil organic matter (SOM) is formed. Yet, research on litter decay and SOM formation has been largely uncoupled, failing to provide an effective nexus between these two fundamental processes for carbon (C) and nitrogen (N) cycling and storage. We present the current understanding of the importance of microbial substrate use efficiency and C and N allocation in controlling the proportion of plant‐derived C and N that is incorporated into SOM, and of soil matrix interactions in controlling SOM stabilization. We synthesize this understanding into the Microbial Efficiency‐Matrix Stabilization (MEMS) framework. This framework leads to the hypothesis that labile plant constituents are the dominant source of microbial products, relative to input rates, because they are utilized more efficiently by microbes. These microbial products of decomposition would thus become the main precursors of stable SOM by promoting aggregation and through strong chemical bonding to the mineral soil matrix.