Accelerated Soil Organic Matter Decomposition Research Articles

Warming reduces soil CO2 emissions but enhances soil N2O emissions: A long-term soil transplantation experiment

Climate warming can accelerate soil organic matter decomposition and stimulate soil CO2 and N2O emissions. However, long-term climate warming and land-use changes in relatively high-latitude regions on soil CO2 and N2O emissions remain largely unexplored, posing challenges to climate change research. Therefore, we conducted a long-term soil transplant experiment (8 years) across three relatively high-latitude northeastern regions in China to study the impacts of climate warming and land-use changes (from cropland to grassland) on soil CO2 and N2O emissions. As the temperature increased by 3 °C and 5 °C, the soil CO2 emissions from cropland were reduced by 59.07% and 56.87%, respectively, and those from grassland were reduced by 17.11% and 10.62%, respectively. The experiment duration, soil C storage, soil microbial abundance and soil moisture may be the main factors that explain why warming did not stimulate soil CO2 emissions. Soil N2O emissions increased by 76.57% in cropland and 263.81% in grassland as the temperature increased by 5 °C. Higher soil CO2 and N2O emissions were observed in grassland compared to cropland. Warming promoted aboveground plant biomass and indirectly promoted soil N2O emissions, particularly in grassland. The effects of long-term warming on soil CO2 and N2O emissions exhibited contrasting patterns, with CO2 emissions in relatively high-latitude and cold regions showing sensitivity to climate warming. When taking strategies to enhance soil C sequestration, consideration should be given to whether these strategies will be offset by stimulating soil N2O emissions, which is crucial for mitigating global warming. Overall, the impacts of long-term natural field warming and land-use changes on soil CO2 and N2O emissions and associated controls provide new insights for mitigating climate change.

Fast labile carbon and litter exhaustion under no-tillage after 5-year soil warming

No-tillage continuously contribute organic matter and increase carbon sequestration in the soil. However, global warming may stimulate microbial activity and accelerate soil organic matter (SOM) decomposition, also especially in the topsoil. Understanding the responses of microbial utilization of SOM accumulated under no-tillage to higher temperatures is necessary to develop management strategies to sustain soil fertility. Determine the response of microbial extracellular enzyme activities and SOM decomposition to long-term warming by incorporation of 14C-labelled maize litter to in situ warmed soil from no-tillage and till systems. We compared decomposability of litter and SOM in soils from a 5-year in situ experiment with two tillage systems (Till and No-till) and under two temperature levels: ambient temperature and continuous soil warming (+1.6 °C at 5 cm soil depth). We hypothesized that decomposition of crop litter (14C-labeled maize) at increasing temperature (15, 21, and 27 °C for 59 days) will be more intensive in warmed soil vs. ambient and the effects should be stronger under No-till vs. Till system. As expected, soil from field warming had always higher total CO2 efflux (from 5.5 to 12%) than the non-warmed counterpart in Till and No-till systems. Five-year warming increased temperature sensitivity (Q10) of CO2 efflux for 2-folds under No-tillage vs. Till. Litter decomposition (measured as 14CO2) in No-till with warming was 7.9% greater than in No-tillage without warming. Three extracellular enzymes (β-glucosidase, chitinase, and sulfatase) had higher activities under warming in No-till but not under Till system. Our results demonstrated that the 5-year in situ warming accelerated preferential microbial utilization of labile SOM fractions in No-till notably, and from stable SOM in Till system. Consequently, warming will accelerate SOM decomposition in future and this acceleration will be especially pronounced on the labile SOM under No-tillage systems.

Effects of nitrogen and phosphorus fertilization on soil organic matter priming and net carbon balance in alpine meadows

AbstractFertilizers‐induced priming effects of soil organic matter (SOM) decomposition influences net carbon balance and nutrient release. We hypothesize that very strong limitation of plant productivity and microbial activities by nitrogen (N) and phosphorus (P), common in Tibetan meadows, retard SOM decomposition and turnover. Consequently, N and/or P fertilization will induce priming effects of SOM and have implications for carbon balance. Soils from a nine‐year fertilization experiment (N alone, P alone, NP together, and control) from a Tibetan alpine meadow were used to investigate priming effect of SOM and carbon balance after addition of 13C labeled glucose. N and/or P fertilization acidified soil by 0.5 pH unit, decreased SOM content, and increased total and available N, total P. Regardless of fertilization, glucose addition accelerated SOM decomposition with priming effects of 30–60 μg C g−1 soil during 78 days. Alleviation of N and P limitation by N and NP fertilization lowered the priming effect by 17% and 14%, respectively, but P fertilization increased priming effect by 67%. The negative correlation of priming effect intensity with SOM, nitrate or total N, and microbial biomass contents indicated that fertilization‐induced differences in soil N and the microbial community are responsible for the priming effects. Positive correlation of carbon balance with total N and ammonium contents suggested that soil N accounts for carbon sequestration. Therefore, long‐term N and/or P fertilization accelerate SOM decomposition and reduce SOM storage in alpine meadows, of which P fertilization induces the highest priming effect and the lowest SOM storage.

Chronic Warming and Nitrogen-Addition Alter Soil Organic Matter Molecular Composition Distinctly in Tandem Compared to Individual Stressors

Forest soils are major reservoirs of carbon (C), but these stores are threatened by increasing global temperatures and atmospheric nitrogen (N) deposition. These environmental stressors can alter soil microbial communities and soil organic matter (SOM) biogeochemistry through a variety of mechanisms. To investigate the impact of chronic warming, N-addition, and simultaneous warming and N-addition (warming + N) on forest soils, soil samples from the Harvard Forest Soil Warming and Nitrogen Addition (SWaN) experiment were analyzed after 14 years. Elemental analysis, targeted compound analysis by gas chromatography–mass spectrometry, and 13C nuclear magnetic resonance (NMR) spectroscopy were used to analyze changes in SOM in both the organic and mineral (0–10 cm) soil layers. Overall, changes in the molecular composition and microbial biomass were observed, but the extent of differences was unique to warming, N-addition, and warming + N treatments. Specifically, N-addition slowed SOM decomposition as measured via solid-state 13C NMR, while warming and warming + N accelerated SOM decomposition. Continued SOM decomposition after 14 years with warming + N signified a pronounced change to observations made after 4 and 10 years of experimental treatment. This is also demonstrative of how a two-factor approach leads to a unique molecular-level response that cannot be predicted from experiments with individual stressors alone. This study emphasizes the need to observe environmental stressors in tandem using a combination of molecular-level approaches to obtain a comprehensive understanding of how persistent anthropogenic activity will fundamentally alter forest soil systems.

Frequent carbon input primes decomposition of decadal soil organic matter

Soil organic matter (SOM) decomposition in response to global change represents a critical uncertainty in coupled carbon (C) cycle-climate models. Much of this uncertainty arises from our limited mechanistic knowledge of the effects of organic C input frequency on SOM decomposition. Based on a three-source-partitioning isotopic approach (14C glucose addition to soil continuously labeled by C4 plants over 21 years) and literature review of 86 observations, we assessed the priming of fast- and decadal-SOM decomposition after occasional and frequent (every 12 vs 60 days) labile C input in a grassland soil. Frequent glucose input accelerated SOM decomposition over 200 days, but the occasional input reduced this positive priming by 2.6 times. The positive priming by occasional C input resulted in 139 μg C g−1 soil net C sequestration corresponding to 38% of C input. During the 200 days incubation, the primed fast-cycling C (younger than 21 years) under occasional C addition was 52–94% greater compared with frequent addition. Conversely, the priming of decadal SOM (older than 21 years) by frequent C input was 463 μg C g−1 after the 200 days incubation, which represents 5.3% of the initial decadal-cycling soil C pool. This was 1.5 times higher than by occasional input because of the stronger N mining caused by the continuously high activity of microbial K strategists under frequent C input. Our global scale assessment showed that occasional C input reduces the priming of decadal SOM decomposition by 89–127 Tg per year compared to frequent input and thus, sequestrated an additional 102 Tg C per year. Overall, the vulnerability of decadal SOM to increasing C input frequency thus weakening the C sink of grassland, whilst reduced C input frequency (e.g. by drought) suppresses the SOM priming and reduces the positive feedback of SOM decomposition to global change. This points out the necessity to assess the age of soil C when predicting the consequences of altered soil C input frequency under global change.

Rhizosphere priming effect of four non-woody perennials after leaf senescence and before shoot regrowth

Living roots and its rhizodeposition can stimulate or retard soil organic matter (SOM) decomposition via the rhizosphere priming effect (RPE). However, little is known about the RPE during non-growing seasons. In this study, we measured the RPEs of two perennial grasses (Elymus dahuricus, Stipa grandis) and two legumes (Medicago sativa, Melissilus ruthenicus) after leaf senescence and before shoot regrowth using a 13C natural tracer method. All four species produced positive RPEs at each sampling time, with the magnitude ranging from 40% to 238% compared to unplanted soil. Further, the inter-specific variation in the RPE during the non-growing season was partly explained by the root:shoot ratio, average root diameter and root tissue density. Overall, these results demonstrate that live roots of non-woody perennials could accelerate SOM decomposition even during the non-growing season.

Simulated climate change and seasonal drought increase carbon and phosphorus demand in Mediterranean forest soils

A better understanding of soil biogeochemical responses to the increasing drought predicted in many regions for the next decades by climate models is needed. Extracellular enzyme (EE) stoichiometry provides integrative information about the soil community metabolism and resource availability that is crucial to understand soil microbial and plant strategies in response to those projected changes in soil moisture and resource availability. The aim of this study was to investigate the responses of enzyme allocation to seasonal and experimental drought conditions. We measured soil extractable organic carbon (EOC), extractable total nitrogen (ETN), extractable total phosphorus (ETP) and its organic and inorganic fractions (Po and Pi), together with soil extracellular enzyme activity (EEA) across four seasons, and tested for association between variables using pairwise resource and enzyme ratios, and vector length and angle enzyme integrated indexes in a Mediterranean holm oak forest subjected to the drought projected for coming decades (15% less soil water availability). Seasonal and experimental drought consistently increased levels of EOC and ETN, while decreases in ETP and Pi in autumn amplified the imbalance between soil extractable P with C and N, increasing the probability of P limitation throughout the year. Overall EE stoichiometry and vector angle indicated a higher relative demand for P than C and N. EE stoichiometry and vector length showed that both seasonal and experimental drought increased the relative demand for C than N or P by soil microbial communities, as supported by the observed rise in EOC and ETN. Vector angle showed that the transient seasonal drought increased the relative demand for P over N by soil microorganisms, especially in control plots, possibly due to the lower mobility of P in soil than N. The results show the important role of phosphatases in the recycling of organic P in this ecosystem. Our results indicated that prolonged drought decreased average decomposition of soil organic matter (SOM), due to the negative effect on individual enzyme activities, but accelerated SOM decomposition response to rewetting due to the accumulation of C and N in soil and the relatively higher presence of C-enzymes than those of other EE. Lower levels of P and N enzymatic activities than those of C under drought infer a reduction in nutrient release that may then negatively affect plant and microbial nutrition.

Lower relative abundance of ectomycorrhizal fungi under a warmer and drier climate is linked to enhanced soil organic matter decomposition.

The aboveground impacts of climate change receive extensive research attention, but climate change could also alter belowground processes such as the delicate balance between free-living fungal decomposers and nutrient-scavenging mycorrhizal fungi that can inhibit decomposition through a mechanism called the Gadgil effect. We investigated how climate change-induced reductions in plant survival, photosynthesis and productivity alter soil fungal community composition in a mixed arbuscular/ectomycorrhizal (AM/EM) semiarid shrubland exposed to experimental warming (W) and/or rainfall reduction (RR). We hypothesised that increased EM host plant mortality under a warmer and drier climate might decrease ectomycorrhizal fungal (EMF) abundance, thereby favouring the proliferation and activity of fungal saprotrophs. The relative abundance of EMF sequences decreased by 57.5% under W+RR, which was accompanied by reductions in the activity of hydrolytic enzymes involved in the acquisition of organic-bound nutrients by EMF and their host plants. W+RR thereby created an enhanced potential for soil organic matter (SOM) breakdown and nitrogen mineralisation by decomposers, as revealed by 127-190% increases in dissolved organic carbon and nitrogen, respectively, and decreasing SOM content in soil. Climate aridification impacts on vegetation can cascade belowground through shifts in fungal guild structure that alter ecosystem biogeochemistry and accelerate SOM decomposition by reducing the Gadgil effect.

Revisiting the original reasons for excluding inorganic fertilizers in organic farming—Why the ban is not consistent with our current scientific understanding

This paper reviews the original reasons of the organic farming movement for excluding mineral (inorganic) fertilizers. In this paper, their theories and decision criteria for excluding use of inorganic fertilizers in crop production were revisited. Original reasons for banning inorganic fertilizers were subjected to scientific scrutiny, which was not possible when they were formulated 50–100 years ago due to limited knowledge of the soil-crop system. The original reasons were as follows: Rudolf Steiner, the founder of biodynamic farming, played down the physical role of plant nutrients and pointed out “flow of forces” as being most important for soils and crops. Eve Balfour and Albert Howard, founders of the Soil Association in England, claimed that inorganic fertilizer increases the breakdown of humus in soil, leading to a decline in soil fertility. Hans-Peter Rusch, the founder of biological organic farming, considered inorganic fertilizers to be imbalanced products not matching crop composition and not in synchrony with crop demand. When testing these historical statements as scientific hypotheses, older and modern scientific literature was used for validation. Steiner’s belief about the “flow of forces” has not be verified using current methodologies. The claim by Balfour and Howard that inorganic fertilizers accelerate soil organic matter decomposition is not substantiated by data from long-term field experiments on carbon and nitrogen cycling in soil-plant systems. The statement by Rusch that inorganic fertilizers supply crops inappropriately is difficult to uphold, as the composition, time, and rate of application and the placement of fertilizer in soil or on foliage can be fully adapted to crop requirements. In light of accumulated scientific evidence, the original arguments lack validity. The decision to ban inorganic fertilizers in organic farming is inconsistent with our current scientific understanding. Scientific stringency requires principles found to be erroneous to be abandoned.

Plant roots increase both decomposition and stable organic matter formation in boreal forest soil

Boreal forests are ecosystems with low nitrogen (N) availability that store globally significant amounts of carbon (C), mainly in plant biomass and soil organic matter (SOM). Although crucial for future climate change predictions, the mechanisms controlling boreal C and N pools are not well understood. Here, using a three-year field experiment, we compare SOM decomposition and stabilization in the presence of roots, with exclusion of roots but presence of fungal hyphae and with exclusion of both roots and fungal hyphae. Roots accelerate SOM decomposition compared to the root exclusion treatments, but also promote a different soil N economy with higher concentrations of organic soil N compared to inorganic soil N accompanied with the build-up of stable SOM-N. In contrast, root exclusion leads to an inorganic soil N economy (i.e., high level of inorganic N) with reduced stable SOM-N build-up. Based on our findings, we provide a framework on how plant roots affect SOM decomposition and stabilization.

Litter quality and site characteristics interact to affect the response of priming effect to temperature in subtropical forests

Abstract Forest litter inputs to soil can stimulate the decomposition of older soil organic matter (SOM) via a priming effect (PE). The magnitude and underlying mechanisms driving PE are poorly understood, with especially little know about how litter quality and site conditions affect PE in situ. Further, very few studies have examined PE in tropical and subtropical soils. Here, we established low and high elevation sites (600 vs. 1,400 m a.s.l.) in the subtropical Wuyishan National Park, China, that differed with respect to mean annual temperature (MAT; ∆MAT = 4.2°C), vegetation, soil texture and soil moisture. We conducted a 1‐year field incubation study at these two sites to compare PE induced by adding low‐ and high‐quality 13C‐labelled leaf litter to soils. At the low elevation site, additions of high‐quality (low C/N) litter caused a PE that was 140% greater than the PE observed following additions of low‐quality (high C/N) litter. In contrast, we saw no significant differences in PE between litter types at the high elevation site, perhaps because PE was not limited by substrate quality at this cooler, finer textured and higher soil moisture coniferous site. In addition, we found a negative relationship between home‐field advantage (HFA) for litter decomposition and PE, indicating that specialized litter decomposer community driving HFA may not accelerate SOM decomposition via PE in the same way. In line with our observed strong relationship between PE and the efficiency of priming (PE size per unit of mineralized litter C), PEs induced by the high‐ and low‐quality litters were directed to microbial phosphorus (P) mining rather than nitrogen (N) mining. This interpretation aligns with observed increases in the activity of P acquiring extracellular enzymes, often described as phosphatases (P‐tases), as well as the positive relationship between the PE, P‐tase activity and the activity of C acquiring extracellular enzymes. Overall, this PE study across two contrasting sites highlights the important role of site characteristics and litter quality in regulating PE size. Further, we suggest that MAT may be a dominant driver of soil priming, through both the direct effects of litter quantity on labile substrate supply and the indirect effects of litter quality changes on downstream decomposer communities. A free Plain Language Summary can be found within the Supporting Information of this article.

Molecular changes of soil organic matter induced by root exudates in a rice paddy under CO2 enrichment and warming of canopy air

While crop productivity and carbon flux/partitioning in agroecosystem have been widely addressed, potential changes in organic matter composition through root deposition are still unknown under climate change. In this study, root exudates of rice and rhizospheric topsoil were collected in a paddy field under 6 years of simulated climate changes respective of CO2 enrichment, canopy air warming, and their combination as compared to the control. The molecular composition of root exudates was analyzed with liquid chromatography/mass spectrometry (LC/MS) while soil organic matter (SOM) was analyzed with pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS), along with a phospholipid fatty acid analysis (PLFA) for characterizing soil microbial community. Compared to control, root exudation under elevated CO2 was enhanced but the exudate composition remained unchanged. On the other hand, root exudation rate under warming was unaffected, but the exudate composition was changed. Relative abundance of phenolic compounds in root exudates under warming, was decreased by 26% but that of amino acids was increased by 53%. However, neither root exudation nor the exudate composition was altered under combined CO2 and temperature elevation. Furthermore, increases in abundance of heterocyclic nitrogen compounds, phenolic acids and phenols (by 37%, 65%, and 26%, respectively) in SOM were relevant to the increase in bacterial/fungal (B/F) ratio under elevated CO2. This accumulation was possibly attributed to preferential utilization of the increased root exudates by bacteria. However, the overall reduction of some highly hydrophobic SOM compounds (fatty acids, phenols, and phenolic acids) by 14%, could be induced by accelerated decomposition in line with significant decrease in B/F ratio under warming. Indeed, the decreased relative abundance of phenolic compounds such as p-coumaric acid (by 49%) in root exudates lead to fungal increase, which accelerated SOM decomposition. Yet, the molecular composition of SOM was hardly changed under the combination of elevated CO2 and temperature. Overall, our findings suggested that in a rice paddy CO2 enrichment increased root exudation and B/F ratio, while air warming altered the root exudate composition and decreased B/F ratio, resulting in changes of SOM composition. These results indicate that root exudates are a key component for the regulation of SOM dynamics under climate change scenarios.

Roots of non-woody perennials accelerated long-term soil organic matter decomposition through biological and physical mechanisms

The rhizosphere priming effect (RPE) is increasingly recognized as an important factor in mediating soil organic matter (SOM) decomposition, which influences the CO2 release from terrestrial systems to the atmosphere. However, little is known about the long-term RPE of non-woody perennial species and the physical mechanisms underlying the RPE. Here the RPEs of three non-woody perennials (Stipa grandis, Leymus chinensis, Medicago sativa) differing in root traits and exudation were quantified using a natural 13C tracer method (C3 plant - C4 soil system) during a 476-day experiment. The results indicated that all plant species showed positive RPEs, with M. sativa showing the largest RPE and S. grandis the smallest RPE. Differences in the RPEs between species are likely associated with differences in root biomass, length, and exudates. Furthermore, the positive RPEs of the three species increased continuously with sampling time, indicating the long-term nature of the RPE. Results from structural equation modeling indicated that plant roots accelerated SOM decomposition via three mechanisms. The first mechanism (biological) was through the positive effect on dissolved organic C and microbial biomass C by the presence of the rhizosphere, consistent with the microbial activation hypothesis. The second mechanism (biological) was where plant roots significantly reduced soil mineral N and in turn promoted SOM decomposition, providing support for the microbial N-mining hypothesis. The third mechanism (physical) was where plant roots caused a net positive effect on SOM decomposition through net destruction of macroaggregates possibly as a result of high root length density and the associated drying-rewetting cycles. Overall, this study provides the first experimental evidence for the aggregate destruction hypothesis as a physical mechanism causing the RPE and highlights the importance of multiple mechanisms of the RPE on long-term SOM decomposition.

Dominant extracellular enzymes in priming of SOM decomposition depend on temperature

Decomposition of soil organic matter (SOM) is operated by microbial extracellular enzymes and is dependent on temperature. It remains unclear how input of labile organics interacts with temperature on SOM decomposition. Soil samples collected from a temperate forest were incubated for 3months to quantify SOM decomposition and its priming induced by input of 14C-labelled glucose at five temperatures (from 3 to 31°C with an interval of 7°C). The activities of eight extracellular enzymes were measured four times during the incubation. Increasing temperature accelerated SOM decomposition with a temperature sensitivity (Q10) value ranging from 1.3 to 4.4. Response of priming to temperature was mediated by the activity and variety of extracellular enzymes temporally: (1) priming increased with increasing temperature at the initial stage; (2) subsequently, priming at lower temperature gradually exceeded those at higher temperature; and (3) finally, priming at high temperature was higher than at low temperatures. At higher temperatures (17–31°C), N-acquiring enzymes (leucine amino peptidase and N-acetyl-β-d-glucosaminidase) dominated at the early stage of priming, but C-acquiring enzymes (β-glucosidase and xylanase) dominated at later stages. Priming exhibited a hysteresis at low temperatures (3–17°C) with C-acquiring enzyme (β-galactosidase) dominating at the initial stage, C-and P-acquiring enzymes (α-glucosidase and phosphomonoesterase) at the middle stage, and C-acquiring enzymes (β-galactosidase and xylanase) at the later stage. Consequently, microbial extracellular enzymes were produced in terms of nutrient availability over time depended on temperature. Such acclimation of microbial extracellular enzymes to temperature and nutrient availability should be considered to predict responses of C cycling to global change.

Geothermally warmed soils reveal persistent increases in the respiratory costs of soil microbes contributing to substantial C losses

Increasing temperatures can accelerate soil organic matter decomposition and release large amounts of CO2 to the atmosphere, potentially inducing positive warming feedbacks. Alterations to the temperature sensitivity and physiological functioning of soil microorganisms may play a key role in these carbon (C) losses. Geothermally active areas in Iceland provide stable and continuous soil temperature gradients to test this hypothesis, encompassing the full range of warming scenarios projected by the Intergovernmental Panel on Climate Change for the northern region. We took soils from these geothermal sites 7 years after the onset of warming and incubated them at varying temperatures and substrate availability conditions to detect persistent alterations of microbial physiology to long-term warming. Seven years of continuous warming ranging from 1.8 to 15.9 °C triggered a 8.6–58.0% decrease on the C concentrations in the topsoil (0–10 cm) of these sub-arctic silt-loam Andosols. The sensitivity of microbial respiration to temperature (Q10) was not altered. However, soil microbes showed a persistent increase in their microbial metabolic quotients (microbial respiration per unit of microbial biomass) and a subsequent diminished C retention in biomass. After an initial depletion of labile soil C upon soil warming, increasing energy costs of metabolic maintenance and resource acquisition led to a weaker capacity of C stabilization in the microbial biomass of warmer soils. This mechanism contributes to our understanding of the acclimated response of soil respiration to in situ soil warming at the ecosystem level, despite a lack of acclimation at the physiological level. Persistent increases in the respiratory costs of soil microbes in response to warming constitute a fundamental process that should be incorporated into climate change-C cycling models.

Reconsidering the phosphorus limitation of soil microbial activity in tropical forests

Abstract It has long been believed that soil microbial activity in tropical forest ecosystems is limited by phosphorus (P) rather than nitrogen (N) availability. In this study, we reviewed the methods used to determine the limiting nutrients and evaluated the validity of the widespread P‐limitation hypothesis in tropical forest soils. The most commonly used analysis method entails testing whether fertilization increased microbial biomass or soil respiration. Fertilization using microbial biomass as an indicator was not a satisfactory method because standing microbial biomass does not always signal microbial activity. An increase in soil respiration after fertilization was also an insufficient indicator because the negative response shown by organic matter decomposition to nutrient addition can also be driven by nutrient shortage (nutrient mining). Nutrient amendment can also shift microbial communities towards more copiotrophic organisms, which may exhibit lower microbial respiration rates. We suggest that P addition may accelerate soil organic matter decomposition compared with N, which is independent of the nutrient limitation of soil microbial activity. The negative response of organic matter decomposition to N addition through nutrient mining is driven by N shortage, which is less likely to occur with P. N addition inhibits microbial activity via chemical reactions, whereas P addition may stimulate activity through replacement by C bound to sorption sites in the soil, improving C availability. Thus, the P‐limitation hypothesis must be reconsidered because of the contrasting effects of exogenous N and P addition on soil microbial activity, which could lead to misdetection of P limitation on soil microbial activity in tropical forest ecosystems. We recommend caution in applying the statement that soil microbial activity in tropical forest ecosystems is limited by P until a novel method is established to accurately determine the nutrients limiting soil microbial activity at the ecosystem level. We proposed alternative ways to describe nutrient shortage for soil microbes. A plain language summary is available for this article.

Hydrolase kinetics to detect temperature-related changes in the rates of soil organic matter decomposition

To evaluate the importance of the role of temperature on the decomposition of soil organic matter (SOM), we investigated the SOM decomposition rates and the kinetics of two hydrolases involved in carbon (C) and nitrogen (N) cycling, namely β-1,4-glucosidase (βG) and β-1,4-N-acetylglucosaminidase (NAG), and their sensitivity to temperature in representative temperate forests. Soils were collected from three spatially-separate replicate plots at study sites distributed at heights of 1233, 1060, and 825 m a.s.l. along an elevation gradient on the southern slopes of Laotuding Mountain, Northeast China. Soils were incubated at temperatures between 4 and 40 °C at 6 °C intervals for 7 days in our laboratory. Decomposition rates of SOM responded positively to temperature, and, at the same temperature, were highest in the soil from 1233 m a.s.l. than in the soils from the other elevations. SOM decomposition rates were positively correlated with the maximum activity (Vmax) of βG and the contents of total and particulate organic C, but were negatively correlated with the soil silt and clay contents. The Vmax and the Michaelis constant (Km) of the two hydrolases were positively correlated and were also correlated with increases in temperature, suggesting that the Km values could offset increases in Vmax with increases in temperature. These correlations also highlight the enzymatic tradeoff between the maximum catalytic rate and the substrate binding affinity for the two hydrolases. The catalytic efficiencies of the two hydrolases were highest at 1060 m a.s.l., followed by 1233 m a.s.l., and were lowest at 825 m a.s.l. The catalytic efficiencies were positively correlated with the soil water contents and macroaggregate contents (>250 μm), but negatively with the soil C/N ratios. The temperature sensitivities (Q10) of the SOM decomposition rates were similar at the different elevations (P > 0.05), but generally increased as the temperature increased (P < 0.05). The Q10(Vmax)/Q10(Km) values of βG and NAG increased significantly as the temperature increased from 22 to 40 °C (P < 0.001), and were generally similar between elevations (P > 0.05). Our results suggest that, in spite of the negative effects of increased temperatures on enzyme substrate affinity, increases in hydrolytic activity will lead to accelerated SOM decomposition in temperate forests.

Do rhizosphere priming effects enhance plant nitrogen uptake under elevated CO2?

Numerous studies suggest that rhizosphere priming can mediate decomposition of soil organic matter (SOM), but direct evidence of priming-induced soil SOM decomposition on plant N uptake under elevated CO2 (eCO2) is very rare. By using a continuous dual labelling technique with 13C-depleted CO2 and 15N-enriched soil, we investigated priming of SOM decomposition and its relationship with plant N uptake of C4 and C3 grasses from a grassland ecosystem under eCO2. We observed that eCO2 induced increases in plant biomass, plant N uptake, rhizosphere priming, and total SOM decomposition in both grasses at an early plant life stage. Increased total SOM decomposition was positively related with plant N uptake by both C4 and C3 grasses under eCO2. However, the C3 grass was more dependent on N acquired from rhizosphere priming of SOM than the C4 grass. Our findings highlight that plant N uptake could be enhanced under eCO2 via accelerated SOM decomposition, and rhizosphere priming effects on SOM decomposition could play a more important role in N availability of the C3 grass in comparison with the C4 grass.

Decoupled stoichiometric, isotopic, and fungal responses of an ectomycorrhizal black spruce forest to nitrogen and phosphorus additions

Many northern forests are limited by nitrogen (N) availability, slight changes in which can have profound effects on ecosystem function and the activity of ectomycorrhizal (EcM) fungi. Increasing N and phosphorus (P) availability, an analog to accelerated soil organic matter decomposition in a warming climate, could decrease plant dependency on EcM fungi and increase plant productivity as a result of greater carbon use efficiency. However, the impact of altered N and P availability on the growth and activity of EcM fungi in boreal forests remains poorly understood despite recognition of their importance to host plant nutrition and soil carbon sequestration. To address such uncertainty we examined above and belowground ecosystem properties in a boreal black spruce forest following five years of factorial N and P additions. By combining detailed soil, fungal, and plant δ15N measurements with in situ metrics of fungal biomass, growth, and activity, we found both expected and unexpected patterns. Soil nitrate isotope values became 15N enriched in response to both N and P additions; fungal biomass was repressed by N yet both biomass and growth were stimulated by P; and, black spruce dependency on EcM derived N increased slightly when N and P were added alone yet significantly declined when added in combination. These findings contradict predictions that N fertilization would increase plant P demands and P fertilization would further exacerbate plant N demands. As a result, the prediction that EcM fungi predictably respond to plant N limitation was not supported. These findings highlight P as an under appreciated mediator of the activity of denitrifying bacteria, EcM fungi, and the dynamics of N cycles in boreal forests. Further, use of δ15N values from bulk soils, plants, and fungi to understand how EcM systems respond to changing nutrient availabilities will often require additional ecological information.

Soil carbon dynamics following land-use change varied with temperature and precipitation gradients: evidence from stable isotopes.

Knowledge of soil organic matter (SOM) dynamics following deforestation or reforestation is essential for evaluating carbon (C) budgets and cycle at regional or global scales. Worldwide land-use changes involving conversion of vegetation with different photosynthetic pathways (e.g. C3 and C4 ) offer a unique opportunity to quantify SOM decomposition rate and its response to climatic conditions using stable isotope techniques. We synthesized the results from 131 sites (including 87 deforestation observations and 44 reforestation observations) which were compiled from 36 published papers in the literatures as well as our observations in China's Qinling Mountains. Based on the 13 C natural abundance analysis, we evaluated the dynamics of new and old C in top soil (0-20cm) following land-use change and analyzed the relationships between soil organic C (SOC) decomposition rates and climatic factors. We found that SOC decomposition rates increased significantly with mean annual temperature and precipitation in the reforestation sites, and they were not related to any climatic factor in deforestation sites. The mean annual temperature explained 56% of variation in SOC decomposition rates by exponential model (y=0.0014e0.1395x ) in the reforestation sites. The proportion of new soil C increased following deforestation and reforestation, whereas the old soil C showed an opposite trend. The proportion of new soil C exceeded the proportion of old soil C after 45.4years' reforestation and 43.4years' deforestation, respectively. The rates of new soil C accumulation increased significantly with mean annual precipitation and temperature in the reforestation sites, yet only significantly increased with mean annual precipitation in the deforestation sites. Overall, our study provides evidence that SOC decomposition rates vary with temperature and precipitation, and thereby implies that global warming may accelerate SOM decomposition.